How hard do crystals push when growing under confinement? Real-time measurements of surface forces during hydration of periclase to brucite

Interactions between minerals and reactive fluids in porous rocks and building materials often result in crystallization of new minerals. The forces exerted by minerals growing under confinement on the surrounding matrix can be large enough to cause fracturing. Fractures expose new reactive surfaces, leading to progressive disintegration of the material. These processes can result in severe damage to cultural heritage and modern infrastructure, as well as changes in the rheological properties and weathering of natural rocks. Understanding and controlling volume-expanding mineral replacement reactions in pore spaces is thus an important objective to address both societal and geological issues. While crystallization pressures have been measured at larger scales, nanoscale force evolution during confined mineral growth remains poorly constrained.

Here, we investigate volume-expansive mineral reactions in pores spaces by studying the hydration of MgO (periclase) in the Surface Forces Apparatus (SFA). Hydration of MgO to Mg(OH)­₂ (brucite) causes a volume increase to 220%, yielding high crystallization pressures. We use MgO thin films (~90 nm) prepared by atomic layer deposition as reactive surfaces in our experiments, which are performed at the Flow Laboratory, Njord Center, University of Oslo. With the SFA, we measure distance-resolved adhesive and repulsive forces acting between two MgO surfaces under variable external load and how these change over time as the reaction progresses. Preliminary results indicate evolution of forces from strongly adhesive to repulsive during the hydration reaction, likely due to the presence of amorphous, gel-like precursors in the early stages of the reaction. As a reference for the SFA experiments, we study the hydration of isolated MgO surfaces with Atomic Force Microscopy (AFM). This allows us to compare nucleation and growth rates, as well as microstructure and porosity of Mg(OH)­₂ grown in SFA and AFM.