Pore Structure and Permeability of Alkali-Activated Metakaolin Cements with Reduced CO₂ Emissions

Principal Investigator

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At a Glance

Portland cement is currently the most common type of cement used in concrete manufacture, but it is a significant source of atmospheric CO₂ due to the production process. To counter this, White and her group, including graduate student Anita Zhang, are developing sustainable cements that are alternatives to conventional Portland cement. These cements can reduce CO₂ emissions but with limited in-field evidence of proven long-term performance. By understanding the pore structures of these alternative cements, and linking pore structure to permeability, the researchers aim to create a predictive phenomenological model that can be used to identify the most suitable alternative cement for a specific environmental application. Reducing concrete emissions in the construction industry would have a large impact on overall CO₂ emissions, which aligns with bp’s ambition of helping the world get to net-zero.

 

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Research Highlight

Hard-to-decarbonize industries such as cement face the daunting task of lowering their CO₂ emissions while maintaining product quality and performance. This is particularly challenging for cement and steel, where any change to the chemistry of the material can have long-term, significant ramifications on performance and safety. After water, concrete is the second most consumed resource in the world and is essential to modern infrastructure. However, one key ingredient of concrete, Portland cement powder, currently accounts for approximately 8% of global anthropogenic CO₂ emissions. Alternative cements can be more sustainable options for the production of concrete, thus avoiding a significant portion of CO₂ emissions in the industry. However, our ability to predict long-term performance of these more sustainable materials is severely hampered by the time it takes to obtain pore structure data of the binder material that controls ingress of harmful chemicals such as CO₂ and sulfate (SO₄^2-) and chlorine (Cl^–) ions.

White and her group are utilizing key pore size characterization techniques and beam-bending to investigate the pore structure and permeability of alkali-activated metakaolin cements. Their aim is to further reduce CO₂ emissions without adversely impacting long-term performance (i.e., retain low permeability). In addition to linking changes in permeability with the evolution of nano-sized pores over time, they have also explored a unique approach for lowering activator concentrations (and thus CO₂ emissions). This involves using a small amount of calcium hydroxide to help offset reduced performance at lower activator concentrations. A preliminary life cycle assessment of the CO_2-eq emissions has shown reduced emissions for these novel systems while lower permeability values show the beneficial effects of the calcium hydroxide addition on long-term performance.

Ongoing research is focused on utilizing rapid, nondestructive small-angle X-ray scattering (SAXS) characterization to obtain nanopore structural information. This information will be used to compute the susceptibility of a concrete system to diffusion-controlled degradation processes (i.e., permeability). By connecting SAXS-derived pore structure attributes with intrinsic permeability data for a range of sustainable cement chemistries, White’s research aims to predict permeability of future systems from relatively quick and non-destructive measurements. This stands in contrast to the destructive and cumbersome testing methods currently used for pore structure characterization.