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

The research, conducted by Fatih Kantarci and Moncef L. Nehdi, centers on geopolymer concrete, an alkali-activated alternative to Portland cement concrete that is synthesized from industrial by-products rich in aluminum and silicon, such as blast furnace slag, metakaolin, and fly ash. When these precursors are mixed with highly alkaline solutions like sodium hydroxide, a chemical process called geopolymerization forms three-dimensional Si-O-Al-O polymeric gels that bind aggregates into a solid mass. Depending on the precursor and activator chosen, geopolymer binders can cut CO2 emissions by up to 80 percent compared with Portland cement, while saving roughly 60 percent of the energy and reducing production costs by about 25 percent. Geopolymer concretes have already found their way into road pavements, precast elements, and fire-resistant construction in the United States, Australia, Europe, and India.

Like most cementitious materials, however, geopolymer concrete is inherently brittle and prone to cracking under moderate loads or shrinkage stresses. The established remedy is fiber reinforcement, which enhances crack resistance, tensile strength, ductility, and impact performance while redistributing stresses within the matrix. Among the many fiber types available, the researchers selected glass fiber for its affordability, ease of manufacture, corrosion resistance, and high tensile strength of 1300 megapascals. The glass fibers used in the study were just 6 millimeters long and 15 micrometers in diameter, with an elastic modulus of 72 gigapascals and, crucially, a melting point of approximately 850 degrees Celsius, meaning they retain structural stability throughout the temperature range examined.

The experimental program began with a careful optimization of the geopolymer mix itself. Blast furnace slag from a local plant, with a specific gravity of 2.84 and a cement-like fineness, served as the primary precursor at a dosage of 400 kilograms per cubic meter. The team varied the sodium hydroxide activator concentration across 10, 12, and 14 molar solutions and tested alkali activator solution-to-binder ratios of 0.50 and 0.60. Compressive strength measurements at 7, 28, and 90 days revealed a clear optimum: strength increased as the sodium hydroxide concentration rose to 12 molar, then declined at 14 molar. The researchers attribute the initial gain to higher alkalinity, which dissolves silicon and aluminum links in the raw precursor to form aluminosilicate gels, while the decline at 14 molar reflects inhibited condensation reactions of silicate species and the precipitation of geopolymer gels that ultimately weaken the matrix. Scanning electron microscopy confirmed the story, showing a dense, compact microstructure with low porosity in the strongest mixes and abundant large pores and cracks in the weakest.

With the optimum production parameters established at 12 molar sodium hydroxide and a 0.60 activator-to-binder ratio, the team incorporated glass fibers at volume fractions of 0.3, 0.6, and 0.9 percent. Notably, the concrete was cured entirely under ambient laboratory conditions at 23 degrees Celsius and 55 percent relative humidity, with no steam or heat curing, removing a major barrier to casting geopolymer concrete on real construction sites. After 90 days of curing, the specimens were exposed to temperatures of 150, 300, 450, 600, and 750 degrees Celsius for one hour in a furnace heated at roughly 2 degrees Celsius per minute, then cooled under two contrasting regimes: gradual air cooling inside the opened furnace, or rapid immersion in room-temperature water, simulating the thermal shock that firefighting operations inflict on burning structures.

The results reveal a nuanced interplay between fiber content, temperature, and cooling method. Glass fibers improved compressive strength in both heated and unheated specimens, with the optimum at 0.6 percent by volume. At this dosage, fibers wrapped in geopolymer gel bond strongly to the matrix, bridging cracks, reducing stress concentrations at crack tips, and retarding crack propagation. The residual compressive strength of the fiber-reinforced samples actually increased up to 150 or 300 degrees Celsius, a phenomenon attributed to polycondensation and further densification of the tetrahedral aluminosilicate gels as moisture evaporates, before declining at higher temperatures. Remarkably, after exposure to 750 degrees Celsius, the water-cooled specimen containing 0.6 percent glass fiber retained a compressive strength approximately 33 percent higher than the plain, fiber-free samples. Beyond 450 degrees Celsius, however, the mismatch in thermal expansion coefficients between glass fibers and the geopolymer matrix generated interfacial stresses and microcracks, while partial softening of the fibers, dehydration of the gels, and thermal phase transformations further eroded strength.

The cooling regime proved to be a decisive variable. Water-cooled samples consistently exhibited lower residual compressive and flexural strengths than their air-cooled counterparts, because the steep temperature gradients during rapid quenching induce thermal shock, microstructural damage, and an elevated risk of explosive spalling. Flexural strength, which is particularly sensitive to crack initiation and propagation, benefited even more visibly from fiber reinforcement, since the three-dimensionally dispersed fibers direct crack paths and transfer stresses through a bridging effect that preserves specimen integrity. At all temperatures, the 0.6 percent fiber content delivered the highest flexural values, and the relative improvement from fiber addition was more pronounced in flexure than in compression, underscoring the dominant role of crack bridging in bending behavior.

Complementary measurements of weight loss and water absorption traced the progressive thermal deterioration of the material. Weight losses remained modest at 150 and 300 degrees Celsius, driven by the evaporation of free and absorbed water, but increased sharply after 450 degrees Celsius as thermal stress generated microcracks, and again at 750 degrees Celsius, where thermo-chemical damage degraded the geopolymer gel itself. The fiber-free air-cooled specimen lost 1.3 percent of its mass at 150 degrees Celsius but 7.1 percent at 750 degrees Celsius, roughly a five-and-a-half-fold increase, while 0.3 and 0.6 percent fiber additions reduced these losses by preserving microstructural integrity. Water absorption told a parallel story: values stayed nearly unchanged up to 450 degrees Celsius thanks to the dense matrix, then climbed as thermally induced shrinkage and thermal-shock microcracking opened new transport pathways. The fiber-free water-cooled sample doubled its water absorption from 4.3 to 8.6 percent after exposure to 750 degrees Celsius. Interestingly, the highest fiber dosage of 0.9 percent proved counterproductive, increasing water absorption because of poor workability, uneven fiber dispersion, and fiber balling, a reminder that more fiber is not always better.

Visual and microstructural examinations completed the picture. Sample surfaces brightened to a light brown up to 600 degrees Celsius and darkened to brown-black at 750 degrees Celsius, a coloration attributed to the gehlenite phase identified by X-ray diffraction, which also detected calcium silicate, calcium oxide, akermanite, and ilvaite. Crucially, the glass fibers did not melt even at 750 degrees Celsius, and no specimen fragmented, chipped, or disintegrated under either cooling regime, although water-cooled samples displayed more surface cracks. Scanning electron microscopy after 750 degrees Celsius showed that the fibrous air-cooled sample retained a dense, compact microstructure, while the non-fibrous water-cooled sample exhibited large cracks and spherical pores, with fiber-matrix debonding and increased microcrack density explaining the measured strength losses.

The study’s conclusions carry practical weight for the construction industry’s decarbonization ambitions. An ambient-cured, slag-based geopolymer concrete reinforced with 0.6 percent glass fiber emerges as a promising candidate for fire-resilient structural applications, provided that workability and fiber dispersion are carefully controlled. The findings also deliver a clear warning for fire engineering: the way a structure cools after a fire matters nearly as much as the fire itself, with rapid water quenching inflicting measurable thermal-shock damage that gradual air cooling avoids. The authors point toward future research on long-term durability under diverse service environments and on extending the approach to other fiber types and cooling scenarios, steps that could help carry geopolymer composites from the laboratory into the load-bearing skeleton of sustainable cities.

Subject of Research: Elevated-temperature resistance of glass fiber-reinforced geopolymer concrete under different cooling regimes

Article Title: Elevated-temperature resistance of glass fiber-reinforced geopolymer concrete under different cooling regimes

Article References: Kantarci, F., & Nehdi, M. L. (2026). Elevated-temperature resistance of glass fiber-reinforced geopolymer concrete under different cooling regimes. Cleaner Engineering and Technology, 34, Article 101311. https://doi.org/10.1016/j.clet.2026.101311

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101311

Keywords: geopolymer concrete, glass fiber, elevated temperature, fire resistance, cooling regime, thermal shock, blast furnace slag, compressive strength, flexural strength, sustainable construction, alkali-activated materials, microstructure