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The decomposition of organic waste going to landfills produces methane gas, which is 21 times worse than CO 2 in its global warming potential. Spent coffee grounds (SCG) are one type of organic waste that makes up a significant proportion of the organic waste going to landfills. Therefore, it becomes imperative to look for a recycling solution to transform this waste into a valuable resource. The concrete industry has the potential to contribute significantly to increasing the recycling rate of this waste material. However, due to its high organic content, it is unsuitable to be used directly in structural concrete. Therefore, this experimental project looks at pyrolysing this waste at different temperatures (350 and 500 °C) to identify its suitability in improving the physicochemical and mechanical properties of concrete. The raw and pyrolysed forms of SCG were used as a replacement of fine aggregates (FA; sand) at 5, 10, 15 and 20% volume replacement levels. X-ray fluorescence (XRF), Carbon, Hydrogen, Nitrogen, and Sulfur (CHNS) analysis, laser diffraction particle size analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM) and compressive strength tests were undertaken to investigate the properties of the raw material and their performance in the blended concrete composites. The results show that the leaching of organic compounds from the SCG hinders the hydration reaction of cement particles, thereby significantly hampering the compressive strength of SCG-blended concrete. However, pyrolysing the SCG at 350 °C led to a significant improvement in its material properties, which resulted in a 29.3% enhancement in the compressive strength of the composite concrete blended with coffee biochar. [Display omitted] • Spent coffee grounds (SCG) leach organic compounds that hinder the hydration reaction and compressive strength of concrete. • SCG pyrolysed at 350 and 500 °C show strong bond formation with the cement matrix. • 15 vol% replacement of sand with pyrolysed SCG at 350 °C provides ∼30% improvement in concrete strength. • 100% of 75,000 tonnes of waste SCG produced in Australia can be utilised in structural concrete. [ABSTRACT FROM AUTHOR]

• Wood biochar was used as a replacement of fine aggregates at 10, 20 and 30% replacement levels. • Effect of CO 2 curing of biochar blended concrete was also investigated. • All three replacement levels provide compressive strength higher than the control mix. • CO 2 curing shows a positive influence on the compressive strength of the biochar concrete. The production of concrete is heavily reliant on the continuous mining of natural resources, with the majority component being the natural aggregates which make up about 70–75% of the concrete volume. As the emphasis shifts towards promoting sustainability through recycling all types of waste to create a closed-loop circular economy, it's vital to explore alternative waste materials that can replace traditional raw materials in concrete production. Biochar derived from different organic waste materials has shown to improve the strength properties of concrete. However, the majority of the research has focussed on using biochar as a cement replacement material with very low cement replacement levels. Therefore, this research focuses on significantly increasing the uptake of the pyrolysed form of organic waste (wood biochar) by using it as a replacement of fine aggregates at replacement levels of 10, 20 and 30 vol%. The biochar blended concrete showed an improvement of 63.9, and 45.6% and a reduction of 9.6% in the 7-day compressive strength results at 10, 20 and 30% replacement levels, respectively. At 28 days, the biochar blended concrete samples showed an improvement of 20.1, 22.6, and 16% in the compressive strength results at 10, 20 and 30% replacement levels, respectively. Separately, the 10% WBC blended concrete was also cured in a CO 2 environment for 7 and 28 days. The results showed an improvement of 24.7 and 37.3 in the 28-day compressive strength results with the respective CO 2 curing of 7 and 28 days, compared to the control mix. [ABSTRACT FROM AUTHOR]