Mechanical and abrasion behaviour of cement and alkali-activated material mixes based on oil refinery waste catalyst: investigation and semi-empirical predictive models

Concrete floors in factories, warehouses, and busy public spaces face a constant battle against wear. Every footstep, every rolling cart, and every vehicle tire grinds against the surface, slowly stripping away material and weakening the structure over time. To build floors that last, engineers rely on cement, the binding agent that holds concrete together. However, producing this cement comes with a heavy price: for every ton of cement made, nearly 0.85 tons of carbon dioxide are released into the atmosphere, contributing significantly to climate change. This reality has pushed researchers to look for ways to replace some of the cement with industrial waste, turning a pollution problem into a construction solution. One such waste product comes from oil refineries, where tiny particles called catalysts are used to help turn crude oil into fuel. Once these particles have done their job, they are discarded as "spent catalyst." These particles are rich in aluminum and silica, the same ingredients found in the rocks and sands that make up concrete, suggesting they might be useful in a new life.

A team of researchers set out to test whether these discarded oil refinery particles could be mixed into concrete to create stronger, more durable floors that also help the environment. They focused on two different types of concrete mixtures. The first was a standard cement mix, where the waste particles would act as a filler to replace some of the cement. The second was an "alkali-activated" mix, a more advanced type of concrete that uses a chemical solution to bind the waste particles together without traditional cement at all. The researchers wanted to see how the size of the waste particles and the type of mixture affected the concrete's ability to resist crushing, bending, and the constant grinding of abrasion. They also aimed to create a set of rules that engineers could use to predict how strong a floor would be before they even poured the concrete, based simply on how much waste they added and how fine the particles were.

The team began by gathering their materials: standard Portland cement, river sand, water, and the spent catalyst particles from a refinery in Poland. They carefully sorted the catalyst particles into two sizes, one finer and one coarser, to see if size mattered. They then created a series of test batches. Some batches replaced up to 20 percent of the cement with the waste particles, while others used the particles as the main ingredient in the alkali-activated mix. To ensure the mixtures were workable, they added a liquid plasticizer to help the wet concrete flow easily into molds. After the concrete hardened, the researchers put the samples through a rigorous series of tests. They crushed cubes of the material to measure compressive strength, bent long beams to test flexural strength, and used a specialized machine that rubbed a heavy, abrasive wheel against the surface to measure how much weight the material lost over time. This abrasion test is crucial because it simulates years of foot traffic and vehicle wear in a matter of hours.

The results revealed a clear split in performance depending on which type of concrete was used. In the standard cement mixtures, adding the waste particles worked remarkably well. When the researchers replaced 20 percent of the cement with the spent catalyst, the resulting concrete became stronger and more resistant to wear. The mix with the finest particles achieved a compressive strength of 52 megapascals and a flexural strength of 8 megapascals, while also showing an 18 percent improvement in resistance to abrasion compared to plain cement. The waste particles acted like a sponge, absorbing water and reacting with the cement to fill in tiny gaps, creating a denser and tougher material. However, the story was different for the alkali-activated mixtures. In these mixes, the waste particles did not react as fully with the chemical binder. The resulting material was significantly weaker and suffered from much higher wear, losing weight at a rate nine times higher than the cement mixes after 28 days. The researchers found that the chemical activation process was incomplete, leaving many of the waste particles unreacted and loosely held within the structure, which made the surface crumble easily under friction.

To make sense of these complex results, the researchers developed a set of mathematical models that could predict the performance of the concrete without needing to run every single test. These models were not just random guesses; they were built on physical principles that linked the amount of waste added, the size of the particles, the type of binder used, and the age of the concrete to its final strength and durability. The models proved to be highly accurate, predicting the compressive strength, bending strength, and abrasion loss with an error margin of less than 7 percent. This means that if an engineer knows the specific mix they are planning, they can use these formulas to confidently predict how the floor will perform under heavy traffic. The models confirmed that for standard cement, adding the waste was a win-win, boosting strength and durability. For the alkali-activated mixes, the models highlighted that the current chemical recipes were not yet ready to fully utilize this waste, as the particles remained too inert to provide the necessary binding power.

The study concludes that using oil refinery waste in concrete is a viable path forward, but it requires the right approach. Replacing a portion of cement with these particles creates a high-performance material that is tougher against wear and reduces the carbon footprint of construction. The researchers estimate that swapping out 20 percent of the cement could cut carbon emissions by approximately 170 kilograms for every ton of binder used. However, this benefit is only realized when the waste is used in traditional cement mixes, where it acts as a reactive filler. In the more experimental alkali-activated systems, the technology is not yet mature enough to handle this specific waste effectively. The work provides a clear roadmap for the construction industry: by carefully selecting the type of concrete and the size of the waste particles, builders can create floors that are not only more sustainable but also longer-lasting, turning an industrial by-product into a foundation for the future.