Mechanical properties and self-expansion mechanism of cemented slag- based backfill influenced with biomass ash
This study demonstrates that incorporating biomass ash into cemented slag-based backfill optimizes its mechanical strength and self-expansion properties by promoting C-S-H gel formation at specific critical ratios, while excessive ash content introduces unreacted silica that degrades performance.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the deep underground tunnels of modern mining, safety and efficiency depend on what happens after the ore is removed. When miners extract valuable minerals, they leave behind vast, empty voids that can cause the rock above to collapse. To prevent this, engineers fill these spaces with a mixture of water, cement, and crushed rock waste, creating a solid mass that supports the mine roof. However, a specific challenge arises with certain types of waste rock, known as slag, which naturally expand as they harden. This self-expansion is a double-edged sword. On one hand, it helps the filling material swell and press tightly against the mine roof, eliminating dangerous gaps. On the other hand, this swelling often weakens the material, making it too fragile to support heavy loads or preventing it from holding its shape over time. The goal for engineers has long been to find a way to keep the helpful expansion while stopping the dangerous weakening.
Researchers at the Jiangxi University of Science and Technology and the North China University of Science and Technology set out to solve this puzzle by testing a common byproduct of burning plant matter for energy: biomass ash. They mixed this ash into a cemented backfill made from mining slag to see if it could act as a regulator. The team prepared dozens of cylindrical samples, varying the amount of ash added from zero up to five percent of the total solid weight. They then waited for these samples to cure over periods of seven, fourteen, and twenty-eight days, subjecting them to tests that measured how much they could be squeezed before breaking and how much they stretched before snapping. They also tracked how much the samples grew in size over time. To understand exactly what was happening inside the material, they used powerful microscopes and chemical scanners to look at the tiny structures formed as the mixture hardened.
The results revealed a delicate balance that depends entirely on the precise amount of ash added. When the researchers added a small amount of biomass ash, the backfill became significantly stronger. The ash acted like a fine powder that filled in the tiny gaps between the larger rock particles and reacted with the cement to create a stronger, more cohesive glue. This process made the material denser and less likely to develop large internal cracks. However, the study found that this benefit has a strict limit. If too much ash is added, the material begins to fail. The excess ash introduces more silica, a component that the cement cannot fully use, leaving behind unreacted particles that weaken the structure. The researchers identified a specific "sweet spot" for the ash content. For the material to reach its maximum strength after twenty-eight days, the ideal amount of biomass ash was found to be approximately 2.5 percent of the total weight. For the material's resistance to being pulled apart, the ideal amount was slightly lower, at about 1.7 percent.
Just as the strength followed a curve of rising and then falling, the expansion behavior of the backfill followed a similar pattern. Without any ash, the backfill expanded quite a bit over time, which is useful for filling gaps but bad for structural integrity. As the researchers added small amounts of ash, the expansion rate dropped, meaning the material stayed more stable and did not grow as much. This reduction in expansion was a direct result of the ash helping to create a tighter, more compact internal structure. However, once the ash content passed a critical threshold of roughly 3 percent, the expansion began to rise again. The excess ash disrupted the chemical reactions that usually keep the material stable, causing it to swell more than the control samples.
By examining the microscopic details, the team confirmed that the ash works by promoting the formation of a gel-like substance that binds the rock particles together. When the amount of ash is just right, this gel fills the pores and creates a dense, strong network. When there is too much ash, the network becomes porous and weak because the extra particles prevent the gel from forming properly. The study concludes that biomass ash is a powerful tool for mining engineers, but only if used with precision. It offers a way to tune the backfill material, allowing them to choose between a mixture that expands more to fill a roof or one that stays strong to support heavy loads, simply by adjusting the amount of ash added to the mix. This discovery provides a practical method for improving safety in mines by turning a waste product into a precise engineering tool.
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