Inﬂuence of metal chloride salts on calcium aluinate cement hydration

Advances in Cement Research, 2012, 24(5), 249–262

http://dx.doi.org/10.1680/adcr.11.00012

Paper 1100012

Received 08/03/2011; revised 20/07/2011; accepted 17/08/2011

Thomas Telford Ltd & 2012

Advances in Cement ResearchVolume 24 Issue 5

Influence of metal chloride salts on calciumaluminate cement hydrationUkrainczyk, Vrbos and Sipusic

Influence of metal chloridesalts on calcium aluminatecement hydrationNeven UkrainczykPost-doctoral Researcher, Faculty of Chemical Engineering andTechnology, University of Zagreb, Zagreb, Croatia

Nevenka VrbosAssistant Professor, Faculty of Chemical Engineering and Technology,University of Zagreb, Zagreb, Croatia

Juraj SipusicAssociate Professor, Faculty of Chemical Engineering and Technology,University of Zagreb, Zagreb, Croatia

In this paper the influence of alkali, alkaline earth and transition metals in the form of chlorides salts on the

hydration of commercial iron-rich calcium aluminate cement (CAC) is investigated. The effect on setting time and

mechanical properties is studied. The results of the setting time obtained from the measured temperature

evolution of cement pastes are compared to standard Vicat needle measurements. Addition of alkali metal salts

both accelerates the setting time of CAC and deteriorates long-term strengths in the order of decreasing ionic

radius of the metals. The effect of alkaline earth metal salts on setting behaviour depends on the amount of

addition: low concentrations retard, but high concentrations accelerate setting behaviour. Calcium is found to have

the strongest acceleration effect with the lowest deterioration in strength. An interesting regularity is observed

when plotting the results of both retarded setting times and strengths against atomic number of added transition

metal cation.

NotationA aluminium oxide (Al2O3)

a, b fitting parameters

C calcium oxide (CaO)

CAC calcium aluminate cement

F iron oxide (Fe2O3)

H water (H2O)

K equilibrium constant

m cement mass (g)

M molar mass (g/mol)

PC Portland cement

pK log (K)

R ionic radius

R correlation coefficient

rn nucleation rate (s�1)

S silica (SiO2)

T temperature (8C)

t time (h)

ti time of initial setting obtained by Vicat measurement

tf time of final setting obtained by Vicat measurement

tmax time of peak maxima in calorimetric measurement

t0 time of initial setting obtained by calorimetric

measurement

w mass fraction of added salt relative to weight of

cement (%)

w/c water-to-cement mass ratio

Z atomic number

ª molar concentration of added salt per kg of cement

(mmol/kg)

IntroductionThe duration of the suitable workability of cement-based mater-

ials during placement is defined by setting time. The use of

additives to modify the properties of cement-based materials for

each specific application is a subject of great practical and

financial interest. From practice, it is known that even small

amounts of different organic and inorganic compounds have a

pronounced influence on the setting behaviour and mechanical

properties of cement-based materials. Calcium aluminate cement

(CAC) as a special cement is particularly interesting because of

its versatility in high-performance applications (Bensted, 2002;

George, 1983; Mangabhai, 1990; Mangabhai and Glasser, 2001;

Scrivener et al., 1999), such as those requiring: high early

strength, resistance to chemical attack, resistance to abrasion,

refractory properties and/or low ambient temperature placement.

In comparison to commonly used Portland cement (PC), CACs

show relatively slow setting but rapid hardening characteristics.

These excellent properties make CAC products useful for rapid

repairs of roads, hydraulic dams, industrial floors, pipe linings

and other such purposes (Scrivener et al., 1999).

Hydraulic hardening of CAC is primarily due to the hydration of

CA (cement notation: C ¼ calcium oxide (CaO), A ¼ aluminium

oxide (Al2O3), F ¼ iron oxide (Fe2O3), S ¼ silica (SiO2),

249

H ¼ water (H2O)), but other compounds may also participate in

the hardening process, especially in long-term strength develop-

ment (Ukrainczyk, 2010; Ukrainczyk and Matusinovic, 2010).

The hydration of CAC is highly temperature dependent, yielding

morphologically different hydration products at different tempera-

tures of hydration. At ambient temperature metastabile hydrates,

CAH10 and C2AH8 (Ukrainczyk et al., 2007) transform to the

more stable C3AH6 and AH3 with consequent material porosity,

permeability increase and loss of strength. The transformation is

accelerated by temperature and moisture availability for the

dissolution and re-precipitation processes to take place (Mangab-

hai and Glasser, 2001).

Along with rapid setting, some metal salts result in the rapid

strength development of CACs at very early ages, so they are

advantageously used for rapid repair works of highways and

airport runways (Justnes, 2008). The influence of metal salts on

cement hydration, particularly setting time and compressive

strengths, is also of interest in stabilisation of hazardous waste

containing potentially soluble heavy metals (Jantzen et al., 2010).

Upon water addition to cement a sharp increase of the pH of the

water solution occurs, less for CAC than for PC hydration, which

causes precipitation of highly insoluble metal oxide–hydroxide,

thus opening an attractive route for heavy metal waste stabilisa-

tion. Lastly, the role of foreign ion additions in the nucleation and

growth process of hydration products provides a fundamental

insight into this intricate and enigmatic process. The admixtures

can interfere with hydration of cement in a variety of ways. A

number of different mechanisms have been suggested (e.g.

Currell et al., 1987; Murat and Sadok, 1991; Rodger and Double,

1984) to explain the action of set regulators on cement hydration,

mainly with respect to the dissolution of anhydrous cement, and

the nucleation and precipitation processes for the main hydration

products. The influence of the particular additive on setting

behaviour of CAC is generally not so well understood and

contradictorily statements can be found in the literature, as

detailed below. Part of the controversy could arise primarily from

differences in the cement composition, the measurement tech-

nique employed and the conditions of hydration.

Rodger and Double (1984) studied the influence of alkali,

alkaline earth and transition metal chlorides additions (25 mmol

of salts per kg of cement) on setting of iron-rich CAC pastes

prepared with a water-to-cement ratio (w/c) of 0.5 by measuring

the maximum temperature rise in a semi-adiabatic calorimeter

(T ¼ 22 � 28C, ˜T . 1008C). A similar chemical composition of

CAC is used in the present work (with 10% F (iron oxide)) but

from a different producer, namely Lafarge Ciment Fondu. The

summarised effect on setting time is in the following series:

Liþ ,, Mg2þ , Ba2þ , none , Kþ , Naþ , Sr2þ , Ca2þ:

Furthermore, Rodger and Double reported that all transition

metals chloride salts have a retarding effect on the set of CAC.

Using a semi-adiabatic calorimeter (T ¼ 18 � 28C, ˜T , 208C)

Currell et al. (1987) investigated the setting of iron-rich CAC

(manufactured by Lafarge, with a relatively low iron oxide

content of 6% compared with the CAC used in the present study)

pastes with a w/c ¼ 0.27 in the presence of many additives with

different concentrations. The effects on the setting time may be

summarised as follows: Liþ ,, Naþ , none , Kþ , Caþ ,

Mg2þ , Sr2þ:

Griffiths et al. (1991) studied the setting of iron-rich CAC (Lafarge,

similar chemical composition as in the present work except for

somewhat lower iron oxide content at 10%) pastes (w/c ¼ 0.4) at

different temperatures in the presence of 15 mmol/kg metal chlor-

ides and sulfates, as well as sea water. Using the time corresponding

to the maximal rate of heat generation to compare the setting

behaviour in the presence of chlorides, they reported the following

order at the various temperatures: Mg2þ , Kþ , Naþ , Ba2þ ,

none , Ca2þ at 158C; Mg2þ , Kþ , Naþ , none , Ca2þ ,

Ba2þ at 208C and 308C.

The literature regarding the influence of alkali and alkaline earth

metal chlorides on the hydration of iron-rich CAC is confusing.

The above-stated conflicting results can be summarised as

follows. For alkali chlorides sodium (Na) and potassium (K) are

reported to act as both retarders and accelerators. For alkaline

earth metal chlorides magnesium (Mg) acts as an accelerator,

according to Rodger and Double (1984), and as a retarder,

according to Currell et al. (1987). Only Rodger and Double

(1984) investigated the effect of both strontium (Sr) and barium

(Ba), which were reported to act differently, that is as retarder

and accelerator, respectively.

Although the influence of admixtures on setting behaviour of

iron-rich and refractory Secar 71 CACs, as two different classes

of CACs, has been compared in the literature, care should be

taken because they differ substantially in their properties (princi-

pally chemical and mineralogical composition). Murat and Sadok

(1991) considered the role of foreign cations in the hydration

kinetics of refractory CAC (Secar 71, produced by Lafarge, that

has an aluminium oxide (Al2O3) content of about 70%). They

concluded that all chlorides metal salts (mono-, bi-, tri- and tetra-

valent) have an accelerating effect on the set of Secar 71. This

result differs from most other studies summarised above. Nilfor-

oushan and Sharp (1995) investigated the setting of Secar 71 with

a range of concentrations (12.5–250 mmol/kg) of alkaline earth

metal chloride salts at different temperatures. At 128C all of the

alkaline earth metal chlorides investigated exhibited retardation

of setting, which increased with increasing concentration of the

admixture. At 208C, at low concentrations, they were shown to

accelerate the setting, but they acted as retarders at higher

concentrations. At 288C and 368C, they retarded the setting, the

more so with increasing concentration of the admixture.

In the present paper, the authors have studied the influence of

alkali, alkaline earth and transition metal chlorides additions on

setting behaviour as well as strength development of commercial

iron-rich CAC.

250


Influence of metal chloride salts oncalcium aluminate cement hydrationUkrainczyk, Vrbos and Sipusic

Experimental work

Materials

This paper examines the hydration of three samples of commer-

cial CAC ISTRA 40 taken from a regular production of Calucem,

Pula, Croatia. The cement has the oxide mass fraction composi-

tion listed in Table 1. Physical properties of the cement are given

in Table 2. The effect of alkali, alkaline earth and transition metal

chlorides was studied on cement samples A, E and T, respec-

tively. The main compounds are CA and ferrite phase (C4AF–

C6AF2), with mayenite, C12A7, gehlenite, C2AS, and �-C2S as

minor compounds. All specimens were prepared with a water-to-

cement mass ratio (w/c) that corresponds to a water requirement

for the standard consistency (given in Table 2). The salts used as

additives were commercial Analar grade reagents, dissolved in

freshly deionised water before mixing with the cement. Alkali

metal salts, NaCl, KCl, RbCl and CsCl, were mixed with a mass

fraction (relative to weight of cement) of 0.05% and molar

concentration of 3 mmol/kg of cement, and LiCl from 0.00025 to

0.50% (0.06–118 mmol/kg). Alkaline earth metal salts: MgCl2

6H2O, CaCl2, SrCl2 6H2O and i BaCl2 2H2O, were mixed with

mass ratios (relative to weight of cement) of 0.10, 0.50, 1.0, 2.0

and 5.0%. Transition metal salts, MnCl2 H2O, CoCl2 6H2O,

CuCl2 2H2O and ZnCl2, were studied at 10 and 100 mmol/kg of

cement. To prepare the samples of pastes the additives were

weighed into deionised water according to a specified concentra-

tion and then mixed with cement.

Mechanical properties

Cement paste specimens (40 3 40 3 160 mm) were prepared

according to ASTM C 109 (with a standard consistency).

Bending and compression tests at different ages were done

according to EN 1015-11. Three specimens were tested for each

age.

Setting time measurement

The setting time was determined using the standard Vicat needle

method carried out according to EN 196-3. The experiments used

a standard consistency CAC paste, as given in Table 2.

CAC temperature evolution measurement set-up

The specimens were cast in high-density polyethylene containers

with inner diameter 2R ¼ 31 mm, 50 mm high and 0.7 mm thick.

The high-density polyethylene container was fully filled with the

cement paste (with a standard consistency), continuously applying

vibrations in order to minimise air entrapment. The thermocouple

measuring end was placed exactly at the centre of the container

(r ¼ 0 and z ¼ h/2 as shown in Figure 1). This was designed by

fixing a thin (1.3 mm), wooden support at the axis of the

container. Specimens were carefully sealed with lids and poly-

vinyl chloride electrical insulation tape and placed vertically in

the temperature-controlled water bath T ¼ 208C (�0.038C). Be-

fore mixing, the cement and water components were left to reach

thermal equilibrium (overnight) in a sealed container placed in a

thermostat.

Temperature evolution measurements were also performed simul-

taneously on the specimens tested for setting time, using a

standard Vicat apparatus. After the Vicat mould (DIN standar-

dised truncated conical form: 75 mm in diameter at the base,

65 mm at the top and 40 mm high, made up from non-absorbent

plastic with a glass base plate) was filled with the cement paste,

the thermocouple sensing end was placed at the weight centre,

that is through the axis at half-thickness of the specimen. This

was done by immersing a thin (1.3 mm) wooden support through

the axis of the paste in the mould. Specimens were tested at

laboratory conditions of T ¼ 20 � 28C.

The K-type thermocouples used were 0.2 mm thick with

Sample CaO Al2O3 Fe2O3 FeO SiO2 TiO2 MgO SO3 Na2O K2O Sum

A 37.10 38.47 14.39 2.90 4.43 1.05 0.90 0.20 0.14 0.17 99.8

E 40.2 39.0 11.7 4.3 1.9 n.a. n.a. n.a. 0.13 0.16 97.4

T 39.12 37.78 13.17 2.46 4.42 1.80 0.91 0.30 0.18 0.17 100.3

Table 1. Chemical composition of investigated CAC

Sample .90 �m: % ,40 �m: % Blaine, cm2/g Specific

gravity:

g/cm3

Setting time: min Water-to-cement mass

ratio of standard

consistencyInitial Final

A 3.76 80.50 3407 3.19 264 279 0.240

E 3.70 80.10 3421 3.19 300 322 0.240

T 3.71 80.90 3563 3.20 209 236 0.250

Table 2. Physical properties of investigated CAC

251



grounded, twisted-shielded wiring. An eight-channel data logger

with a 20 bit resolution was used to collect the temperature

measurements. This stores the entire set of temperatures once

every second. The experimental data are simultaneously trans-

ferred to a personal computer. The cold junction temperature held

at room temperature was sensed by a precision thermistor in good

thermal contact with the input connectors (on thermal block) of

the measuring instrument. In order to have accurate cold junction

compensation, any change of its temperature was kept as slow as

possible.

Results and discussion

Alkali metals

Measurements of the Vicat initial and final setting times, that is ti

and tf respectively, were obtained as described in the Setting time

measurement section. The effect of addition of lithium chloride

(LiCl) in a range of concentrations from 0 to 118 mmol/kg on the

setting of CAC is presented in Table 3. The data show that

lithium chloride accelerates the setting time of CAC even for

very low additions of only w ¼ 2.5 3 10�4%, which corresponds

to a molar concentration of ª ¼ 0.06 mmol/kg. As the addition of

LiCl increases, both of the setting time parameters continuously

decrease. With the mass fraction of 0.1% the setting time occurs

during the mixing time, whereas with the fraction of 0.5% the

setting of CAC occurs immediately after the addition of lithium

chloride.

The results on the investigated effect of various alkali metal

chlorides on the setting time of CAC are presented in Table 4. It

is obvious that lithium metal shows a drastic effect of accelera-

tion in comparison to other alkali metals. The reason for this is

discussed next.

The hydration process of CA, as the principal mineral in CAC, is

generally believed to occur through initial dissolution, formation

of a metastable gel and subsequent precipitation (Sorrentino et

al., 1995; Taylor, 1998), principally CAH10: After dissociation of

CA, the metastable gel formed will acquire stability by condensa-

tion of mono-coordinated hydroxyl groups linked to aluminium

to form ox-obridges between two aluminium centres, leading to

the crystalline CAH10: Solid-state nuclear magnetic resonance

Motor

Thermostat

Cooling

T constant�

Pt100

Pt100 datalogger

TC datalogger

PC

Vicatneedle

rh

2R

Figure 1. Schematic diagram of the CAC paste temperature

evolution measurement set-up

w (LiCl): % ª: mmol/kg ti: s tf: s

0 0 15840 16720

0.00025 0.06 13800 15000

0.00050 0.12 10560 11330

0.00075 0.18 9930 10850

0.0010 0.24 9545 10267

0.0025 0.59 4330 5750

0.0050 1.18 3030 3700

0.0075 1.77 1060 1810

0.010 2.36 790 1433

0.025 5.90 338 413

0.050 11.8 270 293

0.10 23.6 F F

0.50 118 I I

Note: F denotes setting of the CAC with lithium chloride during themixing; I denotes setting of the CAC immediately after addition oflithium chloride

Table 3. Comparison of setting time for different quantities of

lithium chloride addition

252



imaging data indicate that aluminium in CA is entirely four-

coordinated, but the hydration products of CA, CAH10 and

C3AH6 contain six-coordinated aluminium (Taylor, 1998), while

C2AH8 also contains four-coordinated aluminium located in the

interlayer of this double-layer hydroxide phase (Ukrainczyk et al.,

2007). Cong and Kirkpatrick (1993) have shown that the hydra-

tion of CAC proceeds by way of conversion of four to six-fold

coordinated aluminium and the occurrence of intermediate

phases, which still need further clarification. The ox-obridge

condensed structure will be affected by alkali metal cations

forming coordination linkage with the hydroxyl groups (Currell

et al., 1987). Of the alkali ions studied, Liþ behaves differently

from the other alkali cations because of its ability to form

tetrahedral symmetry with hydroxyl groups, while Naþ, Kþ, Rbþ

and Csþ will form octahedral symmetry (Wells, 1962). This is

proved by the present experimental results (Table 4), because

lithium ions show a drastic effect, while differences between the

other cations are not great and exhibit a definitive trend. The

setting time of CAC decreases in the following order:

Cs . Rb . K . Na .. Li. This order is in agreement with the

decrease of ionic radius of the metals. The induction period

during the precipitation of CAC hydration products from a

supersaturated solution is a reflection of the nucleation barrier to

the formation of these compounds (Barret et al., 1974). The

accelerating effect of lithium has been attributed to removal of

this barrier, caused by an initially fast precipitation of lithium

hydrometaaluminate (Matusinovic et al., 1994; Rodger and

Double, 1984), which acts as a heterogeneous nucleation sub-

strate, promoting the nucleation of the hydration products.

The influence of alkali metal chloride addition on the flexural and

compressive strength of CAC pastes has been studied. The results

of measurements are given in Figures 2–4. The results of

compressive strength measurement at early ages of CAC pastes

and CAC pastes prepared with different fractions of lithium

chloride are shown in Figure 2. It can be seen that along with

rapid setting, lithium salts cause strength development at early

ages. The precipitation of lithium hydrometaaluminate is respon-

sible for the rise of compressive strengths of CAC samples at

very early ages (t , 10 h) resulting in higher strengths compared

w ¼ 0.05% ª ¼ 3 mmol/kg

ti: s tf: s ti: s tf: s

Lithium chloride 270 293 712 1371

Sodium chloride 8560 8860 10 960 11 293

Potassium chloride 10 727 11 357 12 027 12 927

Rubidium chloride 11 860 12 560 11 965 12 660

Caesium chloride 11 940 12 820 11 940 12 820

Table 4. Comparison of setting times parameters for alkali metal

chlorides additions (0.05% mass fraction and 3 mmol/kg)

LiCl

w 0·050%�w 0·025%�w 0·010%�CAC

60

50

40

30

20

10

00 5 10 15 20

t: h

Com

pres

sive

str

engt

h: M

Pa

Figure 2. Evolution of compressive strength during early ages of

CAC pastes made with different mass fractions of lithium chloride

13

12

11

10

9

8

7

60 5 10 15 20 25 30

t: d

Flex

ural

str

engt

h: M

Pa

CACLiNaKRbCs

w 0·05%�

Figure 3. Effect of alkali metal chlorides on evolution of flexural

strength of CAC pastes

253



to the reference (plain) CAC sample. The increase of the strength

is caused by precipitation of lithium hydrometaaluminate, which

acts as a heterogeneous nucleation substrate and promotes the

nucleation of the calcium aluminate hydrates. After about 15 h

the strength of the specimens with the lithium addition is lower

than the reference strength. A higher mass fraction of LiCl causes

lesser long-term compressive strengths (Figure 2).

Based on the setting time results in Table 3 the mass fraction of

alkali metal chloride addition of w ¼ 0.05% was taken for further

research. The influence of alkali metal chlorides on evolution of

flexural and compressive strength of CAC pastes is presented in

Figures 3 and 4, respectively. It can be seen that all alkali metal

salts decrease compressive and flexural long-term strengths of

CAC, compared to the reference (plain) CAC paste. The extent of

this reduction in strength follows the same order as for the setting

time reduction: Cs , Rb , K , Na ,, Li (which is in agree-

ment with the order of the ionic radius of the metals). It is worth

noting that although the lithium ion exhibits a drastic setting time

reduction (Table 3), the corresponding long-term strength reduc-

tion (Figures 3 and 4) is only somewhat more pronounced in

comparison to other alkali cations. The strength reduction pattern

for lithium has a higher reduction effect, while differences

between the other cations are not great and exhibit a definitive

trend. This is similar, but not as drastic to the setting behaviour.

The strength reduction with alkali addition can be attributed to a

more rapid nucleation and growth of hydration products because

of the smaller crystalline sizes of hydration products and thus

smaller surface contact area, as well as to the higher degree of

the transformation reactions of the metastable hydration products

because of the higher self-heating of the specimens. At ambient

temperature metastable hydrates transform to the more stable

C3AH6 and AH3 with a release of water and a consequent higher

degree of hydrated CA. This transformation is accelerated by

temperature and moist conditions. During the hydration of CAC,

a large quantity of total heat is liberated in the first 24 h of

hydration, which causes a considerable temperature increase of

prepared specimens (Banfill, 1995; Ukrainczyk and Matusinovic,

2010). Such uncontrolled temperature increase of specimens

could also contribute an increase in the amount of the formed

C2AH8 when the lithium ion is added (Matusinovic et al., 1994;

Rodger and Double, 1984).

The results of the present study are in agreement with Griffiths et

al. (1991) who reported that sodium (Na) and potassium (K) act

as accelerators (15 mmol/kg). On the other hand, Rodger and

Double (1984) indicated that sodium and potassium behave as

retarders (25 mmol/kg), while Currell et al. (1987) reported

sodium as an accelerating but potassium as a retarding admixture.

It has to be noted that Griffiths obtained results from conduction

calorimetry at carefully controlled conditions of temperature,

while Rodger and Double (1984) and Currell et al. (1987)

performed semi-adiabatic calorimetric measurements at 22 � 28C

and 18 � 38C, respectively. Owing to a high sensitivity of setting

time variations with temperature, such poorly controlled tempera-

ture conditions could explain the confusing results in the

literature. Furthermore, in Griffiths et al. (1991) and Rodger and

Double (1984), the authors did the comparison analysis of the

setting behaviour according to the time of maximal heat genera-

tion (or temperature) tmax instead of t0, although the parameter t0

better corresponds to the real initial setting time as obtained by

Vicat measurements. Indeed the effect of salt addition may

decrease the hydration rate and postpone tmax as a result of the

flattening and stretching of the shape of the main hydration peak

(as discussed in the Transition metals section). Moreover, in

different boundary conditions, that is semi-adiabatic (Currell et

al., 1987; Rodger and Double, 1984) or semi-isothermal (Griffiths

et al., 1991), the heat (or temperature) evolution curve is

significantly different, resulting in different tmax values.

The setting time of CAC decreases in the following order:

Cs . Rb . K . Na .. Li, which correlates with the ionic ra-

dius of the metals. To correlate experimental results of the

influence of various cation additions on the CAC (Secar 71)

hydration, Murat and Sadok (1991) employed mathematical

modelling based on a modification of the expression of nucleation

free enthalpy (relying on a hypothesis of homogeneous nuclea-

tion), which in the dilute solution approximation leads to the

following single equation

ln (rn) ¼ aþ b=R1:

where rn is the nucleation rate approximated to (1/t0), where t0 is

the experimentally determined setting time, R is the ionic radius

and a and b are fitting parameters. This model (Equation 1) has

been previously verified on experimental results of calcium

sulfate hydration (anhydrite and hemihydrate).

For the alkali metals we have investigated a correlation according

to Equation 1. The results are shown in Figure 5. The lithium ion

80

75

70

65

60

55

50

Com

pres

sive

str

engt

h: M

Pa

0 5 10 15 20 25 30t: d

CACLiNaKRbCs

w 0·05%�

Figure 4. Effect of alkali metal chlorides (w ¼ 0.05%) on

evolution of compressive strength of CAC pastes

254



is omitted from the regression analysis because of its drastically

different behaviour, as already detailed above. As expected, from

Figure 5 it is obvious that constant mass concentration is not an

adequate parameter for the analysis. For w ¼ 0.05% the corre-

sponding molar concentrations for lithium (Li), potassium (K),

sodium (Na), rubidium (Rb) and caesium (Cs) are 11.8, 8.6, 6.7,

4.1 and 3.0 mmol/kg, respectively. This significant increase of

molar concentrations from the reference concentration of

3.0 mmol/kg results in further/higher increase of the setting time.

For a constant molar concentration (3 mmol/kg) the regression

analyses resulted in a reasonably good correlation coefficient

(R ¼ 0.92927).

Alkaline earth metals

Measurements of the initial and final setting time parameters, that

is ti and tf , respectively, was obtained as described in the Setting

time measurement section. In order to compare the results, a

range of the prepared mass concentrations (fractions relative to

weight of cement) were transformed to molar concentrations, as

shown in Table 5. The effect of a range of molar concentrations

from 4.8 to 525 mmol/kg on the setting time is presented in

Figure 6. It can be observed that lower concentrations (0.1% or up to 10.5 mmol/kg, according to Table 5) resulted in a retarda-

tion of the setting behaviour. Addition of 0.5% of salts for

calcium (Ca) and strontium (Sr) brought about setting times close

to that in the absence of any admixture, while barium (Ba) and

magnesium (Mg) showed significant acceleration. A further in-

crease of admixture concentration results in a dramatic accelera-

tion of the setting time in the presence of any of the four salts.

Furthermore, from Figure 6 it can be seen that calcium chloride

(CaCl2) has the strongest acceleration effect in comparison with

the other alkaline earth metal chlorides, while magnesium

chloride (MgCl2) has the weakest acceleration effect. The highest

concentration (w ¼ 5%) of calcium (Ca) and strontium (Sr)

resulted in a flash set during mixing. As can be seen in Table 5

the addition of a constant mass fraction of salts results in

different molar concentrations that complicate a further detailed

analysis. Unfortunately, the present experiments on the influence

of alkaline earth metal chlorides were conducted with a constant

mass concentration, not fixing the molar concentrations.

The results presented here are compared with the available

literature results obtained on Lafarge iron-rich CACs, focusing on

3·0

2·5

2·0

1·5

1·0

0·5

0

�0·5

�1·0

�1·5

ln(1

/)t i

w 0·05%�γ 3 mmol/kg�Linear fit

ln(1/ ) 1·36 24/t ri � � �

R 0·92927�

Cs� Rb�K�

CAC

Li�

0·006 0·008 0·010 0·012 0·0141/ : pmr �1

Na�

Figure 5. Investigation of the correlation according to Equation 1;

ti is measure in hours (horizontal line: value for plain CAC)

w 0.1% 0.5% 1.0% 2.0% 5.0%

ª: mmol/kg

MgCl2 3 6H2O 10.50 52.52 105.03 210.06 525.15

CaCl2 9.01 45.05 90.11 180.21 450.53

SrCl2 3 6H2O 6.31 31.54 63.08 126.17 315.42

BaCl2 3 2H2O 4.80 24.01 48.02 96.05 240.12

Table 5. Calculated molar concentrations in mmol/kg of CAC for

a corresponding mass fraction

MgCaSrBa

400

300

200

100

00 100 200 300 400 500

γ: mmol/kg CAC

Sett

ing

time:

min

Figure 6. Influence of alkaline earth metal chloride addition on

the initial setting time of CAC pastes (horizontal line: value for

plain CAC)

255



the range of concentrations from 15 to 25 mmol/kg in Figure 6

that correspond to the literature experimental conditions (which

are summarised in the introduction). In agreement with the

finding of the present study, calcium (Ca) is reported by all three

groups of researchers (Currell et al., 1987; Griffiths et al., 1991;

Rodger and Double, 1984) to act as a retarder. The present

study’s results also confirm the different behaviours of strontium

(Sr) and barium (Ba) ions as reported by Rodger and Double

(1984). The differences in the literature regarding the effect of

alkaline earth metal chlorides could be attributed to the change of

the set behaviour influence (i.e. acceleration or retardation) with

the molar concentration, as well as sensitivity to different

chemical compositions of the similar iron-rich cements investi-

gated, temperature of hydration and method of obtaining the

setting parameters.

Moreover, it is interesting to compare the present study’s results

to the literature results obtained on a completely different class of

CAC, a pure CAC Secar 71 (produced by Lafarge, that has a high

aluminium oxide (Al2O3) content of about 70%). Nilforoushan

and Sharp (1995) reported an inverted (opposite) trend of the

results in Figure 6, that is contrary to the present findings. They

showed that alkaline earth metal additions in low concentrations

(12.5 mmol/kg and 25 mmol/kg) exhibit acceleration of the

setting, and significant retardation at higher concentrations (from

50 mmol/kg to 250 mmol/kg). Moreover, Murat and Sadok

(1991) reported that all chloride metal salts (mono-, bi-, tri- and

tetra-valent) have an accelerating effect on the setting of Secar

71, which also differs from most other studies on iron-rich CACs

as summarised in the introduction. Although the influence of

additions on setting behaviour of these two different classes of

CAC has been compared in the literature, resting on the hypoth-

esis of the same hydration mechanism of CA mineral, care should

be taken in this interpretation because they differ substantially in

their properties, principally chemical and mineralogical composi-

tion. Refractory Secar 71 CAC, produced by Lafarge, has an

aluminium oxide (Al2O3) content of about 70%, resulting in CA

and CA2 as the main mineral phases with no C12A7, C4AF and

C2S. The results of the current study have shown that the

influence of alkaline earth metal chlorides additions on setting

behaviour of iron-rich CAC and refractory Secar 71 CAC is

completely different, with an opposite effect of salts concentra-

tion on setting behaviour. This suggests that the hydration

mechanism of the two CACs may be quite different. This

difference may be principally explained by the C/A ratio of the

liquid phase during the nucleation period. The dissolution of the

‘pure’ main mineral CA shows the C/A ratio to be in the range

1.05–1.10 (Fujii et al., 1986; Nikushchenko et al., 1973) thereby

inevitably initiating the precipitation of AH3 gel. Even slight

amounts of C12A7 or CA2 that accompany CA in the commercial

cement composition as separated phases induce a variation of the

C/A ratio in the liquid phase. It is known that the presence of

small quantities of C12A7 (in iron-rich CAC) or CA2 (in Secar

71) influence the duration of the induction period: C12A7

accelerates the nucleation, whereas CA2 retards it. This has been

shown by hydrating a series of synthesised CA samples prepared

with C/A ratios ranging from 0.96 to 1.04 (Galtier and Guilhot,

1984). The dissolution of the ‘pure’ C12A7 exhibits a C/A ratio of

exactly 1.71, showing unambiguously the congruency of its

dissolution. Upon hydration of CA2, large amounts of AH3 gel

are produced (Edmonds and Majumdar, 1989) as a first hydration

product. Later, when the lime concentration reaches a sufficient

level, C2AH8 precipitates coinciding with an acceleration of the

hydration reaction rate. Thus, the chemical composition of ce-

ment influences the change of the C/A ratio in the liquid phase.

With an increasing C/A ratio the length of the induction period

diminishes. Because alkaline earth metal cations bear the same

charge they behave in a chemically similar way, thus the addition

of alkaline earth metal salt increases ‘apparent calcium’ concen-

tration, thus modifying C/A ratio in the solution. Increasing the

C/A ratio of iron-rich CAC induces the formation of AFm and

AFt phases that may protect the cement grains surface from

dissolution and results in the deceleration of the hydration.

The mass concentration of w ¼ 1% was chosen to investigate the

influence of alkaline earth metal salt additions on the flexural and

compressive strength of CAC pastes. The results of measurements

are presented in Figures 7 and 8. Alkaline earth metal salts

decrease compressive and flexural long-term strengths of CAC,

compared to the reference (plain) CAC paste. Interestingly,

although CaCl2 (with the second highest molar concentration,

Table 5) results in the strongest acceleration of the initial setting

time (t0 ¼ 69.5 min) the compressive and flexural strengths

exhibit the lowest reduction from the reference ones (with no

admixtures). On the other hand, addition of 1% magnesium

chloride (MgCl2) with the highest corresponding molar concen-

tration (Table 5) results in a much lower acceleration of the

setting time (t0 ¼ 163.0 min) but higher strength reduction than

calcium chloride (CaCl2). Moreover, strontium chloride (SrCl2)

and barium chloride (BaCl2) (with the smallest molar concentra-

tions) show no significant acceleration effect (Figures 7 and 8)

12·512·011·511·010·510·0

9·59·08·58·07·57·06·56·05·5

0 5 10 15 20 25 30

t: days

Flex

ural

str

engt

h: M

Pa

CACMgCaSrBa

w 1%�

Figure 7. Effect of alkaline earth metal chlorides (w ¼ 1%) on

evolution of flexural strength of CAC pastes

256



but exhibit the highest strength reduction (Figures 7 and 8).

These results indicate the role of the foreign cations, which can

replace the calcium in the hydration products and/or even form

novel hydration products that change the rate of hydration

development and influence strength development. Further re-

search is needed to investigate the nature of the hydration

products formed (e.g. on the hydration of the pure phases with

foreign cation additions).

Temperature evolution and setting time measurements

The rapid hydration and the resulting heat evolution allows an

accurate determination of both the induction time t0 and the time

tmax of the maximum temperature (or heat generation) hydration

peak. Setting can be attributed to the interlocking of the hydration

products when their formation takes place to an appreciable

extent. Unlike calorimetry or temperature rise measurements, the

Vicat initial setting time is significantly affected by the water-to-

cement ratio attributable to the consistency of the paste being

tested. This reflects the fact that the setting time is an arbitrarily

defined parameter. In a more fluid paste, a longer time is needed

to reach the required resistance to penetration. As suggested by

Stegemann and Buenfeld (2001) preferable techniques for meas-

uring setting time, although less traditional and indirect, are

isothermal calorimetry, temperature rise and solution conductivity

measurements. In this paper times of initial, t0, and final, tf , sets

for experimental series on transition metals were established from

the obtained time–temperature curves, as illustrated in Figure 9.

Initial set is acquired from the intersection of two straight lines

(Bushnell-Watson and Sharp, 1986): one fitted through the

induction period of the curve and the other fitted through the

inflection point of the rising slope of the main peak. The final set

was approximated as the point of maximal heat generation.

In order to conduct the comparison of the values of setting time

parameters obtained by Vicat method and temperature rise meas-

urements, the temperature of the specimen in a Vicat mould was

measured as described in the CAC temperature evolution mea-

surement set-up section. The experiment on plain cement paste

(without admixtures) was repeated five times. It was found that

the difference between the initial setting time obtained by the two

methods, (ti � t0) was only 1.2 min � 0.6 min. This validates the

further use of the employed alternative method to obtain accurate

values of the setting time parameters. Interestingly, the time of

final set tf was found to be 9 min � 1.5 min below the point of

inflection (Figure 9).

An example of a temperature evolution of a specimen during

simultaneous standard Vicat method measurement is shown in

Figure 9. The influence of the sample mass and heat transfer

boundary conditions can be analysed by comparing Figures 9

and 10 (for the plain CAC). After moulding the cement paste

and fixing of the thermocouple within a few minutes, a decrease

of temperature is occurring owing to the dissipation of the heat

generated by the initial wetting of cement, chemical dissolution

0 5 10 15 20 25 30

t: days

CACMgCaSrBa

w 1%�

Com

pres

sive

str

engt

h: M

Pa

90

85

80

75

70

65

60

55

50

45

Figure 8. Effect of alkaline earth metal chlorides (w ¼ 1 %) on

evolution of compressive strength of CAC pastes

tmax

tf

Inflectionpoint

0 2 4 6 8 10 12 14 16 18 20 22 24t: h

20

22

24

26

28

30

32

34

T:C°

t0

Figure 9. Temperature evolution of specimen during standard

Vicat method measurement

γ(Me ) 10 mmol/kg CAC2� �CACCo Mn

Cu

Zn

CACCoMnCuZn

23

22

21

20

5 10 15 20 25t: h

T:C°

Figure 10. Effect of transition metal chlorides on the temperature

evolution of CAC, ª ¼ 10 mmol/kg of CAC

257



and partial hydration before reaching the induction period with

very small heat generation (Banfill, 1995; Ukrainczyk and

Matusinovic, 2010). The hydration of CAC at temperatures

below ,208C shows the high influence of water-to-cement ratio

on hydration rate and the obtained hydration degree due to the

high stoichiometric water requirement for the formation of

metastable hydration products (Bensted, 2002; Fryda et al.,

2001; Ukrainczyk and Matusinovic, 2010). The cement hydration

is incomplete if there is not enough water for stoichiometric

hydration, so the hydration rate decreases because of the free

water consumption and reaches zero value upon water insuffi-

ciency. In non-isothermal conditions (Figure 9) the temperature

increase is higher due to lower heat loss to the environment. The

‘shoulder’ in Figure 9 before reaching tmax is a consequence of

the significant commencement of the transformation reactions

(Fryda et al., 2001). As the rate of transformation reactions of

metastable products to the stable C3AH6 and AH3 is significantly

increasing with the temperature, the associated free water

liberation allows further reaction of cement and thus increases

the hydration rate (Bensted, 2002; Fryda et al., 2001; Ukrainczyk

and Matusinovic, 2010). This also explains why the time of

maximal temperature rise, tmax, is reached significantly later after

the onset of the acceleration period in Figure 9 than in Figure

10, but with a faster diminution of the hydration rate after the

maximum at tmax:

The high temperature sensitivity of the setting time measurement

of CACs is not adequately addressed in most of the literature.

One point is the poor control of the starting temperature (e.g.

Currell et al., 1987; Rodger and Double, 1984), probably the

result of following the Vicat standard procedure, which is written

for measurements on a PC that does not have such pronounced

temperature sensitivity as CAC. A second point is that, owing to

a rapid hydration of CAC with a corresponding high heat

generation effect (about 20 mW/g in isothermal conditions (Ban-

fill, 1995; Ukrainczyk and Matusinovic, 2010)), the temperature

evolution of the material strongly depends on the heat transfer

boundary conditions around the testing specimen. In different

boundary conditions, that is semi-adiabatic as in work by Currell

et al. (1987) or Rodger and Double (1984) or semi-isothermal as

in work by Griffiths et al. (1991) or Murat and Sadok (1991), the

heat (or temperature) evolution curve is significantly different,

which could result in significantly different tmax values. Some

authors (Currell et al., 1987; Griffiths et al., 1991; Rodger and

Double, 1984) did the comparison analysis of the setting behav-

iour according to the time of maximal heat generation (or

temperature) tmax instead of t0, although parameter t0 better

corresponds to the real initial setting time as obtained by Vicat

measurements. For an isoperibol calorimeter, the evolved heat in

time t equals the accumulated heat in the calorimeter (that results

in the increase of the calorimeter temperature) and the heat loss

to the surroundings (a constant-temperature environment). Con-

duction (semi-isothermal) calorimeters have small temperature

rise of the samples, whereas semi-adiabatic calorimeters have

small heat transfer to the surroundings.

Transition metals

To the best of the current authors’ knowledge, Rodger and

Double (1984) are the only researchers who investigated the

effect of transition metal chlorides on iron-rich CAC (manufac-

tured by Lafarge). They reported the time of maximum tempera-

ture rise of CAC samples (w/c ¼ 0.5 with 25 mmol of salt per kg

of CAC) in semi-adiabatic conditions. The present authors

analysed the results of Rodger and Double against the added

metallic atomic number, as depicted in Figure 11. A well-defined

trend can be observed in the influence of transition metals on

setting behaviour. It is well known that various trends are

observed when studying chemical reactions or some physical

properties within the transition metal group. For example,

enthalpy change accompanying solvation of cations by water

molecules or melting point of pure metals, and so on. The cause

of such behaviour is qualitatively explained by the gradual filling

of the 3d-orbital by electrons taking into account Hund’s rule of

maximum multiplicity. Interestingly, the regularity of the setting

times observed in Figure 11 follows the same principle. In order

to clarify this finding, previously not addressed in the literature,

part of this paper is devoted to investigating further the influence

of transition metals, in the form of chloride salts, on the hydration

of similar (same class) CAC from a different manufacturer

(namely, Istra cement, Calucem).

The equilibrium between precipitated and soluble species is in

fact dependent on the acid/base properties of the precipitate. The

present authors argue that the acid/base properties of the

precipitate cause the observed regularity of setting times plotted

against atomic number in Figure 11. The acidity of the precipitate

can be described by the following chemical reaction in base

media

Me OHð Þ2 cr:ð Þ þ OH� aq:ð Þ $ Me OHð Þ3� aq:ð Þ2:

1000

800

600

400

25 26 27 28 29 30Atomic number, Z

t T(

): m

inm

ax

6

5

4

3

2

pKa

CAC

p curveKa

t T( ) curvemax

Mn2�

Fe2�

Co2�

Ni2�

Cu2�

Zn2�

Figure 11. Variation of time of maximum temperature rise of

CAC samples (see text for details) and acidity of crystalline

Me(OH)2 quantified by pKa value, plotted against the added

cation’s atomic number

258



The values of acidity constants, pKa, are obtained from thermo-

dynamic data (Shkol’nikov, 2005) and plotted in Figure 11. These

values for pKa correspond to the fully crystalline hydroxide

precipitate. The observed discrepancy for copper and zinc could

be explained by the fact that freshly precipitated amorphous

hydroxide has higher solubility and is therefore expected to be

more acidic. Aluminium also participates in this type of acid/base

equilibrium according to the following reaction

Al OHð Þ3 cr:ð Þ þ OH� $ Al OHð Þ4� aq:ð Þ3:

The reaction in Equation 3 has pKa ¼ 5.9 (Nordstrom and May,

1989). Calcium hydroxide has no observable acidic properties.

By comparing the trends of the setting time and pKa change

plotted against atomic number (Figure 11) it can be concluded

that they agree only qualitatively. Generally, the discrepancies in

quantitative agreement can be ascribed to the complexity of the

real chemistry behind the studied process of CAC hydration.

Overall, the suggested explanation is that the more the acid/base

properties (real pKa values) of transition metal precipitates

deviate from the aluminum hydroxide; the setting time of the

CAC is more prolonged. This concept of acid/base reactions and

the influence on the setting time of calcium aluminate cement is

also corroborated by Mohmel and Gessner (1997), who showed

that acid/base properties of corundum filler significantly influence

the setting of CAC.

The influence of transition metal chloride salts on hydration was

investigated by measuring the temperature evolution of CAC

pastes (as described in the CAC temperature evolution measure-

ment section). Results of temperature evolution measurements for

pastes prepared with ª(Me2þ) ¼ 10 mmol/kg are shown in Figure

10. No significant temperature rise is observed during the induc-

tion period owing to a small and constant rate of heat generation

(about 0.3 mW/g (Banfill, 1995; Ukrainczyk and Matusinovic,

2010)). The induction period is followed by the onset of the

accelerated stage of reaction due to massive precipitation of

hydrates. The maximal temperature rise is reached in only a few

hours after the onset of the accelerated stage. With the addition of

the transition metal chlorides the main hydration peak of the

temperature evolution curve becomes flatter and longer (with a

lower value of maximal temperature at tmax) in the following

order: Co , Mn , Cu , Zn. This flattening and stretching effect

of the main hydration peak supports the notion of slower growth

and crystallisation of hydration products. The effect of various

transition metal chlorides on the setting time of CAC is investi-

gated for two concentrations, 100 mmol/kg and 10 mmol/kg. The

concentration of 100 mmol/kg showed no setting behaviour within

30 h of hydration. The influence of transition metal chloride

addition (ª ¼ 10 mmol/kg) on the setting time of CAC pastes is

depicted in Figure 12. The results are plotted against the atomic

number. It is obvious that all investigated transition metal

chlorides act as retarders, which is in accord with results obtained

by Rodger and Double (1984). The retardation effect is higher for

higher concentrations. In agreement with the finding in Figure 11,

the experimental data from the present study reproduced the

defined trend of the change of setting times against the added

cation’s atomic number well. A comparison of the obtained results

with the literature (Rodger and Double, 1984) is depicted in Figure

13. The results are plotted against the reciprocal values of ionic

radii (Shannon, 1976) in order to test the validity of Equation 1,

which relies on an assumption of homogeneous nucleation (Murat

and Sadok, 1991). The results of Rodger and Double have lower

(more negative) values of ordinate, which corresponds to the

longer absolute setting time values (compare Figures 10 and 11),

primarily owing to the much lower water-to-cement mass ratio

employed in this investigation (i.e. w/c ¼ 0.25 as opposed to

0.50). The translation of the results shows very good agreement of

the trend between the present results and the results obtained

previously by Rodger and Double (1984). It is clear that the

Initial setting time, ti

tmax

Zn2�

Cu2�

Co2�

Mn2�

25 26 27 28 29 30Atomic number, Z

Sett

ing

time:

min

200

300

400

500

600

700

Figure 12. Influence of transition metal chloride addition

(ª ¼ 10 mmol/kg) on the setting time of CAC pastes (horizontal

lines: values for plain CAC)

ln(1

/)

t 0

�1·2

�1·4

�1·6

�1·8

�2·0

�2·2

�2·4

�2·6

�2·8

�3·00·0135 0·0140 0·0145 0·0150 0·0155 0·0160 0·0165

1/ : pmr �1

Current paperRodger and Double (1984)Zn

Cu

Ni

Mn

CoFe

Figure 13. Comparison of results obtained in the present study

with those presented in the literature; ti is measured in hours

259



experimental data significantly deviate from the model in

Equation 1.

On the other hand, on the Secar 71 as a different class of CAC,

Murat and Sadok (1991) reported that transition metal chlorides

have an accelerating effect. Again, as discussed in the Earth

alkali metals section, the influence of transition metal chloride

additions on setting behaviour of iron-rich CAC and refractory

Secar 71 CAC is shown to be completely different. This suggests

that the hydration mechanisms of these two classes of CAC are

significantly different (as discussed in the Earth alkali metals

section). Murat and Sadok (1991) have explained the acceleration

effect by an hypothesis of a heterogeneous nucleation process,

the nuclei formed being grains of metallic hydroxide (or hydrated

aluminates for Liþ and Ni2þ ions in the form of double-layer

hydroxide phases) that lead to an increase of the nucleation rate.

In the present case of iron-rich CAC, the retardation effect may

be explained by the massive precipitation of protective gel

(amorphous hydroxides and/or double-layer hydroxide phases)

coatings on the phase boundary surface of cement grains (Rashid

and Turrillas, 1997), which block hydrolysis and inhibit growth

of the crystalline hydration products. The effect of flattening and

stretching of the main hydration peaks observed in Figure 10

could thus be explained by a formation of larger quantities of,

and/or more impermeable, gel coatings that hinder crystallisation

and growth of the hydration products.

The influence of transition metal salt additions on the flexural

and compressive strength of CAC pastes was studied. The results

of measurements of flexural and compressive strength at 1, 7 and

28 days are presented in Figures 14 and 15, respectively.

Horizontal lines represent experimental values of strength for the

plain CAC, which are increasing with age. Generally, the addition

of transition metal salts decreases compressive and flexural

strengths of CAC, compared to the reference (plain) CAC paste.

When comparing Figures 14 and 15 with Figure 12 it can be

concluded that the magnitude of strength reduction for all tested

ages follows the order observed for the setting time retardation.

For example, the cobalt (Co) sample, which has the setting time

the closest to the plain CAC, achieves the smallest reduction in

strength. On the contrary, the zinc (Zn) addition results in the

highest retardation of the setting time and exhibits the highest

reduction in strength.

Integration of the temperature change evolution curve (curves

shown in Figure 10 were subtracted by 208C before integration)

gives an approximation of a relative amount of achieved degree

of hydration. It was observed that the relative degree of hydration

after 24 h has a tendency to lower in the following order:

none . Mn . Co . Cu . Zn. This order is in agreement with

the magnitude of the strength reduction in Figures 14 and 15.

Thus, the strength reduction may be primarily attributed to the

lower hydration degree achieved owing to the retardation of the

hydration reaction rate (Figure 10).

ConclusionsAddition of alkali metal salts accelerates the setting time of CAC

in the following order: Cs , Rb , K , Na ,, Li. This order is

in agreement with the increase of ionic radius of the metals. A

reasonable correlation was observed between setting time and

the ionic radius of alkali metals, excluding lithium (Li). The

accelerating effect of alkali metals has been attributed to the

removal of the nucleation barrier, through an initially fast

precipitation of alkali metal hydration products (e.g. lithium

hydrometaaluminate) that act as a heterogeneous nucleation

substrate, promoting the nucleation of the CAC hydration pro-

ducts. The lithium has a much greater effect on the setting time

than other alkali cations because of its ability to form tetrahedral

symmetry while other alkali cations form octahedral symmetry.

Along with rapid setting, small quantities of lithium salts cause

Flex

ural

str

engt

h: M

Pa

14

13

12

11

10

9

8

7

6

525 26 27 28 29 30

Atomic number, Z

CAC

Mn2�

Co2�

Cu2�

Zn2�

28 days7 days1 day

Figure 14. Effect of transition metal chlorides (ª ¼ 10 mmol/kg)

on evolution of flexural strength of CAC pastes (horizontal lines:

values for plain CAC).

85

80

75

70

65

60

55

50

45

Com

pres

sive

str

engt

h: M

Pa

Mn2�

Co2�

Cu2�

Zn2�

28 days7 days1 day

25 26 27 28 29 30Atomic number, Z

CAC

Figure 15. Effect of transition metal chlorides (ª ¼ 10 mmol/kg)

on evolution of compressive strength of CAC pastes (horizontal

lines: values for plain CAC)

260



strength development of CAC at early ages, so they can be used

for preparation of rapid hardening cement-based materials – for

example, in rapid repair mortars. Alkali metal salts deteriorate/

reduce compressive and flexural long-term strengths of CAC. The

extent of strength reduction followed the same order as for the

setting time reduction.

Effect of alkaline earth metal salts on setting behaviour depends

on the amount of addition. Low concentrations (0.1%) resulted in

a retardation of the setting behaviour. A further increase of

admixture concentration results in a dramatic acceleration of the

setting time. Calcium addition results in the strongest acceleration

of the initial setting time, while the compressive and flexural

strengths exhibit the lowest reduction from the reference material.

Thus the change in the nucleation rate and the corresponding

deterioration of the strengths can be attributed to the role of the

foreign cations, which can replace the calcium in the hydration

products and/or form novel hydration products.

A simple method of analysing CAC hydration based on the

measurement of temperature rise of cement pastes was described.

Setting times were established from the time–temperature curves

obtained. The results of the setting time obtained from the

measured temperature evolution of cement pastes were compared

to the standard Vicat needle measurements. The difference

between the initial setting time obtained by the two methods is

only 1.2 min � 0.6 min. This validates the further use of the

employed alternative method to obtain accurate values for the

initial setting time parameter. The effects of the sample mass and

heat transfer boundary conditions during setting time measure-

ments were analysed.

With the addition of the transition metal chlorides the main

hydration peak of the temperature evolution curve becomes flatter

and longer, and the setting is retarded in the following order:

Co , Mn , Cu , Zn. The retardation effect is higher for higher

concentrations. A well-defined trend, which was observed when

plotting the results of setting times against added cation’s atomic

number, was related to the change in the acid/base properties of

the precipitate. The retardation effect may be explained by the

precipitation of protective gel (amorphous) coatings on the phase

boundary surface of cement grains, which block hydrolysis and

inhibit growth of the crystalline hydration products. Magnitude of

strength reduction for all tested ages follows the order observed

for the setting time retardation. The strength reduction may be

primarily attributed to the lower hydration degree achieved due to

the retardation of the hydration reaction rate.

The influence of alkaline earth and transition metal chlorides

additions on setting behaviour of iron-rich CAC and refractory

Secar 71 CAC is shown to be opposite in nature. This suggests

that the hydration mechanisms of these two classes of CAC are

significantly different.

It is recommended that for further investigations of the admix-

tures a constant molar concentration (represented in added mols

per mass of cement) should be used. This will enable a better

comparison of the results among researchers, even when mixtures

are prepared with different water-to-cement ratios.

AcknowledgementThe authors acknowledge support from the Croatian Ministry of

Science, Education and Sports under projects no. 125-1252970-

2983 ‘Development of hydration process model’.

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Inﬂuence of metal chloride salts on calcium aluinate cement hydration