A new research study from RMIT University has opened up a potential new market in sustainable construction materials, with common clay used to replace some cement to produce low-carbon concrete.

The manufacture of cement – a key ingredient in concrete – is responsible for eight percent of global carbon emissions. By converting low-grade clay into a high-performance cement supplement, the engineers at RMIT University have reduced the environmental impact of cement production.

Given that high-grade kaolin clay, which is best suited for cement replacement, has high demand for producing ceramics, paints, cosmetics and paper, the RMIT team has demonstrated that the cheaper and more abundant illite clay can be mixed with low-grade kaolinite clay to make stronger concrete.

The study, published in Construction and Building Materials introduces a new process, where low-grade illite and kaolin clays are mixed in equal quantities and heated together to 600°C, improving the material's performance.

“Based on this approach, we are able to replace 20% of cement usage using low-grade illite and kaolin combinations, while achieving even better performance of the yield product,” says project lead Dr Chamila Gunasekara from RMIT’s School of Engineering.

The new material is not only strong and durable but also holds more water in a chemically stable form, which points to better long-term reactions that help the structure stay strong.

“Porosity is reduced significantly by 41%, with its compressive strength increased by 15%, where changes in the way iron compounds formed help create a tighter and more compact internal structure,” Gunasekara says.

Study lead author Dr Roshan Jayathilakage says the technique was also more energy efficient.

“Since raw materials are processed together, it streamlines industrial operations and lowers fuel use compared to multiple calcination steps,” Jayathilakage says. “This makes the method not only technically sound but also economically and environmentally scalable.”

An advanced computational tool for analysing and designing concrete, developed in partnership with Hokkaido University, Japan, allows the team to evaluate performance in various activated clays in concrete mixtures, providing detailed insights into their mechanical properties, durability and energy-efficiency.

“By predicting how different clay compositions affect concrete behaviour, engineers are able to better design energy-efficient mixtures tailored for local clay types and specific environmental conditions,” Dr Yuguo Yu from RMIT’s School of Engineering says.

"This virtual tool could enable the construction industry to accelerate the adoption of eco-friendly materials, paving the way of greener transformation for a more sustainable future.”

This research was enabled by the ARC Industrial Transformation Research Hub for Transformation of Reclaimed Waste Resources to Engineered Materials and Solutions for a Circular Economy (TREMS).

Notes:

“A combination technique to improve natural low-grade illite as supplementary cementitious material for concrete” is published in Construction and Building Materials (DOI: 10.1016/j.conbuildmat.2025.141334)

Other relevant research includes “On the hydration of limestone calcined kaolinitic clay cement and energy-efficient production”, published in Cement and Concrete Composites (DOI: 10.1016/j.cemconcomp.2024.105698).

Image: The combined clay mix after being heat treated, ready for use as a cement supplement (Supplied)

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