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• Concrete with a recycled aggregate content greater than 50% has a significant deterioration in its antifreeze performance and cannot meet its frost resistance requirements. • The Weibull model can better describe the damage variation of recycled concrete. • When the recycled aggregate content is 100%, it is only 40% of the antifreeze life of ordinary concrete. • It is suggested that the proportion of recycled aggregate should not be more than 30%. To study the frost resistance of recycled concrete in a sulfate-rich environment, we prepared recycled concrete with 0%, 30%, 50%, and 100% recycled aggregate replacement rate. The physical and mechanical properties of the recycled concrete in the coupled action of freeze-thaw cycles and a sulfate environment for different replacement rates of aggregate was then studied. We analyzed the micro-structure and the reaction products of the recycled concrete. We used a two-factor Weibull distribution model to establish the damage equation, then predict the service life of the recycled concrete. The results show that when the replacement rate of recycled aggregate increases, the physical and mechanical properties of the recycled concrete gradually deteriorate in the later stages of the freeze-thaw test. When the number of freeze-thaw cycles increases, ettringite and gypsum are gradually formed due to the reaction with sulfate. The Weibull model accurately describes the damage variation in the recycled concrete. Through the prediction of the service life of the recycled concrete, we found that the durability was reduced. Given our analysis, we found that the proportion of recycled aggregate should not exceed 30% to meet the requirements for durability design of a concrete structure in China (GB/T 50746-2008). [ABSTRACT FROM AUTHOR]

• The driving damage mechanism for partially immersed cement mortars was examined. • Lowering the RH would accelerate physical sulfate attacks in drying parts. • An intermediate temperature of 35 °C causes more serious degradation than 20 and 50 °C. Concrete structures in salt lakes of western China and South China Sea deteriorate dramatically because of the aggressive salt, high temperature and low relative humidity (RH). In order to study the deterioration mechanism of cement-based materials in these severe environments, the damage progress of mortar specimens that partially exposed to sulfate solutions and under different ambient RH and temperatures was examined. The results indicate that the driving deterioration mechanism for cement mortars that partially exposed to Na 2 SO 4 solution is physical attacks in drying portions, while it is chemical attacks in immersed portions for cement mortars that partially exposed to MgSO 4 solution. Besides, lowering the ambient RH (e.g., from 80% to 50% and 30%) would promote a narrow but supersaturated evaporation interface zone, which accelerates the salt crystallization and eventually surface scaling. While the relatively high ambient RH (80%) causes a broad diffusion zone where the chemical related expansion and cracking formed. Moreover, an intermediate temperature of 35 °C is proved to produce the most severely deterioration when compared to the temperature of 20 and 50 °C. [ABSTRACT FROM AUTHOR]

Over the vast Northwest China, arid desert contains high concentrations of sulfate, chloride, and other chemicals in the ground water, which poses serious challenges to infrastructure construction that routinely utilizes portland cement concrete. Rapid industrialization in the region has been generating huge amounts of mineral admixtures, such as fly ash and slags from energy and metallurgical industries. These industrial by-products would turn into waste materials if not utilized in time. The present study evaluated the suitability of utilizing local mineral admixtures in significant quantities for producing quality concrete mixtures that can withstand the harsh chemical environment without compromising the essential mechanical properties. Comprehensive chemical, mechanical, and durability tests were conducted in the laboratory to characterize the properties of the local cementitious mineral admixtures, cement mortar and portland cement concrete mixtures containing these admixtures. The results from this study indicated that the sulfate resistance of concrete was effectively improved by adding local class F fly ash and slag, or by applying sulfate resistance cement to the mixtures. It is noteworthy that concrete containing local mineral admixtures exhibited much lower permeability (in terms of chloride ion penetration) than ordinary portland cement concrete while retaining the same mechanical properties; whereas concrete mixtures made with sulfate resistance cement had significantly reduced strength and much increased chloride penetration comparing to the other mixtures. Hence, the use of local mineral admixtures in Northwest China in concrete mixtures would be beneficial to the performance of concrete, as well as to the protection of environment. [ABSTRACT FROM AUTHOR]