# Characteristics of Ordinary Portland Cement Using the New Colloidal Nano-Silica Mixing Method
## 3 Results and Discussion
### Pore Structure  
Figure 6 shows the pore structure of the specimens with 0%, 30%, and 50% CNS by MIP analysis at 1-day and 28-days. Figure 6a shows the pore structure analysis of the 1-day samples. As the amount of CNS increases, the graph gradually moves to the left. This indicates that the diameter and amount of pores gradually decrease. The 28-day samples in Figure 6b also show pore changes that were similar to that for 1-day. In addition, the MIP results shown in Figure 6 confirm that the CNS mixing method applied in this study has the effect of reducing the diameter and amount of pores. Figure 6: MIP results, (**a**) 1-day, (**b**) 28-day. Previous researchers have reported that fine nano-silica particles form a dense matrix by filling pores between hydration reactants, thereby improving mechanical performance [5; 6]. The results of this study applying the new mixing-method using CNS show the same effect as the previous studies. This is because nano-silica particles reduce the porosity of the matrix through the nucleation effect, pozzolanic reaction, and filler effect in cement hydration reaction. This effect is further enhanced with increasing amounts of nano-silica particles [7; 8; 21; 29; 30; 31; 34; 44; 56]. Figure 6 also shows that as the amount of CNS increases, the pore size and amount decrease due to the effects mentioned above. There are various studies on the mechanism and effect on the change of pore structure of nano-silica particles. Once a small amount of nano-silica particles is homogeneously mixed with the cement paste, the cement hydration products can react with these particles due to their ultra-large surface area and high energy, and a continuous solid mass forms around the nano-silica particles. The nano-silica particles that were embedded in the cement paste can further promote and accelerate the hydration process due to their ultrahigh reactivity, causing the formation of a compact microstructure with uniformly distributed mass [22]. Therefore, the pores in the reaction products are reduced. However, increases in the amount of trapped air can be expected in PNS that contain pastes in fresh state conditions. Substitution of nano-silica particles has implications for adversely affecting the pores reduction effects. This is due to the fact that nano-silica particles introduce a high gelling tendency, thereby increasing the entrapped air content, as observed through experimental results in [PERSON] et al. [32]. This increase in porosity is caused by the non-uniform dispersion of the nano-silica particles and the abrupt decrease of the fluidity due to the lack of mixing water as the amount of nano-silica particles increases. Therefore, the use of CNS reduces the rapid deterioration of fluidity and induces dispersion of nano-silica particles better than PNS. [PERSON] et al. [57] classified the pores of cement paste into large capillary pores (10-0.05 \(\upmu\)m), medium capillary pores (0.05-0.01 \(\upmu\)m), and gel pores (\(<\)0.01 \(\upmu\)m). The gel pores represent the intrinsic porosity of C-S-H gel. That is, an increase in gel pores means an increase in C-S-H gel, indicating that a dense hydration matrix has been produced. Table 3 summarizes the pore size distribution in the 0%, 30%, and 50% CNS specimens in Figure 6. As the amount of CNS increases, the large capillary pores gradually decrease, while the middle capillary pores and the gel pores increase. This is due to the hydration promoting action and the filler effect associated with the pozzolanic reaction of CPC and OPC particles. On the 28-day, large capillary pores decreased and gel pores increased than 1-day. Thus, the CNS forms a dense OPC mixture matrix. These results are consistent with those observed in previous studies using PNS [29; 30; 43; 56] or CNS [7; 34]. The total porosity also decreased as the amount of CNS increased, with 28-day total porosity being smaller than 1-day. [PERSON] et al. [7] reported the total porosity of samples with 0%, 6%, and 12% addition of CNS to OPC and OPC + Fly ash weights.