Calcium sulfoaluminate clinker hydration at different alkali concentrations

This paper discusses the effect of alkalis on strength development and hydration products in a commercial calcium sulfoaluminate clinker (KCSA). KCSA pastes were hydrated with water and 0.1 M, 1 M, 2 M, 4 M and 8 M NaOH. The hydration products were characterised at 3d, 28d and 90d, by XRD, SEM, DTA/TG and Al-27 MAS-NMR. With 0.1 M, 1 M and 2 M, the main hydration products were similar to those observed in the water-hydrated pastes: ettringite, AH(3) and calcium aluminate hydrates such as CAH(10); Al-27 MAS-NMR analysis revealed different degrees of crystallinity in AH(3). With 4 M and 8 M, ettringite was not detected, AH(3) crystallinity rose and siliceous hydrogarnet, calcium carboaluminate hydrates, thenardite and calcite were formed at higher alkalinity. Sample carbonation, which grew more intense with time and at higher alkalinity, yielded calcium carbonate and an aluminium hydroxide gel inducing cracking.