Effect of alkalis content on calcium sulfoaluminate (CSA) cement hydration

The behavior of sulfoaluminate (CSA) cements hydrated in neutral or alkaline solutions (0 M, 0.1 M, 1 M, 4 M and 8 M NaOH) was studied. 2-d, 28-d and 90-d mechanical strengths were determined, as well as the reaction rate by isothermal conduction calorimetry. The reaction products were characterised with XRD, DTA and SEM. The findings revealed differences in performance depending on whether hydration transpired at pH ≤ 14 (0.1 and 1 M) or pH > 14 (4 M and 8 M). To 0.1 M and 1 M, there is an improvement of mechanical strength than in water-hydrated pastes. The majority reaction products in these conditions were ettringite and microcrystalline aluminium hydroxide. Mechanical strength was lower in the high alkalinity (4 M and 8 M) than in the reference pastes, mainly due to the formation of different reaction products. Ettringite was unstable at 4 M and failed to form at 8 M, whilst thenardite and U-phase were detected in the 28d and later materials hydrated at both concentrations.