## 3 Effect of Ca\({}^{2+}\) Source and Addition Method on Mineralized Products
### Results and Discussion  
3.2.1. Effect of Ca\({}^{2+}\) Source and Addition Method on the Characteristics of Mineralized Products and the Mineralization Efficiency  
The characteristics, mineral production, mineral productivity, and Ca\({}^{2+}\) source utilization rate of the mineralized products from each group are shown in Table 3.  
Figure 7: Mineralization with different Ca\({}^{2+}\) sources and addition methods. It can be seen from Table 3 and Figure 8 that the mineralized product characteristics, mineral production, mineral productivity, and Ca\({}^{2+}\) source utilization rate of Group A and Group C were similar, where both showed higher values than Group B. However, the hardness of the mineralized products was low and their integrity was poor; they appeared as white pastes in solution and white powders after drying. According to the theory described in Section 2.3, in Group A after 48 h of urea treatment, the urea in the mixed solution was almost completely decomposed and the mixed solution was full of CO\({}_{3}\)\({}^{2-}\). At this time, the \(\alpha_{\text{Ca}^{2+}}\cdot\alpha_{\text{CO}_{3}^{2-}}\) in the mixed solution instantaneously reached its maximum value with the addition of the Ca\({}^{2+}\) source, the reaction illustrated in Formula (1) proceeded to the left and the mineral productivity was close to 100%. In Group B, CO\({}_{3}\)\({}^{2-}\) was gradually produced by the decomposition of urea. Although the reaction in Formula (1) proceeded to the left, the \(\alpha_{\text{Ca}^{2+}}\cdot\alpha_{\text{CO}_{3}^{2-}}\) was lower than that in Group A; thus, mineral productivity in Group B was also lower than that in Group A.  
Calcite exhibits a trigonal crystal system, where Ca\({}^{2+}\) ions are located at the top corner and center of each crystal plane and CO\({}_{3}\)\({}^{2-}\) ions are located at the center of each crystal plane and the midpoint of all edges of the crystal cells. Each Ca\({}^{2+}\) coordinates with six CO\({}_{3}\)\({}^{2-}\); that is, the coordination number of Ca\({}^{2+}\) in calcite is 6. Vaterite exhibits a hexagonal crystal system, where Ca\({}^{2+}\) and CO\({}_{3}\)\({}^{2-}\) are layered, similar to calcite, but there are two CaCO\({}_{3}\) molecules in each vaterite crystal cell; thus, the coordination number of Ca\({}^{2+}\) in vaterite is 12 [27; 28]. In Group A, the newly added Ca\({}^{2+}\) combined with CO\({}_{3}\)\({}^{2-}\) in the mixed solution, meeting the coordination number of vaterite; thus, vaterite was generated first. The vaterite crystals were observed to be small, uniform, and powdery. In Group B, while urea was decomposed, the newly produced CO\({}_{3}\)\({}^{2-}\) combined with the existing Ca\({}^{2+}\) in the mineralizing system, meeting the coordination number of calcite; thus, calcite was generated first. The calcite crystals were observed to be large with certain shapes, and the hardness and integrity were also good. In the repair of high-temperature concrete damage, the mineralized products should exhibit a certain hardness and integrity. Although the mineral productivity and Ca\({}^{2+}\) source utilization rate of Group A was much higher than that of Group B, the characteristics of the mineralized products were not suitable; therefore, the optimal method for Ca\({}^{2+}\) source addition is the post-calcium-source method used in Group B. Comparing the different types of Ca\({}^{2+}\) sources in Group B, the mineral productivity and utilization rate were the highest in Group B2; however, the introduction of Cl\({}^{-}\) might cause some problems such as steel corrosion, which has adverse effects on steel-reinforced concrete structures. The mineralized products of Group B were hard and exhibited good integrity; however, Ca(NO\({}_{3}\))\({}_{2}\) is flammable and explosive and represents a potential safety hazard in the repair and service processes. Thus, Ca(NO\({}_{3}\))\({}_{2}\) is not a suitable Ca\({}^{2+}\) source.