Multi-stage synergistic erosion mechanism of cast-in-situ concrete exposed to landfill leachate with varying COD concentrations.

The redevelopment of closed landfills exposes cast-in-situ concrete, a key infrastructure material, to complex landfill leachate immediately upon placement. This results in the simultaneous occurrence of hydration and degradation, posing a severe challenge to its long-term durability. To elucidate the degradation mechanisms of plastic-state cast-in-situ concrete under multi-factor coupling conditions, this work compares the erosive effects of clean water and landfill leachate with CODs of 10,000, 20,000, and 30,000 mg/L on early-age formation and long-term performance. The results indicate that elevated COD levels markedly accelerate concrete deterioration. Under high-COD exposure, earlier surface damage was observed. After 360 days, specimens in 30,000 mg/L COD leachate exhibited a 15.9% loss in peak compressive strength and a 3.43% mass change rate. Microstructural analysis reveals deterioration from synergistic interactions of organic acids with Cl-, SO42-, CO32-, and Mg2+. High COD levels not only expedite the consumption of Ca(OH)2 but also initiate intricate physicochemical reactions. The initial corrosion stage is dominated by organic acids and chloride ions, forming soluble calcium and Friedel's salt that compromise matrix integrity. This leads to an intermediate stage dominated by sulfate and carbonate reactions that generate expansive ettringite, which accelerates deterioration. The process culminates in an advanced stage where organic acids persist, Cl- revert to Friedel's salt, and Mg2+ facilitate M-S-H gel formation. This coupled multi-stage effect ultimately reduce the microstructural density and accelerate macroscopic property loss of concrete. These insights establish a theoretical basis for developing highly durable, low-carbon construction materials suitable for extreme environments such as landfills.