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).
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