The effect of non-free calcium on dissolution-polymerization reaction mechanism of coal gasification slag

Coal gasification slag is an excellent raw material for preparation of cementitious materials. Due to the addition of calcium-based additives, the content of non-free calcium in silicon/aluminum-rich coal gasification slag is relatively high. Thus, the influence of non-free calcium on leaching of elemental and formation mechanisms of cementitious phase were investigated by this paper. The results indicate that non-free calcium primarily disrupts the Al−O−Al bridges in the aluminosilicate glass phase, leading to decreased polymerization degree of slag. The compressive strength of cementitious materials is affected by alkali activation concentration and polymerization degree of slag. Under low-alkali conditions, as the calcium content in the gasification slag increases, the compressive strength of the cementitious material increases firstly and then decreases. Increased calcium promotes leaching of Al, however, large amounts of dissolved calcium tend to form cementitious phases and Ca(OH)₂, which can hinder leaching of ions and formation of gel. In high-alkali environments, non-free calcium enhances the compressive strength of cementitious material. Glass phase in slag was destroyed by OH⁻, and leaching rates of Si and Al in slag with high-calcium increase, which lead to more C-A-S-H in cementitious material. Additionally, elevated alkalinity promotes the crystallization of sodium-minerals such as zeolites and sodalite.