# Characteristics of Ordinary Portland Cement Using the New Colloidal Nano-Silica Mixing Method
## 3 Results and Discussion
### Compressive Strength
As the substitution rate of the CNS increases, the rate of relative strength increase increases. In Figure 4, the highest relative strength growth rate varies with age and CNS substitution rate. The 10% CNS sample had the highest relative intensity increase of 7-day, the 20% CNS sample was 28-day, and the 30% to 50% CNS sample was 1-day. If the CNS substitution rate was less than 20%, it affected the strength of late age from 7-days to 28-days, and the CNS substitution rate from 30% to 50% greatly affected the initial 1-day strength increase rate. In particular, the 1-day relative strength increase rates were 174.5% at a 30% CNS replacement rate, 205.3% at a 40% CNS replacement rate, and 221.8% at a 50% CNS replacement rate. The 1-day intensity increase of the 40% and 50% CNS replacement rate samples is about twice the increase compared to the sample that did not replace the CNS. If the substitution rate of the CNS exceeds 30%, the rate of increase in the daily strength is then sharply improved. The compressive strength results of previous studies using PNS are consistent with reports that nano-silica particles have a significant effect on the initial strength improvement [6; 21; 22; 43; 46]. However, after a rapid increase in intensity at 1-day, the rate of increase in strength from 3-days to 28-days decreases significantly. The decrease in strength increase after 1-day is similar to the trend in strength change shown in previous studies using PNS [26]. Therefore, the strength-improving effect of nano-silica particles is known to have the greatest effect at the beginning of hydration within 1-day [28]. As a result, as shown in Figure 4, the strength increase rate of 30% to a 50% CNS substitution rate was highest at 1-day. However, the remaining 3-, 7-, and 28-day strength increase rates were significantly lower than those at 1-day. [PERSON] et al. [19] analyzed the compressive strength increase rate of the CNS with 0.5%, 1%, 1.5%, 2%, 2.5% and 3% replacement of the binder mass. As a result, the 1-day strength increase rate is the largest and decreases in the order of 3-days, 7-days, and 28-days. Also, as the substitution rate of CNS increases, the rate of the increase in strength increases. Therefore, the rate of increase of strength is high under the condition of early age and high CNS replacement rate. This is consistent with the tendency of increasing the compressive strength of this study. Studies have also showed that, if the nano-silica particles are excessively mixed, the compressive strength decreases [27]. Therefore, the excess SiO\({}_{2}\) nanoparticles can leach out without any further  
Figure 4: Compression strength increase rate according to the CNS replacement ratio. chemical reaction and serve only as filler and can no longer effectively contribute to the compressive strength of binary blended concrete. This situation can also occur due to the fact that the excess of silica can transform the original C-S-H gel (lower Ca/Si ratio) into tobermorite, which exhibits lower strength properties and increases the extent of silicate polymerization [47; 48]. It may also be due to the defect in the dispersion of nano-silica particles that causes weak zones [22]. However, the weight replacement method of mixing water used in this study did not result in decreased strength, even with 50% CNS replacement ratio. This is because the dispersion of nano-silica particles is enhanced by liquid CNS. Compared with the previous studies using PNS, the strength improvement effect was excellent for the CNS ratios that were tested. In addition, the method of this study, which substituted CNS for the weight of mixing water instead of replacing the weight of the existing binder, meant that as the substitution ratio of CNS increased, the strength was improved because there was no weight change of the binder. In this study, the weight water substitution method of mixing water has the effect of reducing the w/b ratio according to the density difference of mixing water and CNS.