Geothermal reservoirs with acidic geofluids and high temperature - high pressure (HTHP) conditions provide some of the harshest environmental conditions in which Portland cement-based materials are used, even though these materials were not developed for such use. However, such conditions are ideal for promoting rapid carbonation of highly mafic minerals, such as olivine. Carbonation is providing the long term stability of a nature-mimicking rock material in a timeframe that can be over a million times faster (i.e., within hours to days) than the thousands of years such reactions require to proceed under natural surface conditions. Leveraging natural carbonation on a rapid timescale inspires development of an entirely new cementing material, called Geologically Activated Cements (GAC), which turns the challenging HTHP conditions into an advantage by providing the necessary acceleration of the hydration and carbonation reactions that turn granular ultramafic raw materials into the cemented rock. To explore the feasibility of generating GACs under simulated HTHP reservoir conditions, a small-scale, well bore-emulating batch reactor was constructed to test the conditions required for effective hydration and carbonation of olivine. To obtain the desired alteration products at the relevant engineering timescales, different combinations of temperature, pressure, fluid chemistry and grain size were explored. Additionally, by flowing fluid through a fractured GAC system, the GAC specimen was observed to have reduced permeability with time. This demonstration of apparent self-healing ability is contrasted with the behavior of a standard Class H cement material. Hence, the GAC exhibits resilience to failure and, therefore, provides the potential to be an important advanced material for the next generation of resilient well bore cementing and plugging systems.