Self-healing mechanisms in concrete cured in CO2-saturated environments: Synergistic effects of biomass forest ash and metakaolin

This study investigates the effect of autogenous self-healing in high-performance ternary concrete mixes incorporating biomass forest ash when exposed to concentration of carbon dioxide (CO2). To analyse this phenomenon, three cementitious systems were studied: i) 100 % Portland cement, ii) 60 % Portland cement, 25 % biomass ash, and 15 % metakaolin, and iii) 60 % Portland cement, 25 % limestone filler, and 15 % metakaolin. The samples, prepared with different initial crack widths, were subjected to four self-healing conditions: i) continuous immersion in tap water (TW), ii) wet-dry cycles in TW, iii) continuous immersion in carbonated water (CW), and iv) wet-dry cycles in CW, over two exposure periods (28 and 90 days). The effectiveness of the process was evaluated through the analysis of surface crack sealing using a digital microscope and the recovery of impermeability through water permeability tests. To understand the chemical mechanisms involved, the self-healing products formed were analysed using SEM. The results showed that the samples self-healed in CW tended to exhibit lower surface sealing efficiency compared to TW, regardless of the mix type, due to the slightly acidic pH caused by the dissolution of CO2 into carbonic acid. However, in the higher pH inside the cracks promoted the precipitation of calcium carbonates, improving internal impermeability even without achieving effective surface sealing. In this context, biomass ash was particularly effective as a water reservoir, promoting more efficient internal curing when combined with metakaolin and achieving better results in terms of internal impermeability than conventional mixes with cement or limestone filler combined with metakaolin.