Once CO₂ is captured and concentrated it needs to be safely sequestered in geologic reservoirs over geologic time scales.  The traditional way of doing this is by injection into deep saline aquifers or in permeable, former hydrocarbon reservoirs; however, there are geographic and geologic barriers to doing this in eastern Pennsylvania.  Here we will investigate a relatively novel solution consisting of direct back-to-rock CO₂ mineralization, using Ca- and Mg-rich waste rock from the aggregate mining industry. This sequestration option, which takes advantage of the natural silicate weathering cycle, has the potential to remove MtCO₂/yr from the atmosphere, a flux that scales well with the current emissions of the Pennsylvania cement industry.  Although Pennsylvania has the correct rock mineralogy and distribution to justify our enthusiasm (Fig. 1a), significant knowledge gaps in reaction rates, and comparative texture and mineralogy of the waste stream mineralizing the carbon remain.  Research projects need to specifically target the reaction rate problem, focusing on the grain size (reaction surface area) and mineralogy of the aggregate waste stream, with an eye on favorable energy conversion processes that may lead to desirable by-products (Fig. 1b).

We have reason to be optimistic that diabase and serpentinite waste generated by the aggregate industry in eastern Pennsylvania is an excellent material for mineralizing CO₂.  The chemistry involves Mg and Ca-rich silicate minerals.  Consider the  silicate mineral diopside, which occurs in diabase aggregate waste at ~30 wt%, to illustrate the process:

                    MgCaSi2O₆ + 2CO₂ + 4H₂O = MgCO₃ (solid)+ CaCO₃ (solid)+ 2H₄SiO₄ (aqueous)    (3)

For every 2 moles of CO₂ (44 g/mole; 88 g), the dissolution of diopside consumes 1 mole of pyroxene (216 g/mole).  That is one needs 216 g of rock waste (assuming it was pure diopside) to neutralize 88 g of CO₂, or ~2.5 g of rock for every 1 g of CO₂.  For example, the Pennsylvania cement industry is emitting ~1.8 MtCO₂/yr, which translates to ~5,000,000 m³ of rock at 30 wt%, diopside to effectively mineralize all of this carbon.  However, diopside is paired with other silicate minerals like labradorite feldspar, which also dissolves congruently and consumes CO₂, so the necessary rock waste volumes would be less. Furthermore, even considering the modest known reaction rates of ~10^-11 moles/m²/s, the dissolution of the Mg- or Ca-rich silicate proceeds over time frames of years to tens of years.  The resulting bicarbonate ion remains in the Earth surface system for decades to centuries, and the final natural mineralization takes place over centuries.  All of these are time frames commensurate with what is necessary to remove anthropogenic  CO₂ from the atmosphere.

Figure 1. (a) Geologic map of east-central and southeastern PA showing the close geographic distribution of the cement industry to the north of Lehigh, and the location of the rocks south of Lehigh where aggregate waste capable of mineralizing the CO₂ are located.  (b) Summary diagram showing the production of favorable by-products possible in the CO₂ mineralization process from: Gadikota, G., 2021, Carbon mineralization pathways for carbon capture, storage, and utilization: Nature Communications Chemistry, 4-23, doi.org/10.1038/s42004-021-00461-x.