Experimental investigation of the pressure of crystallization

Abstract:

Carbonation of peridotite may be an important sink in the global carbon cycle. In some natural systems, 100% carbonation of rocks is attained via reaction-driven cracking processes. Engineered systems that emulate such processes may provide relatively inexpensive CO₂capture and storage [1,2].

Volume changes during fluid-rock reaction lead to stresses in elastic host rocks, known as the “pressure of crystallization”, that can cause fracture. Cracking can maintain or enhance permeability and reactive surface area in a positive feedback mechanism. So far, experiments on peridotite hydration or carbonation have not produced reactive cracking, possibly due to limited reactive surface area in low porosity samples. To address this, we've begun experiments on reaction (1): solid CaO + H₂O = solid Ca(OH)2, with a 100% increase in the solid volume and high initial porosities (Φ).

We cold-pressed CaO powder to form ~20 mm long cylinders 6 mm in diameter, with initial Φ ranging from 0.36 to 0.53. These cylinders were confined in steel, and compressed with an axial load of 0.1 to 4.2 MPa while water was introduced through a micro-porous frit. Without expansion of the total volume, the reaction would stop when Φ = 0, producing Φ = 0, 2∙Φ∙V_Ca(OH)2+ (1-2∙Φ)∙V_CaO.

Instead, in all experiments the volume of cylinders increased with time, maintaining Φ > 20%. Experiments were stopped at reaction extents from 82 to 100% and would probably reach 100% at longer durations. The pressure of crystallization for reaction (1) is then > 4.2 MPa. Experiments performed on boreholes containing demolition mortar, largely CaO, demonstrate that this pressure is sufficient to break rocks [3].

Reaction (1) is rapid, which allowed us to perform numerous experiments but at higher axial loads Ca(OH)₂ may flow viscously. In the future we plan similar experiments on ground, cold pressed olivine + H₂O + CO₂. At elevated temperatures, reaction progress in unconfined powders is >80% in a few hours [4].

1-Kelemen & Hirth EPSL 12; 2-Kelemen et al AREPS 11; 3-Kelemen et al AGUFM 13; 4-Gadikota et al. Phys Chem Chem Phys 14