Mechanical and Microstructural Changes of Biocemented Sand Subjected to an Acid Solution

An experimental study was performed to investigate the effects of an acid solution on the mechanical strength and the microstructure of biocemented sand. The tests were performed on small triaxial samples extracted from a large-scale model. An acid solution composed of hydrochloric acid and Tris buffer with an initial pH of 6.6 was injected in the triaxial apparatus in different amounts. The changes of the physical and mechanical properties of the sample were studied afterwards. Triaxial drained tests with constant confining pressures were performed on the chemically treated samples in order to determine the remaining strength of the samples. Moreover, several scanning electron microscope (SEM) and X-ray microtomography observations were performed on small subsamples in order to identify the changes in the microstructure due to the chemical dissolution. The experimental results point out that the strength of the treated specimens decreases dramatically compared with that of the initial untreated specimens. Typically, a 50% strength reduction has been found for 10% of total calcite dissolution. Furthermore, the microstructural observations have shown uniform calcite dissolution at the pore scale (no preferential locations). The calcite crystal structures were damaged randomly by the chemical solution. A reduction of the spatial densities and sizes of these crystals were found from SEM and X-ray microtomography observations. Overall, no hysteretic effects were observed on the mechanical (strength) and microstructural (contact surface area) properties between the biocementation and dissolution paths.