On the resilience of bio-cemented silica sands in chemically reactive environment

Exposure of geomaterials to an acidic environment is frequently encountered in modern-day geo-energy and geoenvironmental engineering activities, in e.g. incorporation of chemical stimulation for unconventional shale gas exploitation, enhanced geothermal systems, geological carbon sequestration, and the long-term regional stability in carbonate-rich coastal areas. The Multiphysics-involved process for each application is complex and an optimised control calls for a better understanding on the coupling mechanism of the chemical, hydraulic and mechanical fields. This laboratory-based study aims to provide a quantitative calibration and derivation of the key coupling parameters accommodating our recently proposed framework of reactive chemo-mechanics, using a bio-cemented rock-like material as a representative for dissolvable rocks. The advantage of bio-cemented specimens (here by microbially induced carbonate precipitation) over natural rocks lies in their more uniform grain-bond structure and laboratory tunable calcite content. An experimental setup is introduced for investigating the role of calcite content on the mechanical and hydraulic properties of bio-cemented silica sands, followed by uniaxial tests on the bio-cemented specimens immersed in acidic environment to allow a reactive chemo-mechanical setting. Our results show that bio-cemented samples appear to be more "resilient" to an acidified aqueous environment in terms of less strength degradation compared to natural carbonate-rich rocks. Ductile failure mode is observed in the bio-cemented specimens within a certain range of the calcium carbonate content and a brittle-to-ductile transition in the failure mode occurs when the calcite content in the specimen decreases. With the calibrated model and the derived coupling parameters, we further illustrate an example of numerical prediction on the mechanical response of bio-cemented specimens under varying acidic environments and loading rates.