Mitigation of alkali-silica reaction in blast-furnace slag-based alkaline activated material through incorporation of alum water treatment residue

Alkaline-activated materials (AAMs) have been developed as an alternative binder for cement in an effort to reduce the carbon footprint. However, due to the high alkaline contents in AAMs, these materials, particularly ground granulated blast-furnace slag (GGBS)-based AAM, may have a high potential of detrimental alkali-silica reaction (ASR) when amorphous silica-rich aggregates are used. This study utilised alum water treatment residue (AWTR), a waste from drinking water treatment, as an additional precursor in slag-based AAM to mitigate ASR in mortars with high reactive glass aggregate. The results showed that incorporating 40% AWTR in the precursor improved the compressive strength from 47 to 64 MPa and reduced ASR-induced expansion from 0.9% to 0.2% after 28 days of the accelerated test. The characteristics of the ASR gels were analysed using backscattered electron images coupled with energy-dispersive x-ray spectroscopy. Microscopic observations revealed that the ASR products formed in two stages. In the earlier stage, amorphous ASR gels precipitated surrounding the aggregates, exerting pressure on them and the matrix, resulting in initial cracking. In the latter stage, ASR products extruded into the open cracks, and crystalline products were formed, causing additional damage to the matrix. The quantitative elemental analysis demonstrated that incorporating AWTR could slow down silica dissolution by forming an Al layer in the aggregate surface, prevent the ASR gel calcification and reduce the growth of the harmful crystalline ASR products.