Prefabricated building elements based on FGD gypsum and ashes from coal-fired electric generating plants

Mixtures of fly ash, bottom ash and Flue Gas Desulphurized (FGD) gypsum, all solid wastes from coalfired electric generating plants, can be combined with lime and 10% of water to produce a damp powder which can be moulded at a pressure of 20-40 MFa and then steam-cured in less than 1 day at 35-80 °C. The resulting building materials - in the form of bricks, blocks or slabs - produced by this Pressure Forming (PF) process, are stronger and sounder than the corresponding materials produced by a slip casting (SC) process. The physical and mechanical properties of the materials manufactured through the PF process are based on the reaction of amorphous silica and alumina of the ash with lime or lime and gypsum respectively, so that calcium silicate hydrate and ettringite are produced. When the temperature of the steam curing is as low as 35 °C, the hardened material is sound in the air, but it swells and is quickly destroyed by the action of water. This effect can be ascribed to the formation of ill-crystallized ettringite. On the other hand, with thermal treatment at higher temperatures (60-80 °C), the material is stronger and sound even in the presence of water in service. The well-crystallized ettringite fibers, favoured by the higher temperature of the steam curing treatment, are considered to be responsible for the better mechanical performances and the lower change in length. In general, the physical and mechanical properties of the ash-gypsum-lime cementitious system are better than those of the traditional clay-based ceramic products manufactured at temperatures as high as 1000 °C. Therefore, this process based on steam curing at 60-80 °C appears to be very useful for both the re-utilization of solid wastes and the saving of energy in the production of building materials.