Improving frost resistance of concrete mixed with aeolian sand powder based on microscopic characteristics

Abstract: In this study, the utilization of aeolian sand cementitious properties, the development of new cementitious materials, and the durability of concrete under sulfate environment are combined together. Based on the test of compressive strength and splitting tensile strength of aeolian sand powder concrete, the effect of freezing thawing and salt leaching coupling on the microscopic characteristics of aeolian sand powder concrete was studied. The aeolian sand powder concrete specimens (100 mm × 100 mm × 400 mm) with strength grade of C35 and C25 were used to perform the freezing resistance test after immersed in 0%, 3% and 6% magnesium sulfate solution (20±2℃) for 4 d, and the initial dynamic elastic modulus and mass were measured. Then the minimum and maximum temperature of the specimens center were set at -17 and 5 ℃, respectively, each freeze-thaw cycle was ensured to be completed within 3 h, and the transition time between freezing and thawing was 8 min; and the relative dynamic elastic modulus and mass loss rate were measured after 25 freeze-thaw cycles. When the specimen relative dynamic elastic modulus decreased to 60% or the mass loss rate by 5%, the trial was stopped. At the same time, to observe the morphology and hydration products, the solidified slurry was selected at the central part of the test block before and after the freeze-thaw cycle test, then the hydration of samples was terminated by ethanol, and then the samples were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). NMR (nuclear magnetic resonance) analysis was performed for aeolian sand powder concrete (diameter × height: 50 mm × 50 mm), and then its microscopic pore characteristics were analyzed. The results show that the incorporation of aeolian sand powder can significantly improve the frost resistance of concrete, and to increase the strength grade appropriately is beneficial to improve the frost resistance of aeolian sand powder concrete. The aeolian sand powder concrete with strength grade C35 in 6% magnesium sulfate solution was subject to 425 freeze-thaw cycles, the appearance was intact, and the relative dynamic elastic modulus decreased to 43.9%. The relative elastic modulus of aeolian sand powder concrete decreased with the number of freeze-thaw cycles at the beginning and then turned to stable, and finally declined to the destruction. Compared with the reference group, the ettringite (AFt) and the calcined hydrated product gypsum (CaSO4) were produced. With the swelling properties of ettringite and gypsum, cracks in aeolian sand powder concrete due to frost heaving stress were filled, and the number of harmful and more hazardous holes in concrete was 29.78% lower than that in ordinary concrete. In 6% magnesium sulfate solution, the porosity of C35-15 aeolian sand powder concrete was 40.9% lower than that of the reference group, the irreducible fluid saturation was 32.7% higher, the permeability was lowered by 98.7%, the total pore volume was reduced, the proportion of small holes was increased, the impermeability was improved, the density of the structure was significantly improved, and the frost resistance was enhanced. This has very important social and practical significance for the development of aeolian sand industries, environmental governance and improvement, and the application of aeolian sand powder concrete in water conservancy and hydropower projects. It has significant economic benefits in the contemporary era.