Recycling of Waste Incineration Bottom Ash and Heavy Metal Immobilization by Geopolymer Production

Municipal waste incineration ash contains heavy metals, and the safety of its disposal and reuse is an important issue. In this study, we discussed a safe recycling technology for bottom ash (BA) by utilizing the excellent heavy metal immobilization feature of geopolymer (GP). First, the differences in chemical compositions, physical properties, and heavy metal contents of BAs discharged in different months were investigated. Next, the reaction products and strength of the mixture of BA and alkali-activator (AA) solution were examined to clarify the reactivity of BA in the AA solution. We also investigated the effects of the discharge time of BA, ingredients of AA solution, curing method and mixing ratio of BFS on the setting time, strength and heavy metal immobilization capacity of GP mortar using ground granulated blast furnace slag (BFS) and coal fly ash (CFA) as precursors, and BA as fine aggregate, and discussed reaction products and micro-structure of the GP mortar. The main results are as follows: 1) BA contained a small amount of amorphous phase. Hardened GP monolith using BA and AA solution was not dense and had a very low strength. 2) The BFS/CFA-based GP mortar with BA as fine aggregate had a higher strength and a longer setting time when sodium silicate solution (WG) was used as AA solution than when sodium hydroxide was added or used entirely. The GP mortars using the BAs discharged in the warm season had longer setting time and higher strength. The reaction products of the GP mortar with WG solution and BA were mainly C-A-S-H gels. The leaching of heavy metal elements (HME) from the GP mortars increased with decreasing the alkalinity of leachate, but the effect of BA’s discharge season was not found in this study. The HME leaching concentrations from the GP mortars in non-acidic water environment were less than the HME leaching limits specified for recycled construction materials directly contacting with water, thus the GP materials with BA can be used in dry or non-acidic water environment. However, when used in acidic water environment, the BA content in the GP materials should be reduced.