Analysis of Mechanical Properties and Microstructure of Nano- and Micro-SiO2 Materials as Cementitious Composite Binder

This study evaluated the setting time, mechanical properties and microstructure of Portland cement (OPC) by adding SiO2 nano- and micro-particles. The setting time was reduced due to the pozzolanic reaction of the nano- and micro-SiO2, and the compressive strength was increased through a reduction in the porosity of the microstructure. When nano- and micro-SiO2 were used alone, micro-silica was the most effective in reduced the initial and final setting times and developing compressive strength. When two or more nano- and micro-SiO2 were used, a micro-sized binder and a small amount of nano-silica effectively improved performance as the setting time was reduced to 50-52% of that of ordinary Portland cement (OPC). It appears that a small amount of nano-silica could reduce the setting time and increase compressive strength because it caused the pozzolanic reaction and because the nanoparticles filled the pores between the silica fume and cement, which were composed of relatively large particles. This result could also be derived from compressive strength and microstructure analysis. Cement paste containing to nano- and micro-silica increased the strength by approximately 112% compared to OPC. Because nano-binders may cause a reduction in flow due to their large specific surface area, adding chemical admixture needs to be considered during mix design. In addition, the particle size distribution must be considered when nano- and micro-materials are used because an imbalance in particle size distribution can increase the pore size in the microstructure.