Effects of aggregates on the mechanical properties and microstructure of geothermal metakaolin-based geopolymers

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.