Experimental study on solidification/stabilisation of high-salt sludge by alkali-activated GGBS and MSWI bottom ash cementitious materials

Abstract

This study utilises alkali-activated ground granulated blast furnace slag (GGBS) as the primary method for solidification/stabilisation, with municipal solid waste incineration (MSWI) bottom ash (BA) as an auxiliary precursor to assist in the solidification/stabilisation (S/S) of high-salt sludge. The results show that the alkali-activated GGBS and MSWI BA cementitious material can effectively realize the S/S of high-salt sludge. However, having early strength of the alkali-activated GGBS solidified sludge at low GGBS content is difficult. In comparison, the addition of MSWI BA enhances the unconfined compressive strength (UCS) of the alkali-activated GGBS solidified sludge to reach 6.35 MPa at 3d curing age, whereas the UCS of the alkali-activated GGBS solidified sludge with 20% GGBS achieves only 0.31 MPa at 14d curing age. The MSWI BA significantly improves the effect of S/S on high-salt sludge by alkali-activated GGBS, and the solidified high-salt sludge with 20% alkali-activated GGBS and MSWI BA cementitious material exhibits good water stability, the UCS of which retain 2.33 MPa after 28d water immersion. The analysis of the mineral composition, chemical bond transformation, thermal stability and microstructure of the solidified sludge samples indicates that the geopolymer cementitious material generated by alkali-activated GGBS and MSWI BA can effectively cement and wrap sludge particles and soluble salts, and reduce the strength loss of the solidified samples after water immersion. Moreover, the addition of MSWI BA accelerates the polymerization of alkali-activated GGBS, leading to rapid solidification/stabilisation of high-salt sludge in early curing age and reduction of the consumption of mineral resources. The heavy metal leaching concentration of the solidified sludge is much lower than that of the raw MSWI BA, and can satisfy the pollution control standard limit of landfill. The immobilization efficiency of the heavy metals of the solidified sludge is larger than 89%. By using MSWI BA as an auxiliary precursor in alkali-activated GGBS, the co-disposal of high-salt sludge and MSWI BA can be achieved, providing insights for realizing the energy conservation and environment protection purposes of solid waste reclamation and GGBS consumption reduction.