## 3 Results and Discussions
### Manifestation Mechanism of Compressive Strength  
Ettringite, CSH, CH, and unhydrated clinker grains are known to be representative products of the hydration reaction occurring in cement [41]. Etrimigite is a chemical compound of needle-like crystals that improves the initial strength of cement and is finally transformed into monosulfate [Ca\({}_{4}\)Al\({}_{2}\)O\({}_{6}\)(SO\({}_{4}\))\(\cdot\)12H\({}_{2}\)O] of sheet-like hexagonal crystals [44; 45; 46]. Meanwhile, CSH is reported to have a colloidal size and significantly affects the strength of cement [46]. CH exists as sheet-like hexagonal crystals and has a minor impact on the strength of cement because it has a relatively smaller specific surface area than CSH [47]. In addition, CH is less chemically stable because of its higher solubility [47; 48]. However, few studies have examined the initial or long-term effects of hydration products, such as CSH and ettrinigite, on compressive strength [25]. Accordingly, CSH and ettrinigite were deliberately introduced to investigate which was more effective in improving the compressive strength of SRC, and the results are given in Figure 7. The compressive strengths were enhanced by the introduction of hydration products and met the disposal standard only after 7 days of curing when adding hydration products. Moreover, the results indicate that CSH was more effective than ettrinigite in improving compressive strength [25]. The microstructure of SRC was investigated using SEM to investigate the manifestation mechanism of the compressive strength of both materials. Three different types of SRC were manufactured for SEM analyses: (1) with neither substance, (2) with the addition of 5% CSH, and (3) with the addition of 5% ettrinigite, and the results are shown in Figure 8. Without either material, needle-like ettrinigites were observed above the sheet-like CH, gel-shaped CSHs, and some pores. In the second case, gel-shaped CSHs were predominant, and most of the pores were filled with CSHs, compared with the first case. The last case exhibited net-like structures with mixtures of sheet-like CH, gel-shaped CSHs, and ettrinigite-blocked pores. The order of none of the materials > ettrinigite > ettrinigite > ettrinigite to increase the pore volume, and the opposite tendency was observed in the compressive strength values [25]. Therefore, the CSH exhibited a relatively higher contribution to improving compressive strength than ettrinigite because the former was gel-shaped and effectively blocked the inner pore spaces of SRC. Their Raman spectra were obtained to examine the effect of the hydration products on the compressive strength of SRC more closely (Figure 9). The results indicated that the peaks corresponding to CSH and ettrinigite were observed at 650 and 960 cm\({}^{-1}\), respectively, for all the cases tested, and also the spectra of CH and CaCO\({}_{3}\) were obtained [25]. Without adding both substances (Figure 9a), the width and intensity of the CSH peak appeared to be narrower and higher than those of ettrinigite, respectively, indicating the higher crystallinity of CSH than ettrinigite. In addition, identical results were acquired when both substances were added at equivalent amounts (5 wt%)  
Figure 6: The effect of curing methods on the compressive strength of SRC.  
(Figure 9b,c). These results were consistent with the measurement results of the compressive strength of SRC, and the more significant contribution of CSH to the compressive strength of SRC might be due to its higher crystallinity.  
Figure 8: The SEM results on the microstructure of SRC. (**a**) Without the addition of CSH and ettrinigite, (**b**) with the addition of CSH (5 wt%), and (**c**) with the addition of ettrinigite (5 wt%).  
Figure 7: The effect of different types of hydration products on the compressive strength of SRC. Refer to the detailed experimental conditions in Table 3.