# Effect of Coupled Mechanical-Chemical Activation on Hydration Activity of Copper Slag Powder
## 3 Results and Discussion
### Hydration Heat-Release Characteristics  
Figure 5 demonstrates the hydration rate and heat spectrum of the copper slag powder-based composite cement within 120 h. The first exothermic peak appeared immediately after the samples were mixed with water, as shown in Figure 5a, and then the hydration rate decreased rapidly and the hydration reaction entered the induction period [28]. The hydration heat-release rate in the induction period was quite low, implying that the hydration reaction of the composite cementitious system was very slow. When the concentration of Ca(OH)\({}_{2}\) and C-S-H gel in the hydration reaction system reached supersaturation, large amounts of hydration products were formed, and the hydration reaction turned into the accelerated stage [29]. During the acceleration period, the hydration heat-release rate of the composite cementitious system began to increase, and the hydration reaction rate was accelerated. The water in the hydration reaction system was consumed rapidly, and a large number of hydration products were generated in a short amount of time. The migration diffusion barrier of unhydrated particles increased sharply, so the hydration reaction was quickly hindered, and the hydration heat release rate decreased sharply, forming the second exothermic peak with a narrow peak shape and large peak value. The incorporation of copper slag powder also extended the induction period and the arrival time of the second exothermic peak, and the peak value of the second exothermic peak decreased from 9.87 J/(g-h) to 8.12-8.56 J/(g-h), as shown in Figure 5a. This was due to the incorporation of copper slag powder diluting the content of cement in the cementitious system, resulting in a significant decrease in the dissolved Ca\({}^{2+}\) concentration in the cementitious system and the extension of the time for Ca\({}^{2+}\) to reach the oversaturated state. Therefore, the incorporation of copper slag powder prolonged the hydration induction period and delayed the occurrence of the second exothermic peak. On the other hand, due to the low hydration heat of the copper slag powder in the early hydration reaction, the incorporation of copper slag powder reduced the second exothermic peak value of the composite cement system. Figure 4: Effect of coupled mechanical–chemical activation on hydration activity of copper slag powder composite cement samples. (**a**) Compressive strength; (**b**) compressive strength activity index. It can be seen from Figure 5b that the incorporation of copper slag powder reduced the total heat release of the composite cementitious system. The total heat release of the composite cementitious system decreased with the increase in copper slag powder content. The hydration heat release of the composite cementitious system mainly included two parts. The first part was from the rapid hydration reaction of the cement. The second part originated from the slow hydration reaction of the copper slag powder [30,31]. Since the addition of copper slag powder reduced the mass fraction of cement in the hydration reaction system, the higher the content of copper slag powder, the greater the hydration heat reduction of the composite cementitious system. The hydration heat of cement, CS20-90, CA20-90, CS30-90, CA30-90, and CS40-90 at 120 h was 261.5, 218.7, 230.1, 206.0, 227.1, 203.4, and 213.6 J/g, respectively. The hydration heat of the copper slag powder-based composite cement was 16.4%, 12.0%, 21.2%, 13.2%, 22.2%, and 18.3% lower than that of pure cement, and the reduction ratio of the hydration heat was lower than the mass ratio of copper slag in the hydration reaction system, which was due to the fact that the potential hydration activity of the copper slag powder was stimulated in the alkaline environment generated by the cement hydration, and the secondary hydration reaction of the copper slag powder released a certain amount of hydration heat.