Self-healing behaviour of fly ash/slag-based engineered geopolymer composites under external alkaline environments

This study reports on the self-healing behaviour of fly ash and slag engineered geopolymer composites (FA/S-EGC) under external alkaline environments. FA/S-EGC specimens with different pre-damaged strains were subjected to self-healing at different ages and under different healing environments, including air, water, sodium hydroxide (NaOH) solutions of different concentrations and calcium hydroxide (Ca(OH)₂) solution. The self-healing ability of FA/S-EGC was evaluated by crack characteristics, tensile properties and natural frequency-based damage factor. In addition, micro-characterisation techniques including SEM-EDS, XRD and FT-IR analyses were used to reveal the self-healing mechanism of FA/S-EGC. The results showed that FA/S-EGC retained strain hardening and multi-cracking behaviour after healing in an external alkaline environment. The healing products of FA/S-EGC in air, water and alkaline environments consisted mainly of the C(N)-A-S-H gel and calcium carbonate (CaCO₃), and the phase of the healing products did not change despite the different environments. The self-healing ability of FA/S-EGC in saturated Ca(OH)₂ and 7 % NaOH solutions was significantly improved compared to air and water. This is because the low concentration of the alkaline solution promotes the geopolymerisation of unreacted oxides in the matrix, and the Ca(OH)₂ solution also promotes the deposition of CaCO₃ at the cracks. However, when the NaOH concentration was increased from 7 % to 21 %, FA/S-EGC still showed significant self-healing behaviour in the first seven days, but new damage was produced in the matrix as the healing age increased. This is because the high concentration of NaOH solution leads to depolymerisation of C(N)-A-S-H gels. These results provide scientific recommendations for the application of FA/S-EGC in alkaline environments and for the development of novel healing agents.