Enhancement of the environmentally friendly features of calcium sulfoaluminate cements through the use of a calcium looping spent sorbent as a raw material

Reduced CO2 emission, increased energy saving and potential use of several industrial by-products as raw mix components are well-recognized features associated with the calcium sulfoaluminate (CSA) clinker manufacture which, compared with that of Portland clinker, requires less limestone in the kiln feed as well as lower burning temperature, fuel consumption and grinding energy in the cement mill. The above-mentioned environmentally friendly characteristics can be further improved through a total replacement of limestone, on which both the CO2 emission and kiln thermal input are strongly dependent, with a poorly carbonated source of lime. In this work, a CaO-rich spent sorbent, generated during a calcium looping (CaL) process and purged after 60 calcination/carbonation cycles from a 200 kWth dual fluidized bed pilot facility, was tested as a raw material together with other industrial by-products. These were (1) flue gas desulfurization (FGD) gypsum, as a source of CaSO4, (2) anodization mud (AM), as a source of Al2O3 and additional sulfate, (3) fluidized bed combustion (FBC) fly ash and (4) FBC bottom ash or both of them, as a source of CaSO4 plus additional alumina and uncarbonated lime. Eight CSA clinker-generating raw mixes were heated in a laboratory electric oven at temperatures ranging from 1200° to 1350°C: one included only natural materials (40.4% bauxite, 30.4% limestone, 29.2% gypsum) and was taken as a reference term; the others contained CaL spent sorbent in a measure comprised between 20.0 and 28.3% by mass, bauxite (42.6–47.3%) or AM (53.6–65.6%), natural (15.3–30.7%) or FGD (11.6–24.4%) gypsum, FBC fly- (26.4%) or bottom- (17.6%) ash or a mixture of them (13.2 and 8.8%, respectively). Basically, the X-ray diffraction (XRD) analysis on the burning products showed high conversion of reactants and good selectivity toward the main cement component (C4A3$), especially at 1250° or 1300°C. CSA cements based on different synthetic clinkers, investigated by means of both XRD and differential thermal–thermogravimetric (DTA–TG) analyses, generally displayed a similar hydration behavior.