Study on the Durability of Recycled Powder Concrete Against Sulfate Attack under Partial Immersion Condition

In order to make full use of waste recycled fine powder (RFP) in concrete and achieve the goal of carbon neutrality in the concrete industry, the durability of sulfate resistance is an important aspect of evaluating the performance of recycled powder concrete (RPC). Therefore, the durability of RPC under partial sulfate immersion was studied to provide theoretical guidance for understanding the erosion mechanism of RPC. The compressive strength, mass loss, and microstructure change patterns of RPC under partial immersion of 5% Na2SO4 and MgSO4 solutions were analyzed by cubic compressive strength, mass loss rate, SEM-EDS, and XRD. The results showed that the surface crystalline matter of concrete in Na2SO4 solution was mainly white powders, and that of concrete in MgSO4 solution was mainly transparent paste, both of which had a little spalling on the outer surface of the concrete. The compressive strength and mass loss rate of concrete with 20% RFP was relatively good, indicating that concrete with 20% RFP had better durability against sulfate. The compressive strength of the lower part of the concrete partially immersed in Na2SO4 solution was higher than that of the upper part and the strength of the lower part of RPC-2 was 3.11% higher than the upper part at 180 d; The pattern was reversed in the MgSO4 solution, where the strength of the lower part of RPC-2 was 19.74% lower than the upper part at 180 d. Microscopic analysis showed that the hydration products of RPC were mainly gypsum and ettringite, while the RPC produced more hydration products with the promotion of magnesium ion in the MgSO4 solution. The higher the replacement rate of RFP, the more frequent the gypsum-type failures in the concrete.