The　present　work　studies　gel　evolution　and　microstructure　of　geopolymers　synthesized　with　volcanic　ash　(VA)　and　blast　furnace　slag　(BFS).　The　synthesis　parameters　such　as　BFS　proportions　on　geopolymer　formation　were　investigated.　Gel　evolution　and　microstructure　of　the　geopolymers　were　studied　by　FTIR,　X-ray　diffraction　(XRD),　29　Si　NMR　spectroscopy　and　scanning　electron　microscopy　measurements.　Silicate　gels　(N-S-H)　were　mainly　formed　in　VA-based　geopolymers　of　low　compressive　strength　(14.07　MPa).　While　with　VA　and　BFS　each　account　for　50%,　VA-BFS-based　geopolymers　possessed　a　compressive　strength　of　55.6　MPa,　as　well　as　the　homogeneous　C-(A)-S-H　and　N-A-S-H　gels　were　formed.　The　C-(A)-S-H　and　N-A-S-H　gels　show　synergistic　effects　on　the　mechanical　property　of　the　geopolymers.　This　work　provides　a　clue　for　the　synthesis　of　geopolymers　with　superior　mechanical　properties　in　areas　of　architecture.