New Inventory Data Reveal How Clinker Substitutes Could Slash Cement Carbon Emissions

Cement is one of the most familiar materials on Earth, and one of the most climate-damaging. Portland cement production is responsible for roughly 7 to 8 percent of anthropogenic carbon dioxide emissions, most of it released during the manufacture of Portland clinker, the dark grey nodules that give cement its strength. Producing clinker requires heating limestone and other raw materials to around 1450 degrees Celsius, a process that liberates CO2 both from the chemical decomposition of limestone, known as calcination, and from the combustion of fossil fuels in the kiln. Replacing some of that clinker with lower-carbon materials, known as clinker substitutes or supplementary cementitious materials, is widely regarded as the cheapest and most immediate route to decarbonising the sector. A major new analysis published in the Journal of Industrial Ecology now offers the most detailed and transparent set of process-based inventory data yet assembled for these substitutes, and its findings could reshape how engineers, policymakers and companies assess the true carbon cost of green cement.

The research, led by Pippa Edwards and Rupert J. Myers of Imperial College London together with Wilson Ricardo Leal da Silva of Fuller Technologies and Paul Fennell, also of Imperial College, addresses a long-standing blind spot in life cycle assessment, or LCA, the standard method for quantifying the environmental impacts of products across their entire life cycle. Although clinker substitution currently displaces about 25 percent of clinker in Portland cements on average worldwide, and could reduce global cement emissions by up to 1.3 gigatonnes per year according to earlier work, the emissions associated with processing the substitute materials themselves have often been ignored, estimated crudely, or hidden inside pre-aggregated emission factors. The new study argues that this omission can overestimate the climate benefits of composite cements and obscure impacts in other categories, such as particulate matter pollution, that matter for human health and ecosystems.

To close this gap, the team conducted a systematic Scopus literature search and screened 188 articles out of 539 results, ultimately assembling 36 sets of inventory data that they disaggregated into 57 distinct unit process datasets. The materials covered include granulated blast furnace slag from ironmaking, coal fly ash from power stations, natural pozzolans, silica fume, calcined clays, limestone, steel slag, bauxite residue, biomass ash, mine tailings, and recycled concrete fines, in both uncarbonated and carbonated forms. For each material, the researchers traced every processing stage between generation or collection and its point of use in cement, capturing inputs and outputs of materials and energy, and converting everything into consistent SI units. Crucially, the data are presented as unit process diagrams, allowing analysts to swap fuel sources, test grid decarbonisation scenarios, or model new technologies, something impossible with the single aggregated numbers offered by many existing databases.

Data quality was assessed using the pedigree matrix approach, which scores reliability, completeness, temporal correlation, geographical correlation and technological correlation on a scale from 1, the best, to 5, the worst. The results reveal sobering weaknesses. Completeness was the lowest-scoring category, and data collection duration or the number of sites surveyed was not even specified in 19 of the 36 inventory datasets, often because the figures were derived from models rather than operational measurements. Roughly 55 percent of the data were more than 15 years old, meaning many studies of common substitutes rely on figures too outdated to meet current environmental product declaration requirements, which demand generic data less than a decade old. Most life cycle assessments of granulated blast furnace slag, for instance, still draw on a single 2003 survey of United States producers, underscoring how thin the evidence base for the world’s most important clinker substitutes really is.

The study also maps where new data are most urgently needed. For calcined clays, one of the most promising emerging substitutes, there are no inventory datasets based on verified facility operating data, despite 14 operational calcined clay plants existing globally as of 2023. Only a single dataset covers production of recycled concrete fines, and single datasets also describe carbonation of those fines and specialty slag processing steps such as metal recovery and slag carbonation. Furthermore, fewer than half of the unit process datasets included elementary flows in the foreground system, meaning direct releases to the environment such as particulate matter or heavy metals. No dataset reported heavy metal flows at all, even though materials like mine tailings, bauxite residue and raw clays are known to contain them, and no calcined clay dataset accounted for CO2 released from carbonates in the raw clay, a potentially significant omission given that some UK clays contain more than 55 percent carbonate by weight.

On the quantitative side, the researchers ran gate-to-gate and cradle-to-gate life cycle assessments using the IPCC 2021 global warming potential method with ecoinvent background data, comparing coal, natural gas and refuse-derived fuel as process heat sources. The results are striking in their clarity. Calcination, the thermal treatment used to activate clays, had the highest carbon footprint of any unit process, with a median of roughly 370 kilograms of CO2-equivalent per tonne of product, driven almost entirely by fuel combustion. Heat-fired drying followed at around 50 kilograms per tonne. Mechanical processes were far gentler on the climate: milling typically fell between 17 and 32 kilograms of CO2-equivalent per tonne using European electricity, while crushing and quarrying were lower still. The message is that grinding a material to boost its reactivity is a far lower-impact strategy than heating it, although the energy demand of size reduction rises steeply as particles get smaller.

When the team compared cradle-to-gate carbon footprints against reactivity measured with the R3 heat release test, calcined clays emerged as the most reactive substitutes but also among the highest in carbon footprint, a tension that fuel switching could ease. Carbonated recycled concrete fines were the only carbon-negative material in the analysis, thanks to the mineralisation of roughly 210 kilograms of CO2 per tonne of fines into stable calcium carbonates, although the researchers caution this represents a best-case scenario that ignores transport emissions and assumes optimistic mineralisation potentials. Calcined mine tailings, by contrast, showed only modest reactivity gains from calcination, suggesting that lower-impact mechanical processing or vitrification may make more environmental sense for these materials. Notably, the carbon footprint of a given unit process proved broadly similar across different materials, meaning the data can serve as reliable proxies for emerging substitutes processed by conventional means.

Perhaps the study’s most consequential finding concerns composite cements defined under the European standard EN 197. When the researchers calculated the carbon footprints of these cements, they found that the degree of clinker substitution, rather than the type of substitute used, was the dominant driver of emissions. Cements with the lowest clinker content, such as limestone calcined clay cements and very low-clinker slag cements, delivered the lowest footprints even when their substitutes carried relatively high processing emissions. For the roadmap targets of the Global Cement and Concrete Association, which envision average clinker contents of 58 percent by 2030 and 52 percent by 2050, clinker itself will remain the overwhelming contributor to cement emissions regardless of which substitute is chosen. The practical implication is that research and investment should prioritise maximising clinker substitution and clinker reactivity, including emerging strategies such as clinker micronisation, rather than agonising over marginal differences between substitute materials.

The study also confronts an uncomfortable supply problem. Slag cements cannot scale to meet global demand of roughly 4100 million tonnes per year because only 330 to 410 million tonnes of granulated blast furnace slag are available annually, a figure that may shrink further as the iron and steel sectors decarbonise and coal power stations close, threatening fly ash supplies too. Moreover, under economic allocation, the high market price of slag, up to 160 pounds per tonne, could add as much as 480 kilograms of CO2 per tonne in allocated upstream burden, a reminder that accounting choices can transform conclusions. By publishing their complete, disaggregated inventory openly, the authors provide a transparent baseline that can verify commercial data, improve the credibility of environmental product declarations, and guide where the industry should invest in data collection. As cement makers race toward 2050 climate targets, this work delivers something the field has quietly lacked for decades: a rigorous, flexible map of where the carbon really lies in the materials meant to save it.

Subject of Research: Process-based life cycle inventory data for Portland clinker substitute materials used in low-carbon composite cements.

Article Title: Toward process-based inventory data for Portland clinker substitutes

Article References: Toward process-based inventory data for Portland clinker substitutes. (n.d.). https://doi.org/10.1007/s44498-026-00186-x

Image Credits: AI Generated

DOI: 10.1007/s44498-026-00186-x

Keywords: clinker substitutes, cement decarbonisation, life cycle assessment, inventory data, calcined clay, supplementary cementitious materials, recycled concrete fines, blast furnace slag, carbon footprint, EN 197, green cement, Journal of Industrial Ecology