The global production of cement – a key ingredient in concrete – is responsible for 8% of global CO₂ emissions.

Replacing some cement with clay reduces the environmental impact, but the high-grade kaolin clay best suited for cement replacement is in increasingly high demand for ceramics, paints, cosmetics and paper.

Now the RMIT team has demonstrated cheaper and more abundant illite clay can be mixed with low-grade kaolinite clay, to make stronger concrete.

Technological breakthrough toward a low-carbon future

The study published in Construction and Building Materials introduces a new process, where low-grade illite and kaolin clays are mixed at an equal ratio then heated at 600 Celsius.

Processing the two ingredients together, rather than separately, led to several improvements in the material's performance, the study found.

Project lead Dr Chamila Gunasekara, said low-grade illite clay does not normally bind well with cement and water, but that the joint heating, or co-calcination, process greatly enhances illite clay’s binding ability, known as pozzolanic reactivity.

“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,” said Gunasekara, from RMIT’s School of Engineering.

There was an 18% increase in the amount of disordered material in the new clays, which is beneficial for strength and durability. The material 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 said.

These enhancements demonstrate that the co-calcined illite-kaolin blends can match or surpass the performance of traditional kaolin-based substitutes.

Demand for kaolin is steadily growing, with the market projected to be worth US$6 billion by 2032 and its hoped, thanks to this research, a market for illite clay could follow suit.

Study lead author Dr Roshan Jayathilakage said 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 said.

“This makes the method not only technically sound but also economically and environmentally scalable.”

New computational tool to accelerate green transformation

The research also showcases computational advancements in material science.

Underpinning the group’s work is an advanced computational tool for analysing and designing concrete, developed in partnership with Hokkaido University, Japan.

The tool allows the team to evaluate performance in various activated clays in concrete mixtures, providing detailed insights into their mechanical properties, durability and energy-efficiency, where currently available approaches had struggled.

Dr Yuguo Yu, from RMIT’s School of Engineering, said their computational tool enabled a more efficient assessment of material performance, reducing the reliance on extensive laboratory tests.

“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,” he said.

"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.”

Building on collaborations with global partners including European Synchrotron Radiation Facility in France, the RMIT team is continuing to investigate how different clay types and activation techniques influence concrete behaviour at multiple scales, while expanding performance testing in real-world conditions.

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).

Led by RMIT’s Professor Sujeeva Setunge, TREMS brings together top scientists and industry experts from nine Australian universities and 36 state, industry, and international partners to minimise waste and repurpose reclaimed materials for construction and advanced manufacturing.