Red mud modified fly ash based road base materials: Mix design, hydration mechanism, and heavy metal migration behavior

Red mud(RM) and fly ash(FA) are solid wastes generated by the alumina industry and phosphate chemical enterprises during the production of alumina and phosphoric acid, respectively. Their large-scale harmless and resource utilization is of great significance. In this study, advanced scientific testing techniques and research methods were used to prepare cementitious materials and no-fire ceramic granules from solid wastes such as RM, FA and phosphogypsum(PG), which were used to prepare road base materials, and to investigate the mix design, hydration mechanism, and heavy metal migration behavior.The results showed that the optimum mix proportion for red mud-fly ash cementitious materials(RFCM) was determined as follows:RM 40.79 %, FA 36.89 %, PG 17.47 %, PC 4.85 %, and superplasticizer dosage at 1.7 % of the binder material. Under a water-to-binder ratio of 0.18, after 28 days of curing, the flexural strength was 3.6 MPa, and the compressive strength was 26.9 MPa. Based on soil environmental and human health risk assessment results, Cr had carcinogenic risks, while more attention was needed on the risks of Hg, Cd, and Pb. Thus, this study identified Cr, Cd, Hg, and Pb as characteristic heavy metals(HMs). Using the optimal mix proportion of the RFCM as a basis, the optimal mix proportion of unburned ceramsite was determined as follows: RM 22.74 %, FA 20.56 %, PG 9.74 %, PC 2.7 %, and river sand 44.26 %. The density grade of the unburned ceramsite was 600, meeting the requirements of "Lightweight aggregates and its test methods-Part 1: Lightweight aggregates" (GB/T 17431.1–2010) for bulk density, cylinder compressive strength, and water absorption. Substituting aggregates with equal particle sizes for red mud-fly ash based road base materials(RFRBM) and designing the mix proportion for RFRBM, the mix ratio of base course aggregates was 20–30 mm: 10–20 mm: 5–10 mm (unburned ceramsite): 0–5 mm = 17.3: 34.8: 24.5: 23.5, with an optimal dosage of binder material at 12 %. The specimens could meet the requirements of "Technical guidelines for construction of highway roadbases" (JTG/T F20–2015) for unconfined compressive strength and splitting tensile strength. At the same time, the durability of RFGFM was tested and analysed with reference to the standard. Utilizing leaching kinetic models analyzed the leaching behavior of characteristic HMs in the base course system and constructed a risk control system for HMs in RFRBM. Finally, reference limits for HMs were proposed according to the application scenarios of RFRBM.