Durability of Mortars with Fly Ash Subject to Freezing and Thawing Cycles and Sulfate Attack

Destruction of cement composites occurs due to the alternate or simultaneous effects of aggressive media, resulting in the destruction of concrete under the influence of chemical and physical factors. This article presents the results of changes in the measurement of linear strains of samples and changes in the microstructure of cement after 30 freezing and thawing cycles and immersed in 5% sodium sulfate solution. The compressive strengths ratios were carried out at the moment when the samples were moved to the sulfate solution after 30 cycles and at the end of the study when the samples showed visual signs of damage caused by the effect of 5% Na₂SO₄. The composition of the mixtures was selected based on the Gibbs triangle covering the area up to 40% replacement of Portland cement with low and high-calcium fly ashes or their mixture. Air-entrained and non-air entrained mortars were made of OPC, in which 20%, 26.6%, and 40% of Portland cement were replaced with low and/or high-calcium fly ash. Initial, freezing and thawing cycles accelerated the destruction of non- air-entrained cement mortars immersed in 5% sodium sulfate solution. The sulfate resistance, after the preceding frost damage, decreased along with the increase in the amount of replaced fly ash in the binder. Air-entrained mortars in which 20% of cement was replaced with high-calcium fly ash showed the best resistance to the action of sodium sulfate after 30 freezing and thawing cycles.