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Abstract. The aim of the study was to determine the optimal air content in concretes subjected a combination of exposure conditions. Five series of concretes with different air contents were tested. The article reports the experimental results of the combined sulphate expansion and freeze-thaw attack in ordinary Portland cement concrete. Some of the concrete samples were immersed in a 5% sodium sulphate solution for nine months, and then frozen in air and thawed in water. The resistance of concrete to plain freezing and thawing was also tested. In order to compare the degree of concrete damage in both environments, long-term linear deformations (expansion) and compressive strength of concretes were investigated. To explain the complex damaging process the air-void structure in concretes was investigated. The changes in concrete microstructure after storing in sodium sulphate solution were observed under the scanning electron microscope (SEM). The prior sulphate attack significantly accelerated the deterioration of non-air-entrained concrete and air-entrained concrete with low content of air voids. Optimal air entrainment will protect concrete from the harmful effects of the interaction of sulphate attack and freezethaw damage.

The use of fly ash for the production of cement is a typical example of the utilization of by-products (waste) from various fields of production in construction, a significant reduction of CO2 emissions and reduction of energy consumption. However, in order to be able to determine the suitability of this additive in construction, it is necessary to examine the durability of cement composites with fly ash in various, often complicated, destructive environments. There are known publications regarding the durability of concretes with fly ash in individual environments. However, in natural conditions, several destructive environments are common at the same time. The article presents an attempt to reproduce natural conditions. This paper presents the results of sulphate expansion tests of air-entrained (AE) and non-air-entrained (nAE) Portland and fly ash cement mortars subjected to prior freezing and thawing. Despite significant strains experienced during freeze-thaw cycles, unlike the non-air-entrained Portland cement mortars, the non-air-entrained mortars made of fly ash cement did not exhibit any significant expansion when exposed to Na2SO4. For 17 months no expansion was found in the air-entrained mortars made of either of the cement types when immersed in Na2SO4 solution after freezethaw cycles. The results of the SEM and EDS analyses showed that gypsum and ettringite were the sulphate attack products in all the mortars. The highest amounts of ettringite were found in air voids.

The use of low (LCFA) and high (HCFA) calcium fly ashes in the cement industry allows the implementation of European Union proposals on waste management and energy saving. However, the possibility of using HCFA is limited, because the properties of such waste from coal-fired power plants must comply with national regulations. The paper shows the effect of partial replacement of Portland cement (OPC) by these fly ashes on the resistance of the sulphate attack of the mortars immersed in a 5% solution of sodium sulphate. In order to determine the optimal amount of ash additive, a research plan was designed using statistical methods using the Gibbs triangle for mixtures. Samples of control mortars of OPC, binary mixtures of HCFA or LCFA and ternary mixtures of HCFA/LCFA were made. The composition of the blends was designed in accordance with the statistical plan of the experiment for mixtures. The testing program included linear strains, compressive strength and microstructure tests using SEM with EDXA and XRD analysis. The results of laboratory tests and statistical analyzes have shown that fly ash has a positive effect on the sulphate resistance of cementitious composites.