C4A3Š hydration in different alkaline media

This study explored the behaviour of laboratory-synthesised calcium sulphoaluminate (C4A3Š) in alkaline media. C4A3Š was hydrated in three liquid media: water, 8-M NaOH and 4 (wt.%) Na2CO3 added to the C4A3Š + water mix. Hydration kinetics were studied via isothermal conduction calorimetry and 2- and 28-day mechanical strength values were found. The reaction products were characterised with XRD and FTIR. The findings showed that whilst C4A3Š hydration kinetics were accelerated in the presence of alkalis, the resulting pastes had lower mechanical strength than the pastes hydrated with water and exhibited severe decay in some cases. An analysis of the hydration products revealed the presence of ettringite in the water-hydrated C4A3Š pastes, whereas under alkaline conditions the main calcium sulphoaluminate hydrate detected was U phase.