Feasibility Research on Coal Gasification Slag Used as a Filler in Asphalt Mastic
This study demonstrates that low-carbon pulverized-coal gasification slag is a feasible and economically superior alternative to limestone filler in asphalt mastic, offering comparable rheological performance while reducing material costs by approximately 56.3%.
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
Roads are more than just layers of stone and tar; they are complex systems where a sticky, black binder called asphalt holds together a skeleton of crushed rock. To make this mixture work, engineers add a fine powder, usually made from ground limestone, which acts as a crucial glue. This powder, known as filler, fills the tiny gaps between larger stones and interacts with the asphalt to create a paste that is strong enough to resist the heat of a summer sun yet flexible enough to bend without cracking in the freezing cold. However, the world is running out of easy-to-mine limestone, and the process of grinding it creates dust and consumes energy. This has led engineers to look for alternatives in the piles of industrial waste that surround us, hoping to turn a disposal problem into a construction solution.
One such waste product comes from the coal gasification industry, a process that turns coal into a clean-burning gas. This process leaves behind a solid byproduct called coal gasification slag. For years, this material has been a burden, sitting in landfills and occupying valuable land. Researchers in China recently asked a simple but difficult question: could this slag be ground down and used instead of limestone to make better roads? The challenge was that not all slag is the same. Depending on how the coal was processed, the resulting waste could vary wildly in its chemical makeup, how rough its particles were, and how much unburned carbon remained trapped inside. Some might be too sticky, making the road brittle in winter, while others might be too weak to handle the heat of a heavy truck.
To find the answer, a team of engineers gathered six different types of this slag from various industrial plants, along with a sample of standard limestone. They did not simply dump them into asphalt; they first ground each type of slag into a powder so fine that its particle size matched the limestone exactly. This was a critical step, ensuring that any differences they saw later were due to the material itself, not just the size of the grains. They then mixed these powders with hot asphalt to create a paste and tested how each mixture behaved. They subjected the samples to extreme heat to see if they would soften and deform under the weight of traffic, and to freezing cold to see if they would crack. They also looked at how the materials reacted to repeated stress, simulating the constant pounding of tires over years of use.
The results revealed a clear pattern. The researchers found that the more unburned carbon and porous structure a slag sample had, the stiffer the resulting road paste became. While this stiffness was good for resisting the heat of summer, it came at a cost: the material became too rigid for the cold, losing its ability to relax and stretch without breaking. Several of the slag samples were too extreme in one direction or the other, either becoming too hard in the heat or too brittle in the cold. However, one specific type of slag, produced by a low-carbon gasification process, stood out. This material, which had a relatively smooth surface and very little trapped carbon, offered a perfect middle ground. When mixed with asphalt, it provided a significant boost in heat resistance—improving the material's ability to hold its shape at high temperatures by nearly twenty percent—while barely affecting its ability to handle the cold. In fact, its performance in freezing conditions was almost identical to that of the traditional limestone.
Having identified this promising candidate, the team then investigated how much of it should be used. They tested different amounts of the slag powder mixed into the asphalt to find the "sweet spot." They discovered that if the amount of powder was too low, the benefits were minimal. But as they increased the amount, the road mixture became stronger against heat. However, pushing the amount too high caused the mixture to lose its flexibility and become prone to cracking. The researchers determined that the ideal range was to replace the limestone with a volume of slag powder that was between thirty and forty percent of the total volume of the binder. Within this range, the road material gained the best balance of strength and flexibility.
Beyond the physics, the study also looked at the economics. Because coal gasification slag is often a waste product, it is available for free or at a very low cost, whereas limestone must be mined, crushed, and transported, which adds up quickly. The researchers calculated that for a typical highway project, switching to this slag could cut the cost of the filler material by more than half. Even when accounting for the cost of drying and grinding the slag to the right size, the savings were substantial, amounting to tens of thousands of dollars per kilometer of road. The study concluded that this specific type of coal gasification slag is not just a theoretical possibility but a practical, cost-effective, and high-performing alternative to traditional limestone, offering a way to build better roads while cleaning up industrial waste.
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