Uniaxial compaction creep of wet gypsum aggregates

The uniaxial compaction creep behavior of wet, granular gypsum is investigated under both chemically closed (i.e., drained) and open (i.e., flow through) conditions known to favor pressure solution. The experiments were performed using applied stresses of 0.5 to 2.5 MPa and grain sizes of 32-280 mu m, at room temperature, using pore fluids saturated with respect to unstressed sample material. All wet-tested samples crept rapidly. In contrast, control experiments using dry and oil-saturated samples showed no measurable creep. The microstructures developed in the wet tests provide classical evidence for the operation of grain boundary diffusional pressure solution. To enable detailed comparison with theory, it is demonstrated that despite minor effects of the water of crystallization, conventional pressure solution creep models, for closed systems, are applicable to gypsum. However, the mechanical behavior observed in the closed-system experiments does not fully match these models for either dissolution, diffusion, or precipitation control. Nonetheless, independent crystal growth data suggest that precipitation is most likely to be rate controlling. Additional evidence for this was provided by the experiments with through-flowing solution. In such experiments, precipitation can no longer control the rate of deformation, so that dissolution or diffusion are expected to take over as the rate-limiting process, thus enhancing the creep rate. Indeed, a 10 to 30 times increase in compaction creep rate was observed in the flow-through tests, confirming that creep in the closed-system case probably occurred by precipitation-controlled grain boundary diffusional pressure solution.