Evaluation on the effects of Cu(II) on the properties of blast furnace slag–cement composites with adding Cu-bearing solid wastes

Abstract

The effect of heavy metals on the properties and hydration of blast furnace slag–cement composites (BFS-CC) remain unclear. In this study, two BFS-CC (denoted as DBFS-CC and WBFS-CC) were prepared by dry and wet grinding of BFS, respectively. The effect of Cu(II) on BFS-CC’s properties and hydration was investigated by adding representative copper contaminants (CuO, CuCl2, and CuS) to the composites. Adding 1.0wt% CuO and 0.5wt% CuS increased the 3-d compressive strength of DBFS-CC by 14.9% and 5.7%, respectively, but suppressed the 3-d strength of WBFS-CC. This trend reversed at 28-d curing, where adding 1.5wt% CuO, 2.0wt% CuCl2, and 1.5wt% CuS enhanced the compressive strength of WBFS-CC by 23.4%, 6.2%, and 13.6%, respectively, but adversely affected the strength of DBFS-CC. For 28-d hydration, adding CuCl2 decreased the hydration degree of DBFS-CC but enhanced that of WBFS-CC. Adding CuO promoted the hydration degree of both composites, while adding CuS exhibited inhibitory effects. DBFS-CC immobilized CuCl2 better due to a higher hydration degree, while WBFS-CC immobilized CuO and CuS better due to having finer unhydrated BFS particles and a denser matrix. This study not only focuses on the Cu(II) immobilization effect but also reveals the differential effects of Cu(II) species on the hydration process, providing novel insights into heavy metal interactions in BFS-CC systems and their safe disposal.

1. Introduction

Blast furnace slag–cement composites (BFS-CC) have emerged as promising cementitious materials for resourcefully utilizing heavy metal-containing solid wastes [1–4]. Zhang et al. [5] increased the 28-d compressive strength of BFS-CC by 10.8% by replacing 10wt% of BFS with copper slag (CS). Similarly, Liu and Hua [6] reported that the 90-d compressive strength of BFS–steel slag (SS)–ordinary Portland cement (OPC) composite was 5% higher than that of OPC when SS–BFS admixture (with a mass ratio of 3:1) was used to replace 30wt% of OPC. Furthermore, Lang et al. [7] incorporated 30wt% electrolytic manganese residue into BFS–OPC composites, yielding roadbed materials with 44% and 60% compressive strength increased at 7 and 28 d, respectively. Crucially, Mn2+ leaching concentrations met China’s GB 8978—1996 regulatory limit (<2.0 mg·L−1). Disposal of heavy metal-containing solid wastes using BFS-CC has both environmental and engineering benefits while preventing the risk of heavy metal leaching [8–9].

Take copper-containing solid waste as an example, with common types: CS, municipal solid waste incineration fly ash (MSWI FA), and electroplating sludge (EPS) [9–11]. CS is generated during copper pyrometallurgical smelting [12–13], with a production ratio of 2.2 t of CS per ton of copper [14]. Globally, approximately 752 Mt of CS are produced annually [15], containing 0.5wt%–2.0wt% Cu(II) [16]. MSWI FA is derived from municipal waste incineration processes [17]. Each ton of waste incineration produces 30–50 kg of MSWI FA, resulting in China’s annual MSWI FA production of 6–7 Mt [18], with 0.1wt%–0.7wt% Cu(II) content [19]. EPS originates from electroplating wastewater treatment through chemical neutralization, flocculation, and precipitation [20]. China’s EPS output exceeds 10 Mt annually [21], with a Cu(II) content of 1.7wt%–14.5wt% [20].

In addition to the Cu(II) content, the different chemical forms of Cu(II) present in solid wastes may critically determine their interaction with BFS-CC. However, the effect of Cu(II) forms on the properties and hydration of BFS-CC remains insufficiently understood. Comparative analysis revealed distinct Cu(II) forms: CS contains CuS and CuO [11]; MSWI FA contains CuCl2, CuO, and CuS [22–24]; EPS contains CuO and Cu2+ [25]. Based on the above, CuO, CuCl2, and CuS are representative forms of Cu(II) in solid waste.