Key Takeaways & Executive Findings
- •• Synergistic optimization of fractal aggregate grading (dimension 2.65) and CO2 nanobubble technology significantly increases CFB density and mechanical performance, with uniaxial compressive strength and elastic modulus improving by up to 13.46% and 27.47%, respectively. • CO2 nanobubble water enhances hydration and carbonization, promoting the formation of C–S–H and C–A–S–H gels and CaCO3, which strengthen the microstructure of cement–fly ash-based backfill. • The CO2 sequestration capacity of cement–fly ash-based materials treated with CO2 nanobubble water increases by 12.4 wt% to 99.8 wt% across different fractal dimensions, supporting low-carbon mine filling. • The research provides a scientific basis for designing high-performance, low-carbon backfill materials and advancing sustainable green mining practices.
Abstract
Mine filling materials urgently need to improve mechanical properties and achieve low-carbon transformation. This study explores the mechanism of the synergistic effect of optimizing aggregate fractal grading and introducing CO2 nanobubble technology to improve the performance of cement–fly ash-based backfill materials (CFB). The properties including fluidity, setting time, uniaxial compressive strength, elastic modulus, porosity, microstructure and CO2 storage performance were systematically studied through methods such as fluidity evaluation, time test, uniaxial compression test, mercury intrusion porosimetry (MIP), scanning electron microscopy-energy dispersive spectroscopy analysis (SEM-EDS), and thermogravimetric-differential thermogravimetric analysis (TG-DTG). The experimental results show that the density and strength of the material are significantly improved under the synergistic effect of fractal dimension and CO2 nanobubbles. When the fractal dimension reaches 2.65, the mass ratio of coarse and fine aggregates reaches the optimal balance, and the structural density is greatly improved at the same time. At this time, the uniaxial compressive strength and elastic modulus reach their peak values, with increases of up to 13.46% and 27.47%, respectively. CO2 nanobubbles enhance the material properties by promoting hydration reaction and carbonization. At the microscopic level, CO2 nanobubble water promotes the formation of C–S–H (hydrated calcium silicate), C–A–S–H (hydrated calcium aluminium silicate) gel and CaCO3, which is the main way to enhance the performance. Thermogravimetric studies have shown that when the fractal dimension is 2.65, the dehydration of hydration products and the decarbonization process of CaCO3 are most obvious, and CO2 nanobubble water promotes the carbonization reaction, making it surpass the natural state. The CO2 sequestration quality of cement–fly ash-based materials treated with CO2 nanobubble water at different fractal dimensions increased by 12.4wt% to 99.8wt%. The results not only provide scientific insights for the design and implementation of low-carbon filling materials, but also provide a solid theoretical basis for strengthening green mining practices and promoting sustainable resource utilization.
1. Introduction
As global climate change escalates and the need for natural resources rises, mitigating carbon emissions and harnessing waste resources have emerged as critical issues for environmental conservation and sustainable development. Cement–fly ash-based backfill materials (CFB) provide an eco-friendly, cost-effective alternative for mine filling. CFB lowers material expenses and carbon emissions by partially substituting cement with industrial waste fly ash, simultaneously improving structural integrity by filling goaf. Nevertheless, fly ash exhibits poor reactivity, and when its percentage escalates, the mechanical qualities and durability of CFB frequently deteriorate, constraining its application in intricate engineering contexts. Consequently, enhancing macroscopic mechanical characteristics and optimizing microstructure by scientific design and sophisticated modification technologies is a contemporary research priority.
In recent years, mine filling technology, as an important means of solid waste resource utilization and ecological environment protection, has been widely used in goaf support, surface settlement control and reduction of waste rock storage. Typical filling materials such as cement–fly ash-based mixtures are widely used due to their good fluidity and mechanical properties. To improve mechanical strength and fracture toughness, some studies have introduced fiber reinforcement technology and used X-shaped rock mass to reinforce broken pillars. In addition, the combination of tailings from different ore types and crushed stones with varying particle sizes has also been proven to significantly affect the microstructure and strength evolution behavior of the filling body. In mine backfill materials, the composition of aggregate gradation significantly affects the compaction and mechanical properties of cementitious fill materials. Conventional grading design often depends on empirical formulae or screening curves. Recent progress in fractal dimension-based aggregate grading offers a more empirical basis for aggregate design, precisely characterizing the complexity of particle dispersion and enhancing aggregate packing density.
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Xiaoxiao Cao, Haoyan Lyu, Yanlong Chen, Jiangyu Wu, Hideki Shimada, Takashi Sasaoka, Akihiro Hamanaka (2025). CO2 nanobubble-enhanced cement–fly ash backfill: Optimizing aggregate gradation and microstructure. Journal of Mineral Metallurgy and Materials Science. https://doi.org/10.1007/s12613-025-3154-2
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Frequently Asked Questions
What are cement–fly ash-based backfill materials?
Cement–fly ash-based backfill materials (CFB) are eco-friendly mine filling materials that partially substitute cement with industrial waste fly ash. They lower material costs and carbon emissions while improving structural integrity by filling goaf.
How does CO2 nanobubble technology improve backfill performance?
CO2 nanobubble water promotes hydration and carbonization reactions, leading to increased formation of C–S–H and C–A–S–H gels and CaCO3. This enhances the mechanical strength and microstructure of the backfill material, while also improving CO2 sequestration.
What is the optimal fractal dimension for aggregate gradation in this study?
A fractal dimension of 2.65 was found to provide the optimal balance between coarse and fine aggregates, significantly improving structural density and yielding peak uniaxial compressive strength and elastic modulus.
How much does CO2 sequestration increase with this approach?
The CO2 sequestration quality of cement–fly ash-based materials treated with CO2 nanobubble water increased by 12.4 wt% to 99.8 wt% across different fractal dimensions, demonstrating strong potential for low-carbon backfill applications.
What are the practical implications of this research?
The findings provide scientific insights for designing and implementing low-carbon filling materials and offer a solid theoretical basis for strengthening green mining practices and promoting sustainable resource utilization.
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