The potential of storing carbon dioxide in construction materials through mineralisation: a mathematical modelling study - Thesis

[Geoengineering News]

https://ora.ox.ac.uk/objects/uuid:b8cf0d94-f099-44ea-a501-1855d33f967b

Authors: Liyuan Chen

Abstract

The rapid increase of anthropogenic carbon dioxide (CO₂) emissions continues to pose a critical challenge to achieving the temperature stabilisation targets of the Paris Agreement. While decarbonisation of energy systems and industrial processes is essential, it alone is insufficient to achieve net zero and negative emissions. Carbon dioxide removal (CDR) technologies are therefore necessary to address the historical CO₂ already present in the atmosphere. Among available CDR options, Direct Air Capture (DAC) offers a uniquely scalable approach as it extracts CO₂ directly from ambient air and is not geographically constrained by point source emissions. However, DAC must be paired with secure, durable and cost-effective storage to deliver meaningful climate benefits. Geological storage is effective but raises challenges in terms of site availability, transport infrastructure and public acceptance. The utilisation of CO₂ in construction materials such as cement, concrete, recycled concrete aggregates (RCA) and steel slag provides an alternative pathway that enables long term mineralisation of CO₂, simultaneously valorising industrial by products and promoting a circular economy.

Project 1 consolidates existing global knowledge through a comprehensive meta-analysis of 548 datasets concerning the CO₂ curing of cement and concrete, from 35 studies to enable cross-comparison. The analysis quantitatively identifies the most influential process parameters governing carbon dioxide uptake, with water-to-cement ratio, pre-curing duration, partial pressure, and exposure time identified as the dominant variables. A machine-learning-based ensemble regression model, which combines Random Forest, Gradient Boosting, Support Vector Regression, and Ridge Regression, achieves high predictive accuracy (R² = 0.81), thereby providing a reliable tool for forecasting carbon dioxide uptake under diverse curing conditions. The results establish a quantitative foundation for the optimisation of curing protocols, achieving a predicted carbon dioxide uptake capability in the range of approximately 5-30%. Overall, this project clarifies the variability observed across previous experimental studies, provides a data-driven framework for the optimisation of carbon utilisation in cementitious materials, and establishes a predictive model capable of reliably estimating carbon dioxide uptake performance.

Project 2 evaluates the carbon removal performance of combining DAC with the carbonation of RCA. A process model was developed to examine the effects of key operational parameters on carbonation behaviour and energy use, and this was integrated with a life cycle assessment to determine the overall climate impact under realistic operating conditions. Two representative scenarios were considered: using high-purity CO₂ supplied from an external DAC system, and onsite carbonation with a low-purity CO₂ concentration. The results show that for one tonne of 90% carbonated RCA, the net outcome ranges from a removal of approximately 13 kg CO₂ to a net emission of about 14 kg CO₂, depending on the DAC technology, transport distance, and electricity carbon intensity. When low-purity CO₂ at around 1% concentration is used onsite, the overall carbon removal increases by approximately 70% compared with the use of pure CO₂. Overall, the life cycle assessment confirms that DAC–RCA carbonation can achieve verifiable net CO₂ removal under low-carbon electricity and optimised logistics, providing a quantitative basis for evaluating carbon removal performance in built-environment applications.

Project 3 examines the comparative performance of steel slag carbonation as a carbon removal pathway, focusing on gas–solid and indirect aqueous routes. A process model and life cycle assessment were combined to quantify carbon dioxide uptake, energy demand, and net removal potential under consistent assumptions. The gas–solid route achieved faster reaction rates at elevated temperature and finer particle size, whereas the indirect route exhibited lower reactivity but reduced energy intensity. The life cycle results show that indirect carbonation performs better when the electricity carbon intensity is below 0.192 kg CO₂ per kWh, while gas–solid carbonation is more favourable in higher-intensity grids. Global scenario analysis across 37 countries indicates that approximately 81% can achieve net-negative outcomes, with carbon uptake ranging from about 0.07 to 0.30 kg CO₂ per kg slag depending on process conditions. Overall, the project demonstrates that steel slag carbonation is technically viable and can deliver meaningful net CO₂ removal when deployed within low-carbon energy systems.

Overall, these studies establish a systematic and quantitative framework for evaluating DACU-enabled CO₂ storage in construction materials. They contribute to scientific understanding by harmonising experimental data, building predictive and process models, and integrating techno-environmental assessment with global deployment scenarios. The findings provide actionable insights for researchers, industry stakeholders, and policymakers seeking to design and implement carbon removal strategies that are technically robust, environmentally effective, and economically viable.

Source: University of Oxford