The generation and maintenance of excess pore pressures in dehydrating gypsum aggregates were investigated using experiments and microstructural analyses. A triaxial deformation apparatus, equipped with a pore-fluid system connected directly to the dehydrating sample, was operated in constant fluid volume mode to monitor pore pressure increase under undrained conditions, and in constant pore pressure mode to monitor fluid expulsion under drained conditions. X-ray diffraction and backscatter SEM were used to characterize the spatial relationship among gypsum, the product phase bassanite, and the pores. In addition, we measured the permeability and pore compressibility of the starting material, and explored the influence of effective and pore pressures, temperature and axial load on fluid expulsion. Three stages of fluid expulsion and microstructural evolution during dehydration of an initially low porosity, low permeability gypsum aggregate are defined: 1) Initially fluid is trapped in the sample and high pore pressures are possible because porosity is isolated or in discontinuous networks. 2) An interconnected pore network eventually develops and fluid readily escapes. 3) Fluid expulsion slows down drastically as the reaction nears completion. As a result of coupling between dehydration and porosity production, both the cumulative volume of fluid expelled and the expulsion rate increase with increasing temperature, effective pressure, and axial load and with decreasing pore pressure. Our hydrological and microstructural data, combined with previous mechanical data, provide a better understanding of the relationships among changes in fluid volume and sample porosity, pore pressure excess and the deformation behavior of a dehydrating system where drainage evolves with time.
AGU Index Terms: 5114 Permeability and porosity; 3660 Metamorphic petrology; 8159 Rheology-crust and lithosphere
Keywords/Free Terms: Pore pressure excess, dehydration, experimental studies.
JGR-Solid Earth 96JB02485
Vol. 102
, No. B1
, p. 825