The effect of MgSO4 on thaumasite formation

Normally, MgSO4 is considered to promote thaumasite sulfate attack (TSA) due to pH drop of solution caused by insoluble brucite formation. However, some test results indicate that thaumasite formation (TF) should occur in the solution with relatively high pH. In the current paper, cement, cement + fly ash, cement + limestone powder and high sulfate resisting (HSR) cement pastes were immersed in sealed bottles containing Na2SO4:MgSO4 = 0:5, 1:5, 3:5 and 10:30 at 10 °C. The pH was measured and corrosion products were identified by means of XRD. The results indicated that the existence of relatively low content MgSO4 (even 5%) may not cause significant pH drop under non-exposed condition. The pH was still kept at the level of 13.0, while the increase of MgSO4 could promote TF. The pH fell below 10.0 in the solution containing saturated MgSO4, however, gypsum dominated the mechanism of paste deterioration.