Hisham Hafez

This paper by Working Group 1 of RILEM TC 309-MCP presents a critical review of life cycle assessment (LCA) studies on carbonated recycled concrete aggregates (CRCA). Carbonated products can offer significant environmental benefits, particularly by permanently storing CO ₂ . However, these environmental benefits must be assessed through a comprehensive LCA. A critical analysis of 16 peer-reviewed LCA studies on CRCA reveals several methodological inconsistencies: varied assumptions regarding system boundaries and handling multifunctionality; different end-of-waste criteria and attribution of environmental burdens and benefits among co-products; limited disclosure of key information, including, e.g., technical characteristics of CRCA products, CO ₂ uptake and CO ₂ sources; and unclear interpretation of negative impact results, without proper distinction between avoided emissions and carbon removals. Overall, the Global Warming Potential (GWP) results for unbound CRCA vary between − 48 and 14 kg CO ₂ e/t CRCA within the analysed studies. These findings underscore the need for harmonised LCA methodologies and transparent reporting to enhance the reliability of LCA studies. A set of methodological recommendations is proposed to enhance LCA consistency, aligned with general LCA guidance and best practices, while being tailored to the specific context of carbonated products. The recommendations cover system boundary definition, multifunctionality handling, modelling of CO ₂ flows, reporting of life cycle inventory information, and interpretation of LCA results. This work aims to support both researchers and practitioners in conducting more accurate and comparable LCAs on CRCA and other mineral carbonation products, ultimately contributing to the wider adoption of enforced carbonation within the construction sector.

Meeting the growing demand for urban housing is a global sustainability challenge. Numerous studies have concluded that bio-based materials are lower carbon options compared to concrete while disregarding resource availability and harvesting emissions. This study evaluates low-carbon concrete in comparison to engineered bio-based materials and stabilized earth blocks for three issues: embodied carbon, material supply limitations and production scalability. Bio-based materials could only supply 2(eq.) achievable versus benchmark concrete housing from 2025-2050. Furthermore, engineered biomaterial production would need to scale up faster than low-carbon concrete, while also overcoming greater logistical and social barriers. To meet urban housing needs, low-carbon concrete is not perfect but is the lowest-carbon option that can be scaled-up to meet global demand by 2050.

In the face of two apparently irreconcilable global challenges - housing a growing world population and reducing CO ₂ emissions - we analyse the current, historic and forecast data on the use of construction materials. Today, cement-based materials make up around three quarters of materials used by mass. Historically, we see that cement-based materials use goes through a peak as Gross Domestic Product per capita increases and then falls. This peak of cement use has been particularly pronounced in China, but is now on a downwards path. From now to 2050, three quarters of construction materials demand will be in low- and middle-income countries. We estimate that adopting the best available construction technologies could reduce CO ₂ emissions by about 73% compared to business as usual by 2050. In low- and middle-income countries, the housing and infrastructure needed to achieve the Sustainable Development Goals could be supplied while simultaneously reducing their per capita CO ₂ emissions from structural materials.

Cement production is a major contributor to global emissions, representing 6% of CO 2 energy-related emissions. The existing literature evaluates several interventions in production pathways to reduce energy consumption and carbon emissions. However, a knowledge gap exists regarding the influence of raw material chemical composition on the energy and carbon emissions requirements for clinker production. To address this gap, we have developed an open-source model that estimates the minimum energy requirements for clinker production based on 3738 raw material compositions. The model considers the chemical composition of the input materials and the relevant chemical reactions to calculate theoretical and practical minimum energy requirements, where the theoretical minimum includes the enthalpy of reactions, and the practical minimum also accounts for the energy needed to heat the raw meal. Our findings show that the theoretical minimum energy required for clinker production ranges from 1.60 MJ kg⁻¹ to 1.76 MJ kg⁻¹, while practical minimum energy requirements range between 2.95 MJ kg⁻¹ to 3.11 MJ kg⁻¹. Potential improvements could reduce practical minimums to a range of 1.85 MJ kg⁻¹ to 2.5 MJ kg⁻¹. This study establishes a baseline for potential energy savings and emissions reduction by targeting a more resource-efficient clinker composition.

The foundation industries in the UK were responsible for emitting 42 Mt CO_2eq in 2020, which is approximately 10% of the yearly territorial greenhouse gas emissions. The UK government decarbonisation roadmap issued in 2015 predicted that high-tech strategies such as carbon capture and utilisation, hydrogen and biofuels, as well as electrification of processes are key for achieving the climate mitigating targets by 2050. In this study, a critical assessment was performed on the limitations to achieve these high-tech strategies such as biomass availability, capital investment, and technology readiness. The study is the first to use the UK carbon budget values as the resource limit for the high-tech decarbonisation strategies. The findings show that the significant uncertainty associated with the high-tech scenarios limits their decarbonisation potential by 2050. More importantly, to stay within the mid-century carbon budget for the foundation industries, 20–40% reduction in production, through circular economic strategies such as material efficiency and/or changes in product specifications, is required in order to achieve the decarbonisation targets.