# Characteristics of Ordinary Portland Cement Using the New Colloidal Nano-Silica Mixing Method
## 3 Results and Discussion
### Hydration Products  
Figure 5 shows the XRD results for the 0% CNS and 50% CNS specimens. Representative reaction products included portlandite, calcite, ettringite, and C-S-H gel. 0% CNS hydration reactants are portlandite, ettringite, C-S-H gel, calcite, quartz, and gypsum. The hydration reactants of 50% CNS were similar to 0% CNS. The 50% CNS samples showed little change in peak size, for portlandite, ettringite, and C-S-H gel, which are major hydration reactants in the 1-day and 28-day periods. The gypsum was observed in the 0% CNS specimen; however, it was not observed in the 50% CNS specimen. The nano-silica particles were not observed in 50% CNS gypsum because gypsum and portlandite were consumed by the formation of ettringite by the pozzolanic reaction [8; 20; 27; 31]. The pozzolanic reaction is further promoted as the number of nano-silica particles increased [49; 50; 51; 52]. However, as shown in Figure 5b, there was no change in the size of the C-S-H peak from 1-day to 28-days, even in case of the 50% CNS specimen. This is because the reaction between CNS and OPC occurs mostly before 1-day, which is the initial hydration stage, and then the hydration reaction proceeded very slowly. The reason why the initial hydration reaction is performed rapidly is explained as follows. It is known that the particles in CNS are characteristically composed of mono-dispersed spherical particles consisting of an amorphous SiO\({}_{2}\) core with a hydroxylated surface, making them small enough to remain suspended in a liquid medium without settling [39; 53; 54]. Additionally, the nano-silica particles exhibit a high surface energy, which is known to react easily with other substances. Therefore, the pozzolanic reaction of nano-silica particles is improved during the initial hydration reaction step [6]. This affects the reduction of flow value and setting time as the amount of CNS increases, as in the results already mentioned in Figures 1 and 2. In addition, the increase in the relative compressive strength shown in the Figure 4 is also markedly increased in the 1-day period, so that the initial hydration reaction promoting effect can be confirmed. Figure 5c shows the XRD results after 28-day according to the CNS replacement percentages that ranges from 0% to 50%. The amount of portlandite decreased as CNS increased to 50%. The gypsum has the highest peak at 10% CNS and then decreases to 50% CNS. Thus, gypsum peaks are rarely observed in 50% CNS. Peaks of ettringite from 0% to 50% CNS showed no significant change in size and were almost similar. The decreasing tendency between the ettingite and gypsum peaks is similar. At a substitution rate of less than 20% CNS, nano-silica particles promotes the formation of ettringite and C-S-H gel by the action of pozzolanic reaction and nucleation effects [5; 6; 7; 20; 22; 26; 34; 55]. However, at replacement rates above 20% CNS, nano-silica particles have a major impact on filler effects [5; 6]. Thus, there is little change in ettringite or C-S-H peaks in XRD above 20% CNS. The pozzolanic reaction of these nano-silica particles is most apparent during the initial stage of hydration, which reduces the  
A previous study related to PNS reported that portlandite decreased rapidly during the first 7-day, which then declined slowly after 7-days. This indicates that most of the pozzolanic reactions occurred during the first 7-days [29]. There is almost no change in the ettringite and C-S-H peaks from 3-days to 28-days, as shown in Figure 5b. However, CNS has a faster hydration reaction effect than PNS. Previous studies using CNS have shown that the generation of C-S-H gel is accelerated in the initial hydration stage before 12 h [28]. This means that the initial hydration reaction of CNS is much faster than the PNS. In view of the changes in these reactants, CNS accelerates hydration more rapidly as compared to the powdered nano-silica particles in previous studies.