Technical feasibility of bauxite-enhanced vitrification for utilizing AOD slag as a supplementary cementitious material

Abstract

The utilization of bauxite-vitrified argon–oxygen decarburization (AOD) slag as a supplementary cementitious material is explored as an alternative approach for recycling unmanageable AOD slag and reducing the CO2 emission levels. The results demonstrate that AOD slag can be effectively vitrified by incorporating 15wt% bauxite as the alumina source, facilitating the formation of a stable glass phase. The resulting vitrified product exhibits excellent properties, such as initial and final setting times of 337 and 437 min, respectively, good soundness (<0.05 mm), and a compressive strength of 88.7 MPa. In particular, the robust cementitious matrix effectively encapsulates and immobilizes the Cr ions, thus reducing the total chromium leaching concentration to 0.09 mg/L, which is significantly below the regulatory limit (0.15 mg/L) specified by the HJ/T 301—2007 industrial standard. The leached Cr3+ ions can be easily oxidized in a liquid environment with a pH > 11 at a positive oxidation–reduction potential. The carbonation reaction inhibits the conversion from trivalent chromium to hexavalent chromium, thereby lowering its concentration. These findings suggest that vitrifying AOD slag using Al2O3-rich solid waste as a supplementary cementitious material is a promising and environmentally sustainable method.

1. Introduction

Concrete is one of the most prevalent building materials worldwide, whose annual global consumption is projected to exceed 20 billion tons [1]. However, its widespread use entails significant environmental costs as it leads to high CO2 emission levels. The manufacturing of Portland cement, a key binder in concrete, is the primary source of CO2 emissions associated with concrete production. Cement production is energy-intensive and chemically emissive: Approximately 0.8–0.9 t of CO2 is released per ton of produced cement. Decreasing CO2 emission levels is vital for meeting China’s carbon peak and neutrality goals.

Supplementary cementitious materials (SCMs) are predominantly sourced from abundant industrial solid waste resources [2–4]. The incorporation of SCMs presents a promising approach for reducing CO2 emission levels [5–7] and addressing the issue of industrial solid waste management [8]. The analysis of ground-granulated blast-furnace slag (GGBFS) and carbonated steel slag in concrete manufacturing has gained significant acceptance [9–13]. Majhi et al. [14] found that a combination of 40wt% GGBFS and 50wt% recycled aggregate concrete (RAC) yields a dense, compact cement microstructure with exceptional physical characteristics. Moreover, researchers [15–16] have demonstrated that the inclusion of an appropriate quantity of CaO in RAC leads to enhanced durability and physical properties. Additionally, Cuesta [17–18] successfully formulated self-compacting concrete (SCC) blends using GGBFS. To avoid failures caused by the transverse plastic strain, it is advisable to determine the RCA content based on the serviceability conditions [19]. Furthermore, a regression model was developed to estimate the SCC performance [20]. This model is especially useful for sustainable concrete mix designs in which high proportions of recycled materials are used.

Argon–oxygen decarburization (AOD) slag is a byproduct of the stainless steel refining process, whose annual generation is approximately 10 million tons in China [21]. This slag is generally considered unusable because of its high chromium content and poor physical properties. Mechanical milling is considered a promising method for increasing the specific surface area of the AOD slag and enhancing its reactivity [22]. However, the process often results in high energy consumption and dust generation, in turn necessitating appropriate dust control systems and leading to increased operational costs. Chemical modification involves the addition of stabilizing agents to inhibit undesirable mineral-phase transformations, thereby improving the volume stability and hydration activity of the slag [23–24]. In carbonation treatment, the emitted CO2 is reacted with free CaO and other reactive components in the slag to form stable carbonates. However, the carbonation efficiency is often limited by the slow reaction kinetics and low CO2 diffusion rates, necessitating the optimization of process parameters such as temperature and pressure [25–26].