Potential application of mineral capsules in self-healing cement-based materials under groundwater containing sulfate

Underground engineering is usually exposed to groundwater and is subject to attack by sulfates in groundwater. To realize the crack self-healing and durability improvement of the cement-based materials exposed to groundwater containing sulfate, two types of mineral capsules were produced in this study. One of the mineral capsules used quicklime and metakaolin as healing agents, and the other used quicklime, MgO expansive agent and metakaolin as healing agents. The healing potential of mineral capsules was studied by the core compositions analysis of capsules and the state visualization of capsules in matrix. After healing in water and different concentrations of sulfate environment, the healing efficiency was evaluated through the methods of crack observation, ultrasonic pulse velocity and water absorption. Then the healing mechanism of mineral capsules under different environments was further revealed by the micro-analysis of healing products. Results showed that the mineral capsules had great effectiveness and healing potential. After being mixed into matrix, mineral capsules significantly enhanced the self-healing ability of cracks, especially that with quicklime, MgO expansive agent and metakaolin as healing agents. Compared with water, sulfate environment was more conducive to the improvement of healing efficiency. After healing in 5 % sodium sulfate environment, the recovery rate of ultrasonic pulse velocity and reduction rate of sorptivity coefficient reached 95 % and 90 %, respectively. The coupling effect of mineral capsules and sulfate environment collaboratively promoted the formation of portlandite, C-S-H, ettringite, brucite and hydrotalcite. These products gradually deposited at cracks, thereby realizing the self-healing of cement-based materials under sulfate-containing groundwater.