Experimental Investigation of the Pressure of Crystallization of Ca(OH)2: Implication for the Reactive-Cracking Process

Carbonation of peridotite may be an important sink in the global carbon cycle. Some natural systems attain 100% carbonation. In these systems, all Mg, Ca and Fe is extracted from silicate minerals to form carbonate minerals, perhaps as a result of reaction-driven cracking processes that maintain or enhance permeability and reactive surface area. Without reaction-driven cracking, in situ mineral carbonation could fill pore space and armor reactive surfaces, limiting reaction progress and leaving much of the silicate unaltered (Kelemen and Hirth, 2012).

The reactive-cracking process will occur if stress resulting from anisotropic volume changes during fluid-rock interactions, i.e., the “crystallization pressure”, is sufficient to fracture rocks. The crystallization pressure is usually described as:

P’ = -ΔGr/ΔVs (1)

where P’ is the crystallization pressure, ΔGr is the Gibbs Free Energy of a reaction, and ΔVs is the change in solid volume resulting from this reaction (Scherer, 2004; Steiger, 2005). Kelemen and Hirth (2012) proposed that use of Helmholtz Free Energy of reaction ΔFr may be preferable, yielding:

P’ = -ΔFr/ΔVs = -ΔGr/ΔVs + PΔVr/ ΔVs (2)

where P is the confining pressure and ΔVr is the volume change of a stoichiometric reaction including fluid components.

However, the conditions most favorable for reaction-driven cracking are poorly understood, especially at confining pressures relevant to CO₂ storage. Fundamental uncertainties remain in estimating the crystallization pressure that drives reaction-driven cracking and, 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 explore crystallization pressure in simple systems under controlled laboratory conditions.

We have performed experiments on reaction (1): solid CaO + H₂O = solid Ca(OH)₂, with a 100% increase in the solid volume and high initial porosities (Φ). Using equation (1), the calculated pressure of crystallization of Ca(OH)₂ is 4.1 GPa. However, equations (1) and (2) do not incorporate energy sinks such as exothermic heating and/or thermal diffusion, implicitly assuming that all chemical potential energy is converted into stress. Thus, both equations yield an upper bound for P’.

We cold-pressed CaO powder to form ~20 mm long cylinders 4 to 12 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 27 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%. Hence, the stress exerted by the crystallization of Ca(OH)₂ is higher than the maximum axial load (27 MPa). 27 MPa is also higher than the tensile strengths of most of rocks, suggesting that, at least at low confining pressure, reactive-cracking can happen with this system. The evolution of ΔV is best fit by a power law. The reaction slows over time, either because of a reduction of permeability or consumption of the reactant. The fact that all experiments show the same slope (ΔV = bt^m with m ~ constant) whether the porosity was decreasing or increasing during the reaction, suggests that the latter explanation is correct. Experiments were stopped at reaction extents from 42 to 100% but all would likely reach 100% at longer durations.

To investigate the conditions leading to reaction-driven cracking in this system, we’ve begun a series of higher pressure experiments in a triaxial deformation apparatus. We will map stress and strain resulting from CaO hydration at geologically relevant combinations of confining pressure (10 to 100 MPa), temperature (20 to 400°C), and volume change. The configuration that we chose is a CaO cylinder embedded in a relatively inert, porous, sandstone (i.e., mainly SiO₂). CaO solid + H₂Ofluid = solid Ca(OH)₂  and CaO solid + CO₂ fluid = solid CaCO₃. Both have a big volume change. The configuration with has already been tested without confining pressure, in an experiment in which the first macroscopic fractures appeared after only 3 min. Additional fractures continued to form during the next 15min, after which the sample fell apart.

References:

Kelemen PB, Hirth G, 2012, Reaction-driven cracking during retrograde metamorphism: Olivine hydration and carbonation: Earth Planet. Sci. Lett., v. 345-348, p. 81–89. Scherer GW, 2004, Stress from crystallization of salt: Cement and Concrete Research, v. 34, p. 1613-1624. Steiger M, 2005, Crystal growth in porous materials – I: The crystallization pressure of large crystals: J. Crystal Growth, v. 282, p. 455-469.