sustainability Article The Immobilisation of Heavy Metals from Sewage Sludge Ash in CO 2 -Cured Mortars Erick Grünhäuser Soares 1, * , João Castro-Gomes 1 , Mateusz Sitarz 2 , Tomasz Zdeb 2 and Izabela Hager 2   Citation: Grünhäuser Soares, E.; Castro-Gomes, J.; Sitarz, M.; Zdeb, T.; Hager, I. The Immobilisation of Heavy Metals from Sewage Sludge Ash in CO 2 -Cured Mortars. Sustainability 2021, 13, 12893. https:// doi.org/10.3390/su132212893 Academic Editor: Franco Ajmone Marsan Received: 21 October 2021 Accepted: 17 November 2021 Published: 22 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Centre of Materials and Building Technologies (C–MADE), Department of Civil Engineering and Architecture, University of Beira Interior (UBI), 6201-001 Covilhã, Portugal; jpcg@ubi.pt 2 Faculty of Civil Engineering, Cracow University of Technology, 31-155 Cracow, Poland; mateusz.sitarz@pk.edu.pl (M.S.); tomasz.zdeb@pk.edu.pl (T.Z.); izabela.hager@pk.edu.pl (I.H.) * Correspondence: e.grunhauser@ubi.pt Abstract: Sewage treatment processes are a serious environmental threat due to the difficulties involved in its waste management and disposal. Therefore, one developing trend in sewage sludge processing is its thermal treatment, which generates sewage sludge ash that may contain many environmental pollutants, such as heavy metals. Moreover, due to the European Union requirements that not only focus on the waste generation reduction but also on its reuse and final disposal, it is essential to pursue new applications of such resources, such as the waste-based material incorporation into alternative cementitious materials. Thus, this study evaluated the heavy metals leachability of CO 2 -cured mortars incorporating sewage sludge ash as filler. For this purpose, Portland cement, reactive magnesia, and electric arc furnace slag were used to produce three different CO 2 -cured mortars, which were cured though pressurised accelerated carbonation curing for 24 h. These mortars presented up to 12.7 MPa as compressive strength and their carbonation was confirmed by TG-DTG and FT-IR analyses. Their leachability of heavy metals met the European requirements for all waste materials, including inert materials, and post-industrial wastewater. Therefore, the immobilisation of heavy metals in this binding technology may be considered an effective method to safely manage sewage sludge ash. Keywords: CO 2 -cured mortar; CO 2 adsorption; heavy metal immobilisation; leaching; sewage sludge ash; electric arc furnace slag; carbonated reactive magnesia cement; Portland cement 1. Introduction The construction materials industry, which was once based on a wide range of materi- als appropriate to local conditions and/or to specific needs, has evolved into an industry based on Portland cement-based materials [1]. Such a change of trend has turned Port- land cement-based materials into the most heavily consumed manufactured material in the world in terms of volume, being responsible for about 7% of the total anthropogenic emissions of carbon dioxide (CO 2 )[2]. Thus, worldwide concerns about CO 2 emissions have increased interest in alternative binding technologies and materials to Portland ce- ment (PC) [3,4], such as carbonated-based materials [4], mainly due to their capacity to capture and store CO 2 in their matrices [5]. It has been shown to be feasible to produce CO 2 -cured binders with high strength results from electric arc furnace slag (EAF slag)-, PC- [6], and carbonated reactive magnesia cement (CRMC)-based materials [7], especially if the accelerated carbonation curing conditions are controlled [6] as well as the mixture designs [7]. It was also shown to be practicable to use waste materials in CRMC-based materials [8–12] which may reduce the environmental impacts and production cost of the designed materials [13]. In addition to that, the ash produced by the thermal transformation of sewage sludge is a raw material that may also be used in the production of construction binders, allowing Sustainability 2021, 13, 12893. https://doi.org/10.3390/su132212893 https://www.mdpi.com/journal/sustainability