Alkaline　activation　is　of　great　potential　in　the　solidification　of　municipal　solid　waste　incineration　(MSWI)　fly　ash,　but　the　metallic　aluminum　in　the　ash　inhibits　its　application.　This　work　studies　the　effects　of　residual　metallic　aluminum　on　the　expansion　and　microstructure　of　alkali-activated　MSWI　fly　ash-based　pastes.　Based　on　the　results　obtained,　an　optimized　preparation　process　is　suggested.　Characterizations　of　the　pastes　include　expansion　ratio,　morphology　(SEM),　mechanical　strength　and　microstructure　(XRD　and　FTIR).　It　is　confirmed　that　MSWI　fly　ash　could　be　solidified　through　alkaline　activation　when　using　a　small　amount　of　coal　fly　ash　to　adjust　the　reactive　silica　and　aluminum　ratios.　In　the　optimized　preparation,　sodium　hydroxide　was　added　separately,　so　that　expansion　in　the　pastes　was　significantly　mitigated,　the　formation　of　geopolymer　gel　was　improved　and　the　compressive　strength　of　the　pastes　increased.　©　2019　Elsevier　Ltd