Effect of magnesium and sulphate ions on the sulphate resistance of blended cements in low and medium-strength concretes Available to Purchase

Structural concrete components undergo degradation when exposed to sulphate-bearing environments. Diffusion of sulphate ions through concrete pores causes the formation of expansive products such as gypsum and ettringite in the internal microstructure of concrete. This chemical change leads to a significant reduction of strength in structural components. In the literature it has been reported that in blended cements, because of the reduction of calcium hydroxide, magnesium sulphate ions directly react with the cementitious calcium—silicate—hydrate (C—S—H) gel and convert this into non-cementititous magnesium—silicate—hydrate (M—S—H) gel. However, in the present investigation it was observed that, in low-strength concrete (20 MPa), blended cements show more resistance than ordinary Portland cement (OPC) because of the pozzolanic and dilution effect of added fly ash and slag. In this concrete, the expansive product, namely gypsum, densifies the porous pore structure and does not cause any cracking. In the case of medium-strength concretes (30 and 40 MPa), the pozzolanic and dilution effects of blended cements are not very significant and hence they did not show any increase in sulphate resistance compared with OPC cement. In these medium-strength concretes because of their pore structure, the expansive products cause micro-cracks. The inferior performance of concretes with blended cements due to magnesium ion attack is mainly attributed to the formation of ettringite and gypsum, as in the case of sodium sulphate, and these cause more micro-cracks in comparison with OPC cement. Thermal analysis also confirms this.