# Property Analysis of Slag Composite Concrete Using a Kinetic-Thermodynamic Hydration Model
## 3 Experimental Studies and Modeling
### Modeling
#### 3.3.2 Thermodynamic Equilibrium Model  
GEM-Selektor can be used to analyze the phase assemblage of hydrating products. When using GEM-Selektor, the compound composition of the cement and slag, curing temperatures, and reaction degree of the cement and slag are necessary input parameters. The phase assemblage is one of the output results of GEM-Selektor. GEM-Selektor, which incorporates the thermodynamic database of cement, is used to evaluate the equilibrium reaction of hydrating cement-slag blends [16; 17]. Figure 6(a)-c shows the results of the phase assemblage from GEM-Selektor. First, for plain paste, as the hydration time increased, the contents of the compositions of clinkers deceased. In the early age, the consumption of C\({}_{3}\)S was quick, while at the late age, the reaction rate of C\({}_{3}\)S was slow because diffusion was the dominant process in late-stage hydration. In this study, CSHQ was assumed to be the major type of calcium silicate hydrate (CSH). The mass of CSH is much higher than that of other hydration products, such as CH and ettringite. In addition, the content of capillary water decreased as the hydration proceeded. At later stages, the content of capillary water reached a plateau and a decrease in capillary water was not obvious. Second, for 30% and 50% slag paste, at early stages, the consumption of slag was quick, while at late stages, the reaction rate of slag was slow because diffusion was the control process in the late-stage reactions. With the increase in the slag replacement ratios, the calcium hydroxide content decreased. This was due to the dilution effect and the chemical reaction of slag. With the increase in the slag replacement ratios, the content of hydrol calcite increased. This was because slag has a higher MgO content than cement, which favors the formation of hydrol calcite. At the late stages, 30% slag paste had a higher CSH content than plain paste. This was because slag has a higher content of SiO\({}_{2}\) than cement. Figure 6: Long-term reaction degree of slag. [PERSON] [28] measured the calcium hydroxide content of hydrating blends of cement-slag using thermogravimetric analysis. The water/binder ratio of the paste specimens was 0.5: the slag replacement percentages were 0%, 30%, and 50%; and the curing temperature of paste specimens was 20 \({}^{\circ}\)C. CH was measured from 3 days to 90 days. Figure 8 shows the analysis of GEM-Selektor, which is generally in agreement with [PERSON]'s experiments. For plain specimens, the CH content initially showed a rapid increase, exhibited a slow increase, then finally reached a plateau. For 50% slag paste, at the early stages, the CH content showed an increase because the hydration of cement was the dominant process, while in later stages, the CH content showed a decrease because the chemical reaction of slag became the dominant process. Compared with the plain specimen, the 50% slag specimen showed a much lower CH content. This was because of the consumption of CH from the slag reaction and the reduction in the cement content. Figure 7: Phase assemblage of cement–slag blends. Figure 9a shows the GEM-Selektor calculation results for combined water. For plain paste, at the early stages, the combined water content increased rapidly, while in later stages of curing, an increment in combined water was not obvious. Meanwhile, for blended paste in later stages of curing, the increment in combined water was more obvious than in plain paste. This was because of the reaction of slag in late stages. Moreover, with the increase in the slag replacement percentage, the content of combined water decreased. At later stages, 30% slag paste had a similar combined water content to that of plain paste, while 50% slag paste had a lower combined water content than that of plain paste. Figure 9b shows that for specimens with different mixtures and different ages, the strength of concrete was linearly dependent on the combined water content.