## 4 Discussion  
It is evident from Figure 3 that a large W/C ratio resulted in a longer setting time. In addition to a large W/C ratio, an optimal W/C range enabled SLMKPC to obtain the best mechanical properties with a better operating space. The performance of MKPC was limited by a reasonable W/C range. It is noticeable from Figure 8a that when W/C > 0.4, the slurry was very watery. Due to the presence of excess water in the solidification process, the porosity of the final sample was too large (Figure 7e); thus, the SLMKPC sample had a loose structure and low mechanical properties (Figure 5). The SLMKPC slurry was very dry at W/C < 0.2 (Figure 8c) and could not achieve operational capability due to excessive moisture loss. The optimal value Mg/P was dependent on the W/C ratio [22]. A W/C ratio of greater than 0.2 was sufficient to guarantee a complete reaction of KH\({}_{2}\)PO\({}_{4}\)[43]. Therefore, the optimal W/C range for the best setting performance of SLMKPC was 0.2-0.4. When W/C was between 0.2-0.3, only one exothermic peak appeared in the initial hydration stage (Figure 4). When W/C = 0.2, the moisture content in the SLMKPC slurry was insufficient to allow all CMR and KH\({}_{2}\)PO\({}_{4}\) to react. Due to the residual part of MgO in the CMR, the hydration heat release rate and total amount were relatively low under the condition of the same raw materials. When the W/C ratio increased from 0.2 to 0.3, CMR and KH\({}_{2}\)PO\({}_{4}\) completely reacted with water and reached a critical point,  
Figure 7: SEM images of SLMKPC samples with different W/C ratios after at 28 d of aging: (a) W/C = 0.2, (b) W/C = 0.25, (c) W/C = 0.3, (d) W/C = 0.35, (e) W/C = 0.4.  
resulting in the highest exothermic hydration temperature. The formation of MKPC was accompanied by a typical heat evolution curve consisting of an endothermic valley and two exothermic peaks [16]. The results show that the purity and stability of the raw material can be guaranteed because the raw material of MKPC is commercial MgO. Therefore, two exothermic peaks can be obtained in the W/C of MKPC at different stages. Two exothermic peaks only appeared when W/C was adjusted to a certain proportion in the SLMKPC slurry. When the W/C increased from 0.35 to 0.4, the number of exothermic peaks in the SLMKPC slurry changed from one to two (Figure 4). In the SLMKPC system with only one exothermic peak, a large amount of heat concentrated to release, resulting in a partial performance loss in the later period. Moreover, this exothermic peak had no significant effect on the mechanical properties and microstructure of the material. In contrast, the SLMKPC system with two exothermic peaks (W/C = 0.35-0.4) reduced the exothermic temperature and slowed down the rate of hydration (Figure 5); however, a large W/C ratio resulted in inferior mechanical properties (Figure 5) and a loose microstructure (Figure 7). The typical setting time of an MPC system without a retarder is less than 3 min [16], and it can be extended to above 20 min after the addition of boric acid, mineral ingredients, and other retarders [44]. However, the setting time of SLMKPC without any retarder could not reach above 20 min [34]. When W/C > 0.3, the setting time reached above 10 min. Therefore, in order to convert a large quantity of MR feedstock into usable SLMKPC, the setting time corresponding to W/C = 0.2-0.3 should be considered. According to Equation (1), the amount of MgO in the system decreased with the increase of the MKP content during the reaction. The quantitative analysis of SLMKPC (Figure 6c) revealed that the quantities of MgO and MKP in all SLMKPC samples after 3 d of curing were higher than those after 28 d of curing, and this phenomenon is different from the MKP generation process. Mg\({}_{3}\)B\({}_{2}\)O\({}_{6}\) played a regulating role in the absorption and release of MgO in the system because the boron compound precipitated around MgO grains in the reaction or formed a magnesium borate complex [45; 46].