Microstructure evolution and strengthening mechanism of nano-silica modified magnesium potassium phosphate cement

Abstract

Magnesium potassium phosphate cement (MKPC) is the popular cementitious binder for rapid repair in concrete applications with fast hardening, early strength, and good bonding properties. However, its limitations, such as intense heat release during early hydration, high internal porosity, and brittleness, restrict its broader application. This study employs nano-silica (NS) modification and optimizes key parameters including the phosphate-to-magnesium mass ratio, water-to-binder ratio, borax content, and NS content by orthogonal experiments firstly. Secondly, the optimal mix proportion of NS-MKPC is selected by mean and range analysis. Finally, the effects of NS on the microstructural evolution and macroscopic properties of NS-MKPC are systematically investigated by various characterization techniques such as XRD, SEM/EDS, nanoindentation, and thermogravimetric analysis. The results show that the NS-MKPC with the optimal mix ratio achieved the compressive strength of 99.65 MPa and flexural strength of 9.92 MPa at 28 d macroscopically, representing increases of approximately 74.2 % in compressive strength and 19.2 % in flexural strength compared to pure MKPC, respectively. Microscopic test results show that the incorporation of NS can accelerate the early hydration process of MKPC through filling effect, nucleation action, and chemical bonding. It can promote the uniform growth of hydration crystals (MKP), significantly refine the pore structure, and reduce the residual unreacted MgO particles.