3.2.1. Heat of Hydration
The heat of hydration is an important physical parameter for evaluating the hydration of cement-based materials [
5]. In order to explore the effect of GNPs on the hydration process of SAC, the heat of hydration of SAC with different dosages of GNPs was tested, as shown in
Figure 3. According to the characteristics of hydration exothermic rate change of SAC, the hydration exothermic process can be divided into five reaction stages, as shown in
Figure 3a [
5]. The AB stage is the initial period and the induction period. After the cement particles are in contact with water, a small amount of heat is released. At this time, a small amount of AFt is generated. The hydration rate decreases, and the cement particles enter the induction period. The induction period is short, the hydration enters the acceleration period, and the B-C-D-E stage is the acceleration period. Two exothermic hydration peaks can be observed in the acceleration period, which can be divided into the initial and final stages of acceleration. In the initial stage of acceleration, due to the rupture of the coating layer during the induction period, C
4A
3Š rapidly contacted the solution and generated more AFt. The first exothermic peak corresponds to the formation of AFt and the consumption of C
4A
3Š, hydration entered the late acceleration stage, and a second exothermic peak appeared. Hydration products increased, filling unreacted particles and paste along with the growth of crystals, the paste lost plasticity to a certain extent and the hydration into a deceleration period.
The EF stage is the deceleration stage. Hydration products formed a coating layer along with the hydration, the porosity decreased, the resistance to the passage of water molecules increased, and the hydration heat release rate decreased. Then, the hydration enters a stable stage; at this stage, the exothermic rate of hydration is slow, the porosity of the paste is further reduced, and water molecules can hardly pass through the hydration products.
Figure 3b is the curve of the hydration exothermic rate of samples with and without GNPs. The curves have two exothermic peaks, and the first exothermic peak of G1, G2, G3, and G4 appeared at 1.394 h, 1.344 h, 1.339 h, and 1.389 h, respectively. Adding GNPs promoted the hydration of the matrix, attributed to the nucleation effect of GNPs, and similar conclusions were obtained in other studies [
23]. The second exothermic peaks of G1, G2, G3, and G4 appeared at 5.874 h, 5.783 h, 6.003 h, and 5.755 h, respectively;
Figure 3c shows each group of samples’ cumulative hydration heat release for 24 h, the cumulative heat of each group is 189.3 J/g, 195.1 J/g, 199.7 J/g, and 196.5 J/g, respectively.
3.2.2. Hydration Kinetics
According to the hydration characteristics of cement-based materials, Kondo proposed a kinetic hydration formula, as shown in Equations (2) and (3).
where
α is the degree of hydration,
N is a constant related to the hydration mechanism, and the values differ at different stages.
K is the hydration rate constant, and
t0 is the end time of the induction period. When
N is greater than or equal to 2, the hydration reaction depends on the diffusion rate of water molecules through the product layer, which is controlled by the diffusion–reaction. When
N is 1, the hydration reaction depends on the solid–liquid phase reaction, which is determined by the phase boundary reaction control; when
N is less than 1, the hydration reaction is mainly controlled by crystal nucleation growth.
The Kstulovic–Dabic model divides each stage in detail. The Kstulovic–Dabic model reflects the relationship between the degree of hydration and the hydration heat release time, and the kinetic hydration equation mainly includes three stages [
36].
Crystal nucleation and crystal growth (NG Stage):
Phase boundary reaction (I stage)
Diffusion (D stage)
where
α is the degree of hydration,
K1,
K2, and
K3 are the reaction rate constants of the three hydration reaction processes, respectively,
t0 is the end time of the induction period,
n is the reaction order, and
r is the particle diameter participating in the reaction.
From the above Kondo equation and the Kstulovic–Dabic model, it can be known that the hydration degree of cement-based materials at various stages must be calculated first when calculating the hydration kinetics of cement-based materials at different stages. Knudson proposed a formula to calculate the kinetics of hydration, which can calculate the degree of hydration, as shown in Equations (7) and (8).
where
Q is the cumulative heat release,
Qmax is the theoretical maximum heat release, and
t50 is the time required for the cumulative heat release to reach
Qmax/2.
The Knudson equation shows that the ratio of the heat released at different times during the hydration process to the total heat is the degree of hydration, as shown in Equation (9).
Based on the Knudson equation, the SAC hydration exothermic process was fitted, as shown in
Figure 4, and the fitting results are shown in
Table 4.
It can be seen from
Table 4 that the fitting correlation coefficient
R2 of each group is more significant than 0.97, indicating that the relevant effect is excellent, and it can reflect the hydration heat release. When the hydration exotherm is 24 h, the cumulative heat of each group is 189.3 J/g, 195.1 J/g, 199.7 J/g, and 196.5 J/g, respectively. The
Qmax of each group obtained by Knudsen equation fitting were 216.9 J/g, 222.1 J/g, 227.6 J/g, and 223.3 J/g, respectively. According to Equation (9), the hydration degrees of each group were calculated as 87.27%, 87.84%, 87.74%, and 88.00%, respectively, indicating that the addition of GNPs improves the hydration degree of each group of samples. According to Equation (2), the curve of the hydration accelerated stage of the sample is shown in
Figure 5. It is challenging to perform linear fitting directly on the curve, and the curve of the hydration accelerated stage contains two stages, and there is a turning point.
As shown in
Figure 6 and
Table 5, after 1 h of hydration, the hydration entered the acceleration period, and two exothermic peaks appeared successively at 1.339 h and 5.755 h. The hydration acceleration stage of SAC can be divided into two stages, the acceleration initial stage and the acceleration final stage. In the acceleration initial stage, the
N value is 206.186. The diffusion–reaction mainly controls the hydration process. The clinker dissolves slowly, the Ca
2+ concentration in the solution increases, and the hydration products such as AFt begin to nucleate and crystallize. At this time, it is in the initial stage of crystal growth, the hydration reaction rate is slow, and the formation resistance of hydration products is enormous. With the progress of the hydration reaction, the number of crystal nuclei increases, the Ca
2+ in the cement paste increases significantly, many hydration products are formed. At the exothermic peak, with the continuous growth of AFt crystals, the surface of the cement clinker is gradually covered by the coating layer of hydration products, resulting in the slowing down of the clinker dissolution rate and the increase in the formation resistance of hydration products. It can be seen that after the addition of GNPs, the
N value of each group decreased in the acceleration initial stage, indicating the resistance of the hydration reaction decreased, and the reaction was easy to proceed, suggesting that GNPs promoted the hydration reaction of SAC. In addition, the
K value increased, indicating the hydration reaction rate is faster, and the reaction occurs easier. After adding GNPs, the
K value of each group rises, indicating that the hydration reaction rate of each group is accelerated, and GNPs accelerate the hydration reaction of SAC.
As shown in
Figure 7 and
Table 6, the coating layer is damaged along with the increased osmotic pressure inside and outside the coating layer, and the hydration enters the acceleration final stage. Compared with the acceleration initial stage, the
N value decreases to 17.976, the resistance of the hydration reaction becomes smaller, the exposed cement clinker dissolves more Ca
2+, and hydration products such as AFt have generated again, corresponding to the appearance of the second exothermic peak. However, due to the limited damage to the coating layer, the cement clinker has been dissolved in large quantities, and the amount of Ca
2+ released is limited. On the other hand, the formed AFt also limits the formation and growth of new AFt. Therefore, the second exothermic peak is smaller than the first exothermic peak. It can be seen that after adding GNPs, the
N value decreased from 17.976 to 17.114, indicating that the resistance of the hydration reaction became smaller; GNPs promoted the hydration reaction and generated hydration products. The
K value increased from 0.0099579 to 0.0130089, indicating that the hydration reaction rate increased, and GNPs accelerated the hydration exothermic rate of SAC.
As shown in
Figure 8 and
Table 7, when the hydration enters the deceleration stage, compared with the acceleration stage, the
N value decreases and the
K value increases, which indicates that the hydration reaction rate is faster, and the hydration reaction resistance is small. However, after the acceleration period, the clinker is consumed in large quantities, and the formation of AFt has become stable. This indicates that another hydration reaction quickly occurred in the deceleration period, and the rate is faster. According to the literature, in the deceleration period of SAC hydration, the hydration product wraps the clinker, the ions are difficult to dissolve, the SO
42− is reduced, and part of the AFt would transform and form AFm [
37,
38,
39].
As shown in
Figure 9 and
Table 8, after the hydration enters the stable stage, compared with the deceleration stage, the
N value increases and the
K value decreases, indicating that the hydration reaction resistance is large, and the hydration reaction rate is slow. The diffusion–reaction mainly controls this stage, and the paste porosity is further reduced. The structure is more compact, water molecules can hardly pass through the hydration product, and the paste almost loses its plasticity.