## 3 Results and Discussion
### Effect of Composite Micronized Powder on the Performance of Cementitious Materials
#### 3.2.2 Effect of Composite Micronized Powder on the Microscopic Morphology of Gelling Materials  
Figure 7a shows the microscopic morphology of the pure cementitious mortar specimen cured for 7 d. Figure 7b shows the microscopic morphology of the mortar specimen cured for 7 d, derived from the 1:1 composite of composite micronized powder and cement. As shown in Figure 7a, there are a large number of needle-rod calcium alumina crystals in the pure cement hardened slurry, and the structure is loose and distributed with a large  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Fluidity (mm)**} & \multicolumn{2}{c}{**Activity Index/\%**} \\ \cline{2-3}  & **7 d** & **28 d** \\ \hline
185 & 94.6 & 101.3 \\ \hline \hline \end{tabular}
\end{table}
Table 7: The test results of the optimal ratio of composite micronized powder.  
Figure 6: Exothermic properties of hydration of different cementitious materials: (**a**) hydration exothermic rate and (**b**) cumulative heat release.  
number of pores. As shown in Figure 7b, the microscopic surface structure of the test block doped with 50% composite micronized powder is dense, and the main hydration products, flaky Ca(OH)\({}_{2}\) crystals, are closely packed with fine particles. Only a small amount of pores are distributed around the flaky hydration products and fine particles. It is again shown that the concentration of Ca(OH)\({}_{2}\) in the DG system is higher, and the microstructure of the specimens mixed with 50% composite micronized powder is tighter and has finer and fewer pores compared with the pure cement specimens after 7 d of curing [30].  
Figure 8 shows the microscopic morphology of pure cementitious mortar specimens and specimens mixed with 50% composite micronized mortar cured for 28 d. In comparison with Figure 7, the microstructure of different cementitious materials specimens became more dense as the curing age increased. The main hydration products of pure cement specimens change from the loose needle and rod form attached to the slurry to the form of flocculent gel and flaky crystals cemented to each other; the microstructure of the mortar test block mixed with 50% compound micronized powder changed from the form of accumulation of sheet hydration products and fine particles to the form of continuous sheet lamellae composed of hydration products and unneeded particles. It shows that the vitreous form of SiO\({}_{2}\) and A1\({}_{2}\)O\({}_{3}\) in the composite micronized powder reacts with the cement hydration product Ca(OH)\({}_{2}\) in a secondary reaction to produce continuous dense hydrated calcium silicate, hydrated calcium aluminate and other cementitious substances [31]. After 28 d of curing, the microstructure of the mortar specimens mixed with 50% composite micronized powder was more continuous, and the bond between the cementitious material and the aggregate was tighter when compared with the pure cementitious mortar specimens.  
Figure 8: SEM photos of different gelling materials cured for 28 d: (**a**) OPC mortar and (**b**) OPC mixed with 50% composite micronized powder mortar.  
Figure 7: SEM photos of different gelling materials cured for 7 d: (**a**) OPC mortar and (**b**) OPC mixed with 50% composite micronized powder mortar.