Few　studies　have　focused　on　the　effect　of　mineral　composition　on　the　mechanical　behaviour　and　evolution　of　a　geopolymer　binder　in　artificial　seawater　environment.　In　this　study,　a　geothermal　clay-based　geopolymer　rich　in　micron-size　cristobalite　and　metakaolin　was　compared　with　a　metakaolin-based　geopolymer　in　artificial　seawater.　The　effects　of　the　cristobalite　on　the　mechanical　behaviour　and　microstructure　of　geopolymers　were　characterised　through　compressive　strength　measurements,　x-ray　diffraction　(XRD),　scanning　electron　microscope　(SEM)　and　nuclear　magnetic　resonance　(NMR).　The　micro-size　cristobalite　enhanced　the　compressive　strength　of　the　geothermal　clay-based　geopolymer.　Without　cristobalite,　zeolite　formed　in　metakaolin-based　geopolymer　and　led　to　compressive　strength　decrease　obviously　first　then　increase　slightly.　The　formation　of　the　geothermal　clay-based　geopolymer　gel　was　delayed　in　seawater.　The　cristobalite　in　the　geothermal　clay-based　geopolymer　hindered　the　formation　of　Q4(4Al),　Q4(3Al)　and　Q4(2Al).　©　2019,　©　2019　Institute　of　Materials,　Minerals　and　Mining.　Published　by　Taylor　&　Francis　on　behalf　of　the　Institute.