Effects of accelerated carbonation on carbon dioxide uptake and compressive strength of biomass ash artificial aggregates

Ash disposal is a major cost to most power plants. Utilising ashes in construction products could enable the transition to a circular economy for power plants, bring higher value to the ashes and lower the need for virgin raw materials in the construction industry, while potentially help climate change mitigation via accelerated carbonation. In this study, three accelerated carbonation methods for artificial aggregate manufacturing from two biomass combustion fly ashes were studied: post-granulation CO2 curing in an autoclave, simultaneous tumbling drum granulation and CO2 curing, and carbonated water granulation. The control samples were granulated with water only. The effect on compressive strength was measured from repeated tests with single granules and the effect on carbon dioxide uptake was measured with both quantitative analysis on TG-DSC-QMS graphs and high-temperature oxygen combustion gas analysis of ground samples. It was found that all carbonation methods increased the compressive strength of the granules. Both post-granulation CO2 curing in an autoclave and simultaneous tumbling drum granulation and CO2 curing significantly increased the carbon content of the granules. Based on the results, 1 ton of fresh ash from the power plant could bind up to 166 kg more CO2 when manufactured into carbonated artificial aggregates rather than water granulated aggregates. We note, that the baseline comparison for ash granulation concepts with regards to the climate impact is not zero as the ashes carbonate also naturally in contact with ambient air. More studies on quantitative analysis of natural carbonation of ashes are encouraged.