Fluidized bed combustion wastes as a potential source of ettringite for recycling purposes - a hydration study

The fluidized bed combustion (FBC) technology is very effective in the conversion of solid fuels ensuring also a low environmental impact. A major problem related to the fluidized bed technology is the disposal and/or the utilization of solid wastes, owing to their remarkable quantity and poor quality. In order to find suitable applications fields of these residues, in this work it is explored the possibility of converting them, upon a controlled hydration treatment, into ettringite (6CaO•Al2O3•3SO3•32H2O), a compound having several useful properties. Three types of FBC wastes generated in industrial plants were investigated: 1) FA-Sul, a sulpho-calcic fly ash; 2) FA-Sil, a silico-aluminous fly ash; 3) BA, a sulpho-calcic bed ash. FA-Sul and two binary mixtures (83% FA-Sul/17% FA-Sil and 64% BA/36% FA-Sil) were the most promising, in terms of potential ettringite generation. Hydration tests were conducted at 70°C for 1, 2 and 4 days. Both ettringite and calcium hydroxide (Ca(OH)2) were detected within the hydrated systems, the former being the main reaction product. The kinetics of ettringite formation is related to the sample chemical composition and the conversion degree at 4 days of curing is equal to ~1 for FA-Sul, 0.74 for FA-Sul/FA-Sil and 0.42 for BA/FA-Sil