Micromechanical properties of geopolymers with different calcined clay precursors

Geopolymers and supplementary cementitious materials (SCM) are potential substitutes for ordinary Portland cement (OPC). Calcined common clays appear to be well suited as raw materials for the production of geopolymers. Due to the multiphase composition of common clays, it is necessary to study geopolymers first, which are prepared of individual calcined clay minerals. Nanoindentation was used to investigate the mechanical properties of metakaolinite, metasmectite, and metaillite geopolymers. For geopolymer production Si:Al ratios between 1:1–3:1 and NaOH concentrations ranging from 4 to 10.79 mol/L were used. All geopolymers were prepared without the use of commercial waterglass. All three clay minerals could be constituents of a natural common clay. Additionally, mercury porosimetry was used to determine the porosity and average pore radius of the samples. The porosity of the studied geopolymers (20.3–31.6%) was in the range found for cement pastes. The average pore radius decreased with increasing concentration of NaOH or increasing Si:Al ratio. Hardness and average pore radius of the geopolymers with the same precursor showed a negative correlation. The highest hardness (1.09 GPa) of all geopolymers was reached by one produced with the metasmectite precursor. On average, the metakaolinite geopolymers reached a hardness of 0.19 GPa and a Young's modulus of 4.72 GPa, the metasmectite geopolymers reached values up to 0.60 GPa and 18.88 GPa. The metaillite geopolymers showed an average hardness of 0.26 GPa and an average Young's modulus of 8.43 GPa. A positive correlation of the Si:Al ratio and the hardness was determined. The hardness values found in this study were comparable to hardness values of cement pastes.