Effect of Alkali on the Hydration of Blast Furnace Slag in the Presence of Calcium Hydroxide

Model systems containing blast furnace slag (BFS) and calcium hydroxide (CH) were utilized to investigate the hydration of BFS and its interaction with CH, focusing specifically on the impact of alkali substances, such as sodium hydroxide (NaOH) and water glass (sodium silicate), on the hydration rate and the stoichiometry of products. Hydration experiments were conducted at temperatures of 30, 40, and 50 °C over periods ranging from 1 to 32 days, with varying concentrations of sodium hydroxide. To assess the conversion of CH, thermal analysis techniques were employed, and quantitative energy dispersive spectroscopy (EDS) was utilized to analyze the chemical composition of various phases. Results indicated that the addition of NaOH retarded the early stages of hydration, prompting exploration of several potential mechanisms, including the common ion effect, modulation of the Ca/Si ratio during hydration, and variations in water content of the hydrate gel. Evidence supporting these mechanisms was documented, although some were found to be time-dependent. Notably, variations in the Ca/Si ratio were observed in the early stages, while later stages showed consistency across solution molarity. TGA/DTG analysis suggested that gel phases in strong alkaline conditions evolved differently, exhibiting altered thermal stability and increased water absorption.