Abstract The volume-increase associated with hydration reactions in rocks may lead to reaction-induced fracturing, but requires a stable water film to be present at reactive grain boundaries even when subject to compressive stress. Hydration of periclase to brucite is associated with a solid volume increase of ca. 110 %. Recent experiments on the periclase-brucite system observed that when the effective mean stress exceeds 30 MPa, the reaction rate slows down dramatically. We hypothesize that for the brucite forming reaction to progress, the fluid film between grains must remain stable. If the applied pressure becomes larger than the hydration force, the fluid film will collapse and be squeezed out of the grain contacts. To quantify this effect, we study the behavior of a water film confined between periclase or brucite surfaces subject to compressive stress, by performing molecular dynamics simulations. The simulations are carried out using the ClayFF force field and the single point charge (SPC) water model in the molecular dynamics simulations program LAMMPS. The setup consists of two interfaces of either periclase or brucite surrounded by water. Our simulations show that when the pressure reaches a few tens of MPa, the water film collapses and reduces the water film to one or two water layers, while the self-diffusion coefficient of water molecules by a factor of eight. A water film thickness below two water layers is thinner than the size of the hydration shell around Mg2+-ions, which will limit ion-transport. The observed collapse of the water film to a single layer at a normal pressure of 25-30 MPa might explain the observed slow-down of reaction-induced fracturing in the periclase-brucite system.

中文翻译：

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反应压裂条件下方镁石-水镁石体系承压水分子动力学研究

摘要 岩石中与水化反应相关的体积增加可能导致反应诱导破裂，但即使受到压应力，也需要在反应性晶界处存在稳定的水膜。方镁石水合为水镁石与固体体积增加约。110%。最近对方镁石-水镁石体系的实验观察到，当有效平均应力超过 30 MPa 时，反应速度显着减慢。我们假设，要使水镁石形成反应进行，晶粒之间的流体膜必须保持稳定。如果施加的压力变得大于水合力，流体膜将坍塌并被挤出颗粒接触。为了量化这种影响，我们通过执行分子动力学模拟研究了受压应力影响的方镁石或水镁石表面之间水膜的行为。使用分子动力学模拟程序 LAMMPS 中的 ClayFF 力场和单点电荷 (SPC) 水模型进行模拟。该装置由两个被水包围的方镁石或水镁石界面组成。我们的模拟表明，当压力达到几十 MPa 时，水膜坍塌，水膜减少到一到两层水层，而水分子的自扩散系数增加了 8 倍。两个水层以下的水膜厚度比围绕 Mg2+ 离子的水合壳的尺寸更薄，这将限制离子传输。