Synergistic effects of diatomaceous earth, oxalic acid and ferric sulfate on Ecological Porous Concrete: Mechanisms of alkalinity reduction and enhancement

The high alkalinity of silicate cement in Ecological Porous Concrete (EPC) inhibits plant growth, and synergistically regulating alkalinity and strength is challenging. In this study, a ternary composite alkalinity-reducing system of Diatomite-Oxalic Acid-Ferric Sulfate was proposed. The effects of the composite alkali-reducing system on the pH, mechanical properties, porosity, and permeability coefficient of EPC were systematically analyzed, and XRD, FTIR, TG, and SEM multiscale characterization were used to elucidate the mechanism of alkalinity reduction and strengthening. The results indicated that the composite alkali-reducing system accelerated the cement hydration reaction and significantly consumed Ca(OH)₂, and the pH of the EPC decreased to 8.49 after 56 d. Meanwhile, the iron-rich C-(F)-S-H gel with a low Ca/Si ratio generated in the cementation system synergistically reinforced the matrix structure with AFt crystals. The compressive strength and split tensile strength of EPC at 56 days were enhanced to 14.8 MPa and 1.97 MPa, respectively, under the premise of guaranteeing the porosity (20 %-30 %). This study provides a theoretical basis for the alkalinity-strength synergistic modulation of EPC, which promotes its application in ecological restoration projects.