Principal Investigator
At a Glance
Research conducted by the Sustainable Cements Group promises to advance an understanding of silicate dissolution, a key process involved in several mitigation strategies aimed at reducing industrial CO2 emissions. Postdoc Bastien Wild, part of the White group, is working to understand how silicates (SiO2 -rich solids) dissolve. In particular, he is focused on unravelling the properties of fluid-silicate interfaces across length scales and linking these properties to macroscopic dissolution behavior (i.e., how the solids decompose in aqueous environments). An important, but counterintuitive, finding is that silica-rich layers containing nano-sized pores are seen to form at mineral surfaces when exposed to acidic conditions. This implies that the dissolution rate of the silicate mineral should be much reduced due to the presence of these layers. Yet, unexpectedly high dissolution rates are observed for these systems.
Research Highlight
Scientists are exploring a range of mitigation strategies to reduce industrial CO2 emissions. These include the development and implementation of “green” cements in the construction industry, carbon capture, utilization and storage (CCUS), and the design of long-term repositories for safe storage of radioactive waste. The dissolution of silicates plays an important role in all these specific mitigation strategies, yet the fundamental mechanism(s) controlling silicate dissolution is still being debated.
CMI funding is supporting experiments directed at uncovering the interfacial textural and transport properties for three specific silicate types: alkali-activated slag (AAS) cement, feldspar, and international simple glass (ISG). This project seeks to understand how the formation of amorphous silicarich layers at fluid-solid interfaces, which are seen to exist during dissolution of silicate solids under certain conditions, affects overall dissolution rates at the macroscopic level (Figure 5A-E). Researchers have characterized material properties at several length scales. This includes research at the micron length scale to assess the microstructure of complex materials such as cement or rock. Researchers are also investigating sub-micron length scale to access quantitative data on the thickness and porosity (void space) of these amorphous silica-rich layers.
(A-C) Select interfaces studied in this work, as indicated by the arrows (the blue arrow is on the fluid side of the interface). (A) Depleted coating formed on a sustainable cement sample, (B) diffusive interface formed by sulfate attack of the cement sample, and (C) sub-micrometer thick amorphous silica-rich layer formed on a labradorite feldspar single crystal. Note that the cracks present in (B) are due to the dehydration of the layer, and the change in grayscale (composition contrast) is due to element depletion at the interface. (D) In situ X-ray reflectivity analysis of labradorite feldspar, where oscillations indicate an average thickness of 16 nm for the amorphous silica-rich layer. (E) Amorphous silica nanopore used in MD modeling of amorphous silica-rich layer properties.
Researchers used time-resolved measurements of cation release into solution to capture the dissolution rates of AAS, feldspar and ISG. They showed that the silica-rich layers that formed on AAS samples control how easily cations migrate to the bulk solution, which in turn affects the measured dissolution rate. This is consistent with the distribution of elements in the vicinity of the layers, as measured by electron microscopy. However, for single crystal samples of labradorite (a silicate mineral), the silica-rich layers that were seen to form during exposure to acidic conditions did not influence the overall dissolution rate. This occurred despite these layers containing only very small nanosized pores (as opposed to larger pores). These unexpected findings are consistent with percolation experiments that revealed high transport properties for the corresponding silica-rich layers, even for very thick layers (> 10 µm) that were seen to form over several months of reaction.
The researchers are currently developing and testing hypotheses that explain these novel findings, which differ from current state-of-the-art knowledge of the transport properties of silica-rich layers. In particular, the researchers are exploring the spontaneous generation of dense regions within the porous network of the silica-rich layer, and the convective transport of ions caused by large concentration gradients at the interface.
Overall, this work is advancing our knowledge of the chemical and physical properties of fluid-solid silicate interfaces that are found in negative carbon emission applications involving silicate dissolution, (e.g., enhanced weathering), the manipulation of construction materials, and safe long-term storage of radioactive waste.