Deterioration mechanism understanding of recycled powder concrete under coupled sulfate attack and freeze-thaw cycles

Recycled powder (RP) has great environmental potential as a supplementary cementitious material to replace cement. However, the effect of the microstructure of recycled powder on the pore structure of concrete, which is closely related to the macroscopic properties of concrete, has not been understood. In this paper, we focused on the deterioration mechanism of recycled powder concrete (RPC) under the coupled action of sulfate freeze-thaw cycles. Recycled powder was prepared by mixing recycled brick powder (RBP) and recycled concrete powder (RCP) with different mass ratios to replace cement, and the replacement rates were set to 0%, 15% and 30%. Macroscopic physical-mechanical tests and microstructural characterization under the coupled action of sulfate freeze-thaw cycles were conducted to establish the RPC partition-interface model and damage model. The results showed that with the increase in the number of freeze-thaw cycles, the physical-mechanical properties of RPC deteriorated continuously, the critical pore size increased, the connectivity was enhanced, and the attack products ettringite and gypsum were formed. The deterioration of RPC increased with the increase in the replacement rate of recycled powder, but the increase in the mass proportion of recycled concrete powder mitigated the deterioration of RPC. The pore structure defects and distribution characteristics of RPC accelerated the deterioration process. It was concluded from the damage model that the replacement rate of recycled powder was the most important factor affecting the amount of damage. The research results of this paper provide theoretical support for the engineering application of RPC.