High-value reuse of coal gangue through fiber- reinforced pavement-base composites: Mechanical performance and model-based feasibility

Every year, the global coal industry produces a staggering amount of leftover rock and dirt, a waste product known as coal gangue. This material, which makes up a quarter of the raw coal dug from the earth, is usually piled into massive, unsightly mountains that take up land, pollute the soil, and can even catch fire on their own. For decades, engineers have looked for ways to turn this waste into something useful, often grinding it down to use in road construction. However, simply mixing this crushed rock with cement to make a road base has a flaw: the resulting material can be brittle, meaning it might crack easily under the heavy, repeated weight of trucks. To solve this, researchers have begun experimenting with adding tiny fibers to the mix, much like adding steel rebar to concrete, to make the material tougher and more resistant to breaking.

A team of researchers from China and Australia set out to find the perfect balance for this new type of road material. They created a composite mixture using coal gangue, fly ash (a fine powder from power plants), cement, and chopped basalt fibers. Their goal was to determine exactly how much fiber was needed to make the road base strong without making it too soft or too stiff. They tested mixtures containing different amounts of fiber, ranging from a very small amount to a higher concentration, and subjected them to rigorous laboratory tests. They squeezed the material to see how much weight it could hold before breaking, pulled it apart to test its resistance to cracking, and measured how much it bounced back after being compressed. They also used powerful microscopes to look at the tiny structures inside the material and ran computer simulations to see how a road made of this mixture would behave under real-world traffic.

The experiments revealed a clear sweet spot for the amount of fiber to use. When the researchers added a small amount of basalt fiber, the material became stronger, but only up to a point. The mixture with 0.6 percent fiber by weight proved to be the champion, achieving the highest strength in both compression and tension tests. At this level, the material could withstand a crushing force of nearly 14.5 megapascals after 28 days of curing, a significant improvement over mixtures with less or more fiber. Interestingly, adding more fiber than this optimal amount actually made the material weaker, likely because the fibers began to clump together rather than spreading out evenly. While the 0.6 percent mixture was the strongest, the researchers also discovered a trade-off: as they added more fiber, the material became slightly less stiff, meaning it would flex a bit more under load rather than staying rigid. This suggests that while the fibers make the road tougher against cracking, they also make it slightly more flexible.

To understand how this material would perform in a real highway, the team built a detailed three-dimensional computer model of a road section. They simulated a four-lane highway with a base layer made of their best-performing mixture and watched how the road settled and reacted to moving vehicles. The simulation showed that the amount of fiber in the base layer had a surprisingly small effect on how much the road sank under traffic. Whether the fiber content was 0.2 percent or 0.8 percent, the difference in road settlement was minimal. Instead, the simulations revealed that the speed of the vehicles and the weight of the traffic were far more important factors. When vehicles moved slowly, the road settled significantly more because the heavy load stayed in one spot for a longer time, pressing down harder. Conversely, faster-moving vehicles caused less settlement. Similarly, heavier trucks caused much greater sinking than lighter ones. This indicates that while the material itself is robust, the behavior of the road is driven more by how the traffic moves across it than by tiny variations in the fiber content.

Looking closely at the material under a microscope, the researchers saw how the magic of the mixture actually worked. The coal gangue particles, which are naturally rough and irregular, provided a good surface for the cement to grip. As the mixture aged, the cement and fly ash reacted to form a dense, glue-like substance that filled the gaps between the rock particles. The basalt fibers were found embedded in this hardened matrix, acting like tiny bridges that could hold the material together if a crack started to form. The images showed that after 28 days, the material had become much denser and more solid than it was after just one week, with fewer empty spaces and better connections between all the ingredients. This microscopic evidence confirmed that the fibers were not just sitting there but were actively interacting with the cement paste to reinforce the structure.

Ultimately, this study provides a clear path forward for using coal gangue in a high-value way. By finding the precise amount of fiber needed, engineers can create road bases that are strong, durable, and capable of handling heavy traffic without the brittleness of traditional materials. The research suggests that while the specific mix of ingredients matters, the way we drive on these roads—specifically our speed and the weight of our vehicles—plays an even larger role in how the road holds up over time. This work turns a massive environmental problem into a practical solution, offering a way to build sustainable infrastructure that is both strong and flexible enough to last.