Effect of Nano-SiO2 on the Mechanical Properties, Microstructure, and Hydration Process of Cementitious Materials Incorporating Hydrophobic Admixture

In this study, the effect of nano-SiO2 on the mechanical properties, microstructure, and hydration process of cementitious materials incorporating hydrophobic admixture (CM-HA) was studied through mechanical strength and hydration heat tests, x-ray diffraction (XRD), a scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP). The results showed that the addition of hydrophobic admixture reduced the compressive strength of cement paste by about 18% in 28 days, and the addition of nano-SiO2 increased the strength of cement paste incorporating hydrophobic admixture by about 20% after 7 days, which indicated that the mechanical properties and strength development of CM-HA could be significantly improved by nano-SiO2 due to the nucleus effect, hydration activity, and nano-filling function, although hydrophobic admixture was not beneficial for the strength development due to the delay for hydration process. Results also indicated that nano-SiO2 could significantly accelerate the hydration rate and reduce the porosity of cement, especially the porosity of macropores. This study provided a new research method for alleviating the adverse effects of hydrophobic admixture on the mechanical properties and durability of concrete and implemented the application of nano-materials in hydrophobic materials.