# Characteristics of Ordinary Portland Cement Using the New Colloidal Nano-Silica Mixing Method
## 3 Results and Discussion
### Compressive Strength  
Figure 3 shows the results of compressive strength measurements according to the ages of the test specimens. As the replacement rate of CNS increased from 0% to 50%, the strength was observed to increase at all the measurement ages in Figure 3a. The 1-day strength increased from 15.04 MPa for 0% CNS to 33.37 MPa for 50% CNS, while the 3-day strength increased from 30.51 MPa for 0% CNS to 50.69 MPa for 10% CNS; the 7-day strength increased from 33.72 to 61.33 MPa, followed by a 28-day increase from 40.48 MPa of 0% CNS to 75.41 MPa of 10% CNS. Replacement of CNS exhibited a significant effect on the improvement in strength, and the effect was further increased according to the CNS substitution ratio. In previous PNS studies, the strength enhancement effect of nano-silica particles in the early stages of hydration is already known [6; 20; 22]. In the present study, in which CNS was applied as a weight substitution method for mixing water, the initial strength improvement effect was consistent with previous PNS studies. This is because nano-silica particles promote the hydration of cement and act as a nucleation site in the formation of hydration products [5; 20; 26; 41]. This nucleating effect contributes to the improvement in strength by generating dense hydration products during the early stage of hydration [20; 42; 43]. In addition to the improvement of the microstructure due to the filler effect, the pozzolanic reaction promotes the hydration of the cement, thereby improving the strength [5; 7; 44]. Figure 3b shows the compressive strength results of previous studies using mixing water weight substitution method and binder weight substitution method. [PERSON] et al. [7] used colloidal nano-silica with mean particle size of 35 nm, 50% SiO\({}_{2}\) contents, and pH of 9.5. Two kinds of binders, OPC and OPC + Fly ash, were used, and the compressive strengths of 3-day and 7-day were measured by adding CNS to each binder (not the substitution method). Both the OPC and OPC + Fly ash binding materials increased in 3-day and 7-day compressive strength as the weight substitution rate of CNS increased to 0%, 6%, and 12%. [PERSON] et al. [19] used a CNS with a particle size of 5-40 nm, 40%-41.5% SiO\({}_{2}\) contents, and a pH of 10. CNS was replaced by 0.5%, 1%, 1.5%, 2%, 2.5%, and 3% of the OPC and the 3-day, 7-day, and 28-day compressive strengths were measured. Experimental results show that as the concentration of CNS increases from 0% to 2%, the compressive strength increases at all ages. The highest compressive strength occurs at 2% CNS, after which the strength decreases until 3% CNS concentration is reached. [PERSON] et al. [45] is the result of compressive strength using CNS with pH 10.23, mean particle size 2.1 nm and the specific gravity 1.21. CNS replaced 0%, 1.5% and 3% of  
Figure 2: Setting time of the CNS replacement rate. cement weight. The 1-day, 3-day, 7-day, and 28-day compressive strengths increased with increasing concentrations of CNS. In previous studies using CNS, binder mass displacement methods have increased compressive strength as the concentration of CNS increases [7; 45]. In other studies, however, there was a specific concentration of CNS that produced the highest compressive strength [19]. The specific CNS concentration with the highest intensity is the point where the nano-silica particles achieve a homogeneous dispersion and the pozzolanic reaction, nucleation effect and filler effect are optimal. However, exceeding the optimal CNS concentration, the compressive strength decreases due to the  
Figure 3: CNS replacement ratio and compressive strength according to age (**a**) this study, (**b**) compared with previous studies. drastic loss of fluidity due to uneven dispersion of nano-silica particles and lack of units due to high specific surface area. Figure 4 shows the relative strength increase ratios for each CNS replacement ratio as compared to the strength of the 0% CNS specimen.