Role of Ettringite in the Reuse of Hydrated Fly Ash from Fluidized-Bed Combustion as a Sulfur Sorbent: A Hydration Study

Waste from fluidized-bed combustion (FBC) has a low potential for reuse. One possibility for its recycling lies in a hydration process aimed at reactivating the SO2 sorption ability of the unconverted lime. The formation of ettringite, as well as calcium hydroxide, in the hydrated FBC fly ash to be reinjected into the reactor could be of importance because ettringite is able to play a chemical and physical role in SO2 capture. The aim of this paper is to investigate the conditions under which ettringite is formed by the liquid-phase hydration of FBC waste. To this end, two industrial FBC fly ashes were hydrated at temperatures of 20 and 70 °C, for curing times ranging from 30 min to 96 h. Ettringite concentrations and hydration levels of up to 50 and 74%, respectively, were measured. The experimental data were also employed to set up a simple kinetic model for ettringite synthesis. With respect to the CaSO4 concentration, this reaction was of first order for hydration times up to 8 h and of second order thereafter. In this work, the main characterization techniques employed were X-ray fluorescence, X-ray diffraction, and differential thermal and thermogravimetric analyses. Thermogravimetric analysis was also used for quantitative purposes.