Abstract
We introduce the application of microbial-induced calcite precipitation via the ureolytic soil bacterium Sporosarcina Pasteurii in freeze-dried form, as a means of enhancing overall MICP efficiency and reproducibility for geotechnical engineering applications. We show that the execution of urea hydrolysis and CaCO3 precipitation persist as a “cell-free” mechanism upon the complete breakdown of rehydrated cell clusters. Further, strength and stiffness parameters of bio-cemented sands are determined. Medium-grained bio-cemented sand yields compressive strengths up to 12 MPa while, surprisingly, fine-grained sand yields up to 2.5 MPa for similar bond contents. To understand the observed discrepancies, we undertake a systematic study of the bio-cemented material’s microstructure, by combining a series of microstructural inspection tools. The study extends beyond conventional qualitative and textural characterization and provides with new insight into the material’s peculiar 3D micro-architecture. We apply a new methodology towards quantifying crucial microscopic characteristics such as the particle sizes of the crystalline bond lattice, the bond-grain contacts and particle orientations. Bonds are found to exhibit distinctive geometries and morphologies when MICP applies to different base materials. We thus contribute to the debate on the importance of factors affecting: (i) MICP efficiency, (ii) the mechanical response and (iii) peculiar micro-architecture of bio-improved geo-materials.
Highlights
The quest for novel, performant materials and solutions in engineering problems is driven nowadays by coupling technical innovation with economic efficiency and positive environmental impact
Quite unlike most applications targeting the artificial cementation of soils for improving their overall mechanical properties and bearing capacity[1,2] microbial-induced calcite precipitation (MICP)[3,4,5,6,7] via the ureolytic soil bacterium Sporosarcina Pasteurii[8] is applied via non-erosive, low-pressure propagation of bio-chemical solutions
We focus on the study of kinetics of urea hydrolysis for the case of MICP induced via utilizing lyophilized cells
Summary
The present section comprises a description of the distinctive CaCO3 precipitate behaviours through 3D microstructural characterization. Bonds are found to exhibit a relatively homogenous distribution in the theta-orientation space compared to calcite bonds that precipitate in fine-grained sand (Fig. 2c). The bonds per soil particle ratios are found to range between 4.3 and 3.2 for fine- and medium-grained sand respectively, independently of the overall calcite content. We further provide a qualitative description of precipitation behaviours to understand the evolution of bond geometries with respect to the crucial overall bond content To this purpose we perform observations via time-lapse video microscopy to obtain real-time monitoring of the evolution of MICP within a Polydimethylsiloxane (PDMS)[37,38] physical model of a microporous medium. Such discrepancies are expected given the limitations in the maximum resolution achieved by the micro-CT equipment
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