Feasibility of Utilisation of Coal Mine Tailings in Australian Cement Production

Annually, Queensland and New South Wales discharge 14-16 million dry tonnes of coal tailings, mostly into tailings storage facilities (TSFs). This project aimed to reduce and repurpose coal tailings by investigating their feasibility as both a raw material for clinker production and as a supplementary cementitious material (SCM) in concrete.

Highlights of the findings are as follows:

• The study focused on coal tailings from four sites.
• Pozzolanic reactivity tests (ASTM C1897) showed that heat-treated coal tailings had a higher reactivity index than common SCMs like fly ash. The residence time during heat treatment and the maximum particle size have marginal impacts on pozzolanic reactivity.

• Specific surface area analysis and mortar flow tests indicated that heat treatment around 800°C would provide a workable mortar/concrete mix, balancing reactivity and workability with the treatment cost.

• Water demand tests demonstrated that adding less than 20% extra water to mortar mixes containing 20% coal tailings treated at 850°C achieved comparable flow to control samples, with strength activity indices (SAI) of 84%-85%, classifying these tailings as grade 1 pozzolans according to AS 3582.4.

• Mortar and concrete samples with 20% treated coal tailings showed compressive strength comparable to the control sample containing only Portland cement, while exhibiting superior durability.

• Mortar and concrete samples with 20% treated coal tailings showed superior durability performance with more than 40% reduction in water absorption. This is significant for resting ingress of harmful agents such as chloride.

• Cost analysis indicated that producing concrete with treated coal tailings is economically competitive with, and sometimes cheaper than, conventional Portland cement, although capital investment was not included in the evaluation.

• Clinker mix trials using coal tailings from Sites 3 met Australian standards (AS 3972) requirement (soundness, compressive strength, setting time, chloride and sulphur content, etc.), with estimated conservative fuel savings of up to 30% in clinker production.

These findings suggest that coal tailings can serve as a sustainable, cost-effective alternative for both linker production and SCM use in concrete.