Resistance to Thaumasite form of Sulphate Attack of Blended Cement Mortars

This study concerns the resistance against thaumasite form of sulphate attack on Portland cement reference with high volume ground granulated blast-furnace slag, fly ash and ground basaltic pumice exposed to tap water (5% magnesium sulphate) for ten years. The separate and intergrinding methods, two fineness (250m2/kg and 500 m2/kg) and 30% proportions of each of the different additives were employed in equal amounts by weight. The development of the microstructure and the secondary minerals in the plain and blended cements were studied via polarising microscopy on thin sections and on undisturbed lumps of specimens by scanning electron microscope (SEM-EDAX) analysis. A series of mechanical tests of cement mortars were undertaken on all specimens. The development of the microstructural features and the formation of the secondary minerals in pores were coherent to the increase in sulphate resistance. The presence of thaumasite together with ettringite in some specimens reflects the incomplete transformation phase of hydration. However, despite the numerous studies conducted on the relation of hydration and hydrolysis with reference to cement hardening, the hydration-bound hardening phenomenon coupled with thaumasite morphology and matrix and/or pore space orientations are recommended for further investigation. The use of the pozzolans/ground granulated blast furnace slag and basaltic pumice improved the sulphate resistance of the cement mortars, where specimen E yielded the highest sulphate resistance-highest TSA resistance.