Time-Resolved Insight into Microbially Induced Biocement Formation Using Non-Destructive Ultrasound Testing

Microbially induced calcium carbonate (CaCO₃) precipitation harnesses naturally occurring microbial processes to create biocement, a biologically derived cement. In this process, ureolytic bacteria hydrolyze urea, producing carbonate ions that react with calcium to form CaCO₃. This CaCO₃ acts as a binding agent, bridging adjacent soil granules and creating a cementitious material. Biocement offers a sustainable alternative to traditional concrete in circumstances where heavy equipment is impractical (e.g. remote settings), or where long-term infrastructure is unnecessary (e.g. staging grounds). By enriching existing soil with bacterial and chemical treatments, ground stabilization can be achieved rapidly and efficiently.

Determination of the load-bearing capacity of biocement typically involves crushing the sample to measure unconfined compressive strength. This only provides a low resolution, endpoint measurement. In contrast, this work focuses on implementation of Ultrasound Testing (UT) as a non-destructive means of investigating the formation of biocement over time. UT involves transmitting an ultrasonic pulse through the material and monitoring the output to make determinations about its material properties. This technique has been used in a handful of previous studies, but not extensively to characterize the time-resolved biocementation process or to evaluate the efficacy of various experiment parameters.

Here we show that UT can be used to evaluate the depth profile and relative solidification of biocemented soils in real time. Measurements made during biocementation at regular time intervals and at varying depth, illustrate the importance of accessing greater temporal and spatial resolution, as the internal structure changes dramatically over the course of the experiment. We also explore porosity and percolation characteristics of biocement, as CaCO₃ deposition reduces porosity and increases strength. In conjunction with Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy, we start to gain insight into how the microstructural characteristics impact macroscale properties.