Soil Mix Cutoff Wall Materials with Microcapsule-Based Self-Healing Grout

Abstract

Over the last three decades, cutoff walls using soil mix technology have been developed and deployed to deliver in situ containment of contaminated sites. The aggressive contaminated soil environment imposes significant long-term stresses on wall materials, and underground cracking is very difficult to detect and can compromise the integrity of walls. A recent relevant development is the concept of self-healing materials that can be triggered by damage and self-heal without the need for external intervention. This laboratory study developed, for the first time, a microcapsule-based self-healing soil mix cutoff wall material and demonstrated its performance in the healing processes, recovery of compressive strength, and hydraulic conductivity. The developed microcapsule-based cementitious grout was mixed with a sand soil using a laboratory-scale auger, and the embedded microcapsules were triggered upon cracking and released a sodium silicate cargo that healed the cracks. Micro–computed tomography (micro-CT) scan analysis verified the good survivability and uniform distribution of the microcapsules during the auger mixing process. Scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX) and thermogravimetric analysis (TGA) revealed that the released sodium silicate microcapsule cargo reacted with the cementitious matrix to produce healing products in the form of calcium silicate hydrates. The microcapsule-containing posthealing specimens regained 44% of initial compressive strength and showed a recovered hydraulic conductivity only slightly higher than that of the undamaged specimens. The results demonstrated the great potential of microencapsulated sodium silicate as a self-healing agent for cement mixed soil, which could provide more resilient and reliable soil mix cutoff walls.