Autolytic capsules incorporating alkali-activated slag system for self-healing cementitious composites

The optimization of healing materials used in capsules for effective crack self-healing has garnered significant attention. This study focused on the development of a novel capsule using a full-solid-waste alkali-activated slag system. Instead of traditional cement clinker and chemical agents, healing materials such as red mud, calcium carbide residue, phosphogypsum, and ground granulated blast furnace slag were used. The capsules were autolytic due to the retardation and weak expansion properties of gypsum. The healing mechanisms were mainly attributed to the formation of ettringite and calcium silicate hydrate. Crack width, crack area, and strength recovery tests were used to thoroughly analyze the self-healing behavior of the capsules. Detailed characterizations were conducted to understand the healing products and the evolution process. The results showed that cracks with initial widths less than 350 μm were well-healed, with a healing ratio ranging from 55% to 100% for all cracks (initial widths <600 μm). The cementing characteristics of the healing products were significantly increased by the formation of substantial ettringite, which led to an enhanced strength recovery ratio. Crucially, the composition of the healing products closely resembled the hydrates in the matrix, suggesting that the healed mortar had a high level of system compatibility. These results highlighted the potential of the developed alkali-activated slag capsule in promoting self-healing of cracked cementitious composites.