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To overcome the problems of steel slag (S-Slag) stockpiling and lead (Pb) contamination, in this study, a S-slag-metakaolin based geopolymer (SMBG) was prepared for the immobilization of Pb. The preparation conditions of SMBG were optimized by response surface methodology and immobilization mechanism of Pb was revealed. The optimum conditions for Pb immobilization calculated based on linear regression equations are as follows: S-Slag content 50.3 wt%, liquid–solid ratio 0.31, Baume degrees 51.2°Bé, and modulus 1.39. Under these optimum conditions, the 7-day compressive strength and Pb immobilization efficiency of SMBG were found to be 61.8 MPa and 99.99%, respectively. Subsequently, SMBG with different S-Slag contents were selected for analysis. The characterization analysis results showed that geopolymerization reaction of SMBG containing 50 wt% S-Slag was more complete, and produced more N–A–S–H gels and ettringite. Pb got immobilized in N–A–S–H gels and ettringite by physical encapsulation and ion exchange mechanism. Moreover, Pb reacts directly with silicates to form PbSiO 4 and Pb(OH) 2 precipitates. Further analysis shows that 15–25% of Pb in SMBG was present as precipitate or bound to hydration products, 25–30% of Pb was embedded in the aluminosilicate crystal structure, and the remaining Pb was present as covalent bonds. This study provides an effective solution for the harmless treatment of S-Slag and offers its potential applications in construction field. [ABSTRACT FROM AUTHOR]

The proper treatment of lollingite is of great significance due to its rapid oxidation leading to release of arsenic into the environment. Herein, a green multi-solid waste geopolymer, consisting of red mud, metakaolin, blast furnace slag, and flue gas desulfurization gypsum, was developed. The obtained red mud-metakaolin-based (RMM) geopolymer demonstrated good arsenic retention capability. The results showed that the replacement of SO 4 2− in ettringite with AsO 4 2− via ion exchange, formation of Ca-As and Fe-As precipitates, and physical encapsulation with aluminosilicate gel were the main mechanisms that prevented the release of arsenic. Further dissolution of ettringite in RMM was alleviated by adding a suitable amount of Ca(OH) 2 and controlling the pH of the leachate. TCLP results verified that RMM materials possessed an outstanding ability to stabilize arsenic, with a leaching rate below the permitted value of 5 mg/L for safe disposal. The low leachability of the RMM geopolymers (<0.50 mg/L) is potentially related to the pH buffering capacity of the hydration products at a pH range of 2–5. RMM geopolymers showed a high compressive strength (>15 MPa) and low arsenic leaching concentration (<2.66 mg/L) after 28 days of curing. These results demonstrate the potential of RMM geopolymers to be utilized as an environmentally friendly backfilling cementitious material for sustainable remediation of arsenic pollution. • The evolution pathways of arsenic from lollingite to RMM samples was investigated. • The dissolution of ettringite is correlated with the increase of As leachability. • Arsenic-bearing phases in RMM samples were demonstrated to be stable at pH 2–5. • RMM can be used as environmentally multi-solid waste cementitious material. [ABSTRACT FROM AUTHOR]