As the harsh conditions (high-temperature, high-pressure, the presence of CO2, etc.) of deep-water wells in the Gulf of Mexico (GoM) continually threaten the integrity of traditional well barriers, investigating and understanding what can be done to prevent cement plug damage and/or creating a new cementitious material to remediate wellbore leakage has become urgent and necessary. Inspired by naturally-occurring geochemical processes, we are developing an entirely new cementing material by turning the challenging deep-water conditions of the GoM into an advantage by providing the necessary acceleration of the hydration and carbonation reactions that turn granular ultramafic raw materials into cemented rock, dubbed here as "Geologically Activated Cement" (GAC). A batch reactor was designed and constructed to explore reactions between olivine sand, water and CO2. The reaction products were identified and analyzed by XRD, EDS, solubility test and falling head permeability test, and then a hardened mechanism of GAC was proposed. More importantly, we also demonstrated this proposed magnesium silicate-based, cement-free plugging material that has been mechanically damaged (cracked) heals itself under GoM-like Chemo-Hydro-Thermo-Mechanical (THMC) conditions within a few hours, which is evidenced by the decreasing flow rate through the damaged sample over time. This self-healing provides potential for a resilient system that restores itself after damage, and is in contrast to the behavior measured for class H cement, which, after it is cracked, demonstrates increasing permeability over the hours following the damage.