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Chemical Shrinkage During Hydration Reactions of Calcium Aluminate Cement

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Calcium Aluminate Cement (CAC) is special cement used in versatile high performance applications. Hardening of CAC is primarily due to the hydration of mono-calciumaluminate, in idealized pure form CaAl 2 O 4 , but other minerals also participate in the hydration process especially at higher temperatures and in long term. During the hydration reactions of cement, absolute volume of hydration products formed is smaller than that of its reactants, cement and water, resulting in chemical shrinkage. Chemical shrinkage is the main driving mechanism leading to desiccation and early age cracking of cement based materials. Its evolution is an overall result of complex kinetics of many simultaneous hydration reactions. Even same minerals can have different reaction schemes due to the transformation process of metastable hydration products to the stable ones, which are promoted by temperature. In this paper the hydration of synthetic CaAl 2 O 4 and commercial iron-rich CAC were investigated at 15 and 55 o C by measurements of chemical shrinkage evolution and quantitative powder X-ray diffraction analysis. Design of an experimental configuration for measuring the chemical shrinkage during CAC hydration is discussed. A model is proposed to predict the evolution of chemical shrinkage during hydration. The model is based on the main chemical reaction schemes of the CAC hydration and showed good agreement with the observed experimental results. C 3 AH 6 and AH 3 at temperatures greater than 55 o C. Platy (hexagonal) CAH 10 and C 2 AH 8 [10] are metastable at ambient temperature and convert to the more stable C 3 AH 6 and AH 3 with consequent material porosity and permeability increase and loss of strength [1]. The conversion is accelerated by temperature and moisture availability. Chemical shrinkage occurs during cement hydration reactions because the absolute volume of hydration products formed is smaller than that of its reactants, cement and water (i.e. v hydrates < v cement + v H2O). Chemical shrinkage is generally considered as the main driving mechanism (at microscopic scale) leading to early age cracking and, thus, to the loss of durability of cement based materials at macroscopic scale (e.g. concrete structures) [11-13]. The external (macroscopic) dimensional shrinkage of cement based materials is very similar to the chemical shrinkage until the establishment of a semi-rigid skeleton around setting time [11]. After setting time chemical shrinkage results in vapor filled internal porosity that develops capillary related internal stresses. To quantify chemical shrinkage, cement paste specimen must be kept water saturated so the imbibed external water needed to replace the volume decrease can be measured. Techniques for measuring chemical shrinkage and the discussion on systematic errors and critical design of an experimental configuration are presented in next section (Chemical shrinkage tests). There is a lack of knowledge related to the volume change behaviour of CAC. Recently, Ideker et al. [12,13] studied the early-age volume change behaviour during hydration of low-iron grade CAC.
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Citation: Ukrainczyk N. Chemical Shrinkage During Hydration Reactions of Calcium Aluminate Cement. Austin
J Chem Eng. 2014;1(3): 7.
Austin J Chem Eng - Volume 1 Issue 3 - 2014
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Austin Journal of Chemical Engineering
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Abstract
Calcium Aluminate Cement (CAC) is special cement used in versatile high
performance applications. Hardening of CAC is primarily due to the hydration of
mono-calciumaluminate, in idealized pure form CaAl2O4, but other minerals also
participate in the hydration process especially at higher temperatures and in long
term. During the hydration reactions of cement, absolute volume of hydration
products formed is smaller than that of its reactants, cement and water, resulting
in chemical shrinkage. Chemical shrinkage is the main driving mechanism
leading to desiccation and early age cracking of cement based materials. Its
evolution is an overall result of complex kinetics of many simultaneous hydration
reactions. Even same minerals can have different reaction schemes due to the
transformation process of metastable hydration products to the stable ones,
which are promoted by temperature.
In this paper the hydration of synthetic CaAl2O4 and commercial iron-rich
CAC were investigated at 15 and 55 oC by measurements of chemical shrinkage
evolution and quantitative powder X-ray diffraction analysis. Design of an
experimental conguration for measuring the chemical shrinkage during CAC
hydration is discussed. A model is proposed to predict the evolution of chemical
shrinkage during hydration. The model is based on the main chemical reaction
schemes of the CAC hydration and showed good agreement with the observed
experimental results.
Keywords: Calcium aluminate cement; Hydration; Chemical shrinkage;
X-ray diffraction; Mathematical modeling
C3AH6 and AH3 at temperatures greater than 55 oC. Platy (hexagonal)
CAH10 and C2AH8 [10] are metastable at ambient temperature and
convert to the more stable C3AH6 and AH3 with consequent material
porosity and permeability increase and loss of strength [1]. e
conversion is accelerated by temperature and moisture availability.
Chemical shrinkage occurs during cement hydration reactions
because the absolute volume of hydration products formed is
smaller than that of its reactants, cement and water (i.e. vhydrates <
vcement+ vH2O). Chemical shrinkage is generally considered as the
main driving mechanism (at microscopic scale) leading to early age
cracking and, thus, to the loss of durability of cement based materials
at macroscopic scale (e.g. concrete structures) [11-13]. e external
(macroscopic) dimensional shrinkage of cement based materials
is very similar to the chemical shrinkage until the establishment of
a semi-rigid skeleton around setting time [11]. Aer setting time
chemical shrinkage results in vapor lled internal porosity that
develops capillary related internal stresses. To quantify chemical
shrinkage, cement paste specimen must be kept water saturated so
the imbibed external water needed to replace the volume decrease can
be measured. Techniques for measuring chemical shrinkage and the
discussion on systematic errors and critical design of an experimental
conguration are presented in next section (Chemical shrinkage tests).
ere is a lack of knowledge related to the volume change
behaviour of CAC. Recently, Ideker et al. [12,13] studied the early-age
volume change behaviour during hydration of low-iron grade CAC.
Abbreviations
CAC: Calcium Aluminate Cement; PC: Ordinary Portland
Cement; C: CaO (cement chemistry notation); A: Al2O3; F: Fe2O3; S:
SiO2; H: H2O; CA: CaAl2O4; C12A7: Ca12Al14O33; C4AF: Ca4Al2Fe2O16; Ff:
Ferrite Phase (C4AF); CT: CaTiO3; vH: Specic Volume of Water; vm:
Specic Volume Of Cement Mineral; vhydrates: Volume of the formed
hydration products per 1 g of reacted mineral m; m: cement mineral:
CA, C12A7 or C4AF; h: hydration product: CAH10, C2AH8, C3AH6,
AH3, FH3, CH or C4AFH16; stoichiometric coecient; M: Molar Mass;
ρ: density; CSm: Chemical Shrinkage of fully reacted pure mineral m;
αm: fraction of reacted mineral m; wm mass fraction of the mineral m
in cement; W: Weight; V: Volume; Vpaste: the Volume change of the
cement paste; t: time.
Introduction
Calcium Aluminate Cement (CAC) is special cement used in
versatile high performance applications [1-8] such as those requiring:
resistance to chemical attack, high early strength, refractory,
resistance to abrasion, and/or low ambient temperature placement.
Setting and hardening of CAC is primarily due to the hydration of
CA (cement chemistry notation: C=CaO, A=Al2O3, F=Fe2O3, S=SiO2,
H=H2O), but other compounds also participate in the hardening
process especially in long term strength development [1,2] and at
higher temperatures of hydration. e hydration of CAC is highly
temperature dependent [1-3,9], yielding CAH10 as main products at
temperatures less than 20 oC, C2AH8 and AH3 at about 30 oC, and
Research Article
Chemical Shrinkage During Hydration Reactions of
Calcium Aluminate Cement
Ukrainczyk N*
Faculty of Chemical Engineering and Technology,
University of Zagreb, Croatia
*Corresponding author: Ukrainczyk N, Faculty of
Chemical Engineering and Technology, University of
Zagreb, Marulićev trg 20, 10 000 Zagreb, Croatia, Tel:
3851 4597 229; Fax: 3851 4597 260; Email: nukrainc@
fkit.hr
Received: October 07, 2014; Accepted: October 24,
2014; Published: October 27, 2014
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To the best of our knowledge there are no experimental data showing
the evolution of the chemical shrinkage during pure CA and iron-rich
CAC hydration. Chemical shrinkage evolution is an overall result
of complex kinetics of many reactions of several cement minerals.
Even same minerals can have dierent reaction schemes due to the
transformation process of metastabile hydration products to the stable
ones, whose rate is increasing with temperature. Mounanga et. al. [14]
successfully described chemical shrinkage evolution during hydration
of Portland cement. e information about hydration kinetics of
individual minerals during CAC hydration is not yet fully available
[15]. In literature there is still not yet an adequate cement paste model
for CAC hydration analog to PC hydration [14]. However, in rst
approximation the early age of CAC hydration, especially at lower
temperatures (T < 20°C) could be described by hydration reactions
of the principal minerals [15,16]. is paper comes within the scope
of a larger study conducted to build a model for CAC hydration that
requires complementary experimental results on evolution of various
properties. For this non-destructive continuous experimental tests are
desired, such as calorimetry [7,15,17], chemical shrinkage, thermal
properties [9], ultrasonic wave propagation [18,19], which need to be
combined with destructive tests to determine the chemically bounded
water [10,20], fractions of reacted cement minerals [15], hydration
products formed [8], density of the cement paste solids (volume
fraction of the solids) [18], porosity, and strength [8,17,18].
In this paper the hydration of synthetic CaAl2O4 and commercial
iron-rich calcium aluminate cement were investigated by Quantitative
Powder X-Ray Diraction (QXRD) analysis and measurements
of chemical shrinkage evolution. A model is proposed to predict
the evolution of chemical shrinkage during hydration based on the
main chemical reaction schemes of the CAC hydration. e model
predictions of the chemical shrinkage hydration are compared to the
experimental results.
Chemical shrinkage tests
ere are three basic techniques for measuring chemical
shrinkage: dilatometry, pycnometry and gravimetry [11,21,22]. In
dilatometry test procedure a drop of water level in a capillary tube
above a specimen is monitored manually or automatically [23].
Pycnometry consists of a pycnometer lled with paste and water on
top. At dierent ages water is added to rell the pycnometer, and the
weight increase relates to the total volume change. In gravimetric
test procedure the volume change due to water imbibition in the
hydrating specimen is obtained by measuring submerged weight
according to Archimedes law. is paper employed the gravimetric
method that is relatively simple to automate and is described in detail
in section Chemical shrinkage measurement.
Recent review of chemical shrinkage methods in cement based
materials at early ages is given in [11]. e test methods for measuring
chemical shrinkage have experimental diculties that must be
adequately accounted for in order to avoid systematic errors in results.
First, the specimen must be thin enough (especially for specimens
with low water to cement mass ratio, H/CAC) to easily imbibe the
clear water above the cement paste and thus avoid the creation of
vapor lled internal porosity created by chemical shrinkage [11,23].
In other words, the rate of water supply (or transfer) from the
top to the bottom of the specimen must be higher than the rate of
chemical shrinkage. For Portland cement paste with a low water to
cement ratios, this scale eect is not signicant for sample thickness
inferior to 10 mm during the rst 24 h of hydration [14]. Second,
the specimen must be de-aired, because the entrapped air bubbles
going from water to paran change the submerged weight during
the measurement. ird, the amount of the clear water above the
specimen, and its chemical composition inuence the results,
especially in initial stage. Larger amount of clear water above the
cement paste accelerate the initial reaction because of the dilution
of the cement paste pore solution [11]. To obtain measurements on
water to cement ratios closer to practical values with lower dilution
one should employ paran oil as buoyancy uid. Moreover, as the
chemical shrinkage begins instantly upon water contacting cement
(during initial mixing), the measurement results depend on the delay
of the starting time. e data have to be carefully referenced to the
time of starting the measurements which is about ten minutes aer
mixing the cement with water due to sample preparation. And lastly,
the temperature gradients of the sample should be kept low. is is
important if the measurements of chemical shrinkage of cements
(especially calcium aluminates) are performed at higher temperatures
when the reactions rates, and thus the heating rates are high. During
the hydration of CAC a large quantity (70-90 % [6-9,15]) of heat is
liberated within one day that may cause a considerable increase of
temperature in CAC based material. All the aforementioned eects
must be considered when designing an experimental set-up and
interpreting the results of the chemical shrinkage measurement.
Materials and Methods
Commercial CAC ISTRA 40 was taken from a regular production
of Istra Cement, Croatia (CALUCEM Group). e cement has the
oxide mass fraction composition listed in Table 1. Physical properties
of used cement are given in Table 2. e main compounds are CA
(45%) and ferrite phase (C4AF, 21%), with mayenite (C12A7, 5%),
gehlenite (C2AS) and β-C2S as minor compounds. For the syntheses
of CA, ferrite phase and C12A7, precipitated calcite (CaCO3 analytical
grade purity, Kemika), gibbsite (Al(OH)3, Sigma-Aldrich) and Fe2O3
have been wet homogenized in planetary mill (FRITSCH, Pulverisette
5, α-Alumina pot and grinding balls) in the required stoichiometric
mole proportion, dried at 105 oC and then red at 1350 oC for 3 h
in an air atmosphere electric furnace. Synthetic minerals were milled
in a ring agate mortar and sieved below 40 μm. XRD analysis of the
synthesised CA (Figure 1) shows its high crystallinity and purity, with
only small traces of detected C12A7 and corundum. Specic surface
of the prepared CA used for chemical shrinkage measurements was
4960 cm2/g (Blaine). Decarbonated and deionised water is used
and exposure of the samples to the (CO2) atmosphere is kept to a
minimum.
CaO Al2O3Fe2O3FeO SiO2TiO2MgO SO3Na2O K2O Sum
37.1 38.5 14.4 2.9 4.4 1.1 0.9 0.2 0.14 0.17 99.81
Table 1: Chemical composition of investigated CAC.
>90
μm,
%
<40
μm,
%
Blaine,
cm2/g Specic gravity,
g/cm3
Setting
time,
min Standard
consistency, %
Initial nal
3.76 80.50 3508 3.20 298 329 24.0
Table 2: Physical properties of investigated CAC.
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Chemical shrinkage measurement
In this study, chemical shrinkage measurements are carried out
using a gravimetric method [11,21,23]. e change in buoyancy
was continuously monitored for samples suspended in paran oil.
e illustration of the experimental setup is given in (Figure 2). e
samples are prepared in a thin glass sample holder (2r = 29 mm
and h = 50 mm). Firstly, on 8 g of cement the deionized water was
added with a medicine dropper to obtain water to cement mass ratio
of H/CAC = 0.5 and H/CA = 0.95 for Istra 40 and synthetic CA,
respectively. is paste was mixed by applying vibrations. Secondly,
more deionized water was added with a medicine dropper to form a
layer of clear water above the cement paste. e overall H/CAC ratio
was H/CAC = 1.0 and H/CA = 1.5 for the Istra 40 and the synthetic
CA, respectively. As the measurement necessities the presence of the
clear water above the cement paste the overall water to cement ratio
was chosen accordingly to compensate the volume changes (water
uptake) that are expected during measurement time. e prepared
specimens (cement pastes without the clear water above it) are 5 mm
thick. irdly, specimens were de-aired by placing the lled sample
holder in a desiccator. Air was evacuated from the desiccator by using
a vacuum pump. During the de-aeration the desiccator was vibrated.
Lastly, the paran oil is added, initially with a medicine dropper to
form a layer above the water, followed by pouring larger quantities of
paran to almost completely ll the sample holder.
e sample holder is hung on a balance (sensitivity 0.1 mg) by a
thread and immersed in a laboratory glass (400 mL) lled with paran
oil (Figure 2) at the required curing temperature (thermostated 15
L water bath with ± 0.05 °C). Balance is connected to the Personal
Computer (PC) via RS232 protocol employing acquisition soware.
e measurement started 10 min aer initial water–cement contact
with a sampling rate of 15 s. Appropriate corrections were applied
to account for a temperature stabilization of the sample during
rst minutes of the measurement. is was done by subtracting
the baseline obtained by measuring the response of the hydrated
sample (stabilized at room temperature) when immersed in a curing
temperature of paran oil.
e density of used paran oil, ρpar was measured (to be 0.850
g/cm3 at 15 °C) by applying Archimedes method and utilizing glass
sinker (part of standard equipment for density measurements by
KERN ALS 220-4 balance):
ρpar =(WairWpar)/Vs + ρair (2.1)
where: ρair is air density [0.0012 g/cm3], ρpardensity of
the paran oil [cm3/g], Wair weight of the sinker in air [g], Wpar
weight of the sinker submerged in the paran oil [g], VS – volume of
glass sinker [10.4920 cm3].
Likewise, using the Archimedes law the chemical shrinkage [CS,
cm3/g] evolution during cement paste hydration is calculated:
CS(t)=∆Vpaste (t)/Wcem=(W(t) – W(24))/(ρpar Wcem) (2.2)
where: Vpaste is the volume change of the cement paste [cm3],
t is time, Wcem is weight of cement in paste [g], W(t) is weight of
the submerged paste at time t [g], W(24) is an initial weight of the
submerged paste 24 min aer cement-water mixing [g], and ρpar is
density of the paran oil [cm3/g].
At the end of chemical shrinkage measurements the specimens
were removed from the sample holder and crushed to a ne powder.
e hydration was blocked and free water removed by addition of
acetone (2-propanon). is was done by grinding and mudding the
sample in three doses with acetone in agate mortar and CO2 free
atmosphere.
Powder X-ray diffraction
e composition of CAC and hydrates formed was investigated
by powder X-Ray Diraction (XRD). Shimadzu diractometer
XRD-6000 with Cu Kα1,2 radiation was used (the scan step was 0.02°
with collection time of 1 for qualitative and 4 s for quantitative
analysis). For Quantitative X-Ray Analysis (QXRD), the prepared
hydrated samples were additionally red at 500 oC (for 10 min) to
decompose the hydrates to amorphous oxides and water vapor.
Expected amorphous oxides aer heating are C2A(am.) from C2AH8,
CA(am.) from CAH10, A(am.) from AH3 and C3A(am.) from C3AH6.
By this method, proposed in [15], the interferences of the hydration
products are excluded from the diractograms. is enables a direct
determination of the degree of hydration of individual minerals upon
comparing the quantities in hydrated and initial (non-hydrated)
cement samples. e temperature for decomposition was chosen
by inference to thermogravimetry and XRD analysis of hydrated
samples [10,15].
CAC quantitative X-ray diraction using the adiabatic principle
10 15 20 25 30
0
1000
2000
3000
4000
5000
syntetic CA
CA
C
12
A
7
corundum
Relative intensity
2
θ,
o
CuK
α1,2
Figure 1: XRD analysis of the synthesised CA sample (traces of C12A7 and
corundum were detected).
(a) (b)
Figure 2: Experimental set-up to measure chemical shrinkage: (a) scheme
(b) a photo of the sample holder immersed in parafn oil.
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with auto ushing as proposed by Chung [24] is proven to be a suitable
method [25]. Here, the matrix-ushing method [24] was applied as it
enables to directly quantify relative portions in sample including both
the crystalline and amorphous components by analyzing for only
those components of interest. Here only CA, C12A7 and C4AF, while
quantication of other minerals (e.g. pleochroite, C2AS, C2S, FeO,
CT) and amorphous phases was and can be omitted. e criterion for
this method is that the reference material must have the same level of
crystallinity (the same level of perfection or imperfection in crystal
structure) as the component in the sample. e Full-Width at Half-
Maximum (FWHM) of reections in an X-ray diraction pattern was
compared as an indication of the crystallinity of the components.
FWHM for CA was measured from the peak comprising three close
reections at 30.06o , 30.1o and 30.17o and corrected by subtracting
it with the width of this reections (0.11o). e obtained FWHM
were 0.17o, 0.16o, 0.12o and 0.16o for rutil (reection at 27.4o), C12A7
(33.3o and 18.1o), CA and C4AF (12.2o), respectively. is indicates
no signicant dierences in their crystallinity. It is important to note
that the possible eect of the observed dierence is lowered by using
peak areas instead of peak heights to dene/quantify diraction peak
intensity. In the matrix-ushing method the relationship between
the peak intensity (peak area) of the characteristic X-ray reection
Ii is directly proportional to the weight fraction of the component
by the factor ki which contains the mass absorption coecient of the
total sample. Experimentally determined ki values hold only for the
detecting system and for no other. Reasonable standard materials
for such a study are rutile (TiO2) [15] or corundum (Al2O3) [25].
e rutile used in this study had a narrow particle size distribution
around approximately 0.4 μm, which would reduce microabsorption
eect. XRD analysis of chosen standard rutile showed no traces of
anatas. In this work, CA, C12A7, C4AF in CAC and red hydrated
samples are quantied based on the matrix-ushing method by
Chung. e ki values were determined by mixing of pure phase and
standard mineral rutile (TiO2) in a 50:50 weigh ratio. Pure synthetic
minerals were milled in a ring agate mortar and sieved below 40 μm
to maximize the number of particles analyzed, to improve powder
homogeneity and packing characteristics, and to minimize micro
absorption-related problems. Each sample prepared for QXRD (0.7 g)
was mixed with a xed amount of rutile (0.14 g), followed by grinding
and homogenization in an agate mortar under acetone. Appropriate
corrections for peak overlap were meticulously applied by inference
to the (measured) intensities of the pattern due to pure phases.
Modeling methodology
If knowing the densities of components (Table 3) and hydration
reaction stoichiometry (eqs 1-7 presented in Table 4) the chemical
shrinkage of pure minerals (in cm3/g of mineral) for complete
hydration can be calculated as:
CSm= vH + vmvhydrates (8)
Where the vm and vH is the specic volume of the cement mineral
and water, respectively, and vhydrates is the volume of the formed
hydration products per 1 g of reacted mineral calculated as:
hydrates m m hydrates
h h
hh
M
vM
υ
υ ρ
 
= 
 
(9)
where the sum is for all hydration products (h),
υ
and M
are stoichiometric coecient and molar masses of the reaction
components, respectively, and
h
ρ
is the density of hydration
product [5,26,27] (Table 3). e volume of the water required for the
stoichiometric hydration of the mineral m is:
H H
Hm m H
M
vM
υ
υ ρ
=
(10)
Table 4 summarizes theoretical values of total chemical shrinkage
for complete hydration of mineral upon individual reaction
stoichiometry at T = 15 and 55 °C. Furthermore, the evolution of
chemical shrinkage could be calculated by multiplying the total
chemical shrinkage by the degree of reacted mineral.
To model the chemical shrinkage evolution of CAC paste we need
to know the hydration reaction stoichiometry (thermodinamics) and
reaction kinetics of all cement minerals. is information is not yet
fully available due to the complexity of reactions involved during
hydration of commercial CAC. However, in rst approximation
principal hydration reactions can be considered. From these data, the
chemical shrinkage (CS in cm3/g of CAC) versus overall hydration
degree (α) of the principal minerals, m can be expressed as:
m
m
( ) m m
CS CS w
α α
=
(11)
Where: CSm is the chemical shrinkage of fully reacted pure
mineral calculated by eq. 8, αm is the fraction of reacted mineral (m),
and wm is the mass fraction of the mineral in cement. To predict the
chemical shrinkage of CAC by eq. (11), in this paper the hydration of
CA, C4AF and C12A7 is considered upon reaction schemes as shown
Component ρ / g cm-3 M / g mol-1
CA 2.98 158.1
C12A72.85 1387
C4AF 3.73 485.9
CAH10 1.72 338.1
C2AH81.96 358.2
C4AFH16 2.2 774.1
C3AH62.52 378.3
AH32.44 156.0
FH32.20 213.7
CH 2.24 76.1
Table 3: Densities and molar masses of the components [5,26,27].
Mineral Reaction eq. Eq. no. Temperature,
oCCS, cm3/g of
mineral
CA CA + 10 H → CAH10 (1) 15 0.2317
2 CA + 11 H → C2AH8 + AH3(2) 15 0.1821
3 CA + 12 H → C3AH6 +
2 AH3(3) 55 0.2113
C12A7
C12A7 + 60H → 5C2AH8 +
2CAH10
C12A7 + 33H → 4C3AH6 +
3AH3
(4)
(5) 15
55 0.1879
0.2140
C4AF C4AF + 10H → C3AH6 + CH
+ FH3
C4AF + 16H → C4AFH16
(6)
(7) 55
15 0.0658
0.1343
Table 4: Theoretical values of chemical shrinkage for complete hydration of
mineral upon individual reaction stoichiometry [1,27].
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in Table 4. At 15 oC (t < 35 h) only metastabile hydration products
are formed, while at 55 oC and sucient time the conversion is
practically completed leaving only stable hydration products. To
test the proposed model against the measured chemical shrinkage
the amount of reacted CA, C4AF and C12A7 was quantied by QXRD
analysis.
In the case of the iron rich CAC, the most hydraulic phases are
CA and C12A7, while C2AS, C2S and ferrite phase (nominally C4AF)
are considered to have no signicant reactivity at early ages (rst 48
hours) below 20 °C [1,5,15]. However, although C4AF reactivity has
usually been disregarded in studies of CAC hydration, because of its
high quantities (up to 30 %) one should be careful in considering its
contribution.
Results and Discussion
XRD analysis on samples obtained from hydrated specimens
cured at dierent conditions (Figure 3) conrmed the hydrate
compositions expected from the literature [1,5,8,15]. e main
hydration products observed on specimens hydrated at 15 oC was
CAH10 with traces of C2AH8 and AH3. Transformed samples hydrated
at 55 oC gave C3AH6 and AH3 as hydrated products. Main diraction
peaks of metastable hydration products and AH3 are fairly broadened,
indicating poor crystallinity, while that of the C3AH6 product showed
good crystallinity. e QXRD analysis of investigated CAC gave the
mass proportions of CA, C12A7 and C4AF to be 45 %, 5 % and 21 %,
respectively. From the QXRD analysis the degree of hydration of CA,
C12A7 and C4AF was obtained and presented in Table 6. It can be
observed that the fraction of reacted cement minerals (CA, C12A7 and
C4AF) signicantly increased with temperature.
Results of the chemical shrinkage evolution during the hydration
of the cement samples (Figure 4) indicate the kinetics of the cement
hydration. When cement and water rst come in contact the initial
hydration is attributed mainly to the cement wetting and dissolution
processes. During the induction period a small rate of hydration
reaction is observed. Induction period is followed by the onset of the
accelerated stage of reaction due to massive precipitation of hydrates.
e hydration rate is than again decreasing due to consumption
of the reactants and mass transfer limitations [15]. e reason for
slower nucleation activity of the CA15 than CAC15 (Figure 4) could
be explained by the lack of germination sites for the nuclei to start
the precipitation of the metastabile hydration products. On the other
hand, at 55 oC, the CA55 has higher nucleation activity than CAC55,
which can be attributed to the higher specic surface of the synthetic
CA. Both the nucleation period and the reaction rate aer the onset of
the massive precipitation are higher at higher temperature.
Less amount of chemical shrinkage is evolved during the initial
(dissolution) stage for CAC than for Portland cement (compared
with the experimental results from [11]). is was also observed by
calorimetric measurements [7,8,15]. e maximal CAC hydration
rate at 15oC (inection point of the CAC15 curve in Figure 4 at tmax =
4.3 h), dCS/dtmax is 25 10-3 cm3/(g h) which is about ten times greater
in comparison to chemical shrinkage results on PC (dCS/dtmax,PC ~
2.5 10-3 cm3/(g h) [11]). Aer 35 h of hydration at 15oC chemical
shrinkage of CAC (0.096 cm3/g) is more than 2.5 times higher than
for PC (about 0.035 cm3/g [11]). If temperature gradients are ensured
to be low (below ~ 1°C) the higher hydration rate and achieved
hydration degree could enable more accurate measurements of CAC
chemical shrinkage in comparison to PC (Table 5). As expected, the
obtained values of chemical shrinkage for synthetic CA15 are even
higher (Figure 4 and Table 6), about 2.2 times that for CAC15, with
a maximal rate dCS/dtmax of 0.055 cm3/(g h). e lower values of
chemical shrinkage for commercial CAC hydration than synthetic
CA can be explained mainly by a high amount of slow reacting
minerals in the CAC sample. Moreover, CAC has a lower specic
surface than CA sample.
Results of the QXRD analysis and the comparison of predicted
and measured chemical shrinkage is presented in Table 6. One can see
that the model eq. (11) based on the reaction equations (1-7) (Table
4) is in good agreement with the obtained experimental results. e
reactivity of other phases but CA, C12A7 and C4AF has been disregarded
in the simplied stoichiometric model to describe chemical shrinkage
of CAC. At 15°C it was assumed that C2AH8 originates only from
hydration of C12A7 (eq. 4), as the conversion of CAH10 to C2AH8 is very
slow at T15°C [1-3]. Calculated amount of formed C2AH8 in cement
paste by reaction eq. 4, considering water to cement mass ratio of 0.5,
0 10 20 30
C
3
AH
6
C
3
AH
6
Ff
AH
3
AH
3
C
3
AH
6
C
3
AH
6
C
3
AH
6
55
o
C
C
3
AH
6
C
2
AH
8
CAH
10
CAH
10
2
θ
,
o
CuK
α1,2
Relative intensity
15
o
C
CA
C
12
A
7
CA
CA
CA
Figure 3: XRD analysis of the hydrated CAC pastes.
0 5 10 15 20 25 30 35
0.00
0.05
0.10
0.15
0.20
CAC15
CA15
CAC55
CA55
Hydration time, h
CS , cm
3
/g cement
Figure 4: Experimental results of chemical shrinkage evolution.
Sample
label Cement sample description T, °C H/c
CA15 CaAl2O415
55
15
1.5
CA55 CaAl2O41.5
CAC15 CAC ISTRA 40 1.0
CAC55 CAC ISTRA 40 55 1.0
Table 5: Experimental plan for investigating the hydration of the cement samples.
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5% of C12A7 in CAC of which 60% has reacted, is 2.6%, in agreement
with its relatively very low diraction peak. However, at longer times
and at higher temperatures conversion is signicant, and to calculate
the chemical shrinkage the ratio of reactions (eqs. 1-3) should be
determined, e.g. from ratio of formed hydration products, which is
quite challenging. More data are needed to validate the used model
for predicting the chemical shrinkage evolution during continuous
hydration, especially at higher temperatures. e presented modeling
methodology can be extended to develop more complex models to
describe all three reactions of CA hydration scheme simultaneously,
and even to include other hydraulically active minerals (e.g. C12A7,
C4AF and C2S). QXRD analysis of cement is dicult as the multiple
phases result in substantial peak overlap. ere is also diculty in
securing suitable pure phase reference standards. For better accuracy,
these concerns may be addressed using the quantitative Rietveld
analysis of all CAC phases. More accurate quantitative data on
hydration of dierent CACs should be used to additionally test the
proposed model for predicting the evolution of chemical shrinkage
during CAC hydration.
Conclusion
Chemical shrinkage test methods have experimental diculties
(outlined in section Chemical shrinkage test) that must be adequately
accounted for in order to avoid systematic errors in experimental
results. From the chemical shrinkage results obtained the following
can be stressed out. Aer 35 h of hydration chemical shrinkage of CAC
is more than 2.5 times higher than for Portland cement but about 2.2
times lower than for synthetic CA. Lower chemical shrinkage evolved
during the measurements of commercial CAC than synthetic CA can
be attributed mainly to a high amount of slow reacting minerals in
the CAC sample, and also to dierence in the specic surface of the
cement samples. e reason for the faster nucleation activity of the
CAC than CA at 15 oC could be explained by more germination sites
and the seeded heterogenous nuclei eect. On the other hand, at 55 oC,
the synthetic CA has higher nucleation activity than the commercial
CAC, which can be attributed to the higher specic surface of the CA.
Both the nucleation period and the reaction rate aer the onset of the
massive precipitation are higher at higher temperature.
Chemical shrinkage can be calculated using the stoichiometry of
the individual hydration reactions, the density of hydration products
and reactants and the degree of conversion for each reaction. Model
predictions of chemical shrinkage based on the main reaction scheme
of the CA hydration are in reasonable agreement with the experimental
results. More data are needed to additionally validate the used model
for predicting the chemical shrinkage evolution during continuous
hydration, especially at higher temperatures when initially formed
metastable hydration products transform to stabile ones. e
presented modeling methodology can be extended to develop more
complex models to describe all reactions simultaneously, and even
to include other hydraulically active minerals (e.g. C12A7, C4AF and
Sample αC12A7 αFf QXRD
αCA
CS , cm3/g of sample Deviation, %
Model Measured
CA15 - - 0.72 0.167 0.166 0.5
CA55 - - 0.97 0.199 0.200 - 0.4
CAC15 0.6 0.08 0.80 0.091 0.096 - 4.9
CAC55 1.0 0.5 0.95 0.108 0.107 0.9
Table 6: Results of the QXRD analysis and the comparison of predicted and
measured chemical shrinkage (at hydration time th = 35 h). C2S). For such models more quantitative data on hydration of each
mineral are needed.
Acknowledgement
e authors acknowledge support from the Croatian Ministry
of Science, Education and Sports under project’s no. 125-1252970-
2983 “Development of hydration process model” and thank Calucem,
Croatia for providing CAC samples.
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Citation: Ukrainczyk N. Chemical Shrinkage During Hydration Reactions of Calcium Aluminate Cement. Austin
J Chem Eng. 2014;1(3): 7.
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