Contents lists available at ScienceDirect Journal of CO 2 Utilization journal homepage: www.elsevier.com/locate/jcou Mechanical and environmental properties of carbonated steel slag compacts as a function of mineralogy and CO 2 uptake P. Librandi a,b , P. Nielsen b, ⁎ , G. Costa a , R. Snellings b , M. Quaghebeur b , R. Baciocchi a a Laboratory of Environmental Engineering, DICII, University of Rome “Tor Vergata”, Via del Politecnico 1, 00133 Rome, Italy b Sustainable Materials Management, VITO, Mol, Belgium ARTICLE INFO Keywords: Steel slag Carbonation Compressive strength Microstructure Leaching and mineralogy ABSTRACT Accelerated carbonation is a treatment for converting alkaline industrial residues into added-value products and storing CO2 in solid form. This work investigated the influence of reacting phases and CO2 uptake on micro- structure development, mechanical properties and the environmental behavior of carbonated compacts pro- duced from Basic Oxygen Furnace (BOF) and Electric Arc Furnace (EAF) slags, characterized by a different mineralogy. The compacts were cured under a 100% CO2 atmosphere at 50 °C and pressure of 1.3 or 10 bar for 15 min to 4 h. The BOF slag reacted very fast in the first 30-60 min due to the complete conversion of portlandite to calcite, amorphous calcium carbonate and aragonite, and continued to react over time due to the presence of slower reacting Ca-silicate phases. For the EAF slag, rich in Ca-silicates, the CO2 uptake was lower, and increased only slightly over time at 1.3 bar and became almost stable after 15 minutes at 10 bar; the EAF slag products however presented a higher compressive strength than the BOF slag ones, because of the different phases in- volved in the carbonation reaction. For the BOF slag, portlandite dissolution caused the formation of voids, only partially filled up by the reaction products. For the EAF slag, formation of a carbonate and amorphous silica layer around the reacting silicates yielded a denser matrix. pH and Ba leaching decreased for both types of slag, whereas V release increased due to the dissolution of reactive phases such as dicalcium silicates, which initially contained this element. 1. Introduction Accelerated carbonation (or carbon mineralization) has been de- fined as a process that can allow to bridge CO 2 storage and CO 2 utili- zation [1]. In fact, through carbonation, CO 2 reacts with alkaline earth metal oxide bearing phases, such as Ca/Mg (hydr)-oxides and silicates, forming thermodynamically and chemically stable solid carbonates, such as calcite, dolomite and magnesite [2]. In addition, this reaction may be tailored to manufacture products that may be employed in construction applications, such as structural materials (e.g., cements, concrete, aggregates and mortars), non-structural materials (e.g., ag- gregates or fillers for road-base, erosion, sea or flood protection bar- riers), and high purity Ca- and Mg-based carbonates that may be used for different applications (e.g., as additives for paints and polymers) [3]. Hence, this CO 2 utilization process may be exploited to generate eco- nomic value while reducing raw material use and associated CO 2 emissions, which are quite relevant considering in particular the cur- rent environmental footprint of the construction industry. Specifically, a 2016 market assessment determined that carbonate mineral-based construction materials could reach annual revenues of $1 trillion and have the potential to consume 3–6 GT of global CO 2 emissions by 2030 [4]. Another interesting feature of this process is that alkaline industrial residues may be employed as feedstock of reactive phases instead of natural ores to fix gaseous CO 2 into solid carbonates [5]. These residues are typically available in the proximity of CO 2 source emissions, present a high reactivity even at mild operating conditions and, differently from natural ores, may also present suitable grain size without pre-treatment [5,6]. The chemical composition and mineralogy of industrial residues are generally process- or even plant-specific and may vary over time as a function of the feed material being treated, processing conditions and/or the type of manufactured product. In addition, industrial re- sidues may also contain elements of potential environmental concern, therefore the leaching behavior of products manufactured from these materials must be carefully evaluated, besides the technical perfor- mance required for the foreseen application. Steel slag is one of the most suitable candidates to be treated by carbonation for obtaining added-value products while permanently https://doi.org/10.1016/j.jcou.2019.05.028 Received 15 February 2019; Received in revised form 3 May 2019; Accepted 14 May 2019 ⁎ Corresponding author. E-mail address: peter.nielsen@vito.be (P. Nielsen). Journal of CO₂ Utilization 33 (2019) 201–214 2212-9820/ © 2019 Elsevier Ltd. All rights reserved. T