Correlation between microstructural evolution and mechanical properties of α -quartz and alumina reinforced K-geopolymers during high temperature treatments

Geopolymer material based K2O–Al2O3–SiO2 systems were investigated for high temperature applications. High viscous geopolymer paste was prepared using standard grade metakaolin dissolved in potassium hydroxide/potassium silicate solution (volume ratio, 1∶1). The alkalinity of the solution was predetermined to optimise the dissolution of amorphous aluminosilicate. Quartz sand and alumina (25, 50 and 75 wt-%) were successively introduced as filler to form refractory concretes. RGPS4, RGPS2 and RGPS34 for sand and RGPA4, RGPA2 and RGPA34 for alumina were produced. The samples obtained were cured in sealed plastic containers for 7 days at room temperature before sintering at 200, 800, 1000, 1100 and 1200°C. The heating rate was 5°C min−1, and the dwelling time at peak temperature completes the sintering time to 4 h. Unvaried mechanical strength, up to 800°C, was observed: ∼25 MPa for biaxial and ∼20 MPa for uniaxial four-point bending strengths. This behaviour was ascribed to the maintenance of the structure of the geopolymer materials within the temperature interval considered. Above this temperature, all the samples presented significant densification; the most important increase in mechanical strength was ∼65 and ∼60 MPa respectively for biaxial and uniaxial four-point bending strengths. This was accompanied by a decrease in porosity. The dense structures observed at 1000–1100°C were affected by a further increase in temperature. Microcracks, vitrification and deformation appeared at 1200°C, which can be considered as detrimental to the mechanical properties. The mechanical properties and the microstructural evolution were found to be influenced by the amount of fillers added. Si and Al rich geopolymer concretes were found promising for the development of potential thermoresistant materials through the cold process.