Thaumasite formation in limestone filler cements exposed to sodium sulphate solution at 20 °C

This paper presents a microstructural analysis of mortars made with OPC (C3A=6%) and two SRPCs (C3A < 2% and C3S=40% and 74%) containing 20% of limestone filler. Specimens analysed were immersed in Na2SO4 solution (5% w/w or 0.352 M) with pH control during two years at 20 ± 2 °C. The evolution of attack was determined using XRD semi-quantitative analysis on the material obtained by wearing in layers by millimetre to millimetre of the specimens. Complementary SEM and EDS studies were carried out to confirm the presence of thaumasite. Results show that OPC and high-C3S SRPC containing 20% limestone filler were found to be more susceptible to sulphate attack than the corresponding plain cement. The attack was characterised by the inward front leading first to the formation of ettringite, later formation of gypsum and finally thaumasite formation, when the decalcification of the mortar lead to the breakdown of C-S-H, providing the required silica. The reaction sequence in Portland limestone cements is essentially the same as in plain Portland cements. The main change is that thaumasite is formed at later stages with decomposition of the ettringite formed during the firsts stage of attack. In SRPC with low C3S, the attack was limited to the first millimetres and the thaumasite was not detected.