© 2021 Authors. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/). Oil Shale, 2021, Vol. 38, No. 2, pp. 155–176 ISSN 0208-189X doi: https://doi.org/10.3176/oil.2021.2.04 © 2021 Estonian Academy Publishers * Corresponding author: e-mail mai.uibu@taltech.ee The composition and properties of ash in the context of the modernisation of oil shale industry Mai Uibu (a)* , Kadriann Tamm (a) , Regiina Viires (a) , Janek Reinik (b) , Peeter Somelar (c) , Lembi Raado (d) , Tiina Hain (d) , Rein Kuusik (a) , Andres Trikkel (a) (a) Laboratory of Inorganic Materials, Department of Materials and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia (b) National Institute of Chemical Physics and Biophysics, Akadeemia tee 23, 12618 Tallinn, Estonia (c) Department of Geology, University of Tartu, Ravila 14A, 50411 Tartu, Estonia (d) Building Lifecycle Research Group, Department of Civil Engineering and Architecture, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Received 14.11.2019, accepted 16.05.2021, available online 10.06.2021 Abstract. The modernisation of the oil shale (OS) industry in Estonia over the last decade has also led to signifcant changes in the composition and binding properties of the ash generated as a by-product. These changes also infuence the environmental impact that the ash can have. In the current investigation, oil shale ash (OSA) samples were collected from different points along the ash separation systems of a large pulverized fuel combustion plant (LCP), various points in a circulating fuidized bed (CFB) combustion plant, and from an oil shale pyrolysis plant that utilises oil shale to produce liquid fuels. The results obtained can be used for optimising the deposition conditions, as well as guiding utilisation-recycling processes for ash generated from changing oil shale composition and characteristics. When it comes to developing the circular economy aspects of oil shale utilisation, mapping out the properties of OSA is crucial. Keywords: oil shale ash, circular economy, binding properties, leaching. 1. Introduction Harmonised European environmental regulations are in a state of constant development. Air emissions, energy effciency levels, resource effciency, use