Binding products from hydrothermal reactions of coal fly ash with lime and calcined gypsum

A mixture composed by 23.6% coal fly ash, 30.4 % plaster of Paris and 46.0% calcium hydroxide by mass was submitted to hydrothermal treatments carried out with a 0.8 water/solid mass ratio for curing times ranging from 1 hour to 14 days, at 70° and 85°C. It has been found that fly ash generates (i) calcium silicate hydrate, in a few hours, from the combination of part of calcium hydroxide with its SiO2 amount and (ii) ettringite, at curing times longer than 1 day, by the reaction of its Al2O3 amount with calcium sulfate dihydrate, generated by the fast hydration of calcined gypsum, and residual calcium hydroxide. Both calcium silicate hydrate and ettringite are binding products potentially useful for adding strength and water insolubility to gypsum-based lightweight building materials. Hydrothermal treatments performed for aging times longer than 7 days caused, particularly at 85°C, a partial thermal decomposition of ettringite.