Self-Healing Capacity of Cemented Paste Backfill in Response to Internal Sulfate Exposure

This paper experimentally investigates the effect of internal sulfate exposure on the self-healing capacity of cemented paste backfill (CPB). CPB specimens were prepared with different sulfate concentrations of 0, 5,000, 15,000, and 25,000 ppm and cured under ambient conditions for an initial curing period of 7 days. Following the initiation of the cracks within the CPB matrix at various pre-cracking levels (i.e., 0%, 75%, and 90% of ultimate compressive strength in the pre-peak phase), the studied specimens were subjected to various self-healing periods of 7, 28, and 90 days. Self-healing performance was evaluated by observations of crack closure, mechanical strength tests, hydraulic conductivity measurements, and assessments of physical properties (i.e., porosity and void ratio). Self-healing products were identified using X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and thermogravimetric analysis and derivative thermogravimetry (TG/DTG) techniques. The experimental results show that sulfate significantly affects the self-healing performance of CPB. The highly sulfated specimens (i.e., 25,000 and 15,000 ppm) generally exhibit a more pronounced self-healing efficiency in terms of strength and hydraulic conductivity recovery during the first 28 days of self-healing, followed by an opposite performance during the 90 days of the self-healing period. Conversely, specimens with low sulfate concentrations (i.e., 5,000 ppm) consistently displayed positive effects on self-healing performance throughout the entire self-healing duration. The inhibition of cement hydration by sulfate ions and the amount of formed expansive minerals (i.e., ettringite and gypsum) emerged as critical factors underlying these observed behaviors. Furthermore, the combination of ettringite, gypsum, C─ S─ H, Ca(OH)2, and CaCO3 was identified as the primary self-healing products in the examined specimens. The results presented in the paper hold significant implications for the design, mechanical stability, and durability of CPB structures exposed to sulfate ions, offering valuable guidance for engineering practice.