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The material prepared by the superadiabatic combustion of thermal plant fly ash mixed with a carbonate-containing component was hydrated and then carbonated for a long time to obtain a composite which had a compressive strength of ∼180 MPa and Young's modulus of ∼100 GPa owing to the formation of a mixture of aragonite and calcite in the pores of the composite. Heat treatment of the composite in the range 20–750°С leads to structural changes associated with a number of consecutive processes: dehydration, the aragonite → calcite polymorphic transformation, dehydroxylation, and decarbonation. Each of these processes influences the mechanical properties of the resulting material. The temperature of the aragonite → calcite transformation in the composite is shown to be higher than that in pure aragonite. The transformation causes no dramatic decrease in the mechanical strength of the composite because of the polycondensation of the silicate and aluminosilicate constituents.