Unraveling the Atomic-Scale Surface Chemistry of Wollastonite (CaSiO₃)

Abstract

Wollastonite (CaSiO3) is a mineral that inherently contains calcium and silica, the essential components of Portland cement. Traditional cement production involves the decomposition of limestone (CaCO3), an industrial process that emits significant amounts of CO2 into the atmosphere [1]. In contrast, wollastonite spontaneously reacts with atmospheric CO2, offering the potential for carbon sequestration by its use as a new-generation cement [2]. To deepen the current understanding of such reactions, we investigated the atomic-scale details of wollastonite surfaces and their interaction with H2O and CO2 molecules. Using a combination of atomically resolved non-contact atomic force microscopy (AFM) in ultrahigh vacuum (UHV), ab-initio density functional theory (DFT) calculations, and AFM simulations [3], we reveal the detailed atomic structure of its most stable (100) termination and its surface chemistry. The cleaved surface exposes calcium atoms and silica tetrahedra. These act as preferential adsorption sites for water molecules that adsorb molecularly and form a rectangular pattern. According to our DFT calculations, the first H2O molecule per unit cell adsorbs without a barrier in a novel configuration not reported in previous literature [4], and it establishes the active site framework essential for subsequent CO2 adsorption. Our work provides insights into the hydration process of wollastonite and how CO2 binds to the surface, forming stable carbonates.