# Characteristics of Ordinary Portland Cement Using the New Colloidal Nano-Silica Mixing Method
## 1 Introduction  
Studies in the field of advanced construction materials, such as the use of nanoparticles as cement admixture, have reported significant improvements in the properties of traditional building products. In previous research, various nanoparticles were mixed with cement, and their performance was evaluated [1; 2; 3; 4]. Nanoparticles, such as nanoclay, Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and CaCO\({}_{3}\) were evaluated; however, SiO\({}_{2}\) was the most commonly studied [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Nano-silica denotes that small particles comprise an amorphous SiO\({}_{2}\) core with a hydroxylated surface, which makes the substance insoluble in water. The size of the particles can vary between 1-100 nm; therefore, they are small enough to remain suspended in a fluid medium without settling. Parameters, such as specific surface area, particle size, and size distribution can be controlled using the synthesis technique [3]. Nano-silica particles have a high surface area to volume ratio that provides high chemical reactivity. They behave as nucleation centers, contributing to the development of the hydration of ordinary Portland cement (OPC) or cementitious materials [19]. The addition of nano-silica into mortar and concrete mixtures accelerates the hydration process, improving the strength and microstructure characteristics of the OPC [8; 13; 19; 20; 21; 22; 23; 24; 25]. The increase in the initial hydration rate caused by the pozzolanic reaction increases the amount of calcium-silicate-hydrate (C-S-H) gel [26; 27; 28]. Cement-based materials containing nano-silica particles demonstrate an improved performance due to the combined effects of the filler, nucleation, and pozzolanic reaction [29; 30]. Most of the previous research have focused on the powdered nano-silica (PNS) replacement percentages of less than 5% by binder weight [8; 9; 21; 22; 31], whereas a few studies have increased this fraction to reach 12% [27]. The binder weight replacement method was used, which displayed a significant decrease in fluidity due to the high specific surface area of the PNS. A large amount of mixing water and/or superplasticizer was required to counteract this effect and ensure sufficient fluidity [19; 22; 24; 32; 33]. Therefore, previous studies using PNS and the binder weight replacement method are difficult to apply with a high substitution rate of more than 12% due to the ensuing rapid decrease of fluidity. Some studies have reported the results of binder weight replacement using CNS instead of PNS [7; 19; 26; 28; 34]. As a result, the mechanical behavior of OPC with higher percentages of PNS has not been well-reported. A study on the mixing method and mechanical and microstructural characteristics for replacing CNS by 15% or more is still insufficient. Therefore, a new mixing method is needed to increase the substitution rate of CNS. Following the development of the mixing method, the next step will be to study the mechanical and microstructural characteristics of the matrix of high CNS replacement rates. Therefore, for the development of cement using nano-silica particles and various nano-particles, a mixing method that can apply various substitution ratios is needed. In particular, a method to improve sudden initial fluidity degradation using nano particles is more important. This is important for solving problems such as a reduction in mechanical performance and durability degradation due to rapidly decreased fluidity. Binder weight replacement of nano-silica reduces the amount of binder. This may also affect the formation of hydration reactants and the pore structure. In addition, the rapid decrease in fluidity due to the mixing of nano-silica forms a dense matrix together with the binder reduction effect caused by the binder replacement method. Therefore, there is a need to consider how to apply the effects of nano-silica without reducing the amount of binder.