Effect of Blast Furnace Slag and Fly Ash on Autogenous Self-Healing in Tailings-Based Cementitious Composites

The incorporation of supplementary cementitious materials (SCMs), such as blast furnace slag (BFS) and fly ash (FA), provides a promising strategy to optimize cemented paste backfill (CPB) by reducing operational costs, enhancing durability, and reducing the carbon footprint. This study systematically investigates the effects of partially replacing cement with BFS or FA on the autogenous self-healing behaviour of the CPB system. Four binder blend ratios (i.e., cement/SCM ratio of 100/0, 80/20, 50/50, and 20/80) were evaluated based on crack closure observations, uniaxial compressive strength, hydraulic conductivity, and porosity-related parameters. Results show that the appropriate BFS contents promote self-healing efficiency at early and long-term healing stages compared with cement-only CPB, attributed to the secondary latent hydraulic reactions between BFS and calcium hydroxide, leading to the formation of C-(A)-S-H gels and microstructural densification. The pre-cracked PCI/BFS:50/50 specimens exhibited the highest healing efficiency at 28 days of self-healing, with strength exceeding the uncracked control by 17.2% and hydraulic conductivity recovery reaching 81.8%, whereas at 90 days, superior long-term performance was observed for the PCI/BFS 80/20 mixture, achieving a 21.4% strength increase relative to the uncracked control and 96.2% recovery in hydraulic conductivity. In contrast, FA incorporation resulted in progressively reduced self-healing efficiency with increasing FA content due to its intrinsic physical characteristics and delayed pozzolanic reactivity. While the PCI/FA 80/20 mixture recovered strength comparable to the uncracked control, higher FA contents (50/50 and 20/80) exhibited significantly inferior strength recovery (13.3% and 23.2% lower than controls, respectively) and limited hydraulic conductivity recovery. Across both SCM systems, the formation and sufficiency of self-healing products, such as C-(A)-S-H, calcite, and ettringite, primarily govern the overall self-healing performance. These findings provide critical insights for designing CPB with improved autogenous self-healing capacity and optimized binder formulations for field applications.