Competition Between Crystallization-Induced Expansion and Creep Compaction During Gypsum Formation, and Implications for Serpentinization

Deformation caused by reaction-driven volume increases is an important process in many geological settings. The interaction of rocks with reactive fluids can change permeability and reactive surface area, leading to a large variety of feedbacks. Gypsum (CaSO(4)2H(2)O) is an ideal material to study these processes. It forms rapidly at room temperature via bassanite (CaSO4[1/2]H2O) hydration and is commonly used as an analog for rocks in high-temperature, high-pressure conditions. We conducted uniaxial deformation experiments on porous bassanite aggregates to study the effects of applied axial load sigma(a) on deformation during the formation of gypsum. While hydration of bassanite involves a solid volume increase, gypsum exhibits significant creep compaction when in contact with water. These two processes occur simultaneously. Samples exhibited an initial phase of deformation followed by a slower secondary phase. A particular value of sigma(a) separates expansion from compaction during each of the deformation phases. At sigma(a)150kPa, samples expanded initially; for sigma(a)230kPa, samples compacted initially. Up to sigma(a)approximate to 3.2MPa, samples expanded after compacting initially, while for sigma(a)3.6MPa, no further deformation or continued compaction occurred. This behavior implies that crystallization-induced stresses depend on porosity and reaction extent such that larger stresses cannot be generated by the reaction. We explain aspects of the observed behavior with a model that predicts strain evolution using kinetic relationships for the reaction and creep rates and consider the implications of our results for reaction-induced fracturing during serpentinization.