Sol-Gel synthesis of silica nanoparticles and their role in predicting cement mortar strength at early ages

Abstract

Cement is one of the most fundamentally used material all over the world. Among the many different properties of cement, compressive strength conventionally determined at the age of 28 days remains the most important parameter to assess cement quality. Waiting for 28 days to obtain a test result urges cement manufacturers to set quality parameters higher than the minimum accepted by norms, which means higher unnecessary manufacturing costs. The aim of this study is to assess the performance of cement mortar when nano silica (Nano-SiO2) is used as supplementary cementitious material (SCMs) and to find a fast curing regime that allow us to predict the 28 days compressive strength of cement mortar at an early age. A simplified Sol-Gel method for the synthesis of Nano-SiO2 from sodium silicate was established, using sulfuric acid as hydrolyzing agent.5 This method allows a high yield (about 95%) production of Nano-SiO2 particles of 99% purity. A series of tests were conducted to study the effect of curing temperature and nanosilica addition on the strength development. X-Ray Fluorescence Spectroscopy (XRF) was used to check the chemical composition of the cement used, and the purity of synthesized Nano-SiO2. X-Ray Diffraction analysis (XRD) allowed us to monitor the variation of Alite, Portlandite and amorphous phase during hydration period. XRD was also used to study the nature of the synthesized nano silica. The produced silica particles are of amorphous nature and have an average hydrodynamic diameter of ~135 nm as determined by Dynamic Light scattering (DLS) analysis. Pozzolanic reactivity of the synthesized nano silica was found to be higher than that of microsilica, and lower than that of commercial nano silica having a diameter of~20 nm. Optimum conditions for strength enhancement (at early ages) were found to be a curing temperature of 70 °C and nanosilica/cement substitution of 3% respectively. Combining both optimums allowed us to estimate the ultimate 28 days compressive strength at 72 hours with a Coefficient of Variation (COV) less than 2.5%.