We investigated a novel processing technique of municipal solid waste incineration (MSWI) ashes. By using two thermodynamic driving forces – a strong alkaline activator and a compaction pressure, the otherwise relatively nonreactive MSWI ashes could form strong solids, with relatively high flexural and compressive strengths. The produced material was dense, with a low defect density. The effects of the compaction pressure range, the alkaline activator amount, and the MSWI fly ash to bottom ash ratio were examined. We also used 5 wt% class-C fly ash, 5 wt% class-F fly ash, and/or 1 wt% epoxy as additives. This study may open a door to advanced MSWI ash upcycling approaches, useful to environmental preservation and sustainability.