Red mud waste to stronger cement: Sulphate activation delivers 7.33 MPa compressive strength

China’s growing red mud waste problem could offer an opportunity for the construction industry, as new research has demonstrated how aluminium industry residue can be converted into stronger cementitious materials. The study found that sulphate activation using flue gas desulphurisation gypsum achieved a compressive strength of 7.33 megapascals (MPa), more than double that of untreated mixtures. Researchers also identified the fractal geometry of microscopic pores as a potential indicator of material strength.

The findings could support efforts to develop low-carbon construction materials from industrial waste, including red mud generated during alumina refining. However, the results also highlight a key engineering trade-off: the activation method that delivered the highest compressive strength did not provide the greatest energy absorption capacity.

Red mud presents an opportunity for industrial waste recycling

China’s alumina industry has accumulated more than 1.5 billion tonnes of red mud, a highly alkaline residue produced during the extraction of aluminium from bauxite ore. Growing stockpiles present environmental and safety challenges, including potential risks to nearby soil and water.

Red mud contains silicon, aluminium, iron and calcium, making it a potential raw material for cementitious products. However, its high alkalinity, complex mineral composition and low inherent reactivity have restricted its practical use.

A research team led by Jiawei Zhang and Guochao Yan investigated how different activation methods could improve the performance of red mud-based cementitious materials. The researchers incorporated red mud into a composite system containing coal gangue powder and ordinary Portland cement, then evaluated three activation pathways.

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These included alkali activation using sodium silicate solution, thermal activation through calcination at temperatures ranging from 500°C to 900°C, and sulphate activation using flue gas desulphurisation gypsum, an industrial by-product of coal-fired power plants.

The study, titled Pore structure fractality affects strength and energy dissipation behavior in red mud-based cementitious materials under different activation pathways, was published in Case Studies in Construction Materials in 2026.

Sulphate activation achieves the highest compressive strength

The experimental results showed substantial differences in compressive strength depending on the activation method. The untreated control mixture reached 3.60 MPa after 28 days, with its strength relying primarily on ordinary Portland cement hydration.

Sulphate activation produced the strongest material, while thermal and alkali activation also improved performance compared with the control.

Source: Research published in Case Studies in Construction Materials.

Alkali activation increased compressive strength by 54 per cent compared with the control. The sodium silicate solution raised the alkalinity of the pore solution and supplied soluble silicate, encouraging the dissolution and repolymerisation of aluminosilicate species.

Thermal activation at 800°C delivered a compressive strength of 6.08 MPa, representing a 69 per cent improvement over the untreated mixture. However, increasing the calcination temperature to 900°C reduced the strength of the resulting material.

Sulphate activation outperformed both approaches. Mixtures containing 4 per cent flue gas desulphurisation gypsum achieved 7.33 MPa, more than twice the control mixture’s compressive strength. The researchers also recorded early strength gains of up to 262 per cent after three days.

The findings indicate that industrial by-products can play an important role in improving the mechanical performance of red mud-based cementitious materials.

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Why gypsum improved the material’s strength

Microscopic and crystallographic analyses helped explain the performance of the sulphate-activated mixtures.

X-ray diffraction identified distinct reflections associated with ettringite, a needle-shaped calcium sulphoaluminate mineral. Energy-dispersive spectroscopy also detected locally high concentrations of calcium and sulphur.

Scanning electron microscopy revealed that the sulphate-bearing crystals intergrew to form dense, honeycomb-like structures with relatively few large voids. These structures connected gel phases into a continuous, load-bearing network.

The other activation methods produced different microstructures. Alkali-activated samples contained spider-like networks of sodium-rich aluminosilicate gel surrounding residual particles. Thermally activated samples formed cotton-like, flocculent products that created a relatively continuous matrix.

These differences in microstructure help explain why the activation methods produced different mechanical properties.

Fractal pore geometry offers a new way to assess cement strength

One of the study’s key contributions was its use of fractal analysis to examine the pore structure of red mud-based cementitious materials.

Conventional measurements, including specific surface area, pore volume and pore-size distribution, describe the amount of void space within a material and the dimensions of its pores. However, they provide limited information about the geometric irregularity of pore surfaces.

To investigate this aspect, the researchers applied the Frenkel-Halsey-Hill model to nitrogen adsorption data and calculated the fractal dimension of each sample after 28 days.

The resulting values ranged between 2 and 3, as expected for rough surfaces. More importantly, the fractal dimension increased in the following order: untreated, alkali-activated, thermally activated and sulphate-activated materials.

This ranking matched the order of compressive strength, suggesting a close relationship between pore-surface complexity and the ability of the material to withstand loads.

The pore-volume results provided further evidence that total porosity alone cannot explain strength differences.

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Thermal activation produced the lowest total accessible pore volume, at 0.1271 cubic centimetres per gram. Yet, it did not deliver the highest compressive strength. The sulphate-activated material had a slightly higher pore volume of 0.1324 cubic centimetres per gram but achieved substantially greater strength.

The findings suggest that the geometry and surface characteristics of pores can be as important as their total volume. They also build on earlier observations that larger pores can disproportionately weaken cementitious materials.

Fractal pore characterisation could therefore offer engineers an additional tool for assessing the performance of industrial waste-based binders, complementing conventional porosity measurements.

Stronger cement does not necessarily absorb more energy

Although sulphate activation produced the highest compressive strength, the research revealed a different ranking for energy dissipation.

The team analysed stress-strain curves obtained from uniaxial compression tests, separating the input energy into stored elastic