Hydrothermally-targeted synthesis of Al-substituted tobermorite using MSWI fly ash with industrial SiO₂ for potentially high-quality utilization

Al-substituted tobermorite (TOB) can be synthesized as a value-added product through hydrothermal treatment (HT) using incineration fly ash (IFA) with an exogenous additive. However, extreme conditions and low purity limit its further use. To solve these issues, we explored the potential of industrial SiO₂ (i-SiO₂) in synthesizing Al-substituted TOB using IFA from two megacities: Shanghai (IFA-H) and Shenzhen (IFA-Z). Results showed that vast majority of Ca, Si, and Al in IFA and i-SiO₂ can be converted into Al-substituted TOB, allowing for targeted synthesis with low i-SiO₂ mass. Besides, the experimental condition, e.g., 0.4 mol/L of NaOH concentration, 160 °C of reaction temperature, and 24 h of reaction time, appears to be milder than the previous relevant studies. Both Al-substituted TOBs had Cl⁻ contents below 5.0 wt%, and total immobilization efficiencies for typical heavy metals exceeded 98.0 wt%. The cation exchange capacities (CECs) for Al-substituted TOBs from IFA-H and IFA-Z were 424.0 cmol/kg and 241.3 cmol/kg, respectively, comparable to commercial zeolite and indicating potential for high-quality use. The higher CEC of the Al-substituted TOB from IFA-H may be due to its higher Cl⁻ content. The HT solution, rich in Cl⁻, can be reused. The HT process utilizing i-SiO₂ can reduce economic cost by nearly half compared to current resource utilization approach. This study aims to enhance the resource utilization of IFA.