High-resolution microstructural study of calcite crystals precipitated through bio-cementation under different conditions

Bio-cementation is a new, environmentally-friendly soil-reinforcement process. It is used for civil engineering purposes, such as the fabrication of construction materials, as well as the preservation of monuments. This process uses bacterial activity, mainly that of Sporosarcina Pasteurii, that is capable of  hydrolysing the urea present in the medium, leading to the precipitation of CaCO₃ (calcite) crystals between sand grains, therefore binding them together, and reinforcing the soil. The macro and micro (contact scale) mechanical properties of bio-cememted sand have been extensively studied. However, the microstructure of the precipitated calcite crystals remains undiscovered, which induces mechanical differences under different conditions of cementation. The goal of this study is to investigate the microstructure of biogenic calcite, issued from bio-cementation of sand, and how it varies under different cementation conditions. For this, high resolution synchrotron diffraction imaging at the ESRF was performed, utilizing scanning 3DXRD (s-3DXRD) on ID11 and Dark-Field X-ray Microscopy (DFXM) on ID03. For this, the main experiment was performed on three samples that consist of 3D printed resin cells in which cementation was performed under different conditions, by varying the substrate on which the calcite was grown (between sand grains and glass beads), as well as varying the salinity of the medium. After each cementation cycle, and for each sample, layer measurements were acquired using s-3DXRD. A significant difference was observed between the sand and glass bead cases: the precipitated crystals on the glass beads were much smaller than those precipitated on the sand grains. DFXM measurements showed defects that are only present in the case of high concentration of NaCl in the medium, which could potentially alter the mechanical properties of the material. These two complementary techniques allowed for an in-depth study of the microstructure of the precipitated calcite crystals.