Thermal behaviour and microstructural evolution of metakaolin and meta-halloysite-based geopolymer binders: a comparative study

Two calcined clays (halloysite and kaolinite clays at 700 degrees C) were used as solid precursors for geopolymer synthesis. This study compares the physicochemical properties of the both resulting geopolymer series heated at 200, 400, 600 and 800 degrees C. The end specimens were characterized using Optical Dilatometer, XRD, FTIR, MIP and SEM analyses. Results revealed that the flexural strengths were 18.10 and 21.74 MPa for meta-halloysite- and metakaolin-based geopolymers, respectively. After subjected to high temperatures, the flexural strength drastically decreased from 18.10 +/- 1.06 to 6.7 +/- 0.23 MPa and 21.74 +/- 1.20 to 4.63 +/- 0.24 MPa, respectively. The maximum shrinkage recorded on metakaolin and meta-halloysite-based geopolymers was 14 and 16% around 950 degrees C, respectively. The thermal conductivities decreased with increase in heating temperature from 0.78 to 0.19 Wm(-1) K-1 and 0.96 to 0.26 Wm(-1) K-1, respectively. This reduction is linked to the additional voids and microcracks that occurred within the geopolymer network. The cumulative intrusion in both geopolymers increased with increase in heating temperature up to 600 degrees C, leading to the degradation of geopolymer network that affected the mechanical strength evolution. Both synthesized geopolymer series are potential candidates for insulation materials or refractory applications.