Study on the erosion resistance of recycled mortar with microwave-activated waste concrete powder under semi-immersed in sulfate environment

The increasing scarcity of resources of building materials has led to growing attention on the use of recycled powder (RP) from waste concrete as a novel supplementary cementitious material. However, its application in sulfate environments is limited by its high porosity and water absorption rate. This study utilizes microwave activation technology to enhance the reactivity of RP and investigates its partial replacement of cement in mortar, focusing on the sulfate resistance in a semi-immersed environment. In the experiment, the mortar was divided into an evaporation zone and an immersion zone. The changes in appearance, mass, compressive strength, and elastic modulus of the mortar in these two zones when exposed to a 5%wt sodium sulfate solution were studied. The deterioration mechanism was elucidated by combining the changes in phase composition and micro-morphology. The study found that the incorporation of activated recycled micro-powder reduced the porosity of the mortar, weakening the capillary action and the phenomenon of salt crystallization. During the sulfate attack, chemical damage mainly occurs in the immersion zone, while the evaporation zone is subjected to the dual effects of physical and chemical damage. When the mortar is in the semi-immersed state for 10 months, the compressive strength of the evaporation zone in both the non-activated and activated groups decreased by 61.25% and 25.73%, respectively compared to pre-immersion values. In addition, the relationships between the contents of ettringite and thenardite and the compressive strength were quantified, and the results are also consistent with the changes in the appearance. Finally, the Weibull life prediction method was applied to assess the damage level of the mortar, providing a theoretical basis for repair before structural failure, with the aim of extending the service life of the structure. • A systematic study was conducted on the sulfate resistance of mortar mixed with microwave-activated recycled micro-powder. • Revealed the degradation mechanism of cement-based materials in immersion and evaporation zones. • Establishment of life prediction for cement-based materials based on Weibull life prediction method.