Baked or Beaten? Australia’s Waste Clays Reveal a Hidden Trade-Off in Green Cement

Concrete is everywhere, and so is its carbon problem. Portland cement, the glue that holds the modern built environment together, is responsible for roughly 5 to 7 percent of global carbon dioxide emissions, a footprint driven by the ferocious heat needed to decompose limestone and fuse clinker in roaring kilns. In Australia the situation is particularly stark: 97 percent of the country’s clinker production runs on coal and gas, churning out approximately 975 kilograms of CO2 for every tonne of clinker. More than half of those emissions come from the chemistry of limestone calcination itself, and about a quarter from the energy that feeds the process. As fly ash and slag from heavy industry become scarcer, researchers are racing to find abundant, locally available substitutes, and one of the most promising candidates is sitting underfoot: ordinary, low-grade clay.

A new study from RMIT University, published in Environmental Science and Pollution Research, has put five Australian natural waste clays through one of the most rigorous environmental audits yet attempted for cement alternatives. Led by Ishan Basnayaka and Chamila Gunasekara, the team did something that earlier studies rarely bothered to do: they normalized every environmental impact to the actual compressive strength each binder achieved, measuring impacts per megapascal rather than simply per kilogram of material. This performance-normalized life cycle assessment matters because a green binder that crumbles under load is not green at all. A cement substitute must deliver both lower emissions and adequate mechanical performance, and only by dividing environmental burden by strength can researchers make a genuinely fair comparison between materials that behave very differently in a testing machine.

The five clays were chosen to represent the messy reality of geology outside the laboratory. Rather than relying on pristine, high-grade kaolin, which is increasingly contested by the ceramics and paper industries, the researchers sourced two kaolinite-dominant natural mixed clays known as Red Sealing Clay and Blended Sealing Clay, an illite-dominant clay, a kaolinite-illite blend called Hanson White Clay, and a montmorillonite-and-carbonate bentonite. Each clay was dried, crushed, and sieved to 75 micrometers, then activated by one of two competing routes. The first was calcination, heating the clay to between 600 and 900 degrees Celsius to drive off structural hydroxyl groups and unlock pozzolanic reactivity. The second was mechanochemical activation, an entirely different philosophy: instead of heat, high-shear grinding in a ball mill mechanically fractures the clay particles, dramatically increasing their surface area and reactivity without ever lighting a flame.

The activated clays then replaced 30 percent of general-purpose cement by mass in paste mixes with a water-to-binder ratio of 0.4, cured for 28 days and crushed in a universal testing machine according to Australian standards. The results revealed a clear hierarchy. The calcined Red Sealing Clay binder reached 58.6 megapascals, the only mix to beat the reference cement’s 57.5 megapascals. At the other end of the scale, the mechanochemically activated bentonite managed just 36.1 megapascals, a mere 63 percent of the reference strength. Across the board, every calcined clay outperformed its mechanically activated counterpart, a pattern that would prove decisive once the environmental accounting began.

That accounting, performed in SimaPro using the Ecoinvent and AusLCI databases within a cradle-to-gate boundary, produced a headline finding with real significance for the construction industry. The calcined binders achieved up to 18.6 percent lower global warming potential than general-purpose cement, with the calcined Red Sealing Clay blend delivering the lowest carbon footprint of just 0.0129 kilograms of CO2 equivalent per functional unit. This figure aligns closely with previous studies of calcined clays worldwide, which reported reductions of roughly 15 to 18 percent at similar replacement levels. The mechanism is straightforward: replacing cement directly avoids the emissions from limestone decomposition, and the lower temperatures required for clay dehydroxylation, theoretically just 1145 kilojoules per kilogram for pure kaolinite, cut the energy bill compared with clinker production.

But here is where the story takes a twist worthy of attention. When the researchers examined abiotic depletion potential, a measure of non-renewable mineral resource consumption, the picture inverted dramatically. Mechanochemically activated clays showed up to 255 percent higher resource depletion than ordinary cement, driven primarily by the electricity consumed during grinding and the manufacture and wear of steel grinding media. Because more than 80 percent of Australia’s electricity grid is still powered by fossil fuels, the mechanically activated binders also exhibited fossil fuel depletion up to 59.4 percent above the cement baseline, despite using no direct fossil fuel combustion in the activation step itself. This is a textbook case of what environmental scientists call burden shifting: solving one problem, carbon emissions, while quietly creating another, in this case resource exhaustion and upstream pollution from the grid.

The endpoint analysis, conducted with the ReCiPe 2016 method, extended the pattern to human health, ecosystems, and resource scarcity. All calcined clay binders performed better than cement on human health, measured in disability-adjusted life years, and on ecosystem damage, measured in species-years, with calcined Red Sealing Clay again leading. Yet the mechanochemically activated bentonite increased human health burdens by 39 percent over the cement baseline. In the resource scarcity category, expressed in dollars of future extraction cost, nearly every clay binder exceeded cement, with mechanochemically activated mixes showing increases above 55 percent. Only the calcined bentonite binder achieved a 13 percent reduction in resource scarcity, thanks to its relatively high strength and short transport distance.

Sensitivity analysis added crucial nuance about the levers available to engineers. Cutting clay transport distances to 100 kilometers reduced resource depletion by 3.0 percent, human health impacts by 0.7 percent, and ecosystem damage by 0.5 percent for every 100-kilometer saving, a reminder that road freight, which emits far more CO2 per tonne-kilometer than rail or shipping, matters enormously for bulk materials. More striking were the energy scenarios. Switching mechanochemical activation to Australia’s projected 2050 electricity mix reduced impacts by up to 8.5 percent, and a fully decarbonized grid of hydropower and wind cut endpoint impacts by up to 17.4 percent. On the calcination side, replacing coal with natural gas improved human health and ecosystem scores but inflated resource impacts by up to 50.9 percent, while biomass fuel delivered reductions of up to 6 percent across categories without the resource penalty.

The broader lesson from this study is that there is no single greenest cement substitute, only the right substitute for the right place. Calcination emerges as the more environmentally favorable activation route for Australian natural mixed clays under the current coal-heavy grid, offering superior strength and the largest carbon reductions. Yet mechanochemical activation could become genuinely competitive as the electricity grid decarbonizes, since its environmental fate is tethered almost entirely to the carbon intensity of the electrons feeding the mill. For builders, cement manufacturers, and policymakers, the message is clear: the sustainability of low-carbon binders depends not just on the percentage of cement replaced, but on clay mineralogy, activation method, energy source, and transport logistics, all of which must be optimized together. As the world searches for ways to decarbonize the most consumed industrial product on Earth, this Australian study shows that the humblest materials, dug from waste deposits and activated by fire or friction, could carry a substantial share of that transition, provided we count every burden, not just the carbon.

Subject of Research: Performance-normalized life cycle assessment of calcined and mechanochemically activated Australian waste clays as low-carbon cement substitutes

Article Title: Performance-normalized life cycle assessment of calcined and mechanochemically activated clays in Australia as cement substitutes

Article References: Basnayaka, I., Gunasekara, C., Law, D., Jayathilakage, R., & Setunge, S. (2026). Performance-normalized life cycle assessment of calcined and mechanochemically activated clays in Australia as cement substitutes. Environmental Science and Pollution Research, 33(28), 14408-14423. https://doi.org/10.1007/s11356-026-38183-z

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38183-z

Keywords: cement, clay, life cycle assessment, calcination, mechanochemical activation, supplementary cementitious materials, global warming potential, burden shifting, sustainable construction, Australia, decarbonization, compressive strength