Enhancement of selectivity toward ettringite during hydrothermal processes on fluidized bed combustion wastes for the manufacture of preformed building components

Due to its lime, alumina and calcium sulfate contents, fluidized bed combustion (FBC) waste is worthy of consideration as a raw material for the hydrothermal synthesis of building elements based on ettringite (6CaO$Al2O3$3SO3$32H2O). Two FBC waste samples (a fly and a bottom ash) were hydrated, alone and in a mixture, at temperatures between 40 and 85 C for curing times ranging from 2 hours to 7 days, and were submitted to X-ray diffraction and differential thermal-thermogravimetric analyses. It has been found that: (a) the FBC fly ash could be hydrated alone, due to its satisfactory ettringite-generating ability; (b) the FBC bottom ash needed to be used together with other raw materials; (c) progressively better results were obtained by adding (i) FBC fly ash to FBC bottom ash (to give a blend with a 60 : 40 mass ratio), (ii) anodization mud (a by-product of anodized aluminum manufacture, acting as an additional source of alumina and calcium sulfate) in a measure of 10% by mass to the fly-bottom ash blend, (iii) anodization mud (20% by mass) to the FBC bottom ash. Moreover, compressive strength measurements were carried out, according to the EN 196-1 Standard for cements, on the hydrated systems showing the two largest ettringite concentrations, and a maximum value of about 6 MPa was reached at 70 C and 16 hours of curing. The data obtained in this investigation were consistent with an industrial prefabrication of building components.