Effects of highly dispersed nano-SiO2 on the microstructure development of cement pastes

The agglomeration of nano particles in cement systems remains a major challenge for their wide use in construction materials. A method of preparing highly dispersed nano-SiO2 in cement paste is introduced in this paper, and the effects of modified nano-SiO2 (SiO2@PCE) with a core-shell structure on cement hydration and microstructure development were investigated by isothermal calorimetry, quantitative X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and nanoindentation. The results show that well-dispersed nano-SiO2@PCE particles in a cement system can significantly accelerate cement hydration at an early age. The highly dispersed nano-SiO2@PCE particles refine the pore structure of the hardened cement paste. More specifically, the number of harmless and less-harmless pores is increased, whereas the number of harmful and more-harmful pores is reduced. In addition, the proportion of high-density C-S-H and low-density C-S-H in the hardened cement paste at an early age increases by 69% and decreases by 27%, respectively, with the addition of 1.5% nano-SiO2@PCE. Through the seeding effect, cement hydration is accelerated pronouncedly with the addition of highly dispersed SiO2@PCE nano particles. In addition, the stiffness of the paste is enhanced by the pozzolanic reaction between SiO2@PCE and portlandite.