In-situ formation of layered double hydroxides in MgO–NaAlO2-activated GGBS / MSWI BA: Impact of Mg2+ on reaction mechanism and leaching behavior

Leaching problems of municipal solid waste incineration bottom-ash (MSWI BA) have been extensively reported in the literature. The outstanding heavy-metal-ion binding capacity of layered double hydroxides (LDHs) can potentially address this problem. This study aimed to investigate the in-situ formation of Mg–Al LDHs in MgO–NaAlO ₂-activated ground granulated blast-furnace slag (GGBS) and BA blends, and the evolution of pH, ions, reaction products, microstructure, strength, and leaching behavior were characterized. The results indicate that the formation of Mg–Al LDHs is promoted (up to 69.2%) using a higher MgO/NaAlO ₂ molar ratio, which improves the heavy-metal-ion binding capacity. Simultaneously, the compressive strength increases up to 78.4% with higher MgO/NaAlO ₂ molar ratios compared to NaAlO ₂-activated (NA) sample. Al(OH) ₄ ^– reacts with extra Mg ²⁺ ions rather than Ca ²⁺ ions to preferentially form Mg–Al LDHs over Ca–Al LDHs. Meanwhile, the extra Mg ²⁺ ions snatch the Al(OH) ₄ ^– from Si(OH) ₄, increasing the Mg–Al LDH content and reducing the zeolite content. Moreover, the in-situ-formed LDHs absorbed more SO ₄ ^2– than Cl ^–.