Study of the hydrothermal treatments of residues from fluidized bed combustors for the manufacture of ettringite-based building elements

Fluidized bed combustion (FBC) waste is generally unsuitable for making ordinary cements and concretes, and its alternative uses are therefore worthy of consideration. In the present work, FBC waste is investigated as a potentially suitable single raw material for the manufacture of building components based on ettringite, a compound characterized by low density, high fire resistance, significant mechanical strength and usefulness as the main component of preformed lightweight building materials. The hydration behaviour of two FBC waste samples (a fly and a bottom ash) was explored within curing periods comprised between 2 and 24 h at 55 °C, 70 °C and 85 °C. X-ray diffraction and differential thermal analysis were employed as main experimental techniques in order to evaluate the distribution of the hydration products. The role of the raw ash chemical and mineralogical composition, operating temperature and time in the ettringite formation was highlighted. The fly ash was more prone to generate ettringite which, after 2 h-curing time, tended to form and decompose earlier, as the curing temperature and time were further increased. The selectivity of the reactants toward ettringite can be enhanced by the addition of blending components.