In-situ formation of layered double hydroxides in MgO–NaAlO<inf>2</inf>-activated GGBS / MSWI BA: Impact of Mg<sup>2+</sup> on reaction mechanism and leaching behavior: Fid-Bau Portal

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

Liu, Tao / Li, Shaohua / Chen, Yuxuan / Brouwers, H.J.H. / Yu, Qingliang

Abstract

Abstract 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–NaAlO2-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/NaAlO2 molar ratio, which improves the heavy-metal-ion binding capacity. Simultaneously, the compressive strength increases up to 78.4% with higher MgO/NaAlO2 molar ratios compared to NaAlO2-activated (NA) sample. $A l {(O H)}_{4}^{-}$ reacts with extra Mg²⁺ ions rather than Ca²⁺ ions to preferentially form Mg–Al LDHs over Ca–Al LDHs. Meanwhile, the extra Mg²⁺ ions snatch the $A l {(O H)}_{4}^{-}$ from Si(OH)4, increasing the Mg–Al LDH content and reducing the zeolite content. Moreover, the in-situ-formed LDHs absorbed more ${S O}_{4}^{2 -}$ than ${C l}^{-}$ .

Highlights • MgO strongly promotes Mg–Al LDH formation in the NaAlO2-activated system. • Mg²⁺ captures $A l {(O H)}_{4}^{-}$ from Ca²⁺ to produce Mg–Al LDHs instead of Ca–Al LDHs. • Mg²⁺ captures $A l {(O H)}_{4}^{-}$ from Si(OH)4 to generate Mg–Al LDHs instead of zeolites. • In-situ-formed LDHs immobilize more ${S O}_{4}^{2 -}$ than ${C l}^{-}$ via ion exchange.