Effect and mechanism of two different types of waterproof admixtures and silica fume on the hydration and mechanical properties of natural hydraulic lime

In this study, the effect and mechanism of two different types of waterproof admixtures, silica fume and their composite usage on the hydration and mechanical properties of natural hydraulic lime were evaluated. Specifically, X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and scanning electron microscopy were utilized to investigate the phase composition, micromorphology, and chemical state in-depth. The results showed that, combination of two waterproof admixtures with cementitious matrix via forming Si-O-Si bond and calcium-containing compound weakened the contact between water and cementitious matrix, leading to retardation of hydration. However, silane impregnant more or less promoted formation of hydrated product thanks to the reaction between polycondensated silane and Ca(OH)2, and improved mechanical strength of natural hydraulic lime-based mortars. Hydration and carbonation of natural hydraulic lime was inhibited by bulk modification with silicone-acrylic latex. Partial substitution of natural hydraulic lime by silica fume promoted the hydration and hardening process of cementitious system, leading to improvement of mechanical strength. What is more important, composite modification compensated strength weakening which was the common shortcoming caused by bulk modification of cementitious system with waterproof admixtures.