The synthesis and characterisation of an expansive additivefor M-type cements

Part II. Investigation of shrinkage compensation effect during early stagesof hydration process

Tomas Opravil • Petr Ptacek • Frantisek Soukal •

Jaromır Havlica • Jirı Brandstetr • Lenka Opravilova

Received: 2 August 2012 / Accepted: 21 September 2012 / Published online: 18 October 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract The expansion effect of laboratory-prepared

expansive additives for M-type expansive cement was

investigated at the early stage of hydration by the multicell

isoperibolic calorimeter and volumetric technique based on

Archimedes’ principle. The relative volume changes and

heat released during hydration are strongly affected by the

content of lime in the expansive additive due to the influence

of CaO on the kinetics and mechanism of formation of

ettringite. The increasing content of lime favours the for-

mation of monosulphate and its later transformation to

ettringite generating expansion stress. The effect of expan-

sive additive on the behaviour of mortar samples was mea-

sured as linear elongation of test blocks using Graf-Kaufman

dilatometer. Lower or higher content of lime in expansion

additive slightly decreases the 7th-day compressive and

flexural strength of samples while this effect is negligible for

expansive additive with nominal composition of ettringite.

Keywords Expansive additive � Hydration � Ettringite �Monosulphate � Calorimetry � Dilatometry

Introduction

The M-type expansive cements are defined by ASTM C 845

(Standard Specification for Expansive Hydraulic Cement)

[1] as an interground product of Ordinary Portland cement

(OPC) with expansive additives such as Calcium aluminate

cement (CAC) and gypsum CaSO4 � 2H2O; CSH2

� �, or

calcium sulphate hemihydrate CaSO4 � 1=2H2O; CSH0:5

� �.

Alternatively, the expansive additive consisting of high

Calcium alumina cement, gypsum or hemihydrate and

hydrated lime may be added to the fresh concrete during

mixing. The expansion effect can be then controlled by the

amount of additive that is used. Even though all the details on

hydration chemistry of expansive cements are not yet fully

understood, it is generally accepted that the expansion

pressure in the setting cement paste, mortar or fresh concrete

is generated by the formation of ettringite 3CaO � Al2O3�ð3CaSO4 � 32H2O; C6AS3H32; AFtÞ. In expansive M-type

cements, ettringite is formed by reaction of gypsum with

calcium monoaluminate (CaO.Al2O3, CA) in the presence of

water according to Eq. 1 [2, 3]:

CAþ 3C�SH2 þ 2CHþ 24H! C6A�S3H32 ð1Þ

Significant amount of calcium aluminate monosulphate

3CaO � Al2O3 � CaSO4 � 12H2O; C4ASH12; AFm� �

can be

formed along with ettringite at lower values of water–cement

ratio at ambient temperature. The formation of AFm phase

does not contribute to expansion, but its later conversion to

AFt after exposition of concrete to humidity becomes a

process associated with expansion. On other hand, higher

water to cement ratio and temperatures support the formation

of ettringite. This process is very fast and proceeds prior to

the setting and thus without expansion effect. Only a limited

expansion takes place by formation of ettringite during

Notation: The following cement chemistry notation is used in this

article: C ¼ CaO;A ¼ Al2O3;S ¼ SiO2;S ¼ SO3;H ¼ H2O

T. Opravil (&) � P. Ptacek � F. Soukal � J. Havlica �J. Brandstetr � L. Opravilova

Faculty of Chemistry, Brno University of Technology,

Centre for Materials Research CZ.1.05/2.1.00/01.0012,

Purkynova 464/118, Brno 61200, Czech Republic

e-mail: opravil@fch.vutbr.cz

P. Ptacek

e-mail: ptacek@fch.vutbr.cz

123

J Therm Anal Calorim (2013) 112:1401–1406

DOI 10.1007/s10973-012-2727-2

hydration of residual calcium aluminate. In order to obtain

satisfactory results, it is recommended that concrete mixes

made from M-type cements should be heat-cured at a

temperature higher than 70 �C. Over this temperature, AFt is

thermodynamically unstable and AFm is formed alone

without causing expansion. Then, AFm is transformed into

AFt during subsequent curing in water at ambient

temperature. This procedure leads to the evolution of

expansive stresses in the concrete body [2].

The formation of ettringite is sensible to several factors

such as pH, temperature, Al(OH)4-/SO4

2- and Ca2?/SO42-

ratio. Indeed, there are four components, CaO, Al2O3, SO3

and H2O, that form ettringite. Apart from water, three other

may originate from large varieties of reactants. But, the

materials must be either soluble or at least partially soluble.

Sulphate source is generally gypsum, hemihydrate or

anhydrite. Low-soluble anhydrite results in the lack of

sulphate ions necessary for the formation and stability of

ettringite. The kinetics of ettringite formation depends also

on the source of the calcium sulphate [3–9]. Early forma-

tion of monosulfate and its later transformation into

ettringite is a process that can be detected by calorimeter

measurements.

M-type expansive cements are suitable to use at ambient

temperatures for the production of precast reinforced con-

crete products to be heat-cured in the course of production.

Quick setting and unreliability are the main problems of

standard M-type cements. Therefore, the pre-hydrated cal-

cium aluminate cement should be applied instead the non-

hydrate expansive additive, because the Al3? ions are easily

accessible for formation of sulphoaluminate phase. The

phase composition of pre-hydrated material depends on the

temperature as well as other applied conditions [2, 9].

The synthesis and characterisations of expansive additive

for M-type of expansive cements are reported in detail

elsewhere [10]. The present work deals with the hydration

process of pure expansive additive at early period (within

48 h) using multicell isoperibolic calorimeter. Also, the

volume change of pure expansive additive and the effects of

their addition on the length changes of mortar are reported.

Experimental procedure

Investigation of the expansive effect of pure additive

The expansive additives were prepared as the mixture of

calcium aluminate cement (CAC, Secar 51, Lafarge Alu-

minates), anhydrite II (b-CaSO4) that was prepared by

thermal treatment of gypsum at 1,080 �C and calcined lime

(CL 90) as the source of calcium oxide (CaO, C) [10]. The

mass ratio of these constituents for prepared samples is

listed in the Table 1.

The multicell isoperibolic calorimeter described in the

work [12] was used for measurement of hydration heat

of expansive additives. 300 g of paste of the expansive

additive with composition listed in the Table 1 was used

for the calorimetric assessment.

The volume expansion of pure additive was measured

by volumetric technique. The volumetric method that was

applied for investigation of expansion effect of pure addi-

tive is almost identical with a technique that was used by

Loser et al. [11] for measuring the coefficient of volume

expansion of hardened cement pastes and mortars. The

method is based on Archimedes’ principle when the vol-

ume-tested expansive additives increases with time due to

the expansion effect of AFt formation.

The experiment apparatus is shown on the Fig. 1. The

sample of expansion additive slurry (*100 g) was inserted

into the elastic membrane and enclosed without rest of air

in the top part (a). Sample was next hanged up to balance

(b) and mass ma was determined. The mw,t data were then

continuously collected for the first 3 days of curing. The

preparation time of sample should not be longer than

5 min. The measurement can be started immediately after

filling the mould with paste.

The volume of sample (VT) was calculated according to

Archimedes’ law at time (t) as following:

Vt ¼ma � mw;tðTÞ

qwðTÞð2Þ

where ma and mw, t are mass of the sample measured on air

and in the water, respectively; qw is the specific density of

water at given temperature T. From experimental data, the

relative volume change (DVt) at time t measured under

ambient temperature was calculated from measured data

according to Eq. 3.

DVt ¼ 100V0 � Vt

V0

¼ 100 � ma � mwð Þ � ma � mwð Þma � mwð Þ ½%�

ð3Þ

Preparation of mortars

Fresh mortar containing expansive additive was prepared

by mixing components as listed in the Table 2. The content

Table 1 The composition of prepared samples of expansive additives

in the mass ratio of applied constituents

Sample

notationAC:C�S2:C:H The content of CaO is

M-0.17 0.39:1.00:0.17:0.72 Lower to ettringite stoichiometry

M-0.27 0.39:1.00:0.27:0.72 Equal to ettringite stoichiometry

M-0.37 0.39:1.00:0.37:0.72 Higher to ettringite stoichiometry

1402 T. Opravil et al.

123

of the dry expansive additive in mortar is 15 wt.% for

all prepared samples. The letter ‘M’ in the notation of

expansive additive was replaced by character ‘C’ for the

samples of prepared mortars.

The test blocks with size 40 9 40 9 160 mm were fil-

led with the fresh mortar into the moulds. The test pieces

were excerpted from the mould after 24 h and length

changes were then measured by the Graf-Kaufman dila-

tometer (Fig. 2) for the first nine days of hydration.

The relative length change (DLt) was calculated ana-

logically to Eq. 3 as follow:

DLt ¼ 100l0 � lt

l0

½%� ð4Þ

where l0 and lt were, respectively, the initial length and

length at experiment time t of test samples.

Results and discussion

Hydration of pure additive

The hydration courses of pure additives investigated by

isoperibolic calorimeter via measuring of the temperature–

time dependence are depicted in Fig. 3, in which one can

clearly remark the influence of CaO content on the tem-

perature evolution. This effect is due to exothermic slaking

of lime in the mixtures. The mutual relationship between

temperature, time and content of lime in the expansive

additive can be described by exponential function. From

the chemical point of view, the formation of Ca(OH)2 at

earlier period of hydration of additive enhances the pH

and influences the kinetics of ettringite formation [6]. With

different content of Ca(OH)2, hydration reactions exhibit

different kinetics and mechanisms [8, 13].

Figure 3 shows the temperature evolution of hydration.

It is obvious that hydration temperature of expansive

additive is influenced by the content of lime due to high

exothermic reaction of CaO with water. The increasing

temperature shows two positive effects on the expansion

behaviour of sample. First, the formation of AFm phase is

preferred instead of AFt and then expansive stress in the

more rigid material is generated during the later conversion

of AFm to AFt [2]. Second, the decreasing solubility of

Anhydrite II with increasing temperature has also restricted

(a) SampleBalanceThermostat

(b)(c)

(a)

(c)

(b)

Fig. 1 The apparatus used for determination of volume changes of

the pure expansive additive

Fig. 2 Graf-Kaufman dilatom-

eter for measuring of changes

of length of mortar samples

Table 2 The composition of mortars used for preparation of test

pieces for dilatometer

Constituent/g C-0.17 C-0.27 C-0.37

CEM I 42.5 R OPC 340 340 340

Secar 51 CAC 7.82 7.56 7.33

Anhydrit II CS 20.22 19.56 18.95

Lime CL 90 CL 3.44 5.36 7.00

Sand S 1200 1200 1200

Water H 180 180 180

Synthesis and characterisation of an expansive additive 1403

123

the concentration of calcium sulphate ions. The mentioned

factors have the positive influence on the expansion effect

of additive. Then monosulphate is preferably formed and

later transformed into ettringite thus generating expansion

stress.

The temperature reached during hydration of expansive

additive at earlier period should be taken into account for

interpretation of experimental results. Indeed, the setting

behaviour of expansive additive, expressed as the time

dependence of change of the relative volume during the

first 3 days of hydration process, is plotted in Fig. 4.

Foremost, shrinkage of samples takes place at the initial

period of hydration process as the result of absorption of

water by components of expansive additive. The extent and

duration of this initial shrinkage period is proportional to

lime content in the expansive additives. It was observed

that time interval of shrinkage period increases with

decreasing content of CaO in the expansive slurry. Longer

shrinkage period is observed in the sample M-0.17, while

M-0.27 and M-0.37 present short shrinkage period with

rapid volume expansion.

The expansion reached after 3 days of volumetric test is

strongly affected by the content of lime in the expansive

additive. The sample M-0.27 shows approximately 8 times

higher value of reached relative volume change than

expansive additive M-0.17. The higher volume expansion

of sample M-0.27 than sample M-0.37 is in agreement with

observed retardation effect of lime on the rate of ettringite

formation during hydration of expansive additive [10, 14]

as well as effect of temperature on the process [2].

From the combination of results of calorimetric mea-

surements (Fig. 3) and volumetric test (Fig. 4), the expan-

sion behaviour of individual samples can be explained as

follows. Sample M-0.17 shows longer shrinkage following

by low volume expansion. The lack of CaO in sample has not

contributed to the formation of ettringite. Indeed at lower

value of pH, monosulphate is formed instead of ettringite.

The later formation of postponed ettringite can contribute to

expansion or damage of concrete. The fast formation of

ettringite takes place during setting period of sample M-0.27

due to the stechiometric ratio of Ca2?, SO42- and Al3? ions.

The volume expansion is growing rapidly over time. Sample

M-0.37 with excess of lime shows lower volume expansion

in comparison with sample M-0.27. The rate of ettringite

formation is slowed down by the higher content of CaO but

the highest temperature reached during hydration in sample

M-0.37 supports formation of AFm phase without expansion

effect. Therefore, the sample M-0.27 shows higher volume

expansion than M-0.37.

Expansion of mortars with expansive additive

Figure 5 shows linear elongation of mortars test blocks

measured by Graf-Kaufman dilatometer during early stages

of hydration process.

The behaviour of samples is in agreement with the

principle that the expansion process is almost finished

within the first 7 days of hydration [2]. The expansion

20188642020

30

40

50

60

70

80

90

100

110

Time/hours Time/days

Tem

pera

ture

/°C

Tem

pera

ture

/°C

M-0.17M-0.27M-0.37Peak and exponential fit

of temperature–time dependence:

Content of lime in the sample

Content of lime

Content of lime in the sample0.15

y = 0.95 + 114.58 exp (–x/1.26)

0.20 0.25 0.30 0.35 0.40

y = 1.95 – 178 exp (x/0.59)y = –0.30 + 2.22 exp (x/0.24)

0.15 0.20 0.25 0.30 0.35 0.40

0.2

0.4

0.6

0.8

70

60

80

90

100

Tem

pera

ture

Tim

e

M-0.17

M-0.17

M-0.17

M-0.27

M-0.27

M-0.27

M-0.37

M-0.37

M-0.37

100

90

80

70

600.15

0.200.25

0.300.35

0.40 0.10.2

0.30.4

0.50.6

0.70.8

(a) (b)

Fig. 3 Evolution of temperature in the setting sample during early stages of hydration process

7050 60403020100

Time/hours

45

40

35

30

25

20

15

10

5

0

–5

Rel

ativ

e vo

lum

e ch

ange

/%

M-0.17M-0.27M-0.37

0 20 4 6 8 10

2

1

0

–1

–2

Fig. 4 The relative volume changes of pure expansive additive

1404 T. Opravil et al.

123

effect of applied additive varies in the following way:

C-0.37 [ C-0.17 [ C-0.27.

The highest expansion activity of M-0.37 sample and

the positive effect of lime on the process are in agreement

with previous results [10] as well as the above-described

volume changes during hydration of pure additives. On the

other hand, the higher content of CaO improve expansion

of mortar due to retardation effect on the rate of formation

of ettringite hence hydrated mortar can reach sufficient

rigidity to transport expansion stress.

In order to obtain information about the effect of lime on

the mechanism of expansion, data of Fig. 5 were interlaid

by exponential function. Then, the course of experiment

was described by the time dependence of kinetic degree of

conversion (y = f(t), Fig. 6a) that was defined as follows:

y ¼ DLt � DL0

DL0 � DL1ð5Þ

where DL0, DLt and DL? are relative length changes at

beginning, time t and final state of process.

The mechanism of process was found by linearisation

procedure that is based on the transformation of y = f(t)

function dependence into law:

gðyÞ ¼ kt ð6Þ

where the function g(y) represents kinetic equation that

corresponds to the most probable mechanism and k is the

rate constant. The kinetic equations published in the works

[15–18] were used to find the most probable mechanism of

expansion process in this work. The dependence of g(y) on

time provides straight line with the slope k if proper choice

of kinetic equation was made.

The kinetics of expansion process under applied condi-

tion can be described by the JMAYK (Johanson–Mehl–

Avrami–Yerofeyev–Kolgomorov) equation [16, 19]:

y ¼ 1� expð�ðktÞnÞ ð7Þ

where the exponent n depends on the shape of nuclei and

on the dimensionality of their growth, as well as on the rate

of nuclei formation that is usually known as Avrami’s

exponent. The best fit was found for F1 mechanism, i.e.

g(y) = –ln(1-y), where the kinetic exponent n is equal to 1

(Fig. 6b). The determined mechanism cannot be assigned

to simple chemical process; since observed expansion is

the external effect of many multistage processes in the

hardening material, the notation apparent mechanism is

applied.

There is an obvious expansion process caused by

ettringite formation in the sample which can be described

by F1 mechanism for all of the prepared samples (Fig. 6).

However, the value of reaction rate constant is significantly

higher for the sample C-0.27 than for others, which sup-

ports the previously mentioned conclusion that the higher

content of CaO show retardation effect on the rate of for-

mation of ettringite. Therefore, expansive additive should

contain higher amount of lime with regard to composition

Rel

ativ

e le

ngth

exp

ansi

on/%

0.04

0.03

0.02

0.01

0.00

0 2 4 6 8 10

Time/days

c-0.17 = 0.024 – 0.045 exp(-Time/1.593)c-0.27 = 0.021 – 0.045 exp(-Time/1.267)c-0.37 = 0.038 – 0.069 exp(-Time/1.673)

C-0.17 and Exp. FitC-0.27 and Exp. FitC-0.37 and Exp. Fit

Fig. 5 Measuring of length changes of mortar blocks with expansive

additive

543210

Time/daysTime/days

0 2 4

C-0.17C-0.27C-0.37

6 8

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

1.0

0.8

0.6

0.4

Deg

ree

of c

onve

rsio

n

–In(

1 –

y)

0.2

0.0

C-0.17, k = 0.659C-0.27, k = 0.794C-0.37, k = 0.629Linear fit of C-0.17Linear fit of C-0.27Linear fit of C-0.37

(0 < y < 0.95)

(a) (b)

Fig. 6 The time dependence of y (a) and determination of the most probably mechanism of the expansion process (b)

Synthesis and characterisation of an expansive additive 1405

123

of ettringite to slow down the rate of ettringite formation in

hardening cement paste and improve expansion of mortars.

Figure 7 shows that abundance as well as lack of lime

has effect on the values of 7th day compressive and flexural

strength of mortars whereas insignificant effect of M-0.27

expansion additive on the mechanical properties of mortar

was found.

Conclusions

The hydration processes of pure expansive additives and

mortars with content of expansive additives were investi-

gated by the multicell isoperibolic calorimeter and dilato-

metric techniques. The content of lime shows retardation

effect on the rate of ettringite formation, but expansion

efficiency of additive in mortars is increasing with

increasing content of lime in the expansive additive.

The expansive additive with composition corresponding

to ettringite is not worthy for preparation of expansive

cements due to poor expansive effect. Nevertheless, this

expansive additive shows negligible influence upon

mechanical properties of mortars. The excess of lime leads

to higher expansion than its lack. However, higher negative

influence upon comprehensive and flexural strength was

observed. It can be concluded that expansive additive with

higher content of lime is proper for preparation of expan-

sion cements, while lower content of lime is suitable for

shrinkage compensating cements. The expansion behaviour

of mortars containing expansive additives for M-type

expansive cement can be described by JMAYK equation

with n = 1.

Acknowledgements This work has been supported by project of

Ministry of Education, Youth and Sports of the Czech Republic No.

CZ.1.05/2.1.00/01.0012 ‘Centre for Materials Research at FCH BUT’

supported by operational program Research and Development for

Innovations.

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C-0.17 C-0.27 C-0.37 Reference sample

40

35

30

25

20

15

10

5

0

5.1 6.0 4.46.6

36.2

28.6

35.8

29.9

flexuralcompressive

Str

engt

h/M

Pa

Fig. 7 The influence of expansive additive on the 7th day compres-

sive strength and flexural strength of mortar samples

1406 T. Opravil et al.

123