Microstructural Investigation of High-Volume Fly Ash Composites Containing Nano-Calcium Silicate Hydrate Crystals

This paper reports on the effect of addition of nano-calcium silicate hydrate (nano-CSH) seeds on the early-age strength development and microstructural evolution of concrete containing high-volume fly ash (HVFA). Cement replacement levels of 60% and 70% are considered. For each of the replacement levels, 0.5%, 1%, and 1.5% of the mass of fly ash (FA) are replaced with nano-CSH crystals. The early-age compressive strength of concrete was tested at the ages of 1, 3, 7, 14, and 28 days. Microstructural investigations involve thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The results show that addition of 1% and 1.5% nano-CSH seeds in HVFA60% and HVFA70%, respectively, can effectively facilitate the formation of hydration products Ca(OH)2 and C-S-H as well as increase the rate of reaction and early-age strength gain at early ages. It was found that the dosages of nano-CSH seeds is a critical parameter in achieving the desired properties.