Pore scale characterization of lime-treated sand-bentonite mixtures

Lime treatment of soils is a complex process which combines chemical and mechanical aspects of the soil behavior. The investigation presented here aims at understanding the effect of lime treatment of clayey soils by characterizing their microstructure evolution, along curing time, using X-Ray Micro-Computed Tomography (XRμCT) and Mercury Intrusion Porosimetry (MIP). Binary sand-bentonite mixtures are considered as a model material to simplify the soil microstructure and the diversity of phenomena involved in lime treatment. Samples containing 10%, 15% and 20% of bentonite and, respectively 90%, 85% and 80% of sand have been treated with 1% lime and compacted. Results in XRμCT show first that porosity is present at two scales: micropores within the bentonite aggregates and macropores between sand particles and bentonite aggregates. Micropores are shown to be exclusively saturated with water, while macropores are only full of air. Second, XRμCT images on the same sample at different curing times show the migration of lime enriched aggregates diffusing into bentonite during the first weeks of curing. Third, bentonite is shown to shrink progressively and to form clusters around the sand grains. Consequently, the fraction of macropores increases while the micropore size decreases. On the other hand, through MIP, three pore size categories have been determined: micropores, mesopores and macropores. The evolution in time of the three pore size categories seen in MIP confirms the behavior observed by XRμCT.