Optimizing the preparation of steel slag-metakaolin based geopolymers by response surface methodology and in-depth investigation of Pb immobilization mechanism

Abstract

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 PbSiO4 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.