Waterglass impregnation of municipal solid waste incineration bottom ash applied as sand replacement in mortars

Incineration has been recognized as one of the most applied strategies for the processing of the municipal solid waste (MSW). The primary output from the incineration of the MSW is Bottom Ash (BA), whose particles are highly porous and contaminated with heavy metals, chlorides, and sulphates, limiting its application in concrete. For improving the applicability as aggregates, many porous materials are impregnated with a pozzolanic solution, easing their use as building materials. However, this treatment has never been applied to by-products like BA, and therefore the influence of a coating on the leaching behaviour of the by-products has never been investigated. This study analyses the effect of an impregnation treatment based on different sodium silicate amounts on BA particles between 1 and 2 mm. The application of the coating lowers the pore volume of BA by 2.5 times, allowing a sand replacement up to 100% and improving the rheological behaviour of mortars till 38%, compared to uncoated samples. Replacing 50 vol.% of coated BA achieves 22% higher flexural and comparable compressive strength than the uncoated BA samples, thanks to the reduction of the Ca/Si ratio in the reaction products. Finally, in the presence of the coating, the leaching of the following contaminants Cl, Ba, Cu, Zn is reduced by 88, 98, 94 and 97%, respectively, compared to the uncoated BA application. Therefore, the impregnation treatment not only favours the application of higher amounts of BA, but it also improves the final performances of the product both mechanically and environmentally.