CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

In this chapter, a brief review of the findings of earlier investigations on the important

properties / parameters of fly ashes; the available literature on the mechanisms of

lime-fly ash and cement-fly ash hydration processes; influence of fly ash addition on

the properties of concrete, namely, workability and compressive strength , have been

presented. A comprehensive review of the work of earlier investigators on blended

cements; studies on the activation of low-calcium and high-calcium fly ashes and

natural pozzolans, have also been presented. An attempt has also been made to

critically evaluate the status of activation studies on fly ash. Apart form the above,

literature relevant to the work carried out in this study, namely, on mix proportioning

methods; effect of elevated temperature; influence of various aggressive environments

on blended 1 fly ash concretes, have been briefly reviewed and presented.

2.2 FLY ASH - SOURCE, TERMINOLOGY AND CLASSIFICATION

2.2.1 Source

Fly ash is a by-product of the combustion of pulverized coal in thermal power plants.

B r e e different processes, namely, high-temperature combustion (1 500 - 1700"

C); dry combustion (1 100 - 1400" C ) and fluidized-bed combustion (<900° C) are

employed for the combustion of pulverized coal. Ashes obtained from fluidized-bed

combustion (FBC), are not genuine fly ashes, and are of little interest for building

material applications (Wesche, 1991). The dust-collection system (mechanical /

electrostatic precipitator - ESP) removes the fly ash , as a fine particulate system

from the combustion gases, before they are discharged into the atmosphere. The term

'fly ash' is not applied to the residue extracted from the bottom of boilers.

2.2.2 Terminology

The term 'fly ash' appears to be well accepted in English-speaking countries, except

in Britain, where the term 'pulverized fuel ash' (PFA) is used. 'Cendres volantes' (in

French); 'Flug ashe' ( in German) and 'Cenizas volantes' ( in Spanish) are the other

commonly used terms.

2.2.3 <'lassi tic:ltion

Until rcccntly. 'fly ash' \\as ~.c.gardcd as a 'po~/olan'. flo\icvcr., as tligh-calolun1 fly

ashcs which do not rcquire an cxtcrnal sourcc of 111ilc to P I . O ~ L I C C ' C C I ~ I C ~ I ~ ~ ~ ~ O L I S

properties' came to be niorc widcly used, than beforc, thcy are not strictly poi.zolans.

Manz and others (1982) have suggested that high-calciuni fly ashes (the so-called

Class C ashcs) are best distinguished from the low-calcium (Class F) ashes by their

cementing properties. Thus, a general term 'mineral admixtures' has been suggested

to describe all classes of slags, ashes, pozzolans and other cement supplements, with a

further distinct~on being drawn on the basis of their self-cementing capab~lities. The

above form of classification has been proposed as being preferable to the current

division of fly ashes into two classes i.e. according to the rank of coal from which they

originate, which is the current practice for Canadian (CAN3-A23.5-M86:

Supplementary cementing materials and their use in concrete construction) and LIS

(ASTM C618 - 92a: Standard specification for fly ash and raw or calcitied natural

pozzolan for use as a mineral admixture in Portland cement concrete) standards. The

term 'mineral admixture (MA) has been criticized in the past, as it gives a misleading

picture regarding the quantity actually used in concretes. Hence, in Canada, the term

'supplementary cementing materials' (SC,M) has been adopted in specifications,

which describe precisely the role of these materials in most concretes [Malhotra and

Ramezanianpour, (1994)l. Hawever, the terminology, 'high-calcium' and 'low-

calcium' have been used in this study, in general, and Class C and Class F, while

referring / reporting the type of fly ashes actual used by various researchers, in their

investigations.

2.2.4 Pozzolan

Fly ashes exhibit 'pozzolanic activity'. The American Society for Testing and

Materials (ASTM, 1975) defines a pozzolan as 'a siliceous or siliceous and aluminous

material which in itself possesses little or no cementitious value, but, which will, in a

divided form and in the presence of moisture, chemically react with calcium

hydroxide [Ca(OH)2] at ordinary temperature to form compounds possessing

cementitious properties'.

Ovcr the years, rcsearchcrs have tried to characterize fly ash bascd 011 its physical

nature, by dcfining properties, such as, specific gravity. fineness and grain size

distribution and have attempted to relate these properties and parameters to its

reactivity. All these propettics are generally influenced by the efficiency and type of

coal grinding processes adopted at the power plant.

2.3.1 Grain Size Distribution

Minnick and others(l971) , Kokubu (1968) Mehta (1985) 2Valenti and others 1,

(1986)$ Monzo and others (1994) aya (1995) Chopra and others (19793 Joshi P 3 3 (1982) and Ravina (1981) have investigated the role of particle size distributions and

the reactivity of fly ash. The effect of various grain size distributions on

the reactivity of fly ash or on the strength characteristics of fly ash - cement

concretes, have been investigated prominently on low-calcium fly ashes. It is, as such,

generally believed that the particles below 45/11m are responsible for the pozzolanic

effect. But the enhancement in compressive strength appears to be related more to

particle size below 10 to 20 . P 2.3.2 Fineness

Puri (1975) has stated that fineness is one of the principal parameters to be

considered for fly ash to be added to cement, as it influences the rate of development

of mechanical strength and relative values to be attained. However, there is an optimal

fineness above which the increase in strength becomes less significant, due to the

increase in the specific surface. '

Monzo and others (1995); Watt and Thorne (1966); Davis (1949) and Abdun

Nur (1961) have also stated that finer the fly ash the higher is the pozzolanic I activity. Thorne and Watt (1965) found that the specific surface area relates well

with the pozzolanic activity measured by chemical methods. Ravina (1980) showed

that a linear relation was found to exist between the specific surface of pulverized fuel

ash and its pozzolanic activity index.

Sharnla (1990) based on his study of l~ldia f ly ashcs, h21s rct~ortcd n !~ositive

correlation between lllnc reactivity strength and fineness. Ullopra (1979) grouped

Indian fly ashcs into two categories based on Blaine's fineness, f lowever, no such

classification was found to be possible, based on the residues of OOpm sieve.

From the revicw of literature, it is seen that researchers who tricd to relate fineness

alone with the strength 1 reactivity, could not comc to an agreement, as the

contribution of fineness of f ly ash being purely physical in nature. On the other hand,

most researchers agree about the influence of Blaine's fineness on reactivity and

strength development at early ages. The fact that it also gives a good indication on the

presence of finer particles makes it preferable for specifying characteristics of fly ash.

2.3.3 Specific Gravity

Studies by Weinheimer (1944) and Minnick (1959) indicate that specific gravity

varies significantly for particles of different shape, colour and chemical

composition. Compositional fluctuations, especially of iron and carbon contents seem

to cause difference in density (Helmuth, 1987). Investigations carried out by Carette

and Malhotra (1986) and Jarriage (1971) have revealed wide-ranging variations in

the specific gravity i.e. from a low- value of 1.90 for sub-bituminous ash to about

2.98 corresponding to iron-rich ash.

Minnick and others (1971) performed statistical analysis of data reported in five

major experimental studies and used the combined data to obtain relationship for

the specific: gravity in ter~ns of the percentage of Fe203 and sieve 110.325 as: Specific

gravity = 2.23 + 0.016X - 0.0061Y where, X - iron oxide (%); Y -- residue in 45

pm sieve.

Sharma (1990) based on his study of 25 Indian fly ashes, has reported that specific

gravity seems to have no direct influence on reactivity of fly ash, within the range of

specific gravity values of ashes (2.01-2.44) investigated by him .

In general, it appears that specific gravity seems to have no direct influence on the

reactivity of the ash, but, definitely helps in defining the ash quality in terms of the

prcsencc of carbon and iron co~ltents, which arc consitict.cd to bc delctcrious to

concrctc.

2.4 CHEMIC'AL COMPOSI'I'ION OF FLY ASH

2.4.1 Loss on Ignition

Malhotra ant1 Rarnezaniaripour (1994) have reported that the water required for

workability of mortar and concretes, depends on the carbon content of fly ashes; the

higher the carbon content, higher the water needed to produce a paste of normal

consistency and that higher carbon content (2- 10%) is quite common in low -calcium

fly ashes.

Braun and Gebaur (1983) have explained that the properties of fly ash are affected

by the quality of coal fired in the furnace and the combustion process. They found

that the un-burnt carbon in fly ash (determined by loss on ignition ), is one of the

important parameters of the assessment of the quality of fly ash. Minnick (1959,

1961) have reported that relatively high percentage of carbon decreases the pozzolanic

activity. On the other hand, Davis (1949) and Clendenning and Durie (1962) have

found that carbon does not have a deleterious effect on the strength of lime-fly ash

mixes.

Conflicting views on the effect of carbon on pozzolanic activity of fly ash continue to

exist. However, carbon which acts as a dilutant of the active pozzolanic matter in fly

ash is considered as an undesirable constituent.

2.4.2 Oxide Composition

Selvig and Gibson (1956); Abernethy and Peters011 (1969) have investigated the

occurrence of mineral matter in coal, which forms the common constituent of coal

ash and ten maior constituents were detemined from more than 600 ash samples 1 A-+L\

which formed the representative of coal produced throughout the United States. L. Wesche (1991) has reported tha t -~ ro l j e sub-divided the fly ashes into four groups

depending on the percentage of main compounds.

wat t and 'I'horne (1966); 'rhorne and Watt (1965) have observed that the anlourlt

of SiOz or (Si0z-f A l ~ 0 3 ) 111 a fly ash influences the pot~olanrc activity for long

of curing and that higher ultimate strengths are obtained when silica or silica

alumina content is high. However, Aitcin and others (1986) have stated that the

reactivity of a particular fly ash cannot be related only to its (SiO2+AI2O3+Fe2O3)

contents, as the reactivity is quite a complex phenomenon. Sulphates and lime have

been found to play an important role, particularly at early-ages of hydration.

Cabrera and others (1986) have clarified that only silica and alumina that is soluble

in an alkaline environment, can take part in the pozzolanic reaction. Richartz (1984)

has also emphasized the role of soluble silica and has explained that a pozzolanic

reaction can be expected only from soluble silica in alkaline environment. Sharma

(1990) has reported a positive correlation (coefficient of correlation = 0.77) between

soluble silica content and compressive strength obtained from the lime reactivity

test. Joshi and Rasurer (1973) have found correlation between activity and total

amount of soluble (SiO2+AI2 O3 ) in pozzolanic materials . They also observed that

iron had a deleterious effect on compressive strength. Uchikawa (1986) explained that

the pozzolanic activity of fly ash in enhanced with increasing basicity, content of acid

soluble Si02 or A1203, The reactivity was found to decrease with increasing Fe203

content in the glass phases.

Dhir and others (1988) defined a parameter which provided a means of assessment

of the pozzolanic potential index (PPI). According to them, PPI is an indicator of the

potential of an ash for a true pozzolanic chemical reaction leading to long-term

strength gain, while, PA1 (Pozzolanic activity index) relates to the early physical and

chemical surface effects of the ash which dominates in the early-ages of the hydrating

process.

According to Helmuth (1987) much of what has been learned about the effect of

Class F fly ashes in reaction with cement may have to be re-examined with Class C fly

ash, as the calcium content has a large effect on the reaction chemistry.

hlehta (1985) has reported that except for the calcium content, variations in other

chemical constituents of fly ash appeared to have a little effect on its reactivity.

Superior reactivity of high-calcium fly ashes compared to low-calcium ones was

probably due to both the presence of reactive crystalline compounds, such as, C3 A

and the more active calcium alumina silicate glass.

According to Davis and others (1949), lime and magnesia upto 5% in fly ashes are

not harmful.

Helmuth (1987) has stated that soluble alkalis in fly ash particles may accelerate the

reaction. Tenoutasse and Marion (1986) have observed that the durability of

blended cement concrete is influenced by its behaviour in alkaline solution. Mandal

and Sinha (1988) have explained that Na20, K20 and CaO in presence of water

during wet disposal system get converted to NaOH, KOH and Ca(OH)2 respectively,

and this process results in loss of reactivity of the remaining fly ashes.

2.5 MINERALOGICAL COMPOSITION

McCarthy (1987) and McCarthy and others (1988) have emphasized that

mineralogy of the ash controls the release rate of the potentially harmful trace

elements, for disposal. Luke (1961) determined the quantities of crystalline and other

phases present in fly ash using light microscopic techniques and XRD and has found

that the most abundant phase in each fly ash was glass and that mullite, hematite,

magnetite, quartz and calcite were the other phases observed in fly ash.

Watt and Thorne (1965) examined 14 fly ashes by microscopic and XRD techniques

and found that most of them after extraction with water, contained only four

crystalline phases in significant amounts: quartz, mullite, magnetite and hematite.

Diamond and Lopez-Flores (1981) and Mehta (1983) have pointed out the

differences in the composition of glass in low-calcium fly ashes and high-calcium fly

ashes.

Helmuth (1987) based on the results of many researchers commented that glass is by

far the most abundant constituent of fly ash. Chopra and others (1979) found that

~nriiarl fly ashcs are ~~rcdomiuantly rich in cli.ln:t/, mullitc, licmntitc and niagnctic with

differing orders of crvstallin~ty and the glass content 1s of the ordcr of 20-30% only.

Aoki (1985) colnpared thc results of chen~ical analysis and WKD of recent fly ashes

and that of the I-esults of fly ashes about 25 ycars ago and has observcd that crystalline

material in the older fly ashes were niainly composed of -quartz, whereas, the recent

ones, were composed of - quartz and mullite.

2.6 MORPHOLOGY OF FLYASH

Watt and Thorne (1965) examined 14 ashes microscopically and classified ashes

into eight different types, based on shape, colour, crystallinity and texture and

characteristic size range (pm).

Fischer (1978) made a detailed study of four size fractions of fly ash collected at a

Western U.S power plant and developed a classification system based on 1 1

morphological classes, in order to determine the relative abundance of the particle

types in the different size fractions.

Diamond (1982) has explained that most of the fly ashes recovered from electrostatic

precipitators do not contain poorly formed particles having irregular shapes, which

have undergone probably little or no fusion.

Huang and others (1990) in his study on the use of bottom ash in highway

embankments, sub-grades and sub-bases, has reported that the dry bottom ashes have

quite a number of angular particles and a lightly porous surface texture even in fine

ash particles. They were usually light gray to black in colour.

Based on the studies of Indian fly ashes by SEM, Rajendra Kumar (1989) has

observed that most of the particles appear to be solid and spheroidal, but, some are

cenospheric in nature. The surface of both the solid and cenospheric particles are

mostly smooth, but some particles show damaged surface.

Sharma (1990) has classified Indian fly ashes based on the shape of particles, as one

of the parameters,

m po~/olanic activity is niostly rclnted the reaction betwcen the 'rcactivc silica7 of the 4 poz/olan and calcium hydroxitic producing calcium silicate hydratc (C'SFI). 'I'he

alumina in thc pozzolari niay also react in the pozzolan-calcium hydroxide system or

in the pozzolan-cement system. The reaction products may include calcium

aluminate hydratc (CAH), ettringitc (C3A.3CS.t-132 ), gehlenitc (C2AStly) and calcium

mono-sulphate aluminate hydrate (C3A. CS. H I 2 ) . TIIC SLIM of the reactive silica and

alumina in a pozzolan, is the main indication of its pozzolanic activity (Malhotra and

Ramezanianpour, 1994).

Lea (1971) and Sersale (1980), have stated that the pozzolanic activity is indicative

of the lime-pozzolan reaction, but, it is not very well understood. 'Takernoto and

Uct~ikawa (1980) have proposed mechanisms to explain pozzolanic activity and

described it as a diffusion-controlled dissolution.

2.7.1 Lime -Pozzolanic Reactions

Studies on lime- pozzolan systems have indicated the hydrate to be CSH [(Sersale

1980) and Massazza (1976)j. Sersale and Orsini (1969) reported that the main

compounds formed both in suspensions and in lime- pozzolanic pastes are: CSH and

CZ AS H8.

Minnick observed that lime and MgO can react with iron oxide in the glassy phase

of tly ash. lIo~ve\rer, it has been shown that fly ashes that have reacted with lime

showed a progressive increase in the amounts of reacted silica and alumina, but, little

increase in the amount of reacted ferric oxide (Watt and Thorne, 1965),

Luxan and others (1989) in their study on fly ash calcium hydroxide reaction with

lime have reported the formation of calcium aluminium hydrate (C4AS HI3), carbo-

aluminate (C4ACHII) and mono sulpho-aluminate (C4ASHl2). The pressure of CSH,

hydrated calcium silicate was not confirmed by XRD as it was hidden by the main

peak of calcite, They have also reported the presence of quartz, mullite and hematite,

c\,cn after treatment with lirnc. where as, anhydr-itc, porllandite tuici tiec limc, Lverc

a\,bcllt uftcr- tlic trci~tmtnt.

'I'akemota and IJchikawa (1980) have suniniari;lcd the mechanism of tlic paste

hydration of po~rolans k~ith calciun~ hydroxide . As the alumilia diff~ises m ~ m r a p i d l y

than silica and as it generally requires a higher cation concentration for the formation

of hydrates than do calcium silicate hydrates, the CSH tend to form on the glass,

~ ~ l i i l e , aluniitiate hydratcs form at some distance.

The cement hydration and pozzolanic reactions in fly ash Portland cement- water

mixtures do not pr-occed independently. The rate of hydration of cement may be

accelerated or dccclerated by the presence of fly ash, depending on its characteristics.

Some researchers have reported that soluble alkalis on the fly ash particles may

accelerate the reactions, whereas, the soluble organic matter may be adsorbed,

interfering with surface reactions, nucleation and growth processes (Helmuth, 1987).

In the studies on the early hydration of C3S in presence of fly ash, several researchers

[Kokubu (1968) and Mehta (1985)l have reported retardation of the main C3S

hydration heat evaluation peak. Javed and Skanly (1981) found that two chemically

similar Class F fly ashes of different fineness had very similar retarding effects in

water, but, quite different effects in 0.5M NaOH solutions, which they suggested was

a result of (i) chemisorptions of some cat2 ion on fly ash particles which rcduced cat2

concentrations and delayed nucleation of Ca(OH)2 and (ii) the 'poisoning effect' of

soluble silicate and aluminate species on nucleation and crystal growth of Ca(OH)2

and CHS.

Takernoto and Uchikawa (1980) proposed schematic explanations of C3S-pozzolan

and C3A- pozzolan reactions. In C3S- pozzolan system, calcium ions dissolved from

C3S run about freely in liquid and are adsorbed on the surfaces of pozzolan particles.

CSH formed by the formation of C3S precipitates, as the hydrates of high CaISi ratio

on tlic surfi~cc of ' (';S grains and as the poro11~ hydrates of' low ('a;Si I-irtio on the

suld:ltce c)f po//oiati particles. Attack of' the po//olan surfilcc 111 walcr hrings about

gradtral dissoltrtiori of ' N a ' and K ' resulting i n SI anti A1 rich amorphous layer on the

surfaces. For p0/701ans nith low alkalis, destruction of amorphous Si, A1 rich filni

enables ~ a ' ' to move into the inside of the film and precipitate cnlciu~n silicate and

calciu~~i aluminatcs hydrates on the surface of poz~olnti grain.

The hydration of thc C'?A-r)oxzol~ln system was first observed by Uchikawa and

Uchida (1980), i n the presence of calcium hydroxide and gypsum . According to

them, the presence of po;lzolan accelerates the hydration of C7A by adsorbing c a t '

froni the liquid phase and by providing precipitation sites for ettringite and other

hydrates. The C3A-pozzolan reaction system is similar to the C3S-pozzolan reaction.

Ettringite, nionosulplio- aluminates hydrate, calcium-aluminate hydrate and calcium

silicate hydrate are formed on the surface film, outside the pozzolan particles or on the

surface of the hydrated layer of C3A particles, depending on the concentration of ~ a "

and ~ 0 4 " in solution. On thc other hand, investigations carried out by Cordon and

Thorpe (1975) and at EPRI(1993) have revealed that fly ash retards the hydration of

C3A and the degree of retardation depends on the sulphate content of fly ash, the

amount of dissolved alkalis and calcium adsorption capacity.

Diamond and others (1980) have showed the occurrence of the so called 'duplex

films' both on fly ash and cement particles in fly ash-cement system which rapidly

develop in hydrating cement systems around exposed surfaces, such as, sand grains

and coarse aggregates . As thc hydration procccds, thc 'duplex films' may

eventually become bounded by other hydration products to other particles. The

'duplex films' are formed after one day at room temperature.

Yuan and others (1984) studied fly ash- Portland cement hydration products using

XRD, DTA and SEM and have reported the formation of additional CSH. They have

also observed the conversion of ettringite to mono-sulphate at an earlier curing age

in the fly ash - blended pastes, than in control pastes. They have concluded that

I~o;//olanic reaction occurs in fly ash cot~tairiing pastes on a Inore or lcss particlc- by-

pt~~-ticIc I~asis. go\ urtled p ~ . r m a r ~ l y b} the particlc c i ~ c n ~ ~ s t r y .

flalse and others (1984) on thelr studies on cement-fly ash systems (WiS=0.45 to

0.47) have reported that C'a (OH)* content decreases after 14 days, as a result of

poj.zolanic reaction. 'The decrease in C'a(OH)2 in cement followed as a result of

dissolution of fly ash surface and the augmentation of strength by f ly ash.

Xu and others (1993) have reported that fly ash has both cnhanccmcnt and

retardation cffects on ccrnent hydration and also influences the pore system. They

have identified the forniation of additional hydration products , such as CSH, CAH

and calcium alumno sulphate hydrates with the progress of cement hydration. Thiir

rcsults show that the cement hydration is not distinctly accelerated in the presence of

fly ash.

Ogawa and others (1980) and Clifton and others (1978) have reported early

reactions of fly ash, whereas, some have reported no evidence of reaction of fly ash

for 28 days or longer.

Fraay and others (1 989) in their model for pozzolanic reaction of fly ash in concrete

have explained that the 'incubation period' before the start of the pozzolanic reaction

depends on the breaking down of the glassy phase, which in turn, depends on the

alkalinity of the pore water. The alkalinity of pore water in the cement paste will

increase strongly only after some days. Together with early precipitation of reaction

products of cement on fly ash particles, this will prevent the early breaking down, of

the glass network. Later, when the pH of pore water increases (due to continuing

cement hydration with decrease in free water and increase in ~ a + ,K+ and OH -

concentrations), the rate of degradation of fly ash particles will increase, which will

increase the precipitation of CSH like structures originating from the glass network

which has gone into solution and will take place much far away from the fly ash

particle. After some months of hardening , a fine pore structure with a smaller

amount of capillary pores for the fly ash paste, than, for the reference paste results.

\ < \ C I I though, the reactron cllcrni~tr\, b e t ~ c e ~ l iillcn, aiutiirria. calcium oxide and

stilpliiitc In the hydration of OI'C' system has been discussed In detail (Lea, 1071) and

subsequently highl~ghted by Almin and Sarkar (1991), more Insights into the reaction

;unong thc above, were investigated and advanced as a separate theory by

Bhanumathldas and Kalldas (1992). Sal~ent feat~~res of the theoretical concepts, as

proposed by them are givctl hclow.

Lime or cement has to be added to activate fly ash. 'The CaO content of the latter

reacts with the silica of fly ash and forms calcium silicate hydrates of tobertnorite (C-

S-H) group, which are some of the constituents that render strength and insolubility to

the set matrix. The alumina of fly ash has a negligible role to play, as it reacts in the

prcscnce of lime forming only gehlenite (C2ASH8), which is hydraulically an inert

nlineralogical constituent, and hence, normally gypsum has to be further added to

engage the gehlenite phase so as to convert it into ettringite, which is relatively a

strong and water impermeable constituent.

In the case of fly ash-gypsum-lime system, gypsum hydrates first, faster than its

normal rate, in the presence of alumina. While the hydration of gypsum is in progress,

alumina reacts with lime and gypsum to form ettringite, there' by, contributes to

gypsum hydration and hardening of the matrix, in the first 20-30 minutes. While the

hardening is initiated through crystalline transformation of gypsum from 'trigonal to

monoclinic', the same hardening is further substantiated with the ettringite formation.

Though the material seems to be set by physical appearance, the hardening process

keeps continuing, with the further formation of ettringite, utilizing whatever residual

moisture available in the system, To contain this 'ettringite' formation, it is

recommended to cast the elements with the least quantum of water and allow it to be

kept away from further moisturelwatering for one day. However, it may be added that

the acceleration in setting rate and the increase in compressive strength must

therefore, be attributed to the formation of 'ettringite' in the first few days of aging.

i~~oril the ~ ( : C ( ) I I C ~ hour of' i';lstitig, tho 'tohcrrli(>ritc' f i ) s~~~; l f jon ~ 0 1 1 i l ~ 1 ~ ~ 1 ~ t y i~nd

c c i t ~ i i i j ~ i ~ ~ f o r ciily~ togctl~cr. \\herei~s. thc ii~rnliition oi'ettr.ir~gitc ~.cacil!,~g ~ t s opt~rllun~

t i l t tirst i 1 1 1 ~ 1 sccond clay, starts reducrng tion1 t11t.11 oli\\~ards. ' 1%~ or-ystals of

csttringitc hrrn n strong skeleton arot~nd \shich the ('-S-! l gcl, that is Sornlcd slo\r,ly, is

clq-msitcd. So. \vhate\:er water is splashed for curing, i t is absorbed for continuance of

*tohcrniorite' formation, which can be rtttribi~ted as the main cause for the

r~c\~clopli~cnt of strength over aging. Mono-sulphates arc also fc~mmccl with rcgulutcd

availability of ~bater and, wherever ferrite phase is available, calciunl alumina krrites

itrc also for-nicd. Herice, the strength of fly ash which is ~veak in spite of lime

reactivity, is integrated in the presence of gypsuni, by which the formation of

ettririgite is induced with a regulated pace and quantum, accelerating the setting rate

and increasing the early strength, followed by tobermorite to render later-age strength.

Fly ash and linie reaction yield CSH formations, which are less cohesive and weak

when compared to the hydration of OPC. This is where the role of gypsum is crucial

and constructive. Ettringite is one hydrated form in the above process of mineralogical

phase, which may prove destructive if it exceeds the 'threshold quanta', due to its

crystal growth by about 2.5 times (upon hydration), causing dintensional instability to

the set product, leading to cracks in concrete or mortar. This 'hostile role' of ettringite

is availed in a fly ash - gypsum blend, in a 'friendly manner'. The ettringite which

forms in the initial days of hydration pushes forward the CSH crystals, which in turn,

overcomes the bond formation deficiency and attain more area of cohesive bonding, in

spite of slio~tcr crystal formation. This whoIe sequence of achieving high crystal

cohesion without any external hydrothermal treatment is termed as 'crystallo-mineral

combination of setting behavior'. Hence, a weak crystal formation can be made good

to attain a healthy cohesive bond, if, compensatory mineral fom~ation is initiated

through a conducive stoichiometry.

As, it is not possible to give a graphic and sequential formation of CSH and CAH

mineralogy, because of their overlapping and parallel formations through associations

and dissociations, hence, a simplified fonn of the reaction is indicated below:

2.3 ACTIVATION OF YOZZOLAN: NECESSITY, METHODS AND TYPES OF ACTIVATORS

2.9.1 Necessity

Potentially useful natural poz;.olans. derived from volcanic rocks and volcarlic ash, are

generally rcportcd to be 'less reactive' than fly ashes and slags. [Iencc. in order to

increase the use of natural pozzolans in concrete construction. one needs to

compensate for the detri~nental inlluences of increased water dennand and reduced

activity. by proper treat~ncnt i.e. 'by activation'. to enhance greatly the activity of

some types of pozzolana and hence, their perfornlance in cement and concrete.

It has been investigated and shown that fly ash reaction in concrete is initiated only

after one or Inore weeks and during this 'incubation period'. fly ash behaves, more or

less as an inert material, due to the pozzolanic reaction being slower than the reactions

which occur during cement hydration, and serves as a 'precipitation nucleus' for

Ca(OH)2 and CSM gel originating from ceinent hydration. It is also observed that the

subscqucnt pozzolanic reaction is also a slow process (Fraay an0 others, 1989). It

has also been reported that the strength of concrete containing Class F fly ash is

generally low at early-age and even at later-age in some cases, which is attributed to

the slow hydration and incomplete pozzolanic reaction. The hydration behaviour of

low-lime fly ash blended cement is also reported to be inert over temperature ranges

10 - 55" C and that the presence of the above fly ash may actually retard the hydration

of Portland cement (Ma and others, 1995). Replacement of cement, especially in

high-volumes, decreases the rate of early-strength development of concrete, which is a

L,lc,~r ct~\atltalitiigc 111 the i~sc of fly ash, fi,r cerllcrit replacen~tnt purpose (Shi and

1);l). IWS a ) .

I hc '~ncubation pcr~od' and 'retarding effect' wliich have a bearing on the early-age

strength gain devclopmcnt of a fly ash or fly ash-blended system, strongly indicate

the need for an additional component i.e. an 'activator', to accelerate the reaction /

l~yciration of f l y i ~ sh . 'I'hc possibility of fly ash ilctivation niainly lies i n breaking down

~ t s glass phases, 3s the strength rendering prodiicts from the pozi.olanic reaction will

be t'ornied, only. i f thc glass phases of fly ash particles goes into solut~on. The above

'breaking-off' of the glass network appears to be strongly dependant on the alkalinity

ofthe pore water and on the temperature (Fraay and others,l080). Hcncc, it appears,

that 'activation' is necessary for natural po~zolans and fly ash, so as to use them in

'high 1 very high-volumcs' in cement and concrete and to achieve a higher

'performance-to-cost ratio'.

2.9.2 Methods of Activation

Several methods, under the broad classification of mechanical, chemical and thermal

(either individually or in combination) have been attempted to enhance i.e, activate the

pozzolanic reactivity of natural pozzolans, slags and fly ashes and to increase the

strength of pozzolan-containing cement paste or concrete. Various methods of

activation are : ( i ) calcination of pozzolans; (ii) acid - treatment; (iii) prolonged

grinding; (iv) elevated temperature curing and (v) chemical activation. However, some

methods are too-expensive to be adopted and some may not show any signi.ficant

efficiency. Of thc abovc, the last three nlethods are uscd most often, for activating

pozzolans. Recent studies, have hrther shown that the use of chemical activators is

much more effective in terms of imparting high strength: econolnical (i.e. involves M OW? 7 7 I

less energy cost), r it involves simple production process, than, prolonged

grinding or elevated curing temperatures.

2.9.3 Types of Activators

Apart fiom lime and gypsum, alkalis of Li, Na, K, Ca, have been used as chemical

activators to enhance the pozzolanic reactivity of natural and other pozzolans. Of the

above alkali-activators. chlorirtcs. s~~lphntes. siliratcs, oxit Ics ;lnd hydmxidcac nf N:), K

and ('a have bten extensively used as chemical activators. (;encraily, commercially

available and reagcnt grade of the above chemicals, in smaller, quantities have been

widely used. 1,inie ( reagent gradelnatural form / industrial wastes) and gypsum

(available in various forms and industrial wastes like phosphogypsum, flourogypsum

etc.) have been used in 'binary' or 'ternary' blends to activate pozzolans. Lime and

gypsum content when used as 'activators' in a binary / ternary blend, will be

gcncrally, higher than, the content of pure alkali-based activators.

2.9.4 Selection of Chemical Activator

The type of chemical activator to be used depends on the type, chemical and

mineralogical composition of pozzolan; the type and temperature of curing regimes

proposed and the strength-level desired. As the use of chemical activators to enhance

the pozzolanic reactivity has been in focus only recently, comprehensive guidelines

are not available and hence, the results from earlier investigations and new or further

experimental results have to be relied upon, for selecting the type of chemical

activator, for a pozzolan.

2.9.5 Activation by Ca(OH)2

In a fly ash blended-cement system the latter (i.e. cement) acts as an activator. t Activation by Ca(OH)2 mainly lies in facilitating the pozzolanic reaction which

brakes the bonds and dissolution of the 3-D network structure of ash and hence, the

solubility of SiOz in the ash markedly increases. It has also been demonstrated that

treatment of fly ash with Ca(OH)2 at elevated temperature affects both the reactivity

and the kinetics of dissolution (Fraay and others.1989).

2.9.6 Activation by Gypsum

The principle underlying activation of gypsum is based on the ability of the sulfate

ions to react with the alumina, the latter being one of the principal components in fly

ash, which results in the dissociation of the glass structure. It has been suggested that

thc rcactlon between stllt~hate and fly ash also may lead to a dense hvdrate structure

2nd h~gher strength (Aimin and Sarkar, 1991).

2.10 FLY ASH RI,ENDEI) CEMENT

2.10.1 Idow-Calcium Fly ash Based Blended Cement

Hughes (1985) assessed the stability of variously cured OPC and OPC - fly ash pastes

(30(W, ash, by wt. and prepared with W/S = 0.47) upon immersion in 0.7 M Na2S04.

Expansion behaviour of notched beams, permeability and pore size distribution were

monitored. An sulphate resistant Portland cement (SRPC) paste was considered as a

reference for good sulphate resistance. The samples were cured for periods of 1,4, and

12 weeks. It was found that (i) the sulphate resistance of OPC pastes cured at 35OC in

Ca(OH)2 solution was relatively insensitive to the length of curing pnor to immersion

in 0.7 M Na2S04, where as, the resistance of OPC-fly ash pastes, similarly cured,

increased as the period of curing increased and (ii) the entry size of the sulphate

susceptible pores within the OPC-fly ash pastes decreased with curing, whilst those in

the OPC pastes remained essentially constant.

Mills (1986) evaluated the maturity efficiency factors of blended cements, namely,

Portland cement + low - calcium fly ash and Portland cement -t blast firnace slag, at

various ages ranging from 7 to 365 days. The results indicate a decline in efficiency

with increase in fly ash content and a strong dependence on fineness measured by the

(weight) fraction passing through 45 micron sieve. It was also observed that the

permeability of mixes containing fly ash was also substantially lower than those

containing blast furnace slag.

Hootan (1986) investigated potential improvements to the permeability, pore

structure and chloride / sulphate durability of a sulphate-resistant Portland cement

offered by potential replacements with low-calcium fly ash, slag and silica fime

available in Canada. He also tried to investigate the possibility of predicting

permeability fiom mercury intrusion pore size distributions and predicting chloride i

sulphate durability from variations in permeability, pore size distributions and calcium

hydroxide. Based on his extensive experimental investigations he concluded that even

tho11)rh nll thc three supplcnientary cerrientlng rn:ttcriills rcduced the calc~um

h!iirou~(fe ccmcnt ot tilt pastcs. 2090 by volume of s111ca funie was effective,

cl~rnlnatlng calc~urn tiydrou~de completely afier 0 l days of moist curing. Moreover,

from the prcltm~nary an~tlys~s there does not appear to be an accurate way of

predicting pcrrnenbil~ty from porosity or pore size parameters by mercury intrusion for

all the three pastes, considered in the study.

Tcnoutasse and Marion (1986) investigated the selective dissolution of different

Belgian low-calcium fly ashes with water, hydrochloric acid (I1CI) solutions by

chemical and microscop~cal techniques. 'The behaviour of fly ashes was also studied in

lime-saturated solution. The hydrat~on mechanism was investigated as a function of

time, for OPC and OPC' containing 10% to 80% of fly ashes. Their study revealed that

( i ) the pozzolanic activity of fly ash is confirmed by the study of the behaviour fly ash

in lime saturated solution and the determination of free lime in OPC -t fly ash paste

suggests a reaction between Ca(OH)2 and fly ash particles; (ii) the addition of fly ash

to OPC always increases the total porosity, especially for the early hydration period

and for cement blended with fly ash (upto 25%), the total porosity and pore size

distribution (after 3 months) hydration are similar to OPC pastes and (iii) the

solubility of fly ash in different conditions demonstrate the presence of the totality of

the sulphates in the external layers of the fly ash particles, whereas, NazO and K20

are available in a different location.

Papadakis (1999) conducted extended experimental investigations on the activity of

low-calcium fly ash ( FL) in Portland ccmcnt pastcs and mortars. The effect of FL

addition on the main mechanical and durability properties of Portland cement systems

as a hnction of the replacement type ( cement or aggregates), has been clarified.

Development of strengths (i.e. compressive and flexural); heat development; porosity,

chemically-bound water content (referred to as H-content); Ca (OH)z content

(referred as CH - content), as well as microscopy observations on cement and FL - cement pastes and mortars were studied , 'with the assumption that comparable

conclusions derived from pastes and mortars may be also applicable to concrete'. FL

produced in Denmd, a rapid - setting Portland cement, a normal graded sand and a

cnninioti plil~tici/t.r ncrc 1i5cci fix thc nrep;iratlon of' mortar. W/(' cclual to 0.5 and

A,( t . c l t~ i l l to . % . ( I , \\err tisrri fbr the 'control spcc~mcn'. IO'h,, 20'h) ant1 30 'h of FL

wcrc used to rcplncc cenrent anti fine aggregate. Hasod on the abovc 11e concluded the

follow~np: ( I ) h~ghcr strengths wcrc observed when aggregates or cement are replaced

by low-cnlcium fly ash (upto a certain levcl Y,,,,,); ( i i ) 'The strength enhanccment was

observed to be significantly higher (aftcr 2 3 weeks after mixing), in the former

casc. %here as, i n the latter case, thc enhanccment is much lower even aftcr 3 months.

Thc final strength-gain was roughly proportional to the content of active silica in the

concrete volume; ( i i i ) the thcorctical model developed, based on the reaction scheme

proposed can be applied in mix design and for thc performance prediction of fly ash /

Portland cement systems.

Bouzoubaa and others (2001) studied the mechanical properties and durability of

concrete made with a high-volume low-calcium fly ash (45 %) blended cement and a

clinker used for the production of ASTM Type I cement (55%). The results were

compared with those of the high-volume fly ash (HVFA) concrete, In which un-

ground fly ash had been added at the concrete mixer and control Portland cement

concrete. Properties of the above concrete in fresh and hardened states (W/B=0.32)

were compared. It has been concluded that: (i) the HVFA blended cement can be

proportioned to have the same or higher mechanical properties and durability

characteristics to that of concrete (WIC = 0.45) using commercially available ASTM

Type I cement and (ii) the mechanicaI properties of concrete with HVFA blended

cement are superior and (iii) the durability characteristics are comparable to the

control concretes. A similar study made earlier by them [Bouzoubaa and others (1998)

] using HVFA blended cement produced using fly ash with high CaO content (about

13%) and a clinker generally used for the production of ASTM Type 111 cement, have

shown that the mechanical properties and durability characteristics of the above

concrete are comparable or superior to those using ASTM Type 111 (commercial )

cement.

Nisnevich and others (2001) used a highly porous bottom ash as aggregate and a

high-volume of low-calcium fly ash for producing light weight concrete. The studies

HCTC ~ t ~ ~ t ~ ' ~ . l l t r i t t ~ ~ t <)I \ Ioucr!ng the v o l t l i ~ l ~ 01'illr V O I ~ S in thc cement flv ash paste of'

the ticsh concrete itncl thcrehy Iticrcilslng the strcnpth unit dens~ty 01' hardencd

concrctc, 1 hcy have also suggcstcd :I method fhr optimization of the light weight

concrctc nlixture proportions for two given parameters : tiensity and compressive

strength.

Horsoi and others (2001) studied the compressive strength and d~lrability among

other things, of concretes containing Portlanci cement ( 5 0 % ) low-calciun~ fly ash

( 2 5 % ) and ground slag (25'/;1), (all inter-ground to a Blaine's fineness of about 400 - 0 0 rn2 / kg). 7'he influence of a 40% aqueous solution of sulfonated naphthalene - based (SN) super-plasticizer (SP) and a 30% acrylic polymer (AP) aqueous solution

on the above concrete mixtures were studied. It has been concluded that ( i ) the

durability bchaviour does not seem to be significantly improved by increasing the

fineness of the cementitious materials; (ii) SP based on acrylic polymer appears to be

more effective in terms of compressive strength and better durability behaviour

(among other things), in terms of lower C02 penetration and chloride penetration.

Naik and others (2001) conducted investigations on concrete masonry products (i.e.

bricks, blocks and paving stones) incorporating high-volumes of a low-calcium Class

F fly ash obtained from a source in Illinois, USA. The fly ash content was varied

from 20 to 50 % for bricks and block mixtures and 15 to 30 % for paving stone

mixtures. All masonry products were tested for the strength and durability

characteristics. The test results indicated that bricks and blocks with upto 30% fly ash

are suitable for use in both cold and warm climates and the mixtures containing upto

50% fly ash were appropriate for building interior wails in cold regions and for both

interior and exterior walls in warm regions. However, all the paving stone mixtures

with and without fly ash are found to be suitable for normal construction applications.

Horiguchi and others (2001) evaluated the potential use of two different sources of

fly ash (FA) including off-specification fly ash ( both low-calcium ashes) and three

different sources of clinker ash (CA) ( i.e. bottom ash or coal ash) with three levels of

mixture combinations (70 150; 50150 ;30/70 - FA / CA), in CLSM ( controlled low-

stren)rtt~ ~i~ , i t c r~a l \ ) I tic eftcct ot short-term and long-tct-111 (I.c. upto 01 days)

ct~nl\lrc\sl\c' ~trcrigtl i~ ncrc ~ncestigatcd. l tle rcsults rnti~cittctl that ( I ) thcrc IS no

sign~ficant cl~s,~tl~;i~itage of uslrig thcsc ~natcr~itls In ttic physical gropertics iof ( 'LSM

( 1 1 ) 50"rl ot'cl~nkcr suhst~tut~on I S thunti to he an optimum compos~tion for C'LSM

mlutures; ( 1 1 1 ) the strength of' ('1,SM ivlth fly ash and cl~nker ash increases

s~~t i l f icant l j upto 0 1 days.

gidav and others (2001) studied the potential use of Malaysian fly ash-cement (i.e.

I\;['(' normal f'ortland ccnient with 15%) and 30'%,, by weight of low-calcium fly ash)

in concrete, In terms of its influence on elastic and durability properties (resistance to

chloride Ingress, sulfate attack and suppression of alkali-aggregate reactivity). The

ahove fly ash concretes were found to have similar workability, elastic and strength

properties as NPC' concrete. 'I'he fly ash concretes also showed better resistance to

chloride-ion penetration and sulfate attack. The 30 % fly ash blended cement has also

been found to significantly suppress the potential alkali-aggregate reactivity.

Bhanumathidas and Mehta (2001) made a preliminary investigation to compare the

rate of strength development and chloride permeability characteristics of high - volume low-calcium fly ash concrete mixtures, containing 50% high-early strength

Portland cement and with (10% ) and without rice-husk ash (RHA), A uniform water

to cementitious materials (CM) of 0.45 was maintained for all mixtures and

compressive strength at 3 to 9 1 days were determined, after water curing. It was found

that RHA can play a significant role in improving the properties of high-volume fly

ash concrete mixtures and that incorporation of 10% R H A (by wt.) enhances the early

strength and drastically improves the impermeability.

Sheshasayi and others (2001) investigated the effect of cement replacements (OPC - 53grade) with a low-calcium fly ash (obtained from a power plant located in Andhra

Pradesh, India) and silica fume (obtained from a ferro-alloy industry located in

Andhra Pradesh, India), in the production of high-strength concrete. The combined

levels of replacements were upto 56% (restricting the silica fume to a maximum of

16%). Compressive strengths at 28 and 56 days were evaluated and the results

t n c t ~ ~ , , ~ t c * c l 1l1,1t 1 I I ~ I I L . c r l ~ t l l i i t i t l l lei t I ot' rvpliicernent ot' Or'( ' by tllc fly ash rs about

3(iU,, tcvl i~roclt~c~iig l ~ ~ g h - \ t ~ ~ h ~ i g t l ~ concrefc: ( I I ~ ) the opf in iu~~i lcvcl 01 ' r~pI ; i~c rn~n t by

~ ~ I I C > , I ~ U I I ~ C I \ d f ~ j l l t 1 ? ' ' 1 1 . ( 1 1 1 ) t\.~fll t t l ~ iihoic TilllgC' of' rcplactn~cnt Icvcls, concrete

~ t r ~ ~ i g t h h c ) t ' 80 \ l f ' i~ c;111 he ~~rodilcecl tvitll Lli/('M 0.32 and appropr~ate dosages of

conlpi~rablc supcr-plasticr/crs.

Bhanumathidas and Kalitias (2001) investigatcci the role of gypsum or anhydrite

(('aSt ) 4 ) in the strcngth development charactcr~stics of blast filrn~icc slag-blended

ccmcnt. Normal f'ortlantl ctmcnt (NP('- 43 grade), very Ion-culcium fly ash, GGBFS,

lirlic (X,S(!,o purity as ('a(0kl)2, from an acetylene plant), anhydrite ( a by-product from

alumlnium fluoride industry with 06 (54 ptpurity, ground to Blaine's 380 m2 /kg), were

thc materials used fix preparing 50 mm mortar cubes (1:3), with about W/CM = 0.4.

From thu compressive strength rcsults, they have drawn the following intcrprctations:

( i) i t is possible to enhance the strength of high-volume fly ash -Portland cement

mixtures to the level of reference Portland cement at 28 days, when the reactive phase

in fly ash is activated by additional sulfate; (iii) without additional sulfate the

strengths were lower at early-ages. However, the strength is very nearly equal to

control mortar at 60 days; (iv) the rate of strength-gain of fly ash-Portland cement

mortars was slow, upto 14 days.

2.10.2 High-Calcium Fly ash Based Blended Cement

Hootan (1986) investigated the effects of the high - calcium fly ash produced from

Western Canadian lignite coal, on concrete. The above ash has a CaO content of 10 -

14% and high alkali content (expressed as Na2 SO4 ) of 6 to 7.5 %. The high -calcium

fly ash was evaluated at 20 % and 35 % nominal weight replacements of Portland

cement in M25 control mix, containing air-entraining admixture. Two more concrete

mixtures were made using 20% of fly ash from a mixture of lignite and bituminous

coals and 20% of a commercially available pelletized blast-furnace slag. Workability

(slump), air-entrainment, compressive strength (on 150 x 300 mm cylinders at 1 day

to 91 days), tensile strength, fieezing and thawing resistance, permeability and pore

size distributions and the effects on alkali-aggregate reaction were studied on the five

concrete mixes. Based on the results it has been concluded that (i) by using simple

27

weight rcpli~ccment upto 35% of Portland cement by the high-calcium ash and

adj~lsting it' Ch1 for equal slump, it is possible to obtain strength equivalence at 7

days and higher conipressive and splitting - tensile strength, than control mix; ( i i )

aciequatr air contents and air void spacings could be obtained with only minor doses

of air entrailling agent (AEA) , due to low LO1 of ash and hence, resistance to freezing

and thawring cycles as per ASTM procedure was found to be excellent; ( i i i ) the high-

alkali Ie\.rls in the fly ash should not aggravate a potentially deleterious reaction and

( i v ) the high-calcium fly ash, if used, in sufficient quantity, can be used with a high

C3A cenient to perform equivalently to a moderate sulfate resisting Portland cemenr.

Helmuth and others (1986) studied the performance of blended cements made with

tivo controllcd particle size distributions (CPSD)! i.e. 95% finer than 3 1 or 23 m an d

one normally ground cement having Blaine's fineness of 35 m Z / kg. Pastes made with

these cements blended with Class F or Cass C fly ashes or ground granulated slag,

were tested for flow, strength development and drying shrinkage. Three CPSD

blended cements and two nornally ground blended cements as controls were used for

making mortar and concrete for studying the compressive and flexural strengths and

sulphate resistance characteristics. Cement paste, mortar and concretes made with

CPSD blended cements have shown properties approximately equal or superior to

those made with normally ground blended cements of the same compositions. The

major benetit reported by the use of CPSD blended cements is the production of

concretes with early-age properties, comparable, to those obtained by OPC.

Tishmack and others (1999) selected three high-calcium (Class C) fly ashes ( CaO

content ranping from 23% to 28%) derived from Powder River Basin (PRB) coal in

USA, with the objective to determine the effects of fly ash and cement compositional

variations on the relative amounts of 'ettringite' and mono-sulphate formed, as

determined by differential scanning calorimetry (DSC) and XRD. Cement 1 fly ash

pastes were prepared from the above fly ashes with a fly ash content of 25% (by wt).

of solids and a water to cement / fly ash ratio of 0.45). Five different Type I or Type I

/ I11 Portland cements covering range of C, A, sulphate and alkali contents, were used

for preparing the above cement-fly ash pastes. The pastes after mixing were sealed

atlcl pl;icetj In a cllrrtlp roonl ( K i J=IOO"r~: 23 ( ' 1 h r 28 davs arvl n w c tlletl analvied to

determ~nc thc h~cirnt~oti protiucts. I3ascd o n SRL) , l~~a lys~s , the hydr:itlon products

identitied ncre pnrtland~tc, ettringite (an Abt phase), nlono-sulpl.iate and sn~aller

amounts of hen~l-carho-altuninate and rnc~no-carbo-nluln~nate (all AFtn phases).

F;inally they coticludcd the follo~vitlg: ( i ) thc :tniount of ettringitc formed varied with

the individual cement. Only a modest correlation existed w ~ t h correct sulphate contcnt

and that no corrclation with ccrnent C'? A contcnt was observed; ( i i ) DSC analysis

showed that thc ccn'icnt I fly ash pastes generally formed less ettr~ngitc than the

cemcnt control pastes. but, they fonncd niorc of the AFm phases (mainly mono-

sulphate). I t appears that the addition of high-calcium fly ash reduces the SO4 / A12 O3

ratlo in the systeni, thus favouring AFni form?t' ]on.

Hiibert and others (2001) compared the hydration reactions of bindcr systcrns

containing two different fly ashes from brown coal combustion (i.e, high-calcium -

13% arid low-calci~un - 1.3% ashes) and Portland cement in mixture proportions of

60:40; 70:30; 85: 15 - fly ash : cement. The mixtures were hydrated with a water solid

ratio of 0.5 at room temperature. The solid hydration products were investigated ( at

ages ranging from 3 days to 300 days) by si2" MAS NMR spectroscopy, rnolybdate

method, DTA / DSC. It was observed that ( i ) ettringite is a stable hydration product in

all systems investigated and that the amount of additionally formed C-S-H from the

fly ash depends on the reactivity of the ash; (ii) there exists an optimum in the mixture

proportion with cement for every fly ash and (iii) at 300 days of hydration, the degree

of condensation of the silicate anions was found to be independent of fly ash content

and type.

Mathews and others (2001) investigated the effects of cement and sand replacement

of two classes of fly ash (high-calcium and low-calcium) on the strength and

durability characteristics of concrete (three grades namely, M15, M20, M25) made

using two grades of cement (i.e. grade 43 and 53). Effects of cement and sand

replacement (i.e. lo%, 15% and 20%) by the above two types of fly ashes were

compared with the results from control specimens. The results indicated the

dependence of durability characteristics (permeability and sulfate resistance) on the

class 2nd iirlcllcss 01' f l y ash, the grade ot' concrete, the amoullt and type of'

rc\)Iiiccmct~t atid lo a Iesscl. dcgrcc-' un the gracic ul' cclne~lt. MC)I.L'OIGI., highcr

cortlprcssive strength $!.as obtaincd when high-calcium fly ash rather than, low-

calciun~ fly ash, ivas uscci in concrete.

Shashiprakash and 'I'homas (2001) investigated the use of high-calciun~ fly ash

( ( ' a0 content 20 to 23 ?,u) and silica fume to iniprove the sulfate rcsistance of

mortars. Three Portland cements (with varying C3A contcnts), one ultra - - fine fly ash

with moderate CaO content (about 14%) were also used and the binary and ternary

blends were tested for assessing the sulfate resistance. Based on the expansion test of

mortars stored i l l 5% sodium sulfate solution for periods upto 12 months, it has been

concluded, among other things, that ( i ) the performance of high-calcium ashes can be

ilnproved by the addition of small amounts of silica fume (e.g. 3 to 6%); ( i i ) high-

calcium ash appears to perform well when blended with a moderate or low C3A

cement.

Sideris and Savva (2001) investigated five mortar and concrete mixtures. A normal

cement and four blended cements based on two natural pozzolans of volcanic origin

and two Greek lignite high-calcium ( CaO content :17.5% and 14.87%) were used.

Compressive strength development, sulfate and chloride resistance and carbonation

depth were studied for nearly 18 months. The results indicate that (i) the use of natural

pozzolans and lignite fly ash blended cements increase the sulfate and chloride

resistance of mixtures; (ii) there is increase in the compressive strength of mortar and

concrete due to the formation of greater amount of secondary CSH; (iii) carbonation

depth of blended cement mixtures in both curing environments (i.e. directly exposed

to sea and normal curing ) was greater than the one of normal Portland cement

mixture and (iv) the lignite fly ash (pre-treated to increase its fineness and to reduce

the free CaO content) showed the lowest carbonatioil rate.

Heinz and others (2001) investigated the hydration and cement made with Rhenish

lignite fly ashes (Ifa), Germany, as cement addition. Rhenish lfa is characterized by

higher amounts of CaO (29 - 42%); SO3 (about 4.8 to 8.1%) and MgO (8.5% to 1 1.3).

w'nich could contribute to unsoundness in cements. Cement-lfa mixtures(80:20; by

wt.) and llii ccmcrlts ( 10 - 30"0; by wt.) produced by inter-grinding wcrc subjected to

so~iiidncss test, as pcr /IS'[ h1 and DIN stanciastls. I t was li)und that only ~ ~ p t u LO 00

( by nt.1 both the above types ot'cenicnt were sound.

2.1 1 AC"I'IVA'I'1ON OF NA'I'URAL, POZZOLAN

Shi anti others (1994) studied the effect of some sodium based activators (Naz SO4,

NaOH, Na:, COI and Na2 SiO, . 5F120 - all chemical reagent grade) on the strength

developtncnt of two types of blended cements made with a natural pozzolan. The two

blended systems consisted of: (a) natural pozzolana (from Guatemala) and

commercial slaked lime and (b) natural pozzolana and Portland cement. All together

10 types of cement pastes were prepared. Small paste cylinders ( 25 mm x 5 0 mm)

were prepared by casting fr-esh paste using water 1 solid ratio to have normal

consistency of paste and were moist - cured at 25 O C for 3 days, followed by curing

at 50 " C for another 4 days. At the end of 7 days, compressive strength of the blended

cenients were evaluated. Calcium hydroxide content and non-evaporable water

content were determined by the 'thermo-gravimetric analysis'. The above study

resulted in the following conclusions: (i) Pozzolana-Portland cement (PC) pastes

show substantially higher strengths than the Pozzolana-lime (PL) pastes, due to the

high-reactivity of Portland cement; ( i i ) addition of NaOH, Na2 C03 or Na2 Si03

5H20 decreases the strength of PC pastes, but the use of Na2 SO4 as an alkaline

activator increases the strength; (iii) on the other hand, all the activators considered in

this study, have increased the strength of pozzolana-lime (PL) mixtures, at 7 days,

within thc range of 5 60%; (iv) sodium sulphate and sodium hydroxide activators are

particularly efficient for PL pastes; (v) addition of all the four (chosen) activators

accelerate the pozzolanic reaction of the natural pozzolan in both types of blended

cement pastes, as evident from the substantial decrease in the free calcium hydroxide

content, determined by the thenno-gravimetric analysis.

Shi and Day (1994) investigated the compressive strength, bulk (apparent) density,

free lime content and the effect of activators on the compressive strength of lime -

natural pozzolan (un-ground) blends. Commercial grade hydrated lime and reagent

grndc unliytirot~s Na2 SO4 ;IS a ctiernical activator, wcrc usrd. IJn-ground natural

pcw/olan \$as blcndctl with thc hydrated liine in the raiio of SO:2O (by wt) . C'ylindcrs

of' blend (25 mm x 50 riini) cast using water to solid ratio of 0.15,wcrc compacted *

in a stainless steel mould at ~'nrious pressures ranging fiom 20 - 120 MPa and cured in

a fog room at a temperature of 40 " C for 7 to 28 days. Thernio-gravimetric analysis

was carried out to determine the free lime, Ca(OH)2 content. Based on the above

investigations they have concli~ded that (i) i t is possible to obtain satisfactory strength

by using non-ground natural pozzolan blended with lime and water and compacted;

(ii) if a conipaction pressure of 60 MPa is used, 7 days 'moist curing' at 40' C, is

necessary and if, lower compaction pressures are used, correspondingly, longer curing

periods are required; (iii) the bulk density of lime - pozzolan compacts are

substantially less than standard units manufactured with materials, such as, clay and

Portland cement; (iv) the amount of Ca(OHI2 that has reacted is independent of initial

compaction pressure, at 28 days of curing; (v)where only low-compaction pressures

can be applied, a potential exists to increase early strengths significantly (i.e. about

30%), by the addition of 2% Na2 SO4 activator and (iv) the use of un-ground natural

pozzolan and compaction technology results in the elimination of the energy-intensive

grinding process and thus, a substantial reduction in manufacturing costs.

Day and Shi (1994) examined the importance of the fineness of a natural pozzolan in

the strength development of lime - pozzolan cements (LPCs). A natural pozzolan from

Bolivia, was ground to various fineness, blended with lime in the ratio of 80: 20 (by

wt.) and the resulting LPC with water to binder ratio of 0.5, used to make hardened

cement paste cylinders (25 mm ) . In some pastes 4% sodium sulfate or 4% sodium

chloride (chemical reagent grade) activator (by wt. of lime - pozzolan cement) was

used to enhance strength development. Strength of the cylindrical specimens were

determined from 3 to 90 days, during continuous moist curing at 50 O C in lime water

bath. From the experimental results they have shown that (i) there is a good linear

correlation between the Blaine's fineness of the natural pozzolan (ground in a small

ball mill to six Blaine's finenesses ranging 259 to 554 m2 I kg) and compressive

strength at all ages and for all pastes; (ii) the fineness of the natural pozzolan has its

most sigriificarlt effect on cnrly strength gain and ( i i i ) thc addition of chemical

a ~ t l ~ a t o r s Increases, both the 'rate of strength gain' atld thc 'setlsiti\ity o r strength

g a i ~ i ' to fineness.

Shi and Day (1995) compared the strength characteristics and pozzolanic reactivity of

three natural pozzolans ( N P ) (two volcanic ashes and a pumice); otie low-calcium fly

ash (from Alberta, Canada); one high-calciun~ fly ash (LISA) and a blast furnace slag,

based on the rate of strength gain and the rate of consumption of lime (as measured by

thermal analysis ). Thc three natural pozzolans ( two from Guatclnala and one form

Bolivia), even though are derived from widely varied geographical locations, they

show similar chemical composition to the low-calcium fly ash. Reactivity was

measured on pastes made from blends of 20% lime (commercial and hydrated) and

80% of either ground natural pozzolan, fly ash or ground blast furnace slag using

water / solids ratio for normal consistency and the fresh paste specimens (24 mm x

50 mm) were cured in lime water bath at 50' C, until the time of testing (3, 7, 28 and

90 days). Lime -NP and lime-slag blends showed a significantly higher water demand

for a given consistency compared to the lime -ash blends. Due to loss of consistency

caused by the rapid fo~lnation of calcium aluminate hydrates, the high-calcium ash

gave a higher water requirement than the low-calcium ash. Among the six materials

tested by them, the high-calcium ash showed the most rapid strength-gain and lime-

consumption rates. Low-calcium ash lime blend had a much higher rate of strength

gain, than lime-NP blcnds, in spite of no obvious differences in the lime consumption

rate which may be partially due to the difference in the water requirements. Lime-slag

blends showed a low depletion rate of lime and showed higher strength than the

natural pozzolan blends, which is attributed to the cementitious properties of the slag.

Shi and others (1999) investigated the conversion of a waste mud (red-mud) from

alum production into a pozzolanic material, by calcination and the pozzolanic activity

of the calcined mud (at 750' C for 5 hours and then ground to powder in a rotary

ceramic jar, with ceramic balls), was evaluated through strength tests, by blending the

ground mud with hydrated high calcium lime in a proportion of 70 - 30 % (by wt.).

Na: SO, (rcagcnt gratle) and ('a('/: . 2 f f 2 0 (comn~erc~al flakes) werc lrscd as the

chc~n~cal actl\.ators fir the Ilri~c-culcinutf mud n~lxturcb, to ~rrlpr-ctsc the rcact~vity of

the calciricd ~iiud. .l hrec batchcs of pastes, namely, control, control t 2.5 'i/o Na2 S o 4

and control + 5 ')to ('a<'\?. 2 t I z0 , with water - to - solid ratio of 0.625, were used to

prepare specinlens and evaluate the compressive strength after 3,7, 28 and 90 days of

curing. XRI) and SEM analysis have also been carried out on the hardened pastes.

Based 011 thc above studies, they liavc concluded that ( i ) the calcined mud is much

more reactive than the fly ash or tlie natural pozzolan; ( i i ) tlie presence of Na2 S o 4

accelerated the pozzolanic reaction and iniproved the early and late - age strengths of

the lime-calcined mud mixtures; (iii) the addition of CaClz resulted in the depletion of

Ca(0H) after 3 days of hydration and changed the strength development of lime

calcined mud drastically. The strength doubled ( to about I8 MPa at 7 days) when

compared to the activation with Na2 SO4 and showed a slight increase with time

thereafter and (iv) the formation of the solid solution speeds up the dissolution of

meta-kaolinite pal-ticles and the pozzolanic reaction, which enhances the strength of

lime-calcined mud pastes.

Shi and Day (2000 a) examined the effect of two chemical activators, namely,

Na2S04 and CaC12 .2H20 in pozzolanic reaction kinetics in a lime-pozzolan blend,

consisting of 80% ground volcanic ash from Bolivia and 20% of conlmercial hydrated

high calcium lime, (that meets ASTM C- 141 specifications) at 23, 35, 50 and 65 " C.

Strength development, pozzolanic reaction rate relationship between consumed lime

and hydration products, were the indicators chosen by them for evaluating the relative

advantages of the above chemical activators. The dosage of Na2 SO4 or CaC12 . 2H20

was maintained at 4%; (on the weight of lime-pozzolan blends). In order to monitor

the progress of the pozzolanic reaction between the lime -pozzolan blends, they

measured the amount of free lime content in hardened cement pastes, at different ages,

by 'thermo-gravimetric' method. Based on the above study, they concluded that the

addition of the above two chemical activators have increased both the early and later-

age compressive strength of pastes form 35 ' C to 65" C and that the strength of

reference (i.e. control) paste showed much lower strength when compared to the

activated pastes at all temperatures. They have also concluded that addition of

Na2S04 mainly accelerated the consumption of lime during the first day and did not

affect pozzolanic reaction thereafter, while, the introduction of CaC12 . 2H20 had a

significant effect on pozzolanic reaction, over the testing period.

Shi and Day (2000 b) examined the hydration products and the pozzolanic reaction

mechanism in the presence of Na2 SO4 and CaClz . 2Hz0 ( chemical activators) for

lime-natural pozzolan cement pastes. Based on pH measurement of the saturated

solution of activated pastes and on the results of XRD and SEM of hardened pastes,

they concluded: (i) the higher early strength of Na2 SO4 activated pastes was attributed

to both the accelerated pozzolanic reaction and the formation of AFt. As the

temperature increased to over 35 " C, AFm appeared and resulted in a lower strength;

(ii) the introduction of 4% CaC12 . 2H20 decreased the pH values of the solution and

retarded the dissolution of the pozzolan, but, accelerated the dissolution of Ca(OH)2;

(iii) the CaC12 . 2H20 activated pastes formed a denser structure and exhibited a

higher strength than the control or the Na2 SO4 pastes, after a certain period of time

and (iv) the increase in curing temperature accelerated the dissolution of the pozzoIan

and the formation of reaction products in the CaC12 pastes more significantly than

those in the control or Na2 SO4 activated pastes.

Hossain (2000) has reported the results of a research to assess the suitability of using "

PNG (Papua New Guinea) lime stone (LS) in blended cement production. Limestone

powder (LSP) upto 100% was used with Portland cement and volcanic ash (VA)

(found in abundance in PNG due to volcanic activities in the above country). Physical

and chemical properties of LSP and VA and on the fresh and hardened state of

blended cements were carried out. The investigations led to the following conclusions:

(i) normal consistency and setting time of Portland cement are affected by the

replacement of cement by LSP; (ii) Manufacture of Portland limestone cement

(PLSC) similar to Portland cement (PC) and blended Portland fly ash cement (PFAC)

conforming to Australian standards is possible with maximum replacement of upto

40%; ( i i i ) durability of a strilcturc using PLSC should itliprove due to reduction on

capll~ary porosily at rcplaccmcnt of approximately 5% (by LP t.)

Turanh and ErdoGan (2001) studied the effect of Portland -pozzolan cen~ents (one

type of clinker and one type of natural pozzolan i.e. 'trass' having CaO content 13.4%

at 0, 20, 25 and 30 % of pozzolan by wt.) on the various properties of paste, mortar

and concrete. A WICM ratio of 0.45, two different cement contents (250 and 350 kg

im3) and two curing regimes (50 % and 100% RH) were adopted for concrete, until

the various ages of testing (3 to 90 days). They have concluded that (i) the fineness of

cements affects the rate of hydration - finer the cement, faster is the formation of

strength giving CSH gel; (ii) the cement containing 20% and 25 % pozzolan show

strength close to each other, whereas, cement containing 30% , show very slow

strength development; (iii) the change of curing conditions from 50% to 100% RH,

results in significant increase in the compressive strengths of all types of cements and

(iv) upto 25 % pozzolan addition, would give reasonably satisfactory cement and

concrete compressive strengths.

2,12 ACTIVATION OF FLY ASH

2.12.1 Low-Calcium Fly ash Aitein and others (1986) studied seven North American and European fly ashes from

a physico-chemical point of view. The hydration products with lime [commercial type

- 92% Ca(OH),] have been studied for each fly ash, with fly ash-lime ratio 4.0, by

XRD. It has been found that the lime is first fixed as calcium aluminate and ettringite,

in the pozzolanic activity of fly ash. Lime has been found to play a very important

role, particularly, at the early stage of hydration.

Aimin and Sarkar (1991) investigated the possibility of using a much higher Class F

fly ash (FA) replacement ratio (upto 60 % by weight) in order to enhance still further

the activation properties, in terms of micro-structural development between the ages

one day and 90 days, together with mineralogical identification of the reaction

products. They deliberately chose a very low C,A cement to reduce the hydration of

C,A in f;A - cement blends. in orcicr to a~nplify thc role of [ < A alkalis. A cement low

in alkalis and (';A (corresponding to IISI'M 'Type V ccment), a ('lass I; fly ash (low-

calciunl) and gypsum (98.5'%, pure) were used and five diffcrcnt proportions of cement

FA - gypsum blends (low to high volume of ash and witk/without gypsum) were

obtained. The water - to - binder (cement, FA and gypsum) ratlo for all the mixes was

kept constant at 0.465. High-gypsum : FA (i.e. 1 :10, by wt.) was used in the above

experiments. Mortar samples for compressive strength test ( 1 :2 .5 , binder : std. quartz

sand, prisms-40 x 40x 160 mm) were prepared, water cured and tested at one day to

28 days. XRD and SEM analysis were also carried out. Based on their investigations,

they concluded that the use of additional gypsum (3 to 6%) appears to be beneficial in

terms of dissolving the glass phase in FA, but, its optimum concentration in relation to

the proportion and composition of FA and cement needs to be carefully studied in

order to derive the advantage of higher strength.

Beretka and others (1993) investigated the kinetics of hydration and physico-

mechanical properties of C4A3S (calcium sulfo-aluminate) based cement referred as

'special cements'. Industrial process wastes i.e. low-calcium fly ash, blast furnace

slag, bauxite wastes ('fines') and phosphogypsum and limestone, clay (pure and

commercial materials) constituted the raw mix. 25 mm cube specimens were cast at

WIB ratio of 0.5 for determining the compressive strength and XRD and DTA

analysis for understanding the hydration of cement. Based on the experimental results

they have concluded that addition of blast furnace slag or fly ash to the cements

containing C4A3S : - C2S and CS (1.5: 1:l) reduces the quantity of ettringite in the

systems and hence, the one day strength. However, the ultimate (28 days) strength is

not in general, negatively indicated. Further, they have noted that the rate of strength

development was faster for the composition, containing fly ash at all ages and that the

ternary systems (i.e. using fly ash, blast furnace slag and clay), had the highest overall

strength among those investigated by them.

Ma and others (1995) studied the nature of the pore structure which develops when

low-calcium fly ash is activated by Ca(OH)2 and Ca SO4 .2H2 0 under hydrothermal

treatment. They have emphasized the need for a realistic assessment of the pore

structure of activated ash, in order to understand those importatlt physical and

~nechatlic;al properties of' concrete. XKD and SBM were used to identify and

characterize the hydration products and to measure the surface areas and pore

structure of the various samples under hydrothermal treatment, a multi-point BET

(Brunaur-Emmett-Teller) equation was used. The volume of pores with radii of 19 8\ increased with thermal treatment for the low-lime fly ash with Ca(OH)2 . The

pozzolanic reaction produces calcium silicate hydrate (C-S-H) and that was found to

be responsible for the change of the surface area i.e. of fly ash, activated with

Ca(OH)2 at elevated temperature. However, the effects on the pore volume and

surface area for fly ash hydrated with gypsum in the temperature range of 25" to 180"

C. were found to be minimal.

Sarkar and others (1995) carried out studies on the activation of low-calcium fly ash

and hydrated lime mixture using phospho-gypsum (obtained from an industry

producing phosphoric acid), in two stages. In the first stage, phospho-gypsum, wet

hydrated lime and fly ash were mixed in ratio 1 :3: 16 (by weight). The dry above mass

was mixed with 12.5% water (by weight). In the second stage, phospho-gypsum was

'beneficiated' using a proprietary process developed at the Central Glass and Ceramic

Research Institute, Calcutta, India. Water content corresponding to a flow of 75% was

used. Fly ash content was kept constant at 80% (by weight). The lime - to - phosphogypsum ratio was varied to make the remaining 20% of the mix. XRD

patterns of the hydrated pastes (at one to 28 days) revealed that hydrated lime was

consumed to a large extent within the first week of hydration. The soluble part of

phosphogypsum i.e. soluble calcium sulphate and hemi-hydrate in the un-beneticiated

phosphogypsum began to react from the first day, but, gypsun1 which remained intact

upto the seventh day was consumed by the 28Ih day of hydration. 'Ettringite' and

'calcium silicate hydrate' were the major reaction products that were observed. It was

observed that when 15% lime and 5% phosphogypsum (un-beneficiated) were used, a

molar equilibrium between Ca and SO4 is established, allowing re-crystallization of

gypsum, in the above system. They believe that a 'two-stage reaction' is in progress,

where in, Ca and soluble SO4 react and recrystallize to form gypsum, and this

recrystallized pypsurn is a source for flv ash activation at a later stage to form

ettringite. They concluded that ( i ) calcium sulphate helps to activate the fly ash, i.e, to

break up the glassy phase. and the hydration products formed thereafter depend on

the calcium available. I f the amount of lime in fly ash - lime- gypsum can be

optimized. C-S-FI is the principal phase that will contribute to the strength; (ii)

phosphogypsuin and lime with a 1 : 1 weight proportion can be used with 80% fly ash

(by weight) to activate a system capable of providing strength above 10 MPa.

Sherman and others (1995) synthesized cements comprising CIA;^ (calcium sulfo-

aluminate ), C J S ~ ~ ( calcium sulfo silicate) and C: ( anhydrite) in the ratio 1 : 1 :0.5,

from commercial and industrial process wastes, namely, phosphogypsum, low-

calcium fly ash and blast furnace slag and natural materials (lime stone, bauxite and

clay), in a single firing at about 1200" C. The cements thus obtained are found to be

reactive, hydraulic, self - hardening and provide high mechanical strengths at early

ages (35 - 50 MPa in one day, tested wet) and also at later ages (57 - 76 MPa at 28

days, tested wet). The cements are also found to have good dimensional stability,

similar to OPC pastes, which is due to the presence of C J S I ~ . The strength of these

cements at early ages is attributed to the formation of ettringite (C6AS3H32 ) and the

ultimate strength is due to the presence of 'ettringite' as well as the formation of

cementitious phases from the hydration of C J S , ~ . Although, the ettringite, is partially

converted by carbonation, the mechanical strength of the systems are still retained at

about 67 %. They have also opined that 'cements with different physico-mehanical

properties can be designed.

Skvara and Bohunek (1999) have reported that alkali-activation brought about by the

use of NaOH and NazSiO3 solutions is capable of increasing significantly the

reactivity of substances with latent hydraulic properties, such as, fly ash or mixtures of

fly ash and blast furnace slag. It has been reported that very high compressive

strengths of 120 MPa to 170 MPa were achieved with ash-slag mixtures over the

composition range of 70 - 50 wt. % fly ash and 30 - 50 wt. % of slag, activated with

alkali - activator having a silica modulus (Ms = SiOzI Na2 0) of 0.6 and contained 7

wt. % NazO.

Fan arlrl o i l l t rs (1999) p:c.poseil 'I 1 1 ~ i i iiiithod sf fly 9511 d~:i:':ltl~c with adclii;sn o f

Ca(OiI)z and a small cluantity of Na2SiOl (i.c. 3.91%). 111 their studies, a low -

calcium tly ash obtained frorn a power plant in China, was mixcd with C~(0l-1)~ ( in

certain proportion by weight), Na2Si03 and WIS = 3: l and the mixture held at

constant temperatilre of 55 " C, until Ca(OH)2 disappeared. The above sample was wet

ground for 40 min. and dried at a temperature of 120" C, to yield the activated fly ash

(AFA). They have concluded that the activity of the activated fly ash by the above

method has increased, which can accelerate cement early hydration and promote

setting and hardening. They have recommended a composite utilization of AFA and

FA as cement admixtures, based on comparable mortar strength (i.e. with that of

reference mortar) obtained with 5 Oh to 10% AFA addition. With a small addition of

Na2Si03 to the mixture of FA and Ca(OH)2, the reaction between the above mixture

has been accelerated, due to the higher pH value (13.1) realized (by the formation of

NaOH), thus, greatly facilitating silica - alumina glassy chain corrosion. Therefore,

Na2Si03 has played the role of a 'stimulant' in the above activation of (low-calcium)

fly ash with Ca(OH)2.

Singh and Garg (1999) carried out investigations to formulate cementitious binders

by a judicious blending of fly ash with Portland cement, as well as by admixing fly

ash ( Ca0-1.5% only) with calcined phosphogypsum, flourogypsum, lime sludge and

chemical activators (CaC12 and Na2S04). The study showed that cementitious binders

of high compressive strength and water retentivity can be produced and that the

strcngth of masonry mortars increased with thc addition of chemical activators.

Ettringite, C-S-H and C4AHI3 were identified as the major cementing compounds

responsible for strength development in binders. The binders have been found to be

eminently suitable for use in masonry mortars and for making concrete (replacing

OPC upto 25%), without any detrimental effect on the strength. They have opined that

fly ash in the range of 40-70%, can be used in formulating binders along with the

above industrial wastes. CaC12 as an activator, has been found to be responsible for the

increase in the early-age binder strength, but, it has caused a similar effect at 28 days.

$hi anri Qiall (2000) investigated the strcngth and cquilihrl~ul~ water cxtsaction of

hlcndcd ccmcnt (OPC't['tl!, contai11irl.g iiigil-voii~me (coal) fly ash a r ~ d calcium

as activator (i.c. irldustrial grade flake - CaCI2. 2H20) . Thcy have concluded

that addition of 3%) CaClz . 2fI20 has increased the strength of the blended cement

consisting of 50%) OPC and 50% fly ash, by 50 % to 70 % and increased the strength

the blended cement consisting of 30% OPC and 70 % fly ash, by approximately

100%. However, increase in the CaC12. 2H20 dosage from 3 to 5%. has dccrcased the

strength of cenlent pastes. Based on the 'water equilibrium extraction tcst' and pH

measurement they have shown that the presence of CaC12 accelerated the pozzolanic

reaction between fly ash and lime and that no 'free lime' exists, after 7 days of

hydration. They have opined that the 'extraction method' can also be used to monitor

the pozzolanic reaction in blended cement.

Behera and others (2000) have attempted to develop blended cements using fly ash,

obtained from a thermal power plant of Orissa (India), in activated form. The powdery

low-calcium fly ash (75%) was mixed with 10% lime sludge waste, 10% semi -

plastic clay and 5% coke breeze powder and granulated using water as a binder. The

above fly ash is then used to produce activated fly ash (aggregate) by sintering at 1260

O C', The above activated fly ash was used to replace Portland cement clinker (i.e.

20%, 30% ,40%, and 50%) and cement prepared by grinding in a ball mill with 30%

gypsum. They have concluded that upto 40% of fly ash in activated form can be used

for manufacturing blended cements, conforming to Indian standards. However, they

have also opined that the blended cement with 50% activated fly ash can also be used

safely, as it can impart a strength of 33 MPa (at 28 days).

Kolay and Singh (2001) studied the effect of chemical activation of a typical Class F

lagoon ash, with different concentrations of alkali ( NaOH) and for different durations

of activation. Lagoon ash (low-calcium) obtained from a thermal power plant located

at Nagpur, India treated with I N NaOH for 24 hours in a water bath with reflux

system, was termed as the 'activated lagoon ash' (ALA). Both ALA and 'original

lagoon ash' (OLA) i.e. un-activated ash were analyzed by XRD and SEM etc. Based

on the above, they have concluded that (i) alkali activation causes a decrease in silica

content of lagoon ash and it is responsible for etching and hence dissolution of silica

present in the ash and ( i i ) the formation of Napl and hydroxy - sodalite zeolites has

been confirmed by cation exchange capacity (CEC) values, indicating the potential of

alkali activated lagoon-ash for its use in various environmental protection schemes,

namely, heavy metal removal 1 retention from industrial sludge, for treatment of

polluted water and agricultural soils.

Karandikar and others (2001) carried out studies for producing fly ash belite cement

through alkali activation of fly ash, under non - hydro thermal conditions. The

optimum alkali level for alkali activation (i.e. NaOH at 450 " C) was maintained as

4%. They also investigated the optimization of CaO 1 SiOz ratio and the temperatures

of calcinations for the fly ash belite clinkers (FABC), on the basis of compressive

strengths of the resultant cement. Based on the above experimental investigations,

they concluded that (i) it is possible to produce FABC at 800" C, by alkali activation

of fly ash; (ii) the neat strength of 30 MPa can be achieved a 28 days and (iii) the

hydrated cement phases upto seven days are poorly crystallized.

2.12.2 High-Calcium Fly ash

Schlorholtz and others (1984) studied the type of calcium hydroxide (lime) used in

the ASTM C311 - 81: 'Lime Pozzolanic Activity Test7 (LPAT), considering three

types of lime ( a reagent grade, a commercial grade and a mixture of regent grade lime

and MgO) with four fly ashes (three Class C and one Class F). The formation of

crystalline reaction products in the lime test was monitored by XRD. It was found

based on the compressive strength attained using the various limes and the different

reaction products formed as determined by XRD, that impurities in the commercial

lime play a key role in determining the 'pozzolanic activity' of Class C fly ashes.

Luxan and others (1989) studied the pozzolanic activity of ten fly ash samples (nine

low-calcium ashes and one high-calcium ash), from different power stations of Spain,

which use three types of coal. An accelerated method which consisted of putting the

different types of fly ashes in contact with saturated lime solution at 40' C for 7 and

28 days and at the end of the above period, the total alkalinity and CaO concentration

was cstablishcd. ' I h e solid rcsidue was studied by X K D and iR absorption

sp~ctroscopy. XKI) analysrs rcvcalcd the prcsence of CAFI, carbo-alun~inate, mono-

sulpho-aluminate. Howevcr, the presence of CSH could not be confirnled by XRD and

that none of the saniplcs showed any traces of 'ettringite'.

Khedayuri, Yegiriobali and Swadi (1990) studied the ash of the retort residue of the

oil shale form central Jordan to evaluate its pozzolanic activity. Thermo-gravimetric

(TG) analysis were performed on pastes of ash, cement -ash blend and ash-lime

blends. The spent oil-shalc ash used in the experiments was obtained from a 'retorting

process' and subsequent burning at around 650' C, having a very high CaO content

(39.7%). OPC and lime hydrate were the other materials used in the experiments.

Compressive strength development in mortars of the above three types of pastes were

studied using 25.4 mm cubic specimens and under 'standard' and accelerated curing

regimes. The above study provide experimental data on the pozzolanic properties of

(Jordan) oil shale ash leading to the following conclusions: (i) a slight reaction occurs

between ash and lime liberated during cement hydration; (ii) the consumption of lime

by ash in ash - lime pasts, is mainly a diffusion controlled process, obeying the

Ginstling-Brounshtein equation; (iii) oil shale ash (0SA)-lime mortars can attain

moderate strength with 'accelerated curing'; (iv) OSA can be used as an admixture to

replace cement upto 20% (by wt.) and it is also suitable for the production of auto-

claved ash-lime-sand building units; (v) its modest pozzolanic value of lime reactivity

can be improved by finer grinding and (vi) further research is required on the

accelerated hydration of the blended cement pastes.

Roy and others (1992) studied the alkali activation (i.e. hydroxide solutions of

lithilium, sodium and potassium) of Class C fly ash (obtained from a power plant in

Texas). Activator solutions of Li, Na, and K having pH 12.34 (0.02N) to 14.69 (5N),

waterlash ratio of 0.4 were used for preparing samples and cured at room temperature

for 7 to 28 days. The microstructure phase assemblage and phase composition of

activated fly ash were studied by SEM, XRD, energy-dispersive X-ray micro-analysis

and thermal methods. It was observed that microstructure and phase assemblage

depended on p i l . Ettringite was absent beyond pH 14.3 (2N); a hexagonal platc like

crystalilnc f;)rni (striitllng~te, gelilcn~te hydrate, CzASHR and other con-tpoi~nds)

became morc abundant at h~ghcr pH. At higher pH the microstructure was

characteri~cd by high amounts of the plate-like crystalline phase and a dense matrix,

due to higher reactivity of thc glassy phase in fly ash. The effect of time (i.e. age) was

mlnor compared to that of pH. At the same pH, the types and amounts of crystalline

and non-crystalline phases depended on the hydroxide cation.

Papayianni (1993) studied the con~pressive, flexural and dynamic modulus of

elasticity at diffcrent ages for two kinds of grouts (i.e, sanded grouts and slurries)

based on a high-calcium fly ash (HCFA) (from Ptolemaida in Northern Greece), and

Portland cement. Based on the overall effects of HCFA on the above properties, it has

been stated that ground HCFA behaves better in grout mixtures and it is preferable o

be used instead of raw material. In sanded grouts, ground HCFA may replace upto

40% of cement (by wt) without any change in strength and other charaeter~stics of

grouts in fresh state. Considering fly ash - water slurries, ground HCFA could replace

upto 80% of cement (by wt) with the advantage of lower final bleeding and loss in

volume.

Pavlenko and Chayka (1993) studied the physico-mechanical (compressive

strength, initial modulus of elasticity, frost resistance and heat conductivity) and

deformation characteristics (i.e, crecp and shrinkage) of fine-grained cement-less slag

ash concretc, obtained from high-calcium fly ash form Novosibirskaya, (Russia)

power plant, (having good binding properties, high bound CaO content - 30%), slag

sand having a particle size distribution of 0 to 5 mm; silica fume and an air-entraining

admixture (containing the secondary sodium alkali sulfate). Cubes (100mm) and

prisms ( I00 x 100 x 400 mm) were cast and steam cured at 85 - 95 " C using 3 + 10 +

3 hr cycle, for the above studies. It has been concluded that (i) the 'cement less' fine

grained concrete can be used for the construction of one, two -storey buildings, both

for pre-cast and cast-in-situ applications; (ii) the main physico-mechanical and

deformation characteristics of the above concrete also satisfy the coda1 requirements

of fine - grained concretes and (iii) high-calcium fly ash and slag sand seems to be a

promising material of the future, as they contain neither artificial or nat~lral porous

aggregates.

Tishmack and others (1995) sclcctcd four high-calcium coal combustion by -

~roducts (C'CBf') for evaluating the workability of pastes; extent of ettringite

formation; micro-structure; strength of solidified pastes, among other things. The four

high-calcium ashes (CaO content ranging from 17.9 to 41.4%), were obtained from

thermal power stations located in Wyoming, North Dakota, Minnesota, and Kentucky,

USA. Cubes of pastes (5Inlnl) were cast and cured for 7,28, and 91 days. It has been

reported that the highest strength of plain paste of 27 MPa was developed (at 9 1 days)

for Wyoming fly ash and specimens cured for only 7 days developed 22 MPa,

suggesting that early strength gain is characteristic of the above material. Moreover, a

higher variability in the strength developed was observed by them among the fly ashes

studied.

Shi and Day (1995 b) conducted experiments to determine the effect of various

chemical activators on the strength of lime -fly ash pastes, prepared with two types of

'fly ash', namely, a low-calcium and a high -calcium fly ash. The strengths of 80% fly

ash and 20% hydrated lime were used to evaluate the pozzolanic reactivity of the

ashes. Pastes were continuously moist-cured at 50 O C. The results indicate that (i) the

addition of Na2 SO4 and CaClz can increase the pozzolanic reactivity of both types of

ash, resulting in a significant improvement in strength; (ii) the early-age strength

increases with the amount of Na2 SO4 dosage and extent of strength improvement at

later ages depends upon the dosage and type of fly ash used; (iii) strengths at 90 days

and 180 days are significantly improved by the addition of 3- 5% CaC12 activator; (iv)

the use of upto 5% NaCI, rather than, CaC12 did not result in substantial

improvements to strengths; (v) the addition of Na2 SO4 results in the formation of

substantial amounts of 'ettringite' ( AFt), whereas, the addition of CaClz results in the

formation of solid solutions of C 4AH13 - C3 A. CaCI2 . 10 H20 and (iv) for pastes

made with high-calcium ash, Na2 SO4 was the more efficient activator, whereas,

CaClz was the efficient activator for the pastes with low-calcium ash.

Shi (1995) stildied the pozi.olanic reaction kinetics, reaction products and activation

mechatiistn in the prescnce of an optislii~ni dosagc of Na2 SO4 and Ca('lz, for a low-

calcium fly ash ([,FA) obtained frorn Canada and a high-calcium fly ash (I-IFA)

obtained form USA. A conimercial hydrated lime, was used as a lime source for lime

fly ash blends. Reagent grade Na2 SO4 and CaClz . 2H20 were used as chemical

activators ( at 4% of weight of lime -fly ash blend). The above fly ashes were blended

with the hydrated lime in the proportion of 80% and 20% (by wt.) and WIB ratio was

maintained at 0.35 and 0.375 (for LFA and HFA lime blends). Based on the

compressive strcngth obtaincd, results of SEM and XRD analysis, following were the

conclusions: ( i ) the effectiveness of activators depends on the nature of fly ash;

(ii) NaZ SO4 increased the early age strength, but CaClz increased the later-age

strength of lime LFA pastes significantly; (iii) Na2 SO4 increased significantly and

CaClz only slightly, the strength of the lime-HFA pastes at both the early and later-

ages; (iv) activation with Na2 SO4 significantly accelerated the consumption of lime at

early ages, but, slightly at later ages in the lime - LFA pastes; (v) CaClz accelerated

the consumption of lime - LFA pastes and no more free lime was detected at 7 days

and there after; (vi) Na2 SO4 and CaC12 showed an acceleration effect on the lime

consumption in lime-HFA pastes at one day and no effect thereafter. Finally, he has

concluded (based on XRD analysis) that CaC12 was not that effective as Na2 SO4, for

the lime -HFA pastes.

Ma and others (1997) investigated the hydrothermal reactions of two fly ashes (Class

'C' and Class ' F'). Heat evolved during early hydration was measured both for

activated and un-activated ash. Fly ashes were activated with Ca(0H) 2 and Ca SO4

.2H20 ( 10% by wt.) and at temperatures 25" C to 180' C under saturated steam

pressure. The kinetics of the reactions between the fly ash and the above activators

were determined by 'isothermal calorimetry'. X-ray diffraction analysis was used for

identifying the hydration products. Finally, the effects of activators and temperature

on flexure strength were established. The studies revealed that (i) the high-calcium fly

ash exhibited direct cementitious activity with water and showed pozzolanic activity

and formation of ettringite, when reaction occurred in the presence of Ca SO4 .2H20;

(ii) reaction occut-rcd more rapidly at 00' C for high lime ash. (iii) the principal

cementing ptiases fonneci were: CSH, C'AH and ettringite, during the hydrothermal

reactions of high-linle ash and ash- Ca(OH)2 or ash-CaS04 .2H20 mixtures; (iv) CSH

formation was more extensive when Ca(OH)2 was added to high-lime ash, than, when

it was treated with CaS04 .2H20; (v) significant 'ettringite' formation was observed

only when CaS04 .2H20 was admixed and at temperatures 80" C and below and

(vi) reasonably high-strength can be achieved from high-lime fly ash by the hydro-

thermal activation with the above two chemical activators.

Freidin (1998) investigated the properties of high-calcium oil shale fly ash

(HCOSFA) coal fly ash (LCCFA), produced from Israeli power stations, for the

development of strength and hydration of fly ash binder (FAB) i.e. mixtures of the

above two types of fly ashes. High-calcium oil shale fly ashes found in Israel and in

other countries, generally were found to contain a great amount of CaOaee and SO3 in

the form of lime and anhydrite. In order to overcome the negative consequences of

ettringite collapse and loss of strength associated with Israeli HCOSFA (containing

(CaO) =52.3% and (CaO) f,,, =10.1%), mixtures of the above ashes were used and

the influence of the composition and curing conditions (moist air, water and open air)

on the compressive strength of FAB, studied. It was determined that 'ettringite', is the

main variable in the strength and durability of cured systems. He concluded that (i) the

negative effect of ettringite degradation can be reduced by the introduction of LCCFA

to HCOSFA binder; (ii) the combined hydration of HCOSFA and LCCFA gives an

additional amount of high-strength and more stable CSH, which improves the

durability of the hardened system in open air. But, after a certain period of exposure

(from one to six months), a decrease in strength, was observed; (iii) the ratio of

HCOSFA and LCCFA as well as the amount of mixing water, determines the

compressive strength of FAB and (iv) moist air and water conditions are the most

favourable conditions for curing and for the development of compressive strength of

the above FAB.

Freidin (1999) also investigated the strength dcvelopmcnt as well as the

mlcrostructurc and composit~on s f the new forniat~ons of FAD [as reported in f;reidin

(1')98)], mortar specimens (five different compositions) under 'moist' and 'steam

curing' conditions and three exposure conditions (moist air, water and atmospheric

air). Based on a critical evaluation of the strength development and XRD analysis, he

concluded that (i) the durability of binder may be evaluated by monitoring long-term

strength in various conditions; (ii) continuous increase of FAB compressive strength

are observed under moist air and water conditions ; (iii) there is a sharp drop in the

strength of atmospheric air cured FAB after one month and upto 3 to 6 months and

gradual loss in strength after 3 to 6 months.

2.13 ACTIVATION OF FLY ASH USING LIME AND GYPSUM

Bhanumathidas and Ayyanna (1989) reported to have used calcined gypsum to

activate fly ash lime-mixture and they have claimed that a compressive strength upto

24MPa, (@ 28 days) could be obtained when 15% (by wt.) of calcined gypsum

(obtained through refining and process of phosphogypsum, a by-product of

phosphoric acid industries), was used to activate fly ash and lime (from acetylene

plant) mixture (Fa :L - 511).

Murthy and Rao (1992) arid Siddique (1996) studied the flexural behavior of

reinforced concrete beams, using gypsum activated fly ash and lime (FaL-G) as binder

in concrete. It is reported that they used more of the above binder (i.e. 1.5 parts in

place of one part) in concrete. They have observed that the reinforced concrete beams

cast using the above binder resulted in almost the same cracking and ultimate moment

like that of OPC concrete beams. However, FaL-G beams exhibited wide crack widths

and larger deflections (i.e. 40 to 60% higher) than OPC concrete.

Limited mineralogical analysis by XRD carried out by Bhanumathidas, and Kalidas

(1992), indicated the disappearance of ettringite by 28Ih day. This they observed as

credence to the fact that ettringite may have been converted to mono-sulphate in the

fly ash - lime - gypsum system. This encouraged them to observe that the relatively

shorter. less-cohesive and weak reactionary products of fly ash- limc reaction are

bcttered by thc contributory roic played by the addition of gypsum.

Singh and Carg (1995) studied thc feasibility of the fom~ation of cementitious binder

based on (a) calcined phosphogypsunl ( CaS04 . '/2 HzO) fly ash and hydrated lime

(40%, 40% and 20% respectively and referred as binder 'A') and (b) calcined

phosphogyps~~m fly ash, linic and cement, cured at 27' C to 50' C in RH greater than

go%, for different periods. The enhancement in strength with hydration period (upto

90 days) and during temperature, were monitored by differential thermal analysis

(DTA) and SEM. 10% (by wt.) of hydrated lime in binder 'A' was replaced with

portland cement to obtain binder 'B'. All the cementitious binders were uniformly

ground in a ball mill to a Blaine's fineness value of 320 m2 / kg. 2.5 cm cubes of

cementitious binders 'A' and 'B' were cast at 39% and 34% consistency, respectively.

The above study has revealed that (i) the cementitious binder containing equal

proportions of calcined gypsum and fly ash and Portland cement (i.e. binder 'B'),

attained higher strength, than, the cementitious binder containing hydrated lime and

no Portland cement; (ii) the compressive strength of the cementitious binder was

enhanced with increasing curing temperature from 27' C to 50' C and the maximum

strength was attained at 50 O C; (iii) the strength development in binder 'B' is higher

than that of 'A', due to the formation of 'ettringite' and 'tobermorite' and low

consistency. Early-age strength is attributed to the setting of calcined gypsum and

later-age strength may be due to the formation of ettringite and toberrnorite and

(iv) the strength properties of cementitious binder achieved at elevated temperatures

are good enough and hence, these binders can be used for making building blocks etc.

Roja (1996) and Ambalavanan and Roja (1996) investigated the potential of fly

ash, waste lime and waste gypsum to obtain a binder for using it in bricks/ blocks and

as masonry cement. Low-calcium fly ash (from Ennore thermal plant, located near

Chennai, India); four types of waste limes, each obtained from a processing stage of a

petrochemical industry and waste gypsum from a fluoride industry (5-lo%, by wt.)

were used. Beneficiation methods, such as, grinding, calcination and addition of

chemicals have been attempted, for those waste limes, which do not possess sufficient

react~vity. C'ompressive strength development of mortar mixes, using F-L-G binder

over a per~od oi'tlme (1.e. upto 1 LO days) have been studled. X R D was used tu identify

the effect of the various limes, including the method of beneficiation and the two

curing regimes (air and water) in the strength development. Based on their

investigatiotls, they have reported that (i) only the F-L-G mix (68:23:9) containing

milk of lime-reject, gives adequate strength ( i t . about 4 N 1 mm2) for making bricks

and blocks, without any bcneficiation; (ii) grinding has improved the strength of the

above mix only considerably (i.e. 4 N / mm2 to 10 4 N / mm2 ); (iii) calcination

improved the reactivity of pond lime (PL) after beneficiation and reach a mortar

strength of 4 N 1 mm2; (iv) initial air curing followed by water curing seems to be the

best combination to obtain optimum strengths.

Murthy and others (1999) studied the deformation characteristics and ultimate

strength of reinforced concrete beams (150x250~2500 mm) under flexure (i.e, four-

point symmetrically placed concentrated loads). Beams containing minimum amount

of reinforcement (0.3%) and over reinforcement (1.36%) and using three types of

binders, namely, FaL-G (Fly ash from Vijayawada thermal power station, India;

Lime-sludge from an oxy-acetylene plant; calcinated Gypsum from phosphoric acid -

industry, blended 70: 20: 10, (by wt.); FaL-G and OPC blend - by replacing 20% (by

wt.) of FaL-G with OPC and FaL G - silica fume blend - by replacing 15 % of FaL-G

with silica fume, were investigated. M20 grade concrete was proportioned using the

above binders with W/B ratio of 0.44, 0.62 and 0.50, respectively. Based on their

experimental investigations, following conclusions have been drawn: (i) FaL-G

concrete in flexure has yielded ultimate strength equal to that of OPC concrete;

(ii) partial replacement of FaL-G with OPC and silica fume, have slightly improved

the strength characteristics of FaL-G binder; (iii) strength developed at 28 days is

nearly the same for all binders and (iv) partial replacement of FaL-G with OPC or

silica fume, has not decreased the deformation of FaL-G based reinforced beams; (v)

FaL-G cement is worthy of use in structural concrete, pending durability studies.

2.14 PROPOK'I'IONING OF FLY ASH CONCRETE MIXES

2.14.1 Basic Approaches

Ever since the nlajor rcpot-ted practical application on the use of fly ash for the

construction of the Hungary Horse dam and the use of fly ash in concrete for nearly

six decades, the comnlon practice has been to use some plain concrete as a

comparison for the mixture proportion of fly ash concretes. However, the recent

development has been towards considering the components of a fly ash concrete as a

whole and treating i t as a unique material, without reference to an equivalent plain

concrete mixture. Consequent to the above thinking, three basic mix proportioning

approaches have been developed [Malhotra and Rarnezanianpour, (1994)l:

(i) partial replacement of cement -the simple replacement method;

(ii) addition of fly ash as a fine aggregate -the addition method ;

(iii) partial replacement of cement, fine aggregate and water - (a) modified

replacement method and (b) rational proportioning method.

The simple replacement method was commonly used for mass-concreting applications

and requires a direct replacement of a portion of the Portland cement by fly ash

(Washa and Withney, 1953). In the addition method, fly ash is added to the concrete

without a corresponding reduction in the quantity of cement, but, other mixture

adjustments are, generally made, by changing the aggregate content, depending on the

nature of a particular job. An example of this approach is the investigation by Price

(1961). The modified-replacement method and rational proportioning approach, both

require replacement of a part of the cement by an excess (by wt.) of fly ash, with

adjustments made for fine-aggregate and water content. The original form of this

practice has been termed the 'modified-replacement' method. However, in recent

practice, developments have led to the 'rational proportioning' approach.

The modified - replacement method originated with the work of Lovewell and

Washa (1958). Smith (1967) was probably the first to propose a rational method of

proportioning fly ash concrete by modifying the conventional mix proportioning

procedure, to obtain values for cement content and watedcement ratio by introducing

a f ly as11 'cenienting-efficiency factor' (k), which was assutned to be unique for each

fly ash. ' I he drawback of the abovc method was pointed out by Munday (1983).

Cannon (1968) repol-ted research carried out on the methods of proportioning fly ash

concrete mixtures to obtain equal strength to those of conventional control mixtures.

Cannon en~ployed Abrams' law and a factor that accounted for the relative costs of fly

ash and concrete. Rosner (1976), Ghosh (1976) and Popovics (1982) extended the

above concept to develop niixture proportions for fly ash concrete.

ACI (1981), CAN3-A23.5-M82 a Canadian standard based on Ghosll (1976)

approach, are the standard guidelines available for proportioning pozzolan cements. In

U.K., Munday and others (1983) proposed a procedure for obtaining any desired

strength at 28 days, which requires the collection of data, for a fly ash source.

Variation in fly ash properties may induce variations in the water needed, that are not

considered in the basic mix proportioning procedure. To overcome the above

problem, Olek and Diamond (1989) adopted the mix proportioning method originally

proposed by Cordon and Thrope (1975) for plain concrete and used it for fly ash

concrete mixtures. According to Olek and Diamond (1989), the slump is adjusted by

keeping the paste composition constant, but, increasing or decreasing the proportion

of aggregate to paste, until, the required slump is attained. Although, the above

proposed method avoids the dependence on published proportioning data based on fly

ash from other sources, it still requires a few series of trial mixtures.

Gopalan and Haque (1985) presented a method for the design of fly ash concrete to

achieve a specified compressive strength at 7 and 28 days, based on Abrams' law. The

proposed method was claimed to be simple and easy to use. The above method also

quantified the variation of the constants kl and k2 in Abrams' law, unlike, the

approach proposed by Ghosh (1976), so that the design can be accurately extended to

any desired value, using design charts developed by them.

Hansen (1992) proposed a number of modifications and extensions to the British

DOE method for the 'design of normal concrete mixes', which have made it possible

to extend the 11sc o f the [)OE method to dcsign wet lean concretes and controlled low-

~trcngth rnatcriais (CLSbl) 7'he properties of both thesc types ot concretc are

cons~derably in~proved by the addition of high-volumes of fly ash. The above method

is found to be particularly suitable for the design of high-volume fly ash concretes and

low-strength concretes.

2.14.2 Reproportioning and Generalization of Abrams' Law

Nagaraj and others (1993) proposcd a scientific method for reproportioning concrete

mixes, based on generalization of Abrams' law. The above method was intended to be

a substitute for the trial- and- error approach and hence, can be used to modify the

proportions of a trial mix to suit particular requirements without carrying out

intermediate trials. The proposed method takes into account the physical and chemical

properties of cement and characteristics of aggregates. Based on three chosen

normalization parameters for strength, workability and aggregate-cement ratio, three

equations have been developed that can be used for reproportioning. The above

approach was found applicable to different strength and workability ranges. It has

been stated that it is possible to obtain a wide spectrum of concretes using trial mix

data.

Shashiprakash and others (1994) extended the applicability of the generalized

approach for mix proportioning of plain concrete mixes, proposed by Nagaraj and

others ( 1 993), to proportion fly ash cement concretes mixes for the effective use of fly

ashes. The usefulness of the proposed method has been demonstrated for a few types

of fly ashes obtained from two different sources and deriving a trial mix

proportioning based on ACI method (i.e. ACI 21 1.1 - 1981). Based on the data of the

above trial mix, the proportions of the fly ash concrete mix has been obtained based

on (i) the equation representing the generalized form of Abrams' law (proposed by

Nagaraj and others, 1990), which is of the form SISo.5 = 0.2 - 2.6 log (WIC), where

S= strength for a water - cement ratio, WIC; So,, = the strength for a WIC ratio of 0.5

and (ii) based on two normalized equations to determine aggregate - cement ratios to

satisfy the workability requirements.

3 , . . : IVagas-ai drill 7dli;jda Benu (1996; t f ! - c ; : r i ~ l L G i : tile ;ii;i)jlrn:T)iii[y cf '0+n~r;ii i/~iil~f;1 " :7.f-

Abrams' law' to cover high strength rangc and to makc concrete mix proportioning

simpler and faster. Hascd on the extensive analysis of the British method of mix

design and considering the strength (S) at W/C = 0.5 as the reference state to

reflect the synergetic effects between the constituents of concrete, two generalized

relationships (one for So5 > 30 MPa and another for So 3 0 MPa) have been

obtained, which is of the form (S I So s) = a + b { 1 I (wlc)) .

Ranganath and others (1999) reported the importance of reproportioning of an

aggregate mix for obtaining the desirable workability, when ponded ash was utilized

as a part of fine aggregate in concrete. Ratios of CA I FA that have the least 'vee-bee

time', have been determined and the corresponding mix as per best workability, for a

specified paste volume. It has been shown that the re-proportioned mixes for optimal

workability also correspond very nearly, to the 'least voids' mixtures as determined by

Rao and Krishnamurthy (1993) for similar aggregates and hence, their guidelines

can be used with advantage to obtain the best 'reference concrete mix'. Based on the

results obtained , they have suggested a simple procedure of proportioning mixes

(i.e, re-proportioning CA / FA) which will obviate the necessity of carrying out

extensive laboratory experiments.

2.15 EFFECT OF FLY ASH ON WORKABILITY

Studies carried out by several investigators have confirmed that fly ash improves

workability of concrete [ (Davis and others (1937); Yamazaki (1962); Dhir(1988);

Price (1961); ~arle'(1983); Ravina (1981); Peris Mora and others (1993)l.

On the contrary, a few researchers hence reported that addition of certain fly ashes

decreases the workability and increases the water demand. Brink and Halstead

(1956) reported that some fly ashes reduced the water requirement for test mortars,

whereas, others (generally those with higher water content) showed increased water

requirement usually greater than that of control mortars.

Welsch and Burton (1958) reported less of slump and flow for concretes made with

sotnc l'iusrrai I L I K I f l y C ~ a i l ~ s itscd as partial replacement for cement, when water

content was maintained constant.

Reshi (1973) formed that there is increase in the water requirement of concrete, when

Indian fly ashes were uscd to replace part of cement in cement concrete. However, a

study by Singh (1994) showed an increase in workability of concrete when fine dry

hopper collected fly ash from Dadri power plant (near New Delhi) was used as a part

replacenicnt of fine aggregate upto 30% in cement concrete.

Brown (1982) found that both slump and vee-bee time improved with increased

substitutions and the changes were found to depend on the level of ash substitution

and on the water content. He also observed an increase in workability upto 8%

replacement of sand or aggregate by ash. Further increase in the percentage

replacement caused a rapid decrease in workability.

Results obtained by Sushil kumar (1992) and Mangaraj and krishnamoorthy

(1993) indicate that the use of ponded fly ash as part replacement of fine aggregate in

concrete results in increased water demand for flow, which is due to the effect of

water absorption and increase in the surface area of the combined aggregate. It was

found that the water absorption of ponded fly ash was found to be about 16% (by wt.)

( Mangaraj, 1993) .

2.16 EFFECT OF FLY ASH ON COMPRESSIVE STRENGTH AND ELASTIC PROPERTIES

2.16.1 Compressive Strength

Based on the studies carried out by Hobbs (1993); Gebbler and Klieger (1986);

Lamond (1983); it has been shown that replacement of cement fly ash results in

reduction of early strength of concrete and at later-ages, the fly ash concrete is

reported to achieve substantially higher strength than control concrete.

Langley and others (1984) have reported using 56% of Class F fly ash (by wt.) of

total ccmenr~irous matenais In concrete for s~!uct:lral applications. They have

concluded that high-volumes of ASTM Class F fly ash and low cement constant

prov~de an ecorlomlcal mater~al for strength of the order of 60 MPa at 120 days.

They have also reported that compressive strength, among other things, compare

favorably with normal Portland cement concrete.

Naik and others (1992) have shown that with a 40% Class F fly ash mixture with a

s~per-plast~cizer and W/CM=0.4, compressive strength of 24MPa at 2 days, has been

obtained. Bilodeau and Malhotra (1995) have obtained a compressive strength of

15-18 MPa at one day using high -volume fly ash and Type-111 cement. Galcota and

others (1995) have reported adequate early-age strength and significant increase in

later-age compressive strength for structural applications.

Even though, most of the work reported pertain to Class F fly ash, due to the fact that

they were the first to be examined for use in concrete, Class C fly ash has drawn the

attention of many researchers recently, as they have both pozzolanic and cementations

properties.

2.16.2 Elastic Properties

Published data indicate that fly ash has little influence on the elastic properties of

concrete. Crow and Dunstan (1981) based on the properties of 36 concretes,

containing fly ash at different levels and from different sources concluded that the

elastic properties (modules of elasticity and Poisson's ratio) of concrete containing

both Portland cement and fly ash are similar to those expected with Portland cement

alone. Studies carried out by Ghosh and Timusk (1981) on fly ash concrete

proportioned for equivalent 28 days strength, over a range of modulus of elasticity,

also concluded on the above lines. In a CANMET study conducted by Caretta and

Malhotra (1986) no significant effect of fly ash or type of fly ash on the modulus of

elasticity was noticed. Nasser and Ojha (1990) in a study of Saskatchewan lignite fly

ash reached a similar conclusion - the modulus of elasticity of concretes containing

<20?/;, fly ash was similar to that of the control concrete. At higher percentages, fly

ash reduced botn the cor.rlpressive strength 0 1 conc~c;;e and the r~~odulus of eiasticlty.

Young's modulus (of elasticity) values for fly ash concrete (at 28 days) ranging from

18.8-35.5 GPa and Poisson's ratio ranging from 0.14 - 0.25, have been reported at 28

days in the literature [ Crow and Dunstan (1981) and Caretta and Malhotra

(1986)).

2.17 EFFECT OF CURING REGIMES

Bentur and others (1981) investigated oil shale ash compacts cured in C 0 2

atmosphere to enhance initial and final strength, rather than, by conventional moist

curing. Ash samples produced by burning Israeli oil shale in a fluidized bed were used

and compacts prepared from 1: 1 mixtures of finally ground quartz and cementing

material consisting of ash or OPC or both. The compacts were cured in an atmosphere

of C 0 2 over water (COz curing) or in a moist room . Two curing regimes were

followed, i.e. (1) half an hour curing in C 0 2 and then moist curing until 28 days and

(2) continuous curing for 28 days. Based on the above curing regimes and

compressive strength obtained at half an hour, 1 to 28 days, it has been concluded that

(i) half hour compressive strength of compacts of ash samples can be markedly

increased by C 0 2 curing; (ii) half hour strength of ash and ash-rich systems are far

less sensitive to the initial water content; (iii) the combination of initial COz treatment

followed by moist curing only, is recommended based on technical and economical

advantages and (iv) in the case of C 0 2 curing, the ash and OPC systems are

compatible, in contrast to the case of paste hydration.

Gopalan and Haque (1987) reported the strength of normal and fly ash concretes

cured in a fog room and in an uncontrolled environment. The strength of 91 days air-

cured specimens was less than that of 7 days fog-cured specimens. On air-curing, the

percentage loss of strength increased both with an increase in fly ash content and

curing period.

Day and Shi (1994) sti~dicd the effect of initial curing conditions on the hydration

process u f l'orriar~d -po//oiana cemer~i (P l - ' t ' j cor~ia;n;ilg iOfX natural pozzoian (by

wt). Four Guatemalan natural pozzolans were considered and each Portland pozzolana

blend prepared with a water to solids ratio of 0.35 and cast into small cylinders (25

mm x 50 mm). Three curing regimes were considered ( apart from normal curing

for one day), namely, ( 1 ) curing in saturated lime water for 2,6, and 27 days; (2)

dry~ng in laboratory for 27 days ; (3) curing in saturated lime water for 2,6, and 27

days, followed by drying in laboratory until testing at 28 days, for evaluating the

compressive strength of specimens. They have concluded that ( i ) Portland cement

(PC) can still hydrate and pozzolans can have a significant behaviour for a significant

period after specimens are placed in a drying environment; (ii) the period of initial

water curing prior to drying does not have a significant effect on the total measurable

pore volume, but, it influences the pore-size distribution greatly and (iii) it is not

advisable to usc only strength to evaluate the effect of initial water curing; (iv) PPC

pastes should be kept in moist environment at least for three days and PC pastes for at

least 7 days after casting.

Freidin (1999) studied the firmness of the systems (i.e. mortars) containing high-

calcium coal fly ash and low-calcium coal fly ash in varying proportions (both

obtained from power plants in Israel), as well as, the composition and micro-structure

of new formations after long-term exposure to moist air, water and atmospheric air.

Two curing regimes (moist and steam curing) and three exposure conditions ( moist

air, water and atmospheric air) on five different mortar compositions were studied.

Based on the above, it has been concluded that: (i) there is cotltinuous increasc it1 the

compressive strength of high and low-calcium fly ash binders (FAB), under nloist air

and water conditions; (ii) there are three stages in the strength change of atmospheric

air-cured FAB and that the second and third stages depend on high-calcium oil-shale

fly ash (HCOSFA) content and does not depend on curing conditions; (iii) the

combined hydration of high-calcium oil-shale fly ash and low-calcium coal fly ash

"gives additional amount of high strength and stable CSH and improves the durability

of thc hardcncd systern i n atmospheric air and (iv) a decrease in strength during a

cettilln period ~ ~ ' C S ~ O S L I ~ C to atrtlosptleric air, w85 stlli rloted.

Tokyay (1999) invcstigatcd the relationships between the standard 28-days and

accelerated compressive strength of concrctcs made with OPC and different amounts

of (i.e. 10,20, and 40% - by wt.) two high - calcium (CaO content about 14 - 20%)

and two low-calcium fly ashes (CaO content about 2 - 5 % ) to propose a strength

prediction expression for fly ash concrete strengths at different ages (7,28, and 90

days). Control mixes were designed to obtain the characteristic compressive strengths

of 40, GO, 65 and 70 MPa, respectively. The specimens were subjected to four

different curing regimes: ( i ) standard moist curing (Std.) (23" C, 95 RH) until the time

of test; (ii) autogeneous curing (Auto) specimens sealed in plastic bags and placed in

autogeneous curing chamber for 46 hours and tested at 49 hrs. 15 min; (iii) warm

water (WW) curing - sealed specimens immersed into 35" C water for 24 hrs. and

tested at 26 hrs, 15 min after casting; (iv) boiling water curing (BW) - sealed

specimens kept in moist curing room for 23 hrs. and then immersed in boiling water

for 3.5 hrs and test carried out at 28.5 hrs, 15 min after casting. Based on the

regression analysis between accelerated strength and standard strength (upon moist

curing), a power eqn. of the form Rt = A (Ra )B; where, Rt is the ratio of compressive

strength of fly ash concrete at age 't' to the 28 days compressive strength of the

control concrete; Ra - ratio of accelerated strength of fly ash to the 28 days

compressive strength of control concrete. It has been found that the constants 'A' and

'B' depend on the type of fly ash and age of concrete, but, not influenced by the

quantity of fly ash used. It has been stated that the above relationship is more

dependable for 'autogeneous' and 'warm water curing' than for 'boiling water curing'.

Plante and others (2000) studied the effect of temperature on the compressive

strength of concrete cylinders, used for acceptance of concrete in the field. The

specimens were subjected to various curing situations (i.e. seven in number including

nonnal ambient conditions and various temperature and job-site environmental

conditions) and their compressive strength and micro-structure were evaluated after 7

and 28 days. They have concluded that among other factors influencing the

eonlpressive sl~cngth of concrctc cieiivereci ro i'iz j ~ t : bile, iernjiu-aiur:; is of major

importance; non-compliance to code requirements (CSA- Canadian Standard

Association) pertaining to initial curing temperatures contributes to compressive

strength variations of as much as 10 MPa in certain cases.

Rivera - Villarrcal (2001) investigated the effects of using different types of curing

on the compressive strength of concrete with and without large volumes of fly ash

(FA). Portland cement content was kept at 200 kg / m3in all mixtures, but FA, content

was varied from zero to 33, 43, 50, and 56 % ( by wt. of total binder). It was observed

from intermittent spray - water curing at 35' C in the laboratory (every four hours) for

7 days of FA specimens that, there is increase in the compressive strength upto the

time of testing (i.e. 6 months). Reduced strength was obtained for 3 days intermittent

curing. Higher strengths were obtained as the amount of FA was increased, for a given

quantity of the Portland cement. The FA concrete mixtures cast at 35' C were cured

by covering the specimens with membrane curing compound and placed under

ambient conditions until age of testing, the strengths were lower than reference

concrete by about 20 - 30 % at 28 days, and 30 - 50% at 56 days. The necessity of

'enough curing water' to promote pozzolanic reaction has been emphasized.

Duchesne and Marchand (2001) have highlighted the importance of curing on the

hydration of Portland cement with fly ashes in pastes and mortars with 20% fly ash

cement replacement, (by wt.) and a fixed W/ CM = 0.4. Three types of fly ashes

covering Class C and F and having CaO content ranging from 2.75 % to 28.07% were

used for preparing pastes. The samples were cured under three conditions, namely,

(i) sealed under aluminium film; (ii) immersion in a saturated lime solution and

(iii) immersion in a lime saturated solution containing 0.65 M of a mixture of NaOH

(0.25 M) and KOH (0.40 M), simulating the composition of the cement pore solution.

After curing the specimens in the above environments, ranging from 1 to 365 days,

pore solution of paste samples extracted by the high-pressure method, chemical

analysis (i.e. Na, K, Ca concentrations) and compressive strength of mortars were

determined. It was found that (i) curing of samples with fly ashes in a solution

containing alkalis to concentrations simulating the concrete pore solution is favorable

to the deveiop~ncnt ot hycirairor~ and rr~aite i t pvssrhle io Letter i;uiilpnrc thc behaviour

of samples made In the laboratory with concretes in real situation; ( i i ) sealed method

also make it possible to preserve the alkalinity of the samples and (iii) curing samples

in lime water, reduces the alkalinity of pore solution very quickly, which will harm the

hydration of fly ash, especially for Class F ashes.

2.18 EFFECT OF ELEVATED TEMPERATURE ON CONCRETE

To date, many investigations into concrete strength [ Zhang and others (2000 a);

Castillo and Durani (1990); Terro and Hamoush (1997); Felicetti and

Gambarove (1998); Berwanger and Sarkar (1973); Philavaara (1972) ] and

stiffness [Zhang and others (2000 a); Berwanger and Sarkar (1973)l at varied

heating scenarios, have been conducted. The general conclusion is that the strength

and stiffness of concrete decreases with increasing heating temperature, exposure time

and thermal cycles. Research on the effect of elevated temperature on the 'toughness'

of concrete started in the late 1970 s and since then, more frequent studies have been

carried out by various investigators [ Bazant and Kaplan (1996); RILEM

committee 44 - PHT (1985); Schneider (1988); and Phan and Carino (1998);

Felicetti and Gambarova (1998); Prokoski (1995); Bazant and Prat (1998); and

Felicetti and others (1996)], especially on the qualitative aspects of concrete

toughness at high temperatures. Zhang and others (2000 a and 2000 b) were the first

to study and quantitatively assess the effects of various influencing factors on the

concrete toughness, using 'six toughness indices' proposed by them. Based on a large

amount of thermal exposure testing done by numerous investigators on normal

strength concrete (NSC), it is generally believed that NSC loses approximately 25% of

its original compressive strength when heated to 300 O C and approximately 75 % of

its original strength when exposed to 600' C (Phan, 1996).

Ravina (1981) has discussed the effects of fly ash in concrete cured at moderately

elevated temperatures and its advantages in pre-cast operations. Ravina examined

concrete made with fly ashes of two size fractions from the source (i.e. 30 - 35%

retained on a 45 m sieve and 14-17 % retained on a 45 m s ieve) for making two

series of non-air-entrained mixtures. In the first series, coarse ash alone replaced either

all or 50% : b y ~ 0 1 . 1 of pit sand 2nd in the second series, fiue t7y ash rc;piai;ed 2 ~ 9 6 of

the cement (by wt.) and coarse ash reproduced 50% (by vol.) of the sand. Specimens

initially cured at 23" C for 2 hours are then transferred to a stream chamber, where,

the temperature was raised from 23°C to 75°C (over a 2 hour period) and maintained

at 75°C: for 4 hours and for followed it by cooling the specimens for 22 hours. From

the compressive results obtained, he drew the following conclusions: (i) large

quantities of fly ash in concrete cured at elevated temperatures significantly improve

its compressive strength; (ii) curing coarse and fine fly ash concrete at elevated

temperatures has a significant beneficial effect on the strength of concrete at early and

later ages.

Carette and others (1982) studied concretes with normal Portland cement, slag and

fly ash at sustained temperatures 600 O C . The maximum replacement of cement was

25% in OPC-fly ash mixture. It has been found that in general , incorporation of fly

ash appears not to influence the behavior of concrete at elevated temperatures and loss

of strength and changes in other structural properties occur at approximately the

same temperatures for both types of concrete.

Bhal and Jain (1999) developed expressions for predicting the 'residual strength

factor' ( T) for hot and cold compressive strength, through a best fit curve, based on

published experimental results on the influence of elevated temperatures on

compressive strength of concrete. The values obtained from the expressions developed

by them were compared with the recommendations of Malhotra (1982); Joint

Committee's recommendations (1978) and Ellingwood and Lin (1991). It is found

that the expressions proposed by them are simple and are in better agreement with the

test results in comparison to the above three expressions.

Sharada Bai and others (2000) studied the effect of elevated temperature on

concrete made with blended cement containing fly ash. The behaviour of concrete

specimens made with blended cement was compared with M20 concrete using 43 and

53 grade cements. Four different temperatures (125" C, 200" C, 400" C and 600" C),

three testing conditions (hot, heating and cooling, and heating and water cooling) and

three heating periods (one hour, two hours, and three hours), were considered. The

results obtained were compared with four theoretical expressions for predicting the

residual strength of concrete already available in published literature and it has been

concluded that more accurate theoretical predictions are necessary, wherein, the

duration of heating and the condition of testing are considered as additional variable

parameters.

Hoff and others (2000) have highlighted the residual strength determinations for

three different types of HSC used in oil and gas facilities: nonnal density concrete,

lightweight concrete, and intermediate density concrete called modified normal

density. Cylindrical specimens of the above concrete were exposed to various

temperatures ranging from 100" C to 1100" C, in electrical furnaces. The above range

of temperatures correspond to the anticipated range of temperatures associated with

hydrocarbon (HC) fires. Based on the above study, they have concluded that (i) there

is a slight improvement in residual strength at ZOO0 C when con-qared to ,100° C; (ii)

at exposure temperatures of 300" C and higher, there is a significant loss of strength;

(iii) at temperatures greater than 900" C, all the concretes studied had no structural

integrity; (iv) residual strengths of HSC exposed to 300' C or higher, are not

significantly different than residual strengths for NSC.

Sun and others (2000) carried out experiments to study compressive strength and

micro-structure of high performance concrete (HPC) and nonnal strength concrete

(NSC) subjected to a peak temperature of 800" C. compressive strength, changes in

porosity, pore-size distribution (using mercury intrusion porosimetry - MIP) were

determined after exposure. Low-calcium fly ash (LCFA) (about 28 % of cement + LCF for HPC and about 25% for NSC) were used along with OPC, SF and SP. Three

series of HPC, namely, without fibers, reinforced with 0.2% (by vol.) steel fiber

(aspect ratio= 60, length -25mm) and reinforced with 0.2% (by vol.) polypropylene

fiber ( length=19 mm and aspect ratio = 360) were considered along with NSC. Cube

specilnens (100mm) cured nc-n~!!y for three months were subjected to elevated

temperature upto 800" C, in a computer controlled electric furnace. Based on the

results obtained, it has been concluded that (i) HPC had higher residual strength,

although the strength of HPC degenerated much more than the NSC after exposure to

high elevated temperature; (ii) the variations in pore structure, including porosity and

pore-size distribution, could be due to indicate the degradation o f mechanical

properties of HPC subjected to high elevated temperature and (iii) the relationship

between the strength and the corresponding porosity of concrete can be expressed in

the form fcu =177.45 exp (- 7.102 P), where, fcu - compressive strength of HPC

(MPa); P- porosity (%), with R= 0.964.

2.19 EFFECT OF AGGRESSIVE ENVIRONMENTS ON CONCRETE

Introducing fly ash as a component of concrete, has been shown to influence the

concrete's resistance to chemical attack. Biczok (1964) enumerated from conditions

related to concrete quantity and the constituents of concrete on which the destructive

effects of aggressive waters depend: (1) type of cement used and its chemical and

physical properties; (2) quality of concrete aggregates and their physical properties

and gradation; (3) method used for preparing concrete, the W/C, the proportion of

cement and the placement and (4) condition of the surface exposed to the water. Of

the above, condition 1 relates strictly to the nature of the cementitious binder used,

whereas, conditions 2-4 apply to one or more aspects of the permeability of concrete.

With regard to cement type, two factors are influential in determining the relative

durability of fly ash concrete, namely, (i) the chemical composition of the cement, vis-

a-vis the cernentitious components produced during hydration, has a pronounced

influence on resistance to chemical action; (ii) a combination of chemical conlposition

and physical properties - notably fineness, detentlines the rate at which cement

hydration proceeds.

One of the earliest studies on sulphate resistance of fly ash concrete (blended or plain

fly ash) was carried out by Dikeou (1970) and he col~cludcd that all the 12 fly ashos

(originated from bituminous coals) testcd by him grcattly ilnproved sulphnte

resistance. However, until 1980, not much work was reported.

Krishnamoothy (1976) studied the durability of OPC fly ash concretes (OPC

partially replaced by fly ash - 10% to 40% ) exposed to a few typical environmental

conditions like marine and those encountered in food preservation or dairy industries.

Fly ash blended cement concrete specimens (i.e. 20% cement replacement) were kept

immersed over extended periods (over 3 months) in a simulated sea water medium

containing sodium chloride and magnesium sulphate. Fly ash blended cement concrete

and beams were exposed to carbonic acid, 5% acetic acid and 1.5% lactic acid

mediums. It was found that fly ash blended cement concrete and reinforced beams

maintained its strength properties when exposed to the above medium, where as,

control concrete suffered a relative reduction in strength. It has been concluded that

the perfonnance of fly ash blended concrete and reinforced beams were relatively

better in the above environments than control concrete.

Dunstan (1980) published a report summarizing the results of a five-year study on

sulphate attack of fly ash concretes, including a theoretical analysis of sulphate attack

and its causes. Dunstan's basic postulate is that CaO and Fez O3 in fly ash are the main

contributors to the resistance or susceptibility of fly ash concrete to sulphates. In

order to select fly ashes that can improve the sulphate resistance of concrete, Dustan

proposed the use of a 'resistance factory( R), given by R=(C-5)IF, where C- CaO

content in%; F- Fez O3 in %.

Dunstan (1987) examined the 'R' value as an 'advance indicator' of potential

sulphate resistance of concretes containing fly ash after 12 years of testing and the

results indicated that R value remained a good indicator of potential sulphate

resistance of fly ash concretes. I t was found that for concrete containing 15 - 25% fly

ash with an R value of c 3, the sulphate resistance would be as good as or better than

that of a concrcte with the same content and without fly ash.

Krishnamurthy and Jain (1987) conducted prolonged experiments spanning over a

year on the variation of compressive strength of mortar specimens ( 1 :3) made with

four (ypcs of cement (two OPCs and two fly ash blends) with age of curing in

;;zificislfy prCP3;~d b L ~ f e r a i ~ d t ! t ~ POT.' S~TL!C!L~~C ~ ! ! " ! ! l r + j . .T - %!??c >;?ccI~~:c:Ic

were also cast using sea water. From the results it has been pointed out that (i) curing

of cement mortar specimens in sea water results in deterioration of compressive

strength which is more, when sea water is used as mixing water; (ii) the deterioration

can be attributed to the formation of larger sized pores, which is considered due to

removal of Ca(OH)2 either by leaching or formation of calcium compounds and (iii)

blending of fly ash with cement (about 25 % by wt) inhibits the formation of larger

sized pores, as Ca(OH)2 is stabilized by pozzolanic reaction.

Tikalsky and others (1990) reported the effect of 24 different fly ashes on the

sulphate resistance of fly ash concretes and found that Dunstan's 'R-factor' could, in

general, provide a good indication of the effect of fly ashes on the sulphate resistance

of concrete. They have also found that the effect of a given fly ash on the sulphate

resistance of concrete, was not dependent on the fly ash replacement level (of cement)

within the range of 25-45 % (by vol.). It has been opined that the influence of fly ash

on the sulphate resistance of concrete is not completely understood, and much more

research is needed to establish guidelines on this important aspect of concrete

durability ( Malhotra and Ramezanianpour, 1994).

Sharma and others (1992) carried out laboratory and field investigations on six

blended cement concrete to study their relative performance. Among other things,

change in compressive strength and ultra-sonic pulse velocity were considered to

evaluate the performance of concrete exposed to chemical mediums (4% solutions)

such as ammonium sulphate, sodium chloride, sodium sulphate, sodium chloride,

ammonium chloride, and combinations of 4% solutions of sulphates and chlorides of

sodium and sulphates and chlorides of ammonium. Ordinary Portland cement with fly

ash addition (OPF) and sulphate resisting celllent with fly ash addition (SRF), were

among the six types of cement considered for making concrete. Rased on the results

after one year of immersion in various chemical solutions, it was observed that ( i )

concretes with OPF provided better resistance to sulphate attack; (ii) in chloride

environments, OPF (including three other types of concrete) perform relatively better

than sulphate resistance OPC bascd concrete and ( i i i ) cullcretes ::*ith fllu. :xfi :?..

admixture were the best in their overall performance in aggressive environments

amongst the various cement concretes tested.

Rostani and Silverstrim (1996, 1997) developed an alkali-activated material called,

chemically-activated fly ash (CAFA), which is an activated Class F fly ash. The

resistance of CAFA concrete to chemical attack by acids such as, sulfuric, nitric,

hydrochloric and organic acids was claimed to be far better than that of Portland

cement concrete (Rostami and Silverstrim, 1996). According to Silverstrim and others

(1997), C A F A specimens exposed to 70% nitric acid for 3 months retained its dense

micro-structure.

Naik and others (1998) evaluated the effects of blended fly ash in mechanical

properties and durability of concrete. Three blends of ASTM Class C and Class F fly

as11 were considered with a total fly ash content of 40% of the total cementitious

material. Various mechanical and durability properties were evaluated and compared

with two reference concretes , namely, concrete without fly ash and another with 35%

ASTM Class C fly ash. It was found that all the concretes with or without fly ash

exhibited excellent resistance to abrasion at 28 days and 91 days. At 28 days, the

depth of abrasion was sliglltly higher for the fly ash concretes than the non-fly ash

mixture. At 91 days, blends B2 and B3 showed excellent resistance to abrasion

compared to reference mixtures.

Based on long-term studies initiated and carried out by CANMET on the performance

of fly ash blended concrete in marine environment, it was concluded that concretes

incorporating 25% fly ash from a bituminous sourcc were in satisfactory condition

even after 10 years and with the only exception were the specinlens with W/ ( C t F A )

of 0.0 and hence, it was emphasized that the W/ (C'tFA) ratio should be 0.5 0 .

[Malhotra and Others, 1988 and 19921.

Khaloo and Kim (1999) investigated the influence of seven curing conditions on the

con~pressive strength, splitting tensilc strength and elastic modulus of lightweight

high strength concrete (LWI-ISC') at 7 and 28 days. The curi!~g cunctitiol~s considcrcc!

are : air-cured at 13' C and 24' C (AL and AH); moist-cured at 13' C and 24" C under

polyethylene sheet (PL and PH); moist-cured with 100% humidity at 13" C (PL) and

submerged in water at 13" C and 24" C (SL and SH). The results indicate that (i) the

PH and SH curing conditions are capable of producing the highest compressive

strength at 7 and 28 days and that they are found to be about 12% (i.e. 10 MPa) higher

than the strengths obtained by other curing conditions; (ii) the curing conditions had a

less profound effect on the tensile strength ( at 28 days); (iii) in general, the results for

elastic modulus and compressive strength were compatible and the highest response

belonged to PII, SII and ML curing conditions.

Kumar and Singh (2000) studied the effect of fly ash blended concrete (Class F) in

varying proportions upto 50% of total blend) on the sulphate resistance characteristics,

when the above specimens were exposed in sulphate solution having sulphate

concentration (as SO, -) equal to 10,000 ppm. The reduction in compressive strength

of concrete due to the above exposure was denoted by the strength deterioration

factor (SDF) proposed by Kumar (2000) and given by SDF=(I- ,, 1 , ), where, aR -

average compressive strength (MPa) of concrete specimens after exposure to sulphate

solution for 'T' days; .-average compressive strength (MPa) of concrete specimens

cured in ordinary water for the same 'T' days. Based on the results, it has been

concluded that low amount of fly ash addition (25%) in blended cement concrete

improves the resistance of concrete against sulphate attack and that the performance

is better than plain cement concrete.

Allallverdi and Skvara (2001) studied the mechanism of acid corrosion on geo-

polymeric cements, produced according to the work of Skv ra and Dohull k ( 1004).

Fly ash and blast furnace slag (from power and steel plants of Czech Rep.) were first

ground in a vibration mill to attain higher specific area (650 and 420 m2 /kg,

respectively). The ground ash and slag were mixed in equal proportions. A mixture of

NaOH and Na, SiO was used as the alkali activator, such that, the total Na, 0 content 3 - of the binder amounted to 7% of the binder weight and used for preparing a paste of

acceptable: workability. .After 2.Q days of curing (normal cum elcvated tenaperaturcir.

the specimens were immersed in six different solutions of nitric and sulfuric acids 0 8

with pH values 1,2,3, for 60 days. Based on SEM and chemical composition of

corroded specimens, it has been concluded that: (i) geo-polymeric cements are less

vulnerable to acid attack and (ii) the mechanism and extent of attack depends on the

type and the concentration of the attacking acid, as well as, the chemical composition

of the geo-polymeric cement.

Deepa and Elizabeth (2001) studied the strength and sulphate resistance

characteristics of M25 and M35 fly ash concretes proportioned using the 'cementing

efficiency factor' (K) suggested by Ganesh Babu and Rao (1994, 1996) for various

replacement levels of fly ash (ranging from 15 - 75% and at 7 - 90 days). The

sulphate resistance test was carried out as suggested by Mehta and Gjorv (1975),

(i.e. curing in 4% Na2 SO4 solution at pH = 6.2) and the strength-loss evaluated after

28 days of immersion in the sulphate medium. It was found that no 'adverse effect'

was observed on the high-volume fly ash concrete containing upto 50% cement

replacement by fly ash.

2.20 ABRASION AND EROSION OF FLY ASH CONCRETE

Under many circumstances, concrete is subjected to wear by attrition, scraping or the

sliding action of veliicles, ice, and other objects. When water flows over concrete

surfaces, erosion may occur. In general, regardless of the type of test performed the

abrasion of concrete is usually found to be proportional to its compressive strength

(Neville - 1973). Similarly, at constant slump, resistance to erosion improves with

increased cement content and strength. I t may be anticipated that fly ash concrete that

is incompletely 1 inadequately cured may show reduced resistance to abrasion.

Abdun - Nur (1961) has indicated that abrasion resistance niay be reduced in fly ash

concrete. However, in his work, there is no indication whether atte~npts were made to

compare fly ash concrete and plain concrete at equal strength or equal n~aturity.

Liu (1980) examined the abrasion-erosion resistance of concrete using a 11cwly

developed underwater abrasion test. Performance of fly ash concrete, containing ZS'?,,

(by vol.) replacement of Portland cement, cured Sor 00 days to an average

compressive strength of 49 MPa, was compared with that of a concrete of similar

mixture proportions without fly ash and cured for 28 days to an average compressive

strength of 47 MPa. Little difference in abrasion resistance was found between the two

concretes for test periods 36 hours. However, after 72 hours, the fly ash concrete

had lost about 25% more weight (i.e. 7.6 % - wt. loss) to abrasion - erosion, than, the

control (i.e. 6.1 % loss) concrete.

Carrasquille (1987) examined the abrasion resistance of concretes containing no fly

ash, 15 Oh ASTM Class C or 35% ASTM Class F fly ashes and cast for similar

strengths. I t was found that the resistance to abrasion of concrete containing Class C

fly ash was greater than that of the concrete containing Class F fly ash or no fly ash.

Naik and others (1992) carried out investigations on the compressive strength and

abrasion resistance of concrete containing ASTM Class C fly ash, by replacing cement

in the range of 15 - 17% (by wt.) and with WICM = 0.3 to 0.37. Their results showed

that the abrasion resistance of concretes containing 30% fly ash was similar to that

of control concretes. However, abrasion resistance of concretes, containing > 40% fly

as11 was lower than that of control concrete (without fly ash).

2.21 CONCLUDING REMARKS

Rased on the reported literature on the use of fly ash (low I high-volumes either in

blended I activated form) as a binder in mortar / concrete, following observations have

( i ) Fly ash has been generally considered as a partial replacenlent of cernent, in

varying quantities ranging form 20% to 70%) and to some extent partial

replacement of fine aggregate;

(ii) Studies on fly ash-cement pastes, were focused towards understanding the

hydration process;

(iii) Studies on f ly ash-cement based mortars and concretes were inttnded to achieve

iomp&:ab?.: :)r !lig!jrt stre!lgth characteristics and /or rl~irabilitv than 'cc>nt!oi7 or

'reference' mortar 1 concrete, through judicious choice and combination of

matertais and proper mixture-proportioning approaches;

(iv) The desire to achieve comparable early-strength and the need to convert

industrial wastes, like, fly ash, gypsum, etc. led to the concept of 'activation' of

pozzolans, in recent times, on binary / ternary blends with and without OPC;

(v) Studies on high-calcium fly ash based mortar I concrete, in general, have not

been that extensively carried out and reported, when compared to low- calcium

fly ash-based systems;

(vi) Even the exhaustive investigations carried out at International level and reported,

especially, on the alkali-activation of natural pozzolans and fly ashes, have been

carried out on 'demonstration scale' or confined only upto their application as a

mortar. Moreover, reagent grade chemicals I lime as activators and higher

temperatures (i.e. > ambient conditions) have been used, to achieve sufficient

early - age and later -age strength;

(vii) Studies on the reported innovative blend, based on fly ash ,lime and gypsum

(FaL-G) and its applications, in India and elsewhere, were not comprehensive,

with respect to the type of fly ash, strength and durability characteristics. The

need for the addition of lime to high-calcium fly ash along with gypsum has not

been investigated fi~lly and reported. Moreover, attempts have not been made to

use 'materials as available', nor to activate the pozzolanic reaction and exploit

the full potential of fly ash and other industriai wastes.

From the above observations, it can be seen that there is an immense need to carry out

systematic and comprehensive research on the utilization of high-calcium fly ash and

other industrial wastes, like gypsum, to develop a binder of sufficient strength by

exploiting their inherent pozzolanic / cementing characteristics at normal temperature

and evaluate the strength and durabiiity characteristics of concrete, based on such a

binder, to demonstrate the potential use, in Civil Engineering constructions.