Calcium Sulfoaluminate Cements Obtained from Bauxite-Free Raw Mixes

The most important properties of calcium sulfoaluminate (CSA) cements are regulated by their key-component, C4A3$. Besides this compound, C2S, C5S2$, various calcium aluminates (C12A7, C3A, CA, C4AF, C2AS, CAS2), lime and/or calcium sulfate (in relation to the type of application) can occur in CSA cements, depending on the synthesis temperature as well as the nature and proportioning of raw materials. As far as the manufacturing process is concerned, CSA-based binders show noticeable environmentally friendly features. Compared to ordinary Portland cements, they are characterized by reduced limestone requirement, kiln thermal input and CO2 generation as well as greater usability of industrial wastes and by-products often difficult to reuse. Limestone, bauxite and gypsum are the main natural materials involved in the manufacture of CSA cements and can be usefully replaced by industrial wastes such as blast-furnace slag, coal combustion ash from both traditional combustors and fluidized bed reactors, waterworks slime, red mud, phosphogypsum and flue gas desulfurization gypsum. In particular, fluidized bed coal combustion waste, whose utilization is of critical importance for the success of this technology, has proved to be an excellent source of CaO, SO3, SiO2 and Al2O3 for the synthesis of CSA cements. Unfortunately, its Al2O3 content does not allow a full replacement of an expensive natural resource like bauxite and an integration with Al2O3 rich by-products is therefore needed. In this regard, alumina powders and anodization mud are worthy of consideration. The former are by-products derived from the secondary aluminium manufacture; the latter is a residue generated during the production of anodized aluminium elements. The synthesis process of CSA clinkers obtained from raw mixes (heated in a laboratory electric oven for 2 hours at temperatures ranging from 1150°C to 1300°C) containing limestone, gypsum, fluidized bed combustion waste and/or Al2O3 sources like alumina powders or anodization mud or both, was investigated in this paper, using XRD analysis as main characterization technique. It has been found that a very good selectivity towards C4A3$, even better than that achieved with the use of only natural materials, was displayed by waste-based raw mixes. DTA-TGA analyses showed that CSA cements derived from synthesized clinkers behave, upon hydration, similarly to an industrial CSA cement obtained from natural raw materials.