Carbon sequestration potential and mechanisms of shotcrete for tunnel support in underground metal mines through cement hydration

Abstract

Growing concerns about greenhouse gas emissions from underground mining have intensified the need for carbon reduction strategies at every stage. Shotcrete used in tunnel support presents a promising opportunity for carbon emission reduction. This study investigates the carbon absorption capacity, mechanical strength, and underlying mechanisms of shotcrete when exposed to varying CO2 concentrations during the mine support process. Findings reveal that higher CO2 concentrations during the initial stages of carbonation curing enhance early strength but may impede long-term strength development. Shotcrete samples exposed to 2vol% CO2 for 14 d exhibited a carbonation degree approximately three times higher than those exposed to 0.03vol% CO2. A carbonation layer formed in the shotcrete, sequestering CO2 as solid carbonates. In practical terms, shotcrete in an underground return-air tunnel absorbed 1.1 kg·m2 of CO2 over 14 d, equivalent to treating 33 m3 of contaminated air. Thus, using shotcrete for CO2 curing in return-air tunnels can significantly reduce carbon emissions, contributing to greener and more sustainable mining practices.

1. Introduction

Concerns over greenhouse gas emissions from metal mining have received increasing scrutiny in recent years. As global demand for metal resources rises, interest in reducing the emissions associated with metal production has intensified [1‒3]. According to a report by the International Energy Agency, global anthropogenic CO2 emissions are projected to reach approximately 34.04 Gt in 2021 [1‒4]. The mining industry contributes to global energy consumption and CO2 emissions [5], with China being a major global greenhouse gas emitter. CO2 generated by mine blasting, human respiration, transportation exhaust, slow oxidation of carbon rock layers, and fire accidents, is discharged to the surface through return-air tunnels, ultimately affecting the “dual carbon” process of the mine [6‒8].

To address the challenge of mine carbon neutrality, researchers have explored various approaches, including green mining, clean low-carbon processes, efficient resource utilization, and safe disposal [9‒10]. Several scholars have proposed to enable cleaner and more efficient extraction by injecting CO2‒N2 instead of natural gas hydrates, simultaneously sequestering CO2 [11‒15]. Huang et al. [16] found that using vanadium tailings to create mineralization agents for capturing CO2 from waste gas not only reduces carbon emissions but also holds great potential for developing a low-carbon economy. Ho and Iizuka [17] elaborated on the use of seawater as a solvent in the mineral carbonation process for carbon sequestration. Chen et al. [5] proposed cement paste backfill technology for accelerated carbonization in extraction areas to achieve zero carbon emissions. While previous studies have provided valuable insights into carbon peaking and carbon neutralization, challenges remain, including high curing costs, complex operations, and the inability to implement a complete process system in underground mine workings.

Underground mine support is a critical measure to prevent the collapse of the mining face during operations and to provide reliable reinforcement to the excavation roadway, especially in unstable mines. As one of several support methods, shotcrete involves spraying a mixture of aggregates, cementitious materials, and water onto the rock face to reinforce the surrounding rock [18]. Carbonation curing of concrete is a potential method for CO2 mineralization [19], with studies showing that CO2 concentration is a key factor affecting carbonation in concrete [20]. Higher CO2 concentrations can accelerate carbonation [21], though concentrations above a certain threshold may negatively affect the rate and depth of carbonation [22]. Considering these factors, this study uses the carbonation curing of shotcrete in the return airways to sequester greenhouse gases emitted from underground mining. This approach creates a closed-loop system for carbon neutrality, integrating carbon emission, aggregation, sequestration [23], and utilization. It has the potential to improve environmental safety and production processes and to support underground mines in achieving carbon peaking goals.