Oedometric Small-Angle Neutron Scattering: In Situ Observation of Nanopore Structure During Bentonite Consolidation...

Results of oedometric consolidation experiments linked with small-angle neutron scattering (SANS) measurements are presented, using SWy-2 Wyoming bentonite clay in dry and water-bearing N₂ and CO₂ atmospheres. Oedometric SANS involves deforming a porous sample under uniaxial strain conditions with applied axial force and internal pore pressure control, and combines with SANS for in situ observation of pore structure evolution and interaction. Scattering from both interlayer (clay intra-aggregate) and free (interaggregate) pores is observed, showing decreasing pore size with dry consolidation and interactions between interlayer and free pore types with swelling and consolidation. Introduction of dry liquid CO₂ at zero effective stress (axial stress minus pore pressure) produces large shifts in interlayer scatterers, but is reversible back to pre-CO₂ levels upon decreasing pore pressure and increasing effective stress. Introduction of wet liquid CO₂, conversely, produces large but irreversible changes in interlayer scatterers, which are interpreted to be the combined result of CO₂ and H₂O intercalation under hydrostatic conditions, but which diminish with application of effective pressure and consolidation to higher bentonite dry densities. Consideration of CO₂ intercalation in smectite-bearing CO₂ caprocks needs to include effects of both water and nonhydrostatic stress.