By-products from pulverized coal-fired power plantsas raw materials for the manufacture of preformed building components

Hydrothermal reactions among lime, silica, alumina and calcium sulfate are of interest for the production of preformed building components, due to the generation of binding products such as calcium silicate hydrate (CSH) and ettringite (6CaO•Al2O3•3SO3•32H2O). Coal fly ash and flue gas desulfurization (FGD) gypsum can be used together with lime as suitable raw materials for the manufacturing process. A laboratory investigation on the hydration behaviour of two ternary systems containing 35% calcium hydroxide, 25% fly ash and 40% FGD gypsum or natural gypsum (as a reference material) was carried out using X-ray diffraction and differential thermal analyses as main characterization techniques. The amounts of the constituents of both mixtures were nearly stoichiometric for the formation of ettringite and a calcium silicate hydrate with a CaO/SiO2 molar ratio equal to 1.5. Water/solid mass ratio was 0.75. Curing temperatures and times were 55°, 70°, 85°C, and 1, 3, 7 days, respectively. Shorter hydration periods (2, 4, 8 and 16 hours) at 85°C were also investigated. Both ternary blends showed a useful hydration behaviour. At each temperature, the curing length favorably influenced the conversion towards ettringite and CSH, the formation rate of the former being higher than that of the latter. For the natural gypsum-based mixture, the conversion to both reaction products increased with the increase of the curing temperature and the hydration was already started at 85°C and 2 hours of aging. For the FGD gypsum-based mixture, 70°C was the optimum temperature and the maximum conversion was lower than that of the system containing natural gypsum. This behaviour was ascribed to the different composition between natural and FGD gypsum.