Characterisation of Perovskites in a Calcium Sulfo Aluminate Cement

. Calcium sulfo aluminate cement (CSA) is a promising low CO 2 footprint alternative to Portland cement. The phase assemblage of a commercial CSA cement was investigated by a combination of XRD, SEM-EDX and selective extraction techniques. This study focused on the composition of per-ovsite phases present in the cement.

Calcium sulfo aluminate cements (CSA) 1 were developed by the China Building Material Academy in the seventies. CSA have many specific properties compared to Portland cement as fast setting, rapid hardening, shrinkage reduction. This special cement used alone or in combination with calcium sulphates and Portland cement has found applications such as airport runways and roads patching, selfleveling mortars, tile adhesives grouts… (Zhang et al. 1999). This is also a promising low CO2 footprint alternative to Portland cement due to the difference in the amount of energy used to produce CSA cements (lower kiln temperatures and energy at the mill to grind). The main raw materials used for making CSA cements are bauxite, limestone, clay, and gypsum and this leads to a mineralogical composition very different than Portland cement. While many studies have been carried out on the characterization of Portland cement, few are available concerning CSA. In this study, we report a characterization of a commercial CSA using Rietveld quantitative analysis and scanning electron microscopy. 1 Standard cement chemistry notation is used. As

Methods and Approaches
The CSA cement was from a commercial supplier and the chemical characteristics of the cement are given in Table 1.
In order to analyze mineralogical composition, X-ray diffraction was performed on cement with a diffractometer BRUKER D8 Advance. Powder samples were analyzed using an incident beam angle (Cu Ka, k = 1.54 Å) varying between 5 and 70°. Software X'Pert High Score was used to process diffraction patterns and crystals were identified using the Powder Diffraction File database. Rietveld analysis allowed obtaining mass fractions of crystalline phases in the cement.
For the microscopical investigations, powder samples were impregnated using a low viscosity epoxy and polished down to 0.25 lm using diamond pastes. The samples were further coated with carbon (*15 nm) and examined using a Quanta 200 FEG scanning electron microscope (SEM) from FEI coupled to an Oxford Xmax N 80 mm 2 energy dispersive X-ray spectroscopy (EDX) analyser.
In order to improve the characterization of the cement, two different selective dissolution methods were used. In the first method, the silicate phases were removed in a solution of acid salicylic in methanol (Hjorth and Lauren 1971). In addition, a second selective dissolution method was used to get mainly perovskite phases in CSA by removing ye'elimite, anhydrite, gypsum with 5% Na 2 CO 3 solution (Wang 2010). The method was modified to prevent precipitation of CaCO 3 by washing the filtered suspension with 6% acetic acid. The filter paper and contents were placed in an oven at 105°C until a constant weight was reached.

Results and Discussion
The main phases observed in the experimental diffraction pattern (Fig. 1a) are the orthorhombic ye'elimite C 4 A 3 S with small amount of the pseudo cubic form, belite b and a' H-C 2 S and perovskites from the CSA clinker. Anhydrite II CS is also present as mineral addition. Perovskite family has crystal structures related to the mineral perovskite CT. Ferrite phase Ca 2 (Al x Fe 1-x ) 2 O 5 is usually present in CSA and its structure is derived from that of perovskite by the substitution of Al and Fe for Ti, together with ordered omission of oxygen atoms, which causes onehalf of the sheets of octahedral in perovskite to be replaced by chains of tetrahedral (Taylor 1997). To obtain a good Rietveld refinement, it is also necessary to add a perovskite phase CT (Alvarez-Pinazo et al. 2012). The titanium dioxide is present by the use of bauxite, which usually contains some TiO2, as raw materials in the manufacturing process of CSA clinker. The peaks associated with this cubic phase are confirmed in the XRD pattern of the CSA after the extraction of the main phases (Fig. 1b).
However, with the assumed stoichiometry of perovskite CT, the elemental oxide composition TiO 2 calculated from phase content deduced by Rietveld analysis is strongly overestimated in comparison with XRF analysis.
EDX analysis on polished section of CSA (Fig. 2a) (Gloter et al. 2000). We also observed in some grains perovskite lamellae with high amount of Ti on the scale of few micrometers (Fig. 2b).

Conclusions
The phase assemblage of a commercial calcium sulfo aluminate cement has been investigated with a special attention to the ferrite-perovskite phases. The ferrite composition is closed to the brownmillerite Ca 2 (Fe 2-x Al x )O 5 whereas the perovskite shows heterogeneity with important substitution of Ti by Fe, Al and Si.
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