Geothermal　clay　rich　in　kaolinite　was　first　prepared　into　geopolymers　in　the　preparation　of　alkali-activated　cementitious　binders　(AACBs).　Effects　of　silica　sand　and　steel　fiber　aggregates　on　the　microstructure　and　mechanical　strength　of　the　geothermal-metakaolin-based　geopolymer　were　studied.　Measurements　of　compressive　strength,　X-ray　diffraction　(XRD),　Fourier-transform　infrared　spectroscopy　(FTIR),　scanning　electron　microscopy　(SEM),　Brunauer-Emmett-Teller　(BET)　and　nuclear　magnetic　resonance　(NMR)　were　used　to　characterize　the　geopolymers.　With　the　replacement　of　silica　sand,　combination　between　sand　and　geopolymer　gel　was　formed,　the　geopolymers　therefore　showed　compressive　strength　higher　than　34　MPa　with　silica　sand　up　to　50%.　However,　silica　sand　of　coarse　particle　size　(0.3–0.6　mm)　caused　porous　structure　in　the　geopolymers,　hindering　the　increase　in　their　compressive　strength.　With　the　replacement　of　steel　fiber,　the　flexural　strength　and　flexural　fracture　toughness　of　the　geopolymers　were　greatly　improved.　©　2018　The　Authors