Safe environmentally friendly reuse of red mud modified phosphogypsum composite cementitious material

Phosphogypsum (PG) and red mud (RM) are solid wastes produced by chemical enterprises when producing phosphoric acid and alumina. Therefore, the harmless utilization of them is of great significance to the environment. This study explored the effect of the synergistic activity enhancement effect of solid waste on the macroscopic properties and internal microstructure of RMPS, and explored the optimal ratio of composite cementitious materials and ceramic aggregate. Based on the synergistic enhanced activation effect of multiple solid wastes, it is necessary to evaluate the human health and ecological security risks of heavy metals in the prepared red mud modified phosphogypsum system (RMPS) to ensure the environmental safety of the product. The results showed that the optimum ratio of PG, RM, FA, quicklime and cement in the preparation of cementitious materials by RMPS were 46.9 %, 26.5 %, 4.1 %, 12.2 % and 10.2 %, respectively. Under the waterbinder ratio of 0.33, the 14 days compressive strength of the mortar specimen could reach 19.0 MPa, the 14 days flexural strength could reach 4.3 MPa. The optimum ratios of PG, RM, FA, quicklime, cement and river sand in the prepared ceramic aggregate were 36.5 %, 20.6 %, 19.5 %, 3.2 %, 7.9 % and 22.2 %, respectively. The raw materials were mixed and prepared into ceramsite, which was placed in a drying oven and cured at 20 degrees C for 14 days. The 14 days cylinder compressive strength of ceramic aggregate could reach 2.4 MPa, which met the standard requirements of "Lightweight aggregates and its test method-Part 1: Lightweight aggregates" (GB/T 17431.1-2010). With 10 % cementitious materials content, the road water-stable layer materials was prepared by using ceramic aggregate to replace 4.75 mm crushed stone in equal volume. The 7 days unconfined compression strength of the cylinder specimen was 4.7 MPa, and the 7 days splitting tensile strength was 0.6 MPa, which met the strength requirements of the highway water-stable layer in the "Specification for design of highway asphalt pavement" (JTG D50-2017). The risk assessment of heavy metals in RMPS was carried out. The single factor index indicates that more attention should be paid to Hg and Cd. The geo-accumulation index showed that Hg in the system had a high environmental pollution risk. At the same time, in accordance with the result of human health risk assessment, Cr in RMPS can cause carcinogenic effects through oral intake and need to be controlled. Finally, based on the risk assessment model constructed in this study, the limit values of RMPS applied to three heavy metals in road water stability layer were given and the safety and environmental protection control model of heavy metals in RMPS was proposed.