Hydrothermal characteristics of high-slag cement pastes made with and without silica sand

Advances in Cement Research, 2012, 24(1), 23–31

http://dx.doi.org/10.1680/adcr.2012.24.1.23

Paper 900040

Received 19/09/2009; revised 17/05/2010; accepted 22/07/2010

Thomas Telford Ltd & 2012

Advances in Cement ResearchVolume 24 Issue 1

Hydrothermal characteristics of high-slagcement pastes made with and without silicasandAmin, Habib and Abo-El-Enein

Hydrothermal characteristics ofhigh-slag cement pastes madewith and without silica sandMohammed S. AminFaculty of Science, Ain Shams University, Cairo, Egypt

Amr O. HabibFaculty of Engineering, Ain Shams University, Cairo, Egypt

Salah A. Abo-El-EneinFaculty of Science, Ain Shams University, Cairo, Egypt

High-temperature steam curing of different cementitious materials is almost always associated with building

products having improved physicomechanical and binding characteristics. The properties of these autoclaved

products are always governed by the chemical composition and physical state of the formed hydration products

which act as the main binding centres. In the present study, high-slag cement (HSC) and HSC–ground sand (GS)

specimens were hydrothermally treated at pressures of 8 atm of saturated steam for different autoclaving periods.

Compressive strength tests were done on the hydrothermally hardened specimens; then hydration kinetics, phase

composition and microstructure of the formed hydrates were studied using the ground, dried samples. The results of

compressive strength could be related to the phase composition and microstructure of the formed hydration

products. These results were also explained on the basis of reactivity of both HSC and GS towards the hydrothermal

reaction.

IntroductionAbo-El-Enein et al. (1977) studied the morphology and micro-

structure of autoclaved slag–clinker and slag–clinker–sand mix-

tures by using scanning electron microscopy (SEM). The

microstructure of the autoclaved slag–clinker hydration products

displayed nearly amorphous, fibrous and radial plates of calcium

silicate hydrate C–S–H phase, large cubes of hydrogarnet-like

hydrates (mainly as C3ASH4) as well as hexagonal precipitation

of calcium hydroxide (Ca(OH)2). The hydration of slag–clinker–

sand mixture was associated with the formation of semi-crystal-

line low-lime tobermorite, mainly as C–S–H (I), as well as

fibrous plates and crystalline 11 A tobermorite as the main

hydration products. The strength development could also be

related to the microstructure of the formed hydrates.

Several investigations were reported regarding autoclaved build-

ing products obtained by steam curing of different mixtures

including hydraulically active materials, of these: (a) autoclaved

slag–quartz–kaolin products, (b) hydrothermal reaction products

of granulated blast furnace slag, (c) autoclaved clinker and slag-

lime pastes in the presence and in the absence of silica sand and

(d ) autoclaved slag–clinker pastes in the presence and in the

absence of silica sand (Abo-El-Enein et al., 1981, 1985, 1978,

1994). These studies reported the kinetics and mechanisms of the

hydrothermal reactions studied, the phase composition and micro-

structure of the autoclaved specimens as well as the surface area

and pore structure of the hydrothermally hardened building

products.

Nurse (1964) showed that slags of suitable composition, in the

glassy (amorphous) state, were hydrated quite readily, provided

that a suitable activator was present. For Portland cement–slag

mixtures, activation of the slag was accomplished by the calcium

hydroxide liberated during the hydration of Portland cement.

Alexanderson (1979) discovered that the strength was propor-

tional to the ratio of hydrated calcium silicate to calcium

hydrogarnets. The compressive strength increased as the amount

of calcium silicates hydrate increased. At high temperature of

autoclaving, the formation of hydrated calcium silicates was

accelerated. Chiocchio et al. (1975) discovered that the absence

of ettringite can explain the resistance of autoclaved cement

pastes and concretes to sulfate attack. The stability of quartz

(sands) for the production of high-density silicate concretes was

studied (Klimesch et al., 1996; Ostmann and Efimova, 1974). The

specimens were prepared by variation: packing of a mixture of

ground quartz (sand), lime and cement with water, and then

hardened in an autoclave at 8 atm.

The hydration mechanism of hydrothermally hardened slag alite

(C3S) pastes was investigated (El-Didamony et al., 1982). Early

strength development is claimed to be mainly a result of slag alite

hydration, whereas the later strength was attributed to the

hydration products of the granulated blast-furnace slag. A

mechanism of the hydrothermal process involving slag alite and

slag grains was postulated on the basis of the results. Abd-El-

Wahed et al. (1989) investigated the effect of quartz on the

hydration characteristics of hydrothermally hardened granulated

23

slag–slag alite pastes by studying the compressive strengths of

the hardened product, determination of free lime and combined

water contents.

Galal et al. (1993) studied the hydration and microstructural

characteristics of autoclaved cement mixtures prepared from

cement kiln dust, quartz and blast-furnace slag. Isu et al. (1994)

investigated the chemical reactions in autoclaved concrete blocks

containing blast-furnace slag. The effect of activator dosage,

temperature and water/slag ratio on early hydration of alkali-

blast-furnace slag cements activated with sodium hydroxide

(NaOH), sodium carbonate (Na2CO3) and sodium silicate

(Na2SiO3) was studied (Shi and Day, 1996). Xi et al. (1997)

showed that the compressive strength of slag cement paste

containing 67.5 wt% granulated slag decreases with time after

24 h hydrothermal processing, while the cement strength in-

creases and the pore structure densifies when processed under

complete conditions. Extensive studies were carried out on

autoclaved calcium silicate hydrates, including rate of formation

and molar composition of the formed hydrates, surface area and

pore structure of the hydration products and phase composition

and microstructure of the various CSH formed (Abo-El-Enein et

al., 1990, 1992).

Pane and Hansen (2005) investigated the hydration of Portland

cement pastes containing three types of mineral additive – fly

ash, ground-granulated slag and silica fume – using differential

thermal analysis, thermogravimetric analysis (DTA/TGA) and

isothermal calorimetry. Luke (2004), studied the calcium silicate

hydrates formed at elevated temperature and pressure. At 1808C,

the initially formed amorphous calcium silicate gel [C–S–H]

transforms into well-defined crystalline phases, the stability of

which is primarily dependent on the C/S ratio in the calcium

oxide (CaO)–silicon dioxide (SiO2)–water (H2O) system and the

hydrothermal conditions. Hillebrandite [C2SH], a-dicalcium sili-

cate hydrate [a-C2SH] and h-tricalcium silicate [h-C6S2H3] are

predominantly the stable phases in the lime-rich part of the

calcium oxide–silicon dioxide–water system and are typically

associated with high permeability and compressive strength retro-

gression. Gyrolite [C2S3H,2], tobermorite [C5S6H5], truscottite

[C7S12H,3] and xonotlite [C6S6H] have all been reported to

coexist stably in aqueous solution with silica in the silica-rich

part of the calcium oxide–silicon dioxide–water system.

Siauciunas and Baltakys (2004) explained the parameters of

gyrolite hydrothermal synthesis. Primary mixtures consisting of

calcium oxide and amorphous hydrous silicon dioxide

(SiO2:nH2O) or quartz and a sequence of intermediary compound

formation. Kazumichi et al. (2006) studied the hydration of

�-dicalcium silicate, carried out under hydrothermal conditions at

different temperatures from 508C to 4008C. Kourounis et al.

(2007) investigated the properties and hydration of blended

cements with slag. A reference sample and three cements

containing up to 45% w/w steel slag were tested and initial and

final setting time, standard consistency, flow of normal mortar,

autoclave expansion and compressive strength at 2, 7, 28 and 90

days were measured. The hydrated products were identified by

X-ray diffraction (XRD) while the non-evaporable water was

determined by TGA. The microstructure of the hardened cement

pastes and their morphological characteristics were examined by

SEM.

The object of this investigation is to study the physicochemical

and mechanical characteristics of autoclaved specimens made of

high-slag cement in the presence and absence of silica sand.

Experimental workThe starting materials used in this study are high-slag cement

(HSC) as obtained from the National Cement Company and local

ground sand. The chemical composition of the HSC is shown in

Table 1. HSC consists of 25% ordinary Portland cement (OPC)

and 75% granulated blast-furnace slag (GBFS). In fact, both HSC

and sand represent the two reactants of the hydrothermal

solidification process. Four mixes were prepared by mixing HSC

with ground sand (GS) with different weight percentages as

shown in Table 2. The dry mixtures shown in Table 2 were mixed

using ethanol for 1 h in order to assure complete homogeneity of

the mixture.

All dry mixtures were mixed with distilled water using the same

initial water/solid ratio of 0.20 for 3 min. From the paste

produced, cylindrical specimens of 3.14 cm2 cross-sectional area

and 2 cm high were moulded at a pressure of 50 kg/cm2: The

specimens were first cured at 100% relative humidity at room

temperature for 6 h to attain the initial setting. Then the speci-

mens of each mix were autoclaved at 8 atm of saturated steam for

0.5, 2, 6, 12 and 24 hours. At the end of each autoclaving period,

the specimens were dried, after being removed from the auto-

clave, in a carbon-dioxide-free atmosphere at 1058C for 24 h to

remove the free water.

At each autoclaving period, compressive strength tests were

Oxide Silicon

dioxide

(SiO2)

Aluminium

oxide

(Al2O3)

Iron

oxide

(Fe2O3)

Calcium

oxide

(CaO)

Magnesium

oxide

(MgO)

Sulfur

trioxide

(SO3)

Sodium

oxide

(Na2O)

Potassium

oxide

(K2O)

Titanium

dioxide

(TIO2)

Phosphorus

pentoxide

(P2O5)

Loss in

ignition

% 2.38 6.72 3.105 55.74 3.325 2.408 0.145 0.1 0.065 0.035 1.22

Table 1. Chemical oxide composition of HSC

24


Hydrothermal characteristics of high-slagcement pastes made with and withoutsilica sandAmin, Habib and Abo-El-Enein

carried out on the dried specimens. Free lime and combined

(non-evaporable) water contents were then determined using the

ground, dried samples according to the methods reported in

earlier investigations (Aiad et al., 2008; Kondo et al., 1975). The

phase composition of the formed hydrates was identified using

XRD analysis and differential scanning calorimetry (DSC). The

morphology and microstructure of hydrated phases were identi-

fied using SEM.

Results and discussion

Compressive strength

The results of the compressive strength of autoclaved and HSC–

GS mixes (mixes I–IV) are shown in Figure 1; the experimental

points are an average of three measurements with standard

deviation less than 3–4%. Evidently the strength values increases

with increasing age of autoclaving at 8 atm of saturated steam for

all autoclaved mixes (I–IV). The results of Figure 1 show also

that the autoclaved specimens made of the neat HSC (mix I)

possess relatively lower strength values than those of the speci-

mens made of autoclaved HSC–GS mixes. This result is mainly

associated with the formation of hydration products having

relatively weak hydraulic character for autoclaved HSC speci-

mens, although granulated blast-furnace slag is more reactive in

the hydrothermal reaction than ground sand. The partial substitu-

tion of HSC by GS (mixes II–IV) results in an increase of the

strength values of the specimens made of HSC–GS mixes at all

ages of autoclaving. In fact, the free lime liberated from the

hydration of Portland cement fraction of HSC acts as an activator

for hydration of both blast furnace slag (present in HSC) and

silica sand in the hydrothermal reaction. The increase of sand

fraction of autoclaved HSC–sand mixes is accompanied by a

marked decrease in the free lime content, see later in Figure 3; in

addition, the remaining unhydrated parts of sand grains contribute

strongly to the strength of autoclaved HSC–sand specimens.

Furthermore, the formation of hydration products having high

hydraulic character is associated with an increase in the content

of the binding centres in autoclaved HSC–GS mixes (mixes II–

IV). Evidently, it was found that the autoclaved specimens of mix

III, composed of HSC (80%) and GS (20%), possess the highest

strength values at all ages of the hydrothermal solidification

process; a result which is mainly associated with the phase

composition of the formed hydrates, as will be discussed later in

this study. These findings are in agreement with the results

reported in earlier publications (Abo-El-Enein et al., 2002; Salem

et al., 1995).

Hydration kinetics

Chemically combined water content

The results of combined (non-evaporable) water content (Wn, %)

of all autoclaved HSC and HSC–GS mixes (I–IV) are shown in

Figure 2. Obviously, the combined water content values increase

with increasing age of autoclaving for all the autoclaved mixes

(I–IV); this is attributed to the relatively high hydrothermal

interaction at the very early ages of autoclaving (0.5 h) with a

continuous slight increase in the Wn values at the later autoclav-

ing ages (2–24 h). The results of Figure 2 show also that the

autoclaved specimens made of HSC–GS mixes (mixes II–IV)

possess relatively lower combined water contents than those of

the autoclaved specimens made of HSC (mix I); this indicates

that the hydration products of the HSC hydrothermal reaction

possess relatively higher water contents than those of HSC–GS

Mix Weight percentage: %

HSC Sand

I 100 0

II 90 10

III 80 20

IV 60 40

Table 2. Mix composition of HSC–GS dry mixtures

800

600

400

200

0

Com

pres

sive

str

engt

h: k

g/cm

2

100% HSC90% HSC 10% GS�80% HSC 20% GS�60% HSC 40% GS�

0 6 12 18 24Autoclaving age: h

Figure 1. Compressive strength plotted against autoclaving age

for HSC and HSC–GS blends

15

10

5

Che

mic

ally

com

bine

d w

ater

con

tent

: Wn,

%

100% HSC90% HSC 10% GS�

80% HSC 20% GS�

60% HSC 40% GS�


Figure 2. Combined water content plotted against autoclaving

age for HSC and HSC–GS blends

25



hydrothermal reaction, as well as the higher reactivity of

granulated slag than GS towards the hydrothermal reaction.

Free lime content

The results of free lime content (calcium oxide (CaO) %) are

shown in Figure 3 for all autoclaved blends (mixes I–IV).

Evidently, the free lime content decreases with increasing age of

autoclaving for all of the autoclaved HSC and HSC–GS mixes

but with different rates. A marked decrease in the free lime

content was observed during the first 6 h of autoclaving of HSC–

sand blends (mixes II–IV), which reflects the rapid consumption

of the free lime release of the OPC fraction of HSC as a result of

its hydrothermal interaction with silica sand. Obviously, the free

lime content is almost consumed after 2 h of autoclaving of the

sand-rich blend (mix IV); meanwhile, relatively high values of

free lime content are obtained for autoclaved HSC specimens

(mix I) at all ages of autoclaving.

Phase compositionThe phase composition of the formed hydration products was

identified using XRD analysis, DSC and SEM.

X- ray diffraction

The results of XRD analysis obtained for all autoclaved mixes

I–IV after 6 h of the hydrothermal process are shown in Figure 4.

The main phases identified are calcium silicate hydrates (CSH),

(ASTM Card 83-1520); composed mainly of 11 A tobermorite

having relatively lower calcium oxide/silicon dioxide ratios at

later ages of autoclaving, calcium hydroxide (CH) (ASTM Card

04-0733) and quartz (Q) (ASTM Card 05-0494). The remaining

unhydrated parts of C3S and �-C2S (ASTM Card 23-1043),

Æ-dicalcium silicate hydrate (Æ-C2SH) (ASTM Card 03-0566)

could also be distinguished in autoclaved hydration products of

mix I (HSC) while the quartz phase appeared in the diffracto-

grams of mixes II–IV. Evidently, the intensities of the peaks

characterising the portlandite phase (CH) decreases from mix I to

mix III and completely disappears in the specimens made of mix

IV; this is mainly attributed to the hydrothermal interaction of

free calcium hydroxide with quartz in mixes II–IV. The intensity

of the peak characteristic for Æ-C2SH phase decreases for the

autoclaved specimens made of mixes II–IV. The intensities of the

peak characterising the quartz phase (Q) increases from the

autoclaved specimens made of mix II to those of mix IV. The

intensity of the peak characterising CSH in the autoclaved

specimens, after 6 h of the hydrothermal reaction, decreases from

mix I (100% HSC) to mix IV (60% HSC + 40% GS); this is

attributable to the decrease of the amount of free lime liberated

from the Portland cement fraction of HSC with increasing ground

sand (GS) content from mix II to mix IV. Earlier studies on the

hydrothermal interaction between Portland cement and sand

showed that CSH is the main hydration product and the degree of

its crystallinity increases with increasing time of autoclaving

(Abo-El-Enein et al., 2002).

The increase in sand content at the expense of HSC leads to the

formation of less amount of CSH having low C/S molar ratio

with different degrees of crystallinity. The complete consumption

of free lime content of autoclaved HSC–sand specimens (mixes

II–V) is mainly attributable to the deficient amount of Portland

cement fraction in these mixes, Figure 3. This is also confirmed

by the lower Wn values of these HSC–sand mixes, Figure 2.

Differential scanning calorimetry (DSC)

The DSC thermograms of autoclaved specimens made of mixes

I–IV are shown in Figures 5–8, respectively, at different times

(0.5, 6, 24 h) of the hydrothermal process. The thermograms

indicate four endothermic peaks at 105–200, 380–410, 490–510

and 580–5908C. The first endotherm located at 105–2008C

100% HSC90% HSC 10% GS�80% HSC 20% GS�60% HSC 40% GS�


4

3

2

1

0

Free

lim

e co

nten

t: C

aO, %

Figure 3. Free lime content plotted against autoclaving age for

HSC and HSC–GS blends

Cou

nt/s

2 theta: degrees20 30 40

Mix IV

Mix III

Mix II

Mix I

Q

Q

Q

CH

Q

Q

Q

Q Q

Q

Q

CH

CH

CH

CSH

CSH

α-C

SH 2α

-CSH 2

α-C

SH 2

α-C

SH 2

C S3

C S3

C S3

C S3

�-C S2

�-C S2

�-C S2CSH

Figure 4. X-ray diffraction patterns of autoclaved HSC (mix I) and

HSC–GS blends (mix II, mix III and mix IV) after 6 h of autoclaving

26



represents the decomposition of tobermorite-like CSH as well as

the sulfoaluminate hydrates. The enthalpy of this endotherm

decreases from 105.12 to 64.13 and 31.53 J/g at 0.5, 6 and 24 h

of autoclaving of mix I made of 100% HSC and from 61.26 to

60.71 and 47.67 J/g at 0.5, 6 and 24 h of autoclaving of mix II

made of HSC (90%) and GS (10%) (Cong and Kirkpatrick,

1995; Santoro et al., 1984). However, the enthalpy of this

endotherm decreases from 73.35 to 45.42 J/g at 0.5, 6 h followed

by an increase to 87.36 J/g at 24 h of autoclaving of mix III

made of HSC (80%) and GS (20%); while in the case of

autoclaved specimens of mix IV, made of HSC (60%) and GS

(40%), the enthalpy of this peak increases from 50.05 to 77.53 J/

g at 0.5, 6 h followed by a decrease to 43.27 J/g at 24 h of

autoclaving.

The decrease in the enthalpy characterising CSH with increasing

age of autoclaving is mainly attributed to the conversion of the

initially formed high-lime hydrates (CSH-II) to low-lime hydrates

(CSH-I); since high-lime CSH possesses lower water content than

low-lime CSH, also this decrease in enthalpy may be attributed to

the decomposition of sulfoaluminates (Heikal et al., 2004; Kondo

et al., 1975).

The second endotherm located at 380–4108C with lower intensity

may be attributed to the decomposition of the hydrogarnet phase

(C3ASH4) having lower degree of crysrallinity (Klimesch et al.,

2002) which cannot be detected by XRD. The enthalpy of this

endotherm decreases with increasing time of autoclaving for all

autoclaved specimens made of mixes I–IV; it decreases from

3.37 to 1.14 J/g at 0.5 and 6 h and vanishes at 24 h of autoclaving

of HSC (mix I); it has a value of 3.20 J/g at 0.5 h and vanishes at

6 and 24 h of autoclaving of mix II. The enthalpy of this peak

decreases from 2.55 to 0.80 and from 1.54 to 0.58 J/g with

increasing time of autoclaving from 0.5 to 24 h for the autoclaved

specimens of mixes III and IV, respectively.

DSC:mW

Endo

∆T

Exo

Temperature: °C0 200·00 400·00 600·00

0·5 h6 h

24 h

Figure 5. DSC–thermograms of autoclaved specimens of HSC

(mix I) after 0.5, 6 and 24 h of autoclaving

DSC:mW

Endo

∆T

Exo

Temperature: °C0 200·00 400·00 600·00

0·5 h

6 h

24 h

Figure 6. DSC–thermograms of autoclaved specimens of HSC–GS

blend of mix II after 0.5, 6 and 24 h of autoclaving

DSC:mW

Endo

∆T

Exo

Temperature: °C

0 200·00 400·00 600·00

0·5 h

6 h

24 h


blend of mix III after 0.5, 6 and 24 h of autoclaving

DSC:mW

Endo

∆T

Exo

Temperature: °C

0 200·00 400·00 600·00

0·5 h

6 h

24 h


blend of mix IV after 0.5, 6 and 24 h of autoclaving

27



The third endotherm located at 490–5108C is attributable to the

decomposition of calcium hydroxide (CH). The enthalpy of this

endotherm increases from 10.09 to 25.75 and 42.74 J/g with

increasing time of autoclaving from 0.5 to 6 and 24 h for mix I

(Figure 5); it also increases from 8.85 to 32.86 and 45.36 J/g at

0.5, 6 and 24 h of autoclaving of mix II (Figure 6). The increase

in the enthalpy of the endothermic effect characterising CH with

increasing time of autoclaving of mixes I and II is mainly

attributed to the increased degree of crystallinity of free CH and

not related to the amount of free CH; since the total amount of

free lime obtained by solvent extraction (amorphous and crystal-

line parts) decreases with increasing time of autoclaving (Figure

3). However, for the autoclaved specimens made of mixes III

(having 20% GS content), the enthalpy of this endotherm de-

creases from 10.11 to 4.43 J/g at 0.5 and 6 h followed by an

increase to 7.15 J/g at 24 h of autoclaving (Figure 7); it also

decreases from 3.49 to 0.11 J/g at 0.5 and 6 h followed by an

increase to 0.32 J/g for autoclaved specimens made of mix IV

(having 40% GS content). The variations in the values of enthalpy

of the endotherm characterising CH with increasing time of

autoclaving of the silica-rich mixes (mixes III and IV) are mainly

related to the conversion of high-lime CSH at the early stages to

low-lime CSH at the later ages of the hydrothermal reaction

(Kondo et al., 1975). Evidently the free lime contents given in

Figure 3 and Figures 5–8 represent a net effect between the

amount of free lime liberated from the Portland cement fraction

of HSC hydration and the amount of free lime consumed by the

hydrothermal interaction with both the slag fraction of HSC and

sand.

The fourth weak endotherm located at 580–5908C is attributed

to the decomposition of the small amounts of calcium carbonate

(CC�) having a nearly amorphous character; the decomposition

temperature of the (CC�) phase in this investigation is relatively

lower than that of stable (CC�) (Klimesch et al., 2002), which

decomposes normally in the range of 600–8008C depending on

(a) (b)

(c) (d)

Figure 9. Scanning electron microscopy (SEM) of autoclaved HSC

((a) mix I) and HSC–GS blends ((b) mix II, (c) III and (d) IV) after

0.5 h of autoclaving

28



its degree of crystallinity. The enthalpies of this endotherm are

within the range of 0.19–0.93 J/g as a result of carbonation of

the autoclaved specimens made of mixes II, III and IV. This

endotherm is completely absent in the DSC thermograms of the

autoclaved specimens of HSC (mix I); this is mainly attributable

to the discontinuous pore structure of these autoclaved speci-

mens, which hinders the carbonation of the formed hydrates.

Scanning electron microscopy (SEM)

The microstructure of the autoclaved HSC (mix I) and HSC–GS

(mixes II, III and IV) pastes after 0.5 h of the hydrothermal

reaction displayed a mixture of nearly amorphous and microcrys-

talline fibres of calcium silicate hydrates (CSH) as well as

crystalline calcium hydroxide (CH). A more dense structure was

obtained with increasing silica sand content of the paste up to

20% (from mix I to mix III) as a result of formation of excessive

amounts of low-lime calcium silicate hydrates, which is accom-

panied by a discontinuity of the pore system (Figures 9(a)–(c)).

However, autoclaved specimens made of mix IV, HSC (60%)–GS

(40%), possess a more porous structure with a relatively low

compressive strength (Figure 9(d)).

After 24 h of autoclaving, good crystallisation of the formed

calcium silicate hydrates was observed with increasing silica sand

content in the autoclaved mixes (Figure 10). Therefore, various

forms of calcium silicate hydrates were observed; the degree of

crystallinity increases from short fibres (mix I) to interlocking

fibres and plates (mixes II and III) and massive plates in larger

pore spaces (mix IV). The Portlandite phase (CH) completely

disappeared in the microstructure after 24 h of autoclaving

(Figures 10(a)–10(d)).

ConclusionOn the basis of the results obtained in this investigation, the

following conclusions are derived.

(a) (b)

(c) (d)

Figure 10. Scanning electron microscopy (SEM) of autoclaved

HSC ((a) mix I) and HSC–GS blends ((b) mix II, (c) III and (d) IV)

after 24 h of autoclaving

29



(a) The compressive strength results showed that the

autoclaved specimens made of the neat HSC (mix I)

possess relatively lower strength values than those of the

specimens made of autoclaved HSC–GS mixes. The mix

composed of HSC (80%) and GS (20%) possesses the

highest strength values at all ages of the hydrothermal

solidification process.

(b) The results of XRD analysis and SEM micrographs showed

that the main hydrothermal reaction products formed in all

mixes were calcium silicate hydrates and free lime.

(c) Various forms of calcium silicate hydrates were observed; the

degree of crystallinity increases from short fibres (mix I) to

interlocking fibres and plates (mixes II and III) and massive

plates in larger pore spaces (mix IV). The Portlandite phase

(CH) completely disappeared in the microstructure after 24 h

of autoclaving of mix IV.

REFERENCES

Abd-El-Wahed MG, El-Didamony H, Amin AM and Taha AS

(1989) Hydration of granulated slag-alite in the presence of

quartz. Journal of Material Science 24(1): 276–280.

Abo-El-Enein SA, Gabr NA and Mikhail RS (1977) Morphology

and microstructure of autoclaved clinker and slag-lime pastes

in presence and in absence of silica sand. Cement and

Concrete Research 7(3): 231–238.

Abo-El-Enein SA, Mikhail RS, Daimon M and Kondo R (1978)

Structure area and pore structure of hydrothermal reaction

products of granulated blast furnace slag. Cement and


Abo-El-Enein SA, Shater MA and Nawar M (1981) Reactivity of

granulated blast furnace slag, quartz and kaolin in the

hydrothermal reaction. 1 – hydration kinetics and

compressive strength. Il Cemento 78(3): 137–144.

Abo-El-Enein SA, Hanafi S and Nowar M (1985) Reactivity of

granulated blast furnace slag, quartz and kaolin in the

hydrothermal reaction. II-surface properties and phase

composition. Il Cemento 82(1): 49–60.

Abo-El-Enein SA, Hekal EE, Abdel-Khalik M and El-Hosiny FI

(1990) Autoclaved calcium silicate hydrates, I-rate of

formation and molar composition. Il Cemento 87(3): 147–

160.

Abo-El-Enein SA, Hekal EE, El-Hosiny FI and Marusin SL (1992)

Autoclaved calcium silicate hydrates, III phase composition

and microstructure. Il Cemento 89(2): 77–86.

Abo-El-Enein SA, Hekal EE, Gabr NA and El-Barbary MI (1994)

Blended cement containing cement kiln dust. Silicates

Industriels 59(9–10): 265–269.

Abo-El-Enein SA, El-Hosiny FI, El-Simy E and Osman TA (2002)

Hydrothermal characteristics of blended cement pastes

containing silica sand using cement kiln dust as an activator.

Journal of Material Science Technology 18(5): 474–478.

Aiad I, Morsy MS and Habib AO (2008) Influence of some low

formaldehyde superplasticizers on the rheological and

mechanical properties of cement pastes. Silicates Industriels

73(5–6): 71–76.

Alexanderson J (1979) Relations between structure and

mechanical properties of autoclaved aerated concrete. Cement

and Concrete Research 9(4): 507–514.

Chiocchio G, Collepardi M and Turriziani R (1975) Substituted

hydrated calcium silicates obtained in autoclave hydration.

Journal of the American Ceramic Society 58(5–6): 185–188.

Cong X and Kirkpatrick RJ (1995) Effects of the temperature and

relative humidity on the structure of C–S–H gel. Cement and


El-Didamony H, Shater MA, Taher M and Abo-El-Enein SA (1982)

Hydration mechanism of hydrothermally hardened slag–alite

pastes. Zement Kalk Gips 35(8): 432–434.

Galal AF, Abo-El-Enein SA, El-Hosiny FI and Salem TM (1993)

Hydration and microstructural character-istics of auto-claved

cement kiln dust-slag quartz pastes. Proceedings of the 15th

International Conference on Cement Microscopy, Florida,

USA, pp. 300–311.

Heikal M, Morsy MS, El-Shimy E and Abo-El-Enein SA (2004)

Hydration reactivity of granulated slag using lime-rich

industrial waste as an activator on the electrical and

rheological properties of superplasticized blended cement

pastes. L’Industria Italiana del Cemento, 800: 614–624.

Isu N, Sasaki K, Ishida H and Mitsuda T (1994) Mechanical

property evolution during auto-claving process of aerated

concrete using slag-I, Tobermorite formation and reaction

behavior of slag. Journal of American Ceramic Society 77(8):

2088–2092.

Kazumichi Y, Xiulan H, Ayumu O and Koji K (2006) Hydration

of �-dicalcium silicate at high temperatures under

hydrothermal conditions. Cement and Concrete Research

36(5): 810–816.

Klimesch DS, Ray A and Sloane B (1996) Autoclaved cement–

quartz pastes the effects on chemical and physical properties

when using ground quartz with different surface areas. Part 1:

Quartz of wide particle size distribution. Cement and


Klimesch DS, Ray Abhi and Guerbois J-P (2002) Differential

scanning calorimetry evaluation of autoclaved cement based

building materials made with construction and demolition

waste. Thermochimica Acta 389(1–2): 195–198.

Kondo R, Abo-El-Enein SA and Daimon M (1975) Kinetics and

mechanisms of hydrothermal reaction of granulated blast

furnace slag. Bulletin of the Chemical Society of Japan 48(1):

222.

Kourounis S, Tsivilis S, Tsakiridis PE, Papadimitriou GD and

Tsibouki Z (2007) Properties and hydration of blended

cements with steelmaking slag. Cement and Concrete

Research 37(6): 815–822.

Luke K (2004) Phase studies of pozzolanic stabilized calcium

silicate hydrates at 1808C. Cement and Concrete Research

34(9): 1725–1732.

Nurse RW (1964) The Chemistry of Cements (Taylor HFW (ed.)).

Academic Press, London, UK, vol. 2.

Ostmann E and Efimova L (1974) Physicochemical properties of

lignite ash of electric power stations and estimating its

30



suitability for production of ash silicate. Sb. Tr., Gos.

Nauchno-Issled. Proektn Inst. Silik. Betona Avtoklavn. Tverd

8(1): 15–22.

Pane I and Hansen W (2005) Investigation of blended cement

hydration by isothermal calorimetry and thermal analysis.

Cement and Concrete Research 35(6): 1155–1164.

Salem TM, Abo-El-Enein SA, El-Hosiny FI and Nassar YA (1995)

Autoclaved products containing cement kiln dust, cement kiln

dust-slag and cement kiln dust-slag-quartz systems. Il

Cemento 92(2): 77–86.

Santoro L, Valenti G and Volpicelli G (1984) Application of

differential scanning calorimetry to the study of the system

phosphogypsum lime aluminiumhydroxide water. Thermo-

chimica Acta 74(1–3): 35–44.

Shi C and Day RL (1996) Some factors affecting early hydration

of alkali-slag cements. Cement and Concrete Research 26(3):

439–447.

Siauciunas R and Baltakys K (2004) Formation of gyrolite during

hydrothermal synthesis in the mixtures of CaO and

amorphous SiO2 quartz. Cement and Concrete Research

34(11): 2029–2036.

Xi Y, Simer DD and Scheetz BE (1997) Strength development,

hydration reaction and pore structure of autoclaved slag

cement. Cement and Concrete Research 27(1): 75–82.

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Hydrothermal characteristics of high-slag cement pastes made with and without silica sand