Synthesis of special cements from calcium looping spent sorbents

Using as a raw material for making cement a poorly carbonated source of lime, such as the CaO-rich spent sorbent generated in the FB calcium looping (CaL) process, does not imply only a potential decarbonisation of both power generation industry and cement manufacture, but can also result in a number of further remarkable advantages for the cement industry. As a matter of fact, a large decrease of the kiln thermal input, as well as a strongly reduced specific consumption of fuel and raw materials, can be achieved. However, in the manufacture of ordinary Portland clinker, a considerable limitation to the replacement of limestone with a CaL spent sorbent is imposed by its SO3 content which, even if relatively low, can be able (i) to determine operational problems during the burning process (e.g. material accretion and plugging) and (ii) to depreciate the clinker quality. These problems are not encountered in the manufacture of calcium sulfoaluminate (CSA) cements, usually produced from raw mixes containing limestone, bauxite and natural gypsum. This work was aimed at investigating CSA clinkers synthesized in a laboratory electric oven, at heating temperatures ranging from 1150° to 1350°C. They were obtained from a generating raw mix containing bauxite, natural gypsum and, in total replacement of limestone, a CaL spent sorbent; this was purged after 60 calcination/carbonation cycles from a 200kWth dual fluidized bed pilot facility, operated with a commercial limestone coming from South Germany. As for CSA clinkers derived from a raw mix containing only natural materials, both high conversion of reactants and good selectivity toward the main CSA cement component (4CaO·3Al2O3·SO3) were found; the hydration behavior was also similar.