The influence of sodium salts and gypsum on alite hydration

This work aims to clarify the impact of alkali and sulfate on the hydration kinetics and microstructural development of alite. The plain alite system is compared to systems with addition of: i) gypsum, ii) Na2SO4, iii) NaOH plus gypsum and iv) NaOH. In all the cases a clear early acceleration of the hydration rate is observed. Alkalis increase the solubility of aluminate and this further inhibits the silicate reaction at later ages. This inhibition does not take place with sulfate, where ettringite is precipitated lowering the aluminate concentration in solution. Major differences in the morphology of hydrates are observed (C-S-H and portlandite). Sulfate uptake leads to a divergent needle-like C-S-H compared to the convergent needles in plain alite. In the presence of NaOH without sulfate, C-S-H tends towards a foil-like morphology. The highest mechanical strength obtained with the combination of alkalis and sulfate is not dependent on the morphology of C-S-H.