In-situ formation of LDH/zeolite nanocrystals in alkali-activated materials for efficient lead-zinc tailings stabilization.

Lead-zinc tailings (LZTs) pose potential environmental risks due to high content of heavy metals (HMs). However, conventional cementitious materials are employed for LZTs encapsulation suffering from high costs, substantial carbon emissions and secondary pollution. This study proposed an efficient strategy of forming in-situ layered double hydroxides (LDH) and zeolite in alkali-activated material (AAM) for stabilization/solidification (S/S) of HMs within LZTs, utilizing slag as precursor and sodium aluminate (NaAlO2) as alkali activators. Immobilization capacity and mechanisms of the AAMs toward lead (Pb) and chromium (Cr) were investigated. The results revealed that NaAlO2-activated AAM (NA) induced the in-situ formation of LDH, zeolite and calcium aluminate silicate hydrate (C-(A)-S-H) through tetracoordinate aluminum from NaAlO2 with silicoaluminates and calcium dissolved from slag, which was contributed to the immobilization of HMs by ion exchange, surface complexation and electrostatic adsorption. Compared to NaOH-activated AAM (NH) with equivalent alkali content, NA system exhibited superior immobilization capacity for anionic and amphoteric HMs. In Pb-doped NA system, Pb-enriched LDH phases were observed. Under low-alkalinity conditions, Cr incorporation led to the formation of a Cr-U phase within NA. When applied to the solidification of actual hazardous LZTs, NA paste containing 80 wt% LZTs achieved the compressive strength of 5.25 MPa and immobilization efficiency exceeding 96% for HM ions. Besides, carbon emissions were reduced the by 66% in NA system, demonstrating significant energy-saving and economic benefits, as compared to the conventional cement-based materials. This work provides new insights into the development of low-carbon alkali-activated materials for heavy metal remediation.