Synergistic effects of recycled concrete aggregate and spherical nickel slag on abrasion resistance, acoustic attenuation, and environmental efficiency of porous concrete

Cities are constantly reshaping themselves, and with every new building and road, vast amounts of old concrete are torn down and discarded. At the same time, heavy industry produces mountains of waste material, such as the slag left over from smelting metals. Traditionally, these materials end up in landfills, taking up space and posing environmental risks, while the construction industry continues to dig up fresh river stones and gravel to make new concrete. This cycle is unsustainable. Engineers are now searching for ways to turn these waste streams into valuable building blocks, creating materials that not only save resources but also perform better than the traditional options. One such material is porous concrete, a special kind of pavement that looks like a sponge. Instead of a solid, impermeable surface, it is filled with tiny, connected holes that let rainwater soak straight through, reducing floods and recharging groundwater. Because of these holes, it also acts as a natural sound barrier, soaking up the noise of traffic rather than reflecting it back into the air. However, making this material is tricky. The very holes that make it good for water and sound often make it weak, causing the surface to crumble under the weight of tires.

Researchers at Hasanuddin University in Indonesia set out to solve this puzzle by testing a new recipe for porous concrete. They wanted to see if they could mix two types of waste materials together to create a pavement that was strong enough to last but porous enough to work. The team took two specific waste products: recycled concrete aggregate, which is old concrete crushed into small stones, and spherical nickel slag, which is a byproduct of nickel smelting that has been rapidly cooled in water to form smooth, round balls. They created six different batches of concrete. Some used only natural river stones, some used only recycled concrete, some used only the nickel slag, and the rest used various combinations of these materials. To ensure a fair test, they kept the amount of cement and water exactly the same for every batch, changing only the type of stone used. They then put these concrete cylinders through a series of rigorous tests to see how heavy they were, how many holes they contained, how well they resisted being worn away by friction, and how effectively they absorbed sound. They also calculated the environmental cost of making each batch, measuring the carbon emissions and energy required from the moment the raw materials were extracted until the concrete left the factory.

The results revealed a clear story about how the shape and density of the stones changed the concrete. When the researchers used the smooth, heavy nickel slag, the concrete became significantly denser and heavier, but it also lost a large portion of its porosity. The round balls packed together so tightly that there was less space for water to flow through. In contrast, the mixture that combined natural stones with the rough, angular recycled concrete created the most porous structure, with the highest volume of interconnected holes. This high porosity turned out to be the key to acoustic performance. The mixture with the most holes absorbed sound better than any other, reaching a level where it could capture nearly all the sound energy hitting it at certain frequencies. This suggests that for reducing traffic noise, a more open, airy structure is superior. However, this openness came with a trade-off. The most porous mixture was also the weakest when it came to resisting abrasion. When placed in a machine that simulated the grinding action of tires, this mixture lost the most mass, shedding pieces of stone and mortar.

The most surprising finding came from the mixture that combined the recycled concrete with the spherical nickel slag. This specific blend, which the researchers called RS50, achieved the best balance of all. It was not the most porous, nor was it the lightest, but it proved to be the most durable. When subjected to the same grinding test, it lost the least amount of material, holding its shape far better than the other mixes. The smooth, hard spheres of the nickel slag seemed to act as a strong skeleton, locking the rough recycled stones in place and preventing them from being knocked loose. This combination created a structure that was tough enough to withstand wear and tear while still maintaining enough porosity to be functional. From an environmental standpoint, replacing the natural river stones with recycled concrete and industrial slag significantly lowered the carbon footprint and energy cost of the concrete. The recycled materials required far less energy to produce than mining and crushing new stones. However, the study also noted that the cement used to bind everything together remained the largest source of environmental impact, accounting for the vast majority of the carbon emissions in every batch, regardless of the stones used.

Ultimately, the study showed that there is no single "perfect" mixture for every job. If the goal is to build a quiet sidewalk or a noise barrier where sound absorption is the priority, the mixture with the most recycled concrete and natural stones is the best choice, as it creates the most effective sound-absorbing sponge. But if the pavement needs to handle heavy traffic or frequent use where durability is critical, the blend of recycled concrete and spherical nickel slag is the superior option. This combination offers the best environmental performance when durability is taken into account, proving that waste materials can be engineered to work together. By carefully selecting and mixing these industrial byproducts, engineers can create porous concrete that is not only kinder to the planet but also stronger and more effective than traditional materials. The research demonstrates that the path to sustainable infrastructure lies not just in using waste, but in understanding how different waste materials interact to create a whole that is greater than the sum of its parts.