Effect of Curing Conditions on Hydration Reaction and
Compressive Strength Development of Fly Ash-Cement Pastes
Warangkana Saengsoy
Candidate for the degree of Doctor of Philosophy
Supervisor: Prof. Dr. Toyoharu Nawa
Division of Solid Waste, Resources and Geoenvironmental Engineering
Introduction
Fly ash is one of the pozzolanic materials which has
been widely used to effectively improve the various
properties of concrete. The reaction between pozzolan
and calcium hydroxide is referred to as the pozzolanic
reaction. The product of the pozzolanic reaction,
calcium silicate hydrates (C-S-H), is highly efficient in
filling up large capillary spaces due to its lower density
which is lower than that of C-S-H produced from the
reaction of Portland cement and water. Thus, it
improves the strength and impermeability of cementbased materials.
The term for curing of concrete stands for procedures
devoted to hydration reaction of cement and pozzolanic
reaction of the fly ash, consisting of control of time,
temperature, and humidity conditions. Several studies
have undergone to understand the influence of curing on
the compressive strength of the fly ash concrete. In
addition, it has been reported that the fly ash concrete is
more sensitive to curing conditions and requires a
longer curing period than that required by OPC concrete.
However, the mechanism on the effect of curing
conditions on the strength development is yet to be
ascertained. Due to their hydration characteristic, both
pozzolanic reaction and the reaction of Portland cement
require water to complete their reaction. The prevention
of rapid and excessive loss of water occurring during
the evaporation process in the early age is necessary for
the adequacy of water for the slower pozzolanic process.
Objective and Scope of Study
The objective of this study is to control and predict
mechanical properties and durability of fly ash concrete
from the viewpoint of the progress on the hydration of
cement, and pozzolanic reaction of fly ash at any curing
conditions.
The scope of this research is threefold: Firstly, the
existing prediction model of strength development is
applied to hardened pastes cured at different
temperatures and relative humidity curing conditions.
Next, the effect of curing conditions on individual
hydration degree of cement and fly ash in the fly ashcement paste is quantified by the X-ray diffractionRietveld analysis and selective dissolution analysis,
respectively. The prediction model that characterizes the
temperature and relative humidity influences on
hydration of both Portland cement and fly ash is
developed based on the Arrhenius equation. Finally, the
modified gel/space ratio theory is proposed to enhance
the prediction model of strength development and
verified by taking into account the difference in the
density of C-S-H with fly ash addition, curing
temperature, and curing relative humidity.
Methodologies
Materials and Mix Proportion
Ordinary Portland Cement (OPC) and fly ash type II
were used according to JIS R5210 and JIS A6201,
respectively. The paste specimens were prepared with a
water-to-binder ratio (W/B) of 1.00 by volume. The
replacement ratios of cement by fly ash were 0%, 25%,
and 50% by volume. A polycarboxylate-based
superplasticizer was used to control the fluidity of the
paste.
Curing Conditions
In order to investigate the effect of curing temperature,
the paste specimens were cured in water with three
different curing temperatures at 20±2°C, 35±2°C, and
50±2°C.
As for the effect of curing relative humidity, the
specimens were placed in a controlled temperature
chamber at 20±2°C. At three days after mixing, the
specimens were removed from their moulds and cured
in five different environmental conditions until the
required age was reached. The curing conditions were
water curing, sealed curing, and at relative humidity
levels of 60%, 80% and 95%. Relative humidity was
controlled by saturated salt solutions.
Hydration Reaction Measurement
The degree of hydration of the hydrated cement pastes
was estimated with an XRD-Rietveld analysis. In the
case of the pastes that had been incorporated with fly
ash, the degree of hydration was estimated with a
combined method using XRD-Rietveld analysis and
selective dissolution [1, 2].
At the required age, the specimens were broken into
small pieces (2.5-5.0mm). Then, the hydration reaction
was stopped by soaking them in acetone and further
drying at 105°C. Next, they were ground in a ball mill at
a speed of 300rpm for eight minutes.
The X-ray diffraction equipment used in the XRDRietveld analysis was a Rigaku, CuKα X-ray type. The
experiments were performed in the range of 5-70° 2θ at
120
Compressive Strength
(MPa)
40kV, 20mA, 0.02° sampling width and 2°/min scan
speed. The divergence slit, scattering slit and receiving
slit were 1/2°, 1/2° and 0.3 mm, respectively. Standard
corundum, 10% by weight of sample, was used as an
internal reference. The sample and corundum were
mixed in the ball mill at a speed of 100rpm for three
minutes. The software used in the Rietveld analysis for
this study was SIROQUANT (version 3.0).
As for selective dissolution, the unhydrated fly ash
was determined using a solution of 2N HCl and 5%
Na2CO3.
Once the weight of unhydrated cement and fly ash
was obtained, the degree of hydration of cement and fly
ash can be calculated from their hydrated quantity
divided by the unhydrated quantity at 0 days [3].
20 C
35 C
50 C
100
80
60
40
20
Water
FA 50%
0
0
10
20
Age (days)
30
Fig. 2 Compressive strength development of pastes with
50% fly ash cured in water at different
temperatures
100
80
60
Sealed
RH 95%
RH 80%
RH 60%
40
20
0
(1)
where RH is relative humidity (%) and ρ is electrical
resistance (Ω).
Strength Development of Fly Ash Cement
Pastes
There is no significant difference in the strength
development of cement pastes cured in water at
different curing temperatures as shown in Fig. 1. On the
other hand, an increase of the curing temperature
significantly increases the strength development of fly
ash cement pastes as shown in Fig. 2.
Compressive Strength
(MPa)
120
FA 0%
80
60
40
20
20 C
35 C
50 C
Water
0
0
10
40
Age (days)
60
120
FA 25%
80
60
40
Water
RH 95%
RH 60%
20
Sealed
RH 80%
0
0
20
40
Age (days)
60
Fig. 4 Influence of curing methods and ambient relative
humidity on compressive strength of paste with
W/B = 1.0 by volume
Effect of Curing Temperature
100
20
Fig. 3 Change in internal relative humidity of paste with
ambient relative humidity: W/B = 1.0 by volume
100
RH = −0.05( ρ − 327.5) 0.42 + 100
FA 25%
0
Compressive Strength
(MPa)
The internal relative humidity in the pastes was
measured with a ceramic sensor embedded in the centre
of each specimen during specimen preparation. The
dimensions of the ceramic sensor were 1cm in diameter
and 1cm in height. The sensor was connected to the data
logger to record electrical resistance every ten minutes
throughout the experiment. In order to obtain the
relationship between the electrical resistance of the
sensor and the relative humidity, the ceramic sensor was
calibrated using salt solutions. In this study, relative
humidity was calculated based on electrical resistance
as in Eq. (1).
Internal Relative
Humidity (%)
Internal Relative Humidity Measurement
20
30
Age (days)
Fig. 1 Compressive strength development of cement
pastes cured in water at different temperatures
Effect of Curing Relative Humidity
Hydration of ordinary Portland cement consumes the
water, accordingly internal relative humidity of paste
decreases with time even if the paste was cured under
sealed curing condition. It is also observed that when
cured the paste at ambient relative humidity of 95%, the
internal relative humidity of paste is almost the same as
water immersing and sealed curing. In contrast, at
ambient relative humidity below 95%, the internal
relative humidity of paste is decreased with a reduction
in the ambient relative humidity. A typical result of
change in the internal relative humidity of paste cured at
different moisture conditions is shown in Fig. 3. The
reduction of internal relative humidity in paste is
derived from two causes: one is the consumption of
water due to the hydration of cement and pozzolanic
reaction of fly ash, and the other is the evaporation of
Prediction Model of Strength Development of
Pastes
Bernhardt [4] proposed that the rate of strength gain at
any age was a function of the current strength and the
temperature and proposed the mathematical expression
as in Eq. (2).
dS
= f (S ) × k (T )
dt
(2)
where S is the compressive strength, f(S) is a function of
strength and k(T) is a function of temperature.
The strength function was proposed as a hyperbolic
equation as in Eq. (3) [5].
⎡ ⎛ S
f (S ) = S u ⎢1 − ⎜⎜
⎣⎢ ⎝ S u
⎞⎤
⎟⎟⎥
⎠⎦⎥
temperature influence on the strength development but
it can not describe the influence of the relative humidity.
Hence, the model should be modified by taking into
account the influence of the relative humidity.
It is shown in Fig. 6 that the compressive strength is
linearly decreased with a reduction in the internal
relative humidity regardless of replacement ratio of fly
ash. As a result, it is assumed that the relative humidity
effect is linearly related to the rate of strength
development as in Eq. (7).
E
⎛ RH ⎞ − RT
k (T , t , RH ) = ⎜ a
⎟ Ae
⎝ 100 ⎠
(7)
where RH is the relative humidity (%), and a is a
constant.
The modified compressive strength model with
considering the influence of the relative humidity
reasonably estimates the compressive strength of pastes
cured at any conditions as shown in Fig. 7.
45000
Apparent Activation
Energy (J/mol)
water by drying when pastes are exposed at low ambient
relative humidity.
The compressive strength of the pastes cured at
various methods is shown in Fig. 4. There is no
significant difference in the strength development of
specimens cured in water, in sealed curing and in moist
curing at a relative humidity of 95%. However, the
compressive strength slowly develops when the pastes
are exposed at a relative humidity below 80% regardless
the replacement ratio of fly ash, in particular the
compressive strength of pastes exposed at a relative
humidity of 60% is hardly increased after 7days.
40000
35000
FA 0%
FA 25%
FA 50%
30000
25000
3
0
(3)
20
40
Age (days)
60
Fig. 5 Apparent activation energy
k (T , t ) = Ae
−
E = E 0 e −αt
E
RT
(6)
where A is a constant, R is gas constant and equal to
8.3144 J/K mol, and T is curing temperature (K). E is
the apparent activation energy (J/mol). E0 is the initial
apparent activation energy (J/mol). α is a constant and
equals to 0.000615. t is age (days).
The apparent activation energy can be estimated and
it is shown in Fig. 5. It can be seen that the apparent
activation energy increases with increasing fly ash
replacement ratio. The apparent activation energy is
almost constant and independent of age of pastes.
Even though the Eq. (4) can characterize the
R2 = 0.9069
80
60
R2 = 0.9275
40
20
R2 = 0.7869
28 days
0
40
(4)
(5)
FA 0%
FA 25%
FA 50%
100
60
80
100
Internal Relative Humidity (%)
Fig. 6 Relationship between the compressive strength
and the internal relative humidity of pastes with
W/B = 1.0 by volume
Calculated Relative
Compressive Strength
dS
= f (S ) × k (T , t )
dt
120
Compressive Strength
(MPa)
where Su is the ultimate compressive strength.
Because the Arrhenius function estimates the effect
of temperature more accurately than the previous
temperature function. Kim et al. [5] proposed Eq. (4)
and suggested that the function k(T,t) was based on the
Arrhenius equation as shown in Eq. (5). The apparent
activation energy in Eq. (5) was a function of age as in
Eq. (6).
1.4
1.2
1
0.8
0.6
0.4
0.2
0
R 2 = 0.9045
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Measured Relative Compressive
Strength
Fig. 7 A comparison between the calculated and
measured relative compressive strength of pastes
cured at different relative humidity
Effect of Curing Relative Humidity
There is no significant difference in the degree of
hydration of Portland cement in pastes cured at ambient
relative humidity of 95%, sealed condition, and water
immersed condition. However, the degree of hydration
of cement is seen to decrease with a reduction in
ambient relative humidity when the pastes were exposed
to lower ambient relative humidity levels below 80% as
shown in Fig. 10. The influence of curing relative
humidity on the degree of hydration of fly ash also
exhibits the similar tendency as that of cement. It can be
seen in Fig. 11 that the degree of hydration of fly ash is
severely restricted at relative humidity below 80%.
Prediction Model of Hydration Reaction
It has been reported that the exponential function as
shown in Eq. (7) can be used to effectively characterize
the development of the degree of hydration [6, 7].
⎛ ⎡τ ⎤β
α (t e ) = α u exp⎜⎜ − ⎢ ⎥
t
⎝ ⎣ e⎦
⎞
⎟ × 100
⎟
⎠
(7)
where α(te) is degree of hydration at equivalent age te
(%), τ is hydration time parameter (h), β is hydration
shape parameter, and αu is ultimate degree of hydration
(%). The equivalent age can be expressed in the
Arrhenius form as in Eq. (8).
t
t e (Tr ) =
∑
0
⎛E⎛
1
1
exp⎜ ⎜⎜
−
⎜ R 273 + T
273 + Tc
r
⎝ ⎝
⎞⎞
⎟⎟ ⎟∆t
⎟
⎠⎠
(8)
where te(Tr) is equivalent age at reference curing
temperature (h). ∆t is chronological time interval (h). Tc
is average concrete temperature during time interval ∆t
(°C); Tr is reference temperature (°C).
The activation energy of the hydration reaction of
cement and the hydration reaction of fly ash is shown in
Fig. 12. It shows that the apparent activation energy of
cement hydration decreases with increasing fly ash
replacement ratio. It was reported that the acceleration
Degree of Hydration of
Cement (%)
FA 25%
80
60
40
20 C
35 C
50 C
20
Water
0
0
10
20
Age (days)
30
Fig. 8 Degree of hydration of OPC in fly ash-cement
paste cured in water at different temperatures
100
Degree of Hydration of
Fly Ash (%)
An increase of the curing temperature accelerates the
degree of hydration of cement at early age but it has a
small effect at later age of pastes as shown in Fig. 8.
The degree of hydration of fly ash is strongly
influenced by the curing temperature. The increase of
the curing temperature accelerates the degree of
hydration of fly ash both at early age and later age of
pastes as shown in Fig. 9. This finding implies that the
increased strength development of fly ash cement paste
is owing to the accelerated degree of hydration of fly
ash due to the increase of curing temperature.
FA 25%
80
20 C
35 C
50 C
Water
60
40
20
0
0
10
20
Age (days)
30
Fig. 9 Degree of hydration of fly ash in paste with W/B
= 1.0 by volume
100
Degree of Hydration of
Cement (%)
Effect of Curing Temperature
100
FA 50%
80
60
Water
RH 95%
RH 60%
Sealed
RH 80%
40
0
20
40
Age (days)
60
Fig. 10 Influence of ambient relative humidity on
degree of hydration of OPC in paste with W/B =
1.0 by volume
40
Degree of Hydration of
Fly Ash (%)
Hydration Reaction of Fly Ash Cement
Pastes
Sealed
RH 80%
RH 60%
30
FA 50%
20
10
0
0
20
40
Age (days)
60
Fig. 11 Influence of ambient relative humidity on
degree of hydration of fly ash in paste with W/B
= 1.0 by volume
of cement hydration reduced the apparent activation
energy [5, 8]. Therefore, the increase in fly ash
replacement ratio accelerates the degree of hydration of
cement and reduces the apparent activation energy. In
case of the fly ash, the activation energy of the
pozzolanic reaction is higher than that of cement
hydration and it increases with increasing fly ash
replacement ratio. The higher value of the activation
energy of the hydration reaction of fly ash indicates that
the reaction is more sensitive to the temperature than
that of cement.
The reduction of the internal relative humidity due to
drying leads to prevent the hydration reaction of cement
and fly ash. In this study, the influence of the internal
relative humidity on the hydration of cement and/or fly
ash can be expressed by Eq. (9). The preventing
influence of the hydration reaction is modeled by a
coefficient CRH as in Eqs. (10) to (13).
β
⎛ ⎡τ ⎤ ⎞
α (te ) = α u exp⎜⎜ − ⎢ ⎥ ⎟⎟C RH × 100
te
(9)
⎝ ⎣ ⎦ ⎠
C RHC = 0.0051RH + 0.5292 for RH < 95% (10)
C RHC = 1 for RH ≥ 95%
(11)
C RHF = 0.0047 RH + 0.5679 for RH < 95% (12)
C RHF = 1 for RH ≥ 95%
Activation Energy
(kJ/mol)
Cement
Fly Ash
100
80
y = 132.08x + 44.218
60
The degree of hydration is one factor to contribute to the
compressive strength development. However, the
compressive strength could not be directly estimated by
only either the degree of hydration of cement or the
degree of hydration of fly ash: The hydrates such as CS-H gel produced from both cement and fly ash
hydration reactions fill the space and contribute to
strength development. For OPC pastes, assuming that 1
ml of cement will occupy 2.06 ml of space when fully
hydrated [9].
Pipat et al. [10] reported that the density of C-S-H
produced in fly ash-cement paste is different from that
of Portland cement paste as shown in Fig. 14. This
implies the filling effect of hydrates is also different.
Hence taking into account of this effect, the hydration
degree of fly ash-cement system is correspond to about
1.5 times as large as that of Portland cement system.
The volume of the hydrated gel can be obtained as a
function of the degree of hydration of cement and fly
ash and the volume of the hydrated product of cement
and fly ash as shown in Eq. (14).
(13)
where CRHC and CRHF are coefficients of internal relative
humidity for the degree of hydration of cement and fly
ash, respectively. RH is internal relative humidity (%).
It is shown in Fig. 13 that the proposed equations can
be used to estimate the degree of hydration of cement
and the degree of hydration of fly ash.
120
Prediction Model of Strength Development
of Pastes based on Hydration Reaction
R 2 = 0.9993
40
VGel = 2.06Vc
100
+ DfVf
αf
100
R 2 = 0.9052
0
where, VGel is volume of hydrated gel. Vc and Vf are
volume fraction of cement and fly ash, respectively. αc
and αf are degrees of hydration of cement and fly ash
(%), respectively. 2.06 is the volume of the hydration
products of cement at the completion of hydration of 1
ml of cement. Df is the volume of the products of
hydration of fly ash at the completion of reaction of 1
ml of fly ash and equals to 3.0.
Model for Predicting Strength Development of
Pastes at Different Curing Temperatures
0.2
0.4
0.6
Replacement Ratio of Fly Ash
The density of the hydrated gel may change according
to the curing temperature. Therefore, the volume of the
hydrated gel is modified by taking into account the
effect of temperature as in Eq. (15).
Fig. 12 Activation energy of cement hydration and fly
ash hydration
VGel = 2.06Vc
100
αc
100
+ DT D f V f
αf
100
R 2 = 0.9926
60
40
Fly As h
R 2 = 0.9491
20
0
20
40
60
80
100
Measured Degree of Hydration (%)
Fig. 13 A comparison between the calculated and
measured degree of hydration of cement and fly
ash in paste with W/B = 1.0 by volume
Density of C-S-H gel (g/cm3)
Cem ent
80
0
(14)
y = -14.17x + 44.218
20
0
Calculated Degree of
Hydration (%)
αc
2.2
Cement
2
1.8
1.6
Fly Ash
1.4
1.2
1
40
50
60
70
80
Amount of hydrated gel (% by weight)
Fig. 14 The density of hydrated gel
(15)
where DT Df is the volume of the products of hydration
of fly ash at the completion of reaction of 1 ml of fly
ash. DT equals to 1 for curing temperature at 20ºC and
equals to 0.75 for higher curing temperature at 35ºC and
50ºC. These values represent the shrinkage of C-S-H gel
in fly ash cement paste due to higher curing temperature.
The compressive strength of paste is closely related to
the volume of the hydrated gel and the relationship
between them can be estimated by Eq. (16).
f c ' = 676.82VGel 2.96
(16)
where fc’ is the compressive strength (MPa).
Model for Predicting Strength Development of
Pastes at Different Curing Relative Humidity
The compressive strength is governed by not only the
volume of the hydrated gel but also other factors such as
the internal relative humidity.
It is well known that the C-S-H gel shrinks by the
drying, that is the reduction of internal relative humidity
[11]. Hence, Eq. (16) should be rewritten by taking into
account the C-S-H shrinkage. In this study, the
coefficient that shows the influence of shrinkage is
assumed to be expressed as a function of the internal
relative humidity. Therefore, the Eq. (16) can be
rewritten into the following equation.
f c ' = 947.11(C RHSVGel ) 3.34
RH ⎞
⎛
C RHS = 1.0 − ⎜1.0 −
⎟
100 ⎠
⎝
(17)
2.75
(18)
where VGel is the volume of hydrated gel from Eq. (15).
CRHS is coefficient of the shrinkage of the C-S-H gel.
RH is the internal relative humidity (%).
Fig. 15 shows a good correlation between the
compressive strength and the multiplication of the
coefficient of the shrinkage of the C-S-H gel and the
volume of the hydrated gel of the pastes. It is confirmed
that Eq. (17), considering both influence of the degree
of hydration and the internal relative humidity, can be
used to estimate the compressive strength.
Compressive Strength
(MPa)
140
120
100
y = 947.11x3.34
R 2 = 0.94
80
60
40
20
0
0.2
0.3
0.4
CR H S *V Gel
0.5
0.6
Fig. 15 Relationship between the compressive strength
and the multiplication of the coefficient of the
shrinkage of the C-S-H gel and the volume of the
hydrated gel of paste with W/B = 1.0 by volume
Conclusions
The effect of curing conditions on the hydration reaction
and the strength development of pastes has been
investigated and their prediction models were proposed.
The proposed models with considering the effect of
curing temperature and curing relative humidity
reasonably estimate the compressive strength and
degree of hydration of pastes cured at any conditions.
The modified gel/space ratio model with considering
the effect of fly ash addition, curing temperature, and
curing relative humidity was proposed and properly
used to estimate the compressive strength development.
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