Sodium sulphate attack on blended cements under different exposure conditions

The sulphate resistance of ordinary Portland cement, sulphate-resistant Portland cement, and blended cements with different proportions of natural pozzolan and low-lime fly ash were compared. Plain and blended cement mortar specimens were prepared and stored under three different conditions: (a) continuous curing in lime-saturated water, (b) continuous exposure to 5% Na₂SO₄ solution at room temperature, and (c) cyclic exposure to 5% Na₂SO₄ solution at room temperature in which the cycle consisted of immersion and drying exposures (each cycle was composed of immersion in 5% Na₂SO₄ solution for 24 h at 23 ± 2°C, oven drying at 50°C for 23 h, and cooling in air at 23 ± 2°C for 1 h). The sulphate resistance of the cements was evaluated from compressive strength and length change of mortar specimens after up to 1 year of exposure. Microstructural investigations such as X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis were also used to support the explanation of deterioration. This study revealed that under continuous sodium sulphate exposure the length changes of blended cements were less than those of sulphate-resistant Portland cement with 3.6% C₃A content. However, for the mortars exposed to sulphate attack and cycles of immersion and drying exposures, sulphate-resistant Portland cement was found to perform better than blended cements when compressive strengths were considered