Co-disposal of hazardous waste incineration fly ash and bottom ash through alkaline-activation: Effects of added CaO on solidification of heavy metals

Hazardous waste incineration fly ash (HFA) and bottom ash (HBA) are considered hazardous due to their high concentrations of heavy metals, which drew much attention about their safe disposal. Geopolymers have emerged as promising materials for solidifying heavy metals. With HBA having relatively high content of alu-minosilicates, it presents a potential opportunity to convert both HFA and HBA into geopolymer through alkali activation, thus effectively solidifying heavy metals. However, HBA is often regarded as having lower reactivity, necessitating the addition of reactive calcium (Ca) to enhance alkali activation. In this study, we investigated the impact of incorporating a small amount of CaO on heavy metal solidification in geopolymer. The results demonstrate that the addition of CaO can effectively reduce the leaching concentration of heavy metals in geopolymer when the HBA content exceeds 40 wt% and meets the limits specified by the Chinese standard (GB18598-2019). Characterization using transmission electron microscopy (TEM) and micromeritics instrument corp (MIP) reveals that the addition of CaO reduces the porosity of the geopolymer from 23.59 % to 14.30 %, and resulted in a more uniform distribution of hydration products on the particle surface. This enhanced distribution contributes to a stronger encapsulation effect, effectively enveloping and sealing heavy metal ions within HFA and HBA particles. Consequently, the geopolymer incorporating CaO in the HFA and HBA mixture is deemed suitable for the safe treatment mandated by GB18598-2019. This study offers valuable insights for the comprehensive disposal of these hazardous waste types.