Improving foamed mixture lightweight soil containing red mud properties by utilizing nano-SiO2

Red mud (RM) is an aluminum industrial solid waste with a global inventory exceeding 4 billion tons. To utilize RM efficiently, this study used RM as the primary material, with cement, steel slag powder, and desulfurization gypsum as supplements, to prepare foamed mixture lightweight soil containing RM (FMLSR) for subgrade filling. The effect of nano-silica (nano-SiO 2 ) on the fresh properties, unconfined compression strength, durability, and microscopic properties of FMLSR was investigated. The test results indicated nano-silica significantly reduced the flowability of FMLSR. Therefore, to maintain suitable flowability, the nano-silica content should not exceed 1.5%. When the content ranged from 1.0% to 1.5%, nano-silica effectively enhanced the compression strength of FMLSR, reduced its volumetric water absorption, and improved its resistance to dry–wet cycles. Nano-silica can also reduce the environmental risk of FMLSR by significantly lowering the pH of FMLSR leachate. Microanalysis revealed that nano-silica enhanced the density of the skeletal structure of FMLSR by promoting the formation of ettringite, thereby improving its properties. The addition of nano-silica increased the CO 2 emissions of FMLSR, but as nano-silica improves various aspects of FMLSR properties, this increase is acceptable. In conclusion, the suitable content of nano-silica for FMLSR ranged from 1.0% to 1.5%.