Two-phase geopolymerization of kaolinite-based geopolymers

The effect of immersion in alkali solution as a second-phase geopolymerization of kaolinite-based geopolymers on their properties was investigated in this study. The aim of this work was to improve the geopolymer properties by increasing both its resistance to water absorption and mechanical performance. It was found that immersion in alkali solution as a second-phase geopolymerization – or a secondary treatment – at 80 °C for 1 h resulted in a high gain in compressive strength, and formation of secondary geopolymer gel. A dense stable layer, with several micrometers depth, was formed on the surface of kaolinite-based geopolymer without causing swelling or shrinkage. These surface and microstructural modifications led to reduction of water absorption and thus induced high durability. As a result of second-phase geopolymerization, the produced geopolymers exhibited an increase in compressive strength from 46 MPa to 59 MPa, and from 22 MPa to 27 MPa after drying and water immersion, respectively. The water absorption of the geopolymers decreased from 10% (w/w) to 4% (w/w), and the overall shrinkage after curing decreased from 0.18% to 0.01%.