New insights on thaumasite precipitation based on thermodynamic modeling and 3-year exposure of cement pastes to Na2SO4 and MgSO4 solutions

Abstract

Cement pastes were exposed to 0.10 mol/L Na₂SO₄ and MgSO₄ solutions at 20 °C for over three years to investigate the precipitation mechanism of thaumasite and the associated phase evolution. Powder X-ray diffraction, thermogravimetric analysis, Fourier-transform infrared spectroscopy, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy show that samples exposed to MgSO₄ have lower contents of portlandite, calcite, and C-S-H, compared to those exposed to Na₂SO₄. In addition, samples exposed to MgSO₄ contained significant amounts of thaumasite and brucite. Ettringite content is comparable in both conditions, but thaumasite is not detectable in samples exposed to Na₂SO₄. This difference can be attributed to the higher degree of supersaturation of thaumasite in samples exposed to MgSO₄ compared to Na₂SO₄. Thermodynamic modelling was carried out to validate this hypothesis and the predicted phase assemblages show good consistency with experiments. By introducing a critical supersaturation into the thermodynamic phase equilibrium calculations, the model successfully reproduced the selective precipitation of thaumasite in samples exposed to MgSO₄.