Key Takeaways & Executive Findings
- •• Optimal composite alkali activator proportion of 1:2 enhances 3-day compressive strength by 31.25% in gangue cemented rockfill. • The activator proportion strengthens the pozzolanic effect of gangue and inhibits calcium hydroxide agglomeration, leading to a denser microstructure. • Molecular dynamics reveals that 12% sodium substitution in C–S–H improves interlayer adhesion and stress transfer. • The proposed alkali-activated gangue method offers a low-cost, sustainable approach for solid waste recycling and reduced cement consumption.
Abstract
Using cemented rockfill to replace coal pillars offers an effective solution for reducing solid waste while ensuring the safety of gob-side entries. However, achieving the balance among low cost, high waste recycling rates, and adequate strength remains a significant challenge for cemented rockfill. This study used a composite alkali activator to activate gangue cemented rockfill. The compressive strength, scanning electron microscopy, energy dispersive spectrometer, mercury intrusion porosimetry, X-ray diffraction, and thermogravimetric tests were carried out to investigate the effect of the composite alkali activator proportion on the compressive strength, microstructure, and composition of the cemented rockfill. The calcium silicate hydrate (C–S–H) molecular model of cemented rockfill was constructed to explore the fracture evolution of the nucleated molecular structure under tension. The results show that compressive strength initially increased and then decreased with the activator proportion, the optimal activator proportion of 1:2 resulted in a 31.25% increase in strength at 3 d. This reasonable activator proportion strengthens the pozzolanic effect of gangue, and consumes more calcium hydroxide to inhibit its agglomeration, ultimately achieving the densification of microstructure. The activator proportion inevitably substitutes calcium ions with sodium ions in the C–S–H molecular model. The 12% substitution of calcium ions increases the adhesion between silicon chain layers, which is beneficial to the interlayer stress transfer. This work proposes a method for preparing low-cost cemented rockfill from alkali-activated gangue, which can be used for solid waste recycling and reducing cement consumption to achieve low-carbon goals.
1. Introduction
Large accumulations of gangue from coal mining not only occupy land resources but also have the potential to cause collapse and spontaneous combustion, and it also releases hazardous gasses (SO2, CO2, and H2S) into the air [1‒4]. Heavy metal ions from gangue after rainfall further migrate to pollute the groundwater and soil ecosystem [5‒7]. And the surface subsidence caused by goaf is harmful to the ecological environment and people’s safety in the mining region [8‒9]. Using cemented rockfill to fill the goaf may effectively support the overlying strata, alleviate the problem of gangue accumulation, and achieve efficient solid waste recycling while assuring the safety and stability of underground engineering [10‒13].
Overlying strata deformation and safety of underground structures mainly depend on cemented rockfill’s mechanical properties [14‒16]. Ensuring high performance while maintaining low cost has always been the main goal. Previous studies [17‒20] improved the properties of cemented rockfill material by adding external additives (nanomaterials, early strength agents, water reducing agents, activators, etc.). Yeşilmen et al. [21] investigated the effects of nanomaterials on flexural strength, compressive strength, and microstructure, they believed that nano-silica can accelerate the hydration reaction of cement. Roshani and Fall [22] conducted a comprehensive investigation on the rheological properties of cemented paste. Cavusoglu et al. [23] employed sodium silicate as an early strength agent to enhance the early strength and microstructure of cemented rockfill, indicating that adding sodium silicate raised the early strength by 14% and reduced the setting time by 58%. Liu et al. [24] studied the influence of polycarboxylic acid and naphthalene series on the performances of cemented rockfill. We have always focused on enhancing the performance of cemented rockfill through various additives, but the potential of alkali activation on gangue-based cemented rockfill remains underexplored.
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Jiangyu Wu, Wenyu Zhang, Yiming Wang, Feng Ju, Hai Pu, Evgenii Riabokon, Mikhail Guzev, Qian Yin, Dan Ma, Hao Zhang (2025). Effect of composite alkali activator proportion on macroscopic and microscopic properties of gangue cemented rockfill: Experiments and molecular dynamic modelling. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3140-8
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Frequently Asked Questions
What is the optimal composite alkali activator proportion for gangue cemented rockfill?
The optimal proportion is 1:2, which resulted in a 31.25% increase in compressive strength at 3 days compared to other proportions tested.
How does the alkali activator proportion affect the microstructure of cemented rockfill?
A reasonable proportion strengthens the pozzolanic effect of gangue, consumes more calcium hydroxide to inhibit its agglomeration, and ultimately achieves a denser microstructure.
What is the role of molecular dynamics in this study?
Molecular dynamics was used to construct a C–S–H molecular model to explore fracture evolution under tension, revealing that 12% sodium substitution improves interlayer adhesion and stress transfer.
What are the environmental benefits of this research?
The proposed method enables low-cost cemented rockfill production from alkali-activated gangue, promoting solid waste recycling and reducing cement consumption, contributing to low-carbon goals.
What tests were conducted to evaluate the properties of cemented rockfill?
Compressive strength, scanning electron microscopy, energy dispersive spectrometer, mercury intrusion porosimetry, X-ray diffraction, and thermogravimetric tests were carried out.
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