Kaolin-based geopolymers: Effect of mechanical activation and curing process

The present work aimed to investigate the influence of mechanical activation of raw kaolin on the final compressive strength of as-obtained geopolymers regarding the curing profile. A commercial raw kaolin containing 81.5 mass% of kaolin (labeled KBip) was used. Mechanical activation was performed by dry ball-milling of raw kaolin at 250 rpm for 1 h. The curing temperatures were 40 °C, 70 °C or 100 °C for 24 h, 48 h or 72 h. The compressive tests were conducted on geopolymers after the 2nd, 7th, 14th and 28th days of ageing. Mechanical activation was performed to improve mechanical properties. Results showed that without mechanical activation, the optimal curing condition was 24 h at 70 °C and the compressive strength was 15 MPa after 28 days of ageing. Under mechanical activation, improvement of the compressive strength was obtained with a curing time of 72 h at 70 °C (to reach 35% increase) or with a curing temperature of 100 °C (for 76% improvement). The formation of alkaline aluminosilicate gels and new crystalline hydrated phases controlled the strength development of geopolymers while the occurrence of carbonated species was responsible for the degradation of mechanical properties.

► Optimal activation is achieved under dry ball-milling of the raw kaolin 250 rpm for 1 h.
► Optimal curing condition is 72 h at 70 °C and 24 h at 100 °C.
► Final compressive strength can be increased from 15 MPa to 27 MPa.
► Aluminosilicate gels and new crystalline phases govern the consolidation of geopolymers.
► Carbonated phases lead to a transitory degradation of mechanical properties.