Dry CO2 exposure of rock flours: Implications for enhanced rock weathering in moisture limited environments?

Screening of geomaterials for the suitability of enhanced rock weathering (ERW) typically relies on cation contents (Ca + Mg) to assess carbon dioxide (CO₂) sequestration potential, but the applicability of this approach has not been tested under the water limited conditions prevalent across much of Australia. We exposed a range of commercial and quarry derived (ultra)mafic rock flours to pure CO₂ in closed laboratory experiments for 14 days to quantify mineral carbonation in a low moisture environment. These experiments were designed to explore the reactivity of different materials in a controlled environment under extreme CO₂ concentrations with limited moisture availability. Whilst the experiments do not represent a realistic ERW scenario, they do provide preliminary observations for the potential reactivity of minerals with near-pure CO 2 under extremely moisture limited conditions. Controlling factors of mineral carbonation at surface conditions are generally regarded as suitable pH, chemical composition, moisture and surface area, with relatively little regard for the specific mineralogical partitioning of reactive phases. Prior to exposure, rock flours were characterised using x-ray diffraction, inductively coupled plasma mass- and optical emission spectrometry, as well as moisture and pH measurements. Gravimetry and Fourier-transform infrared spectroscopy (FTIR), were utilised to quantify CO 2 sorption and verify the formation of carbonates. Whilst the mass gain after CO₂ exposure was small, it varied eight-fold (0.2-1.6 wt.%) for the various rock flours and increased to nine-fold with the inclusion of a wollastonite reference sample (1.8 wt.% mass gain). Commercial products showed strong correlations between mass gain post CO 2 exposure and Mg, Fe, and Ca concentrations (r = 0.97; 0.90; 0.87, p = 0.001 - 0.023), as well as with ERW amenable mineral phases such as augite (r = 0.98, p = 0.003) and pyroxenes (r = 0.90, p = 0.017). Although the largest mass gain of the experimental suite was recorded on quarry derived materials, no compositional correlations were observed for these samples. Our results show that elemental composition and mineralogy alone do not fully explain potential carbonation of the experimental materials in moisture limited systems. Other factors, such as the mineral crystallinity (amorphous versus crystalline phases) along with surface properties may need to be considered. These findings indicate that while chemistry and mineralogy are essential for understanding the CO 2 uptake potential of different mineral amendments, other factors may play a role in determining the rate at which this process happens.