Recycled EPS Diluted in D-Limonene as a Sustainable Stabilizer for Building Foundation Subgrade: Evaluation of Physical and Mechanical Properties in Fine Clay Soils

Beneath every building, from a humble home to a towering skyscraper, lies a layer of earth that must be strong enough to hold the weight above it. This ground, often a mix of clay and silt, is the foundation of our built world. However, natural soil is not always reliable; it can be too soft, too squishy, or prone to shifting when wet. For decades, engineers have solved this by mixing in cement or lime to harden the dirt. While effective, these traditional methods come with a heavy price tag on the planet, as their production releases vast amounts of carbon dioxide into the atmosphere. At the same time, the world is drowning in plastic waste, particularly expanded polystyrene, the lightweight foam used in packaging and disposable cups. This material is durable to a fault, persisting in landfills for centuries without breaking down. The challenge for modern science has been to find a way to turn this stubborn waste into a useful tool for construction without creating new environmental problems.

A team of researchers from universities in Colombia has taken a novel approach to this problem, proposing a method that turns two distinct waste streams into a single, powerful solution. They investigated whether recycled polystyrene foam, when dissolved in a natural, citrus-based liquid, could be used to strengthen fine clay soils. The goal was to create a new type of ground stabilizer that is not only effective at holding up buildings but also reduces the carbon footprint of construction and gives a second life to plastic waste. By mixing the dissolved foam into the clay, the researchers aimed to create a composite material that is stronger and more rigid than the soil alone, offering a sustainable alternative to the cement-heavy practices of the past.

The experiment began with a specific type of clay soil, one that is common in many regions but often too weak to support heavy structures on its own. The researchers took recycled polystyrene beads, the kind found in packaging, and dissolved them in D-limonene, a clear liquid extracted from orange peels. This solvent is biodegradable and non-toxic, making it a far greener choice than the harsh chemical solvents often used in industrial processes. They created dozens of small, cylindrical samples of this clay, each mixed with different amounts of the dissolved foam and varying concentrations of the orange-scented liquid. These samples were then compacted tightly and left to dry in the air for a week, allowing the solvent to evaporate and the foam to set within the soil matrix.

Once the samples were ready, the team put them through a series of rigorous tests to see how they held up. They measured how fast sound waves could travel through the material, a technique that reveals how dense and continuous the internal structure is. They also subjected the cylinders to immense pressure to see how much force they could withstand before cracking. The results were striking. The samples that contained the highest amount of dissolved foam, specifically a mix where the foam made up 12 percent of the total weight and the citrus solvent was present at 25 percent, performed exceptionally well. These samples could withstand a pressure of nearly 1,010 kilopascals. To put this in perspective, the untreated clay soil used as a baseline could only handle about 123 kilopascals. This means the new mixture was nearly six times stronger than the natural soil, a massive leap in performance.

Beyond just raw strength, the material showed other promising traits. The samples with the most foam became significantly stiffer, meaning they were less likely to bend or deform under load, yet they retained enough flexibility to absorb stress without shattering immediately. The researchers also observed that the presence of the foam helped trap the citrus solvent inside the mixture for longer, allowing for a more uniform hardening process as the liquid slowly evaporated. This encapsulation effect ensured that the foam distributed evenly throughout the clay, creating a cohesive network that bound the soil particles together. Statistical analysis confirmed that the increase in strength was not a fluke but a direct result of the specific proportions of foam and solvent used.

The study suggests that this combination of recycled plastic and natural solvent offers a viable path forward for geotechnical engineering. It demonstrates that waste materials, often seen as a burden, can be transformed into high-performance construction aids. The researchers found that the optimal balance required a specific amount of solvent to dissolve the foam effectively without leaving too much liquid behind, which would weaken the final product. When this balance was achieved, the resulting soil mixture was not only stronger but also lighter, which could reduce the overall load on a building's foundation. This dual benefit of increased strength and reduced weight, achieved through the reuse of waste, points to a more sustainable future for building foundations.

While the study focused on the physical and mechanical properties of the material, the implications extend to the broader environmental impact of construction. By replacing a portion of the cement typically used to stabilize soil with this recycled foam mixture, the construction industry could significantly lower its carbon emissions. The process relies on materials that are already abundant as waste, turning a disposal problem into a resource. The researchers noted that future work could involve testing how temperature affects the drying process to further optimize the removal of the solvent, ensuring the material reaches its full potential. For now, the findings provide a clear, measured proof that a simple mixture of orange peel liquid and recycled foam can turn weak, shifting clay into a solid, reliable base for the structures of tomorrow.