Influence of recycled fine aggregate particle size on wet carbonation for producing sustainable aggregate from construction waste
This study demonstrates that wet carbonation of recycled fine aggregate (RFA) is highly dependent on particle size, where coarser fractions (>1.18 mm) achieve superior pore refinement and uniform carbonation due to slower ion leaching, while finer fractions suffer from surface passivation and increased porosity, ultimately yielding a 25.1% compressive strength improvement in mortars when recombined carbonated RFA is used.
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
Every time a city grows or an old bridge is torn down, it leaves behind a mountain of broken concrete. For decades, the standard way to handle this rubble has been to crush it and use it as a substitute for the natural sand and gravel found in riverbeds. This recycled material is a vital resource for building a sustainable future, but it comes with a flaw. The broken pieces are often coated in a rough, porous layer of old cement paste that makes them weak and thirsty for water. To fix this, scientists have developed a process called wet carbonation. In simple terms, this involves soaking the crushed concrete in water and bubbling carbon dioxide gas through the mixture. The gas reacts with the calcium in the old cement to form a new, solid mineral that can fill the holes and strengthen the material. It is a clever way to turn waste into a better building block while simultaneously trapping carbon dioxide from the air.
However, a new study from researchers at Central South University suggests that this process does not work the same way for every piece of rubble. The team discovered that the size of the crushed particles is the deciding factor in whether the treatment helps or hurts. They found that the method acts like a double-edged sword: it strengthens the larger chunks of recycled sand while simultaneously weakening the finest grains. This finding challenges the common assumption that all recycled concrete benefits equally from carbonation and reveals that the size of the particle dictates a complex chemical battle happening inside the water.
To uncover this hidden dynamic, the researchers took a batch of recycled fine aggregate, which is essentially crushed concrete sand, and sorted it into five distinct size groups. They then treated each group separately, bubbling carbon dioxide through the water and sand mixture for one hour. By keeping the particle sizes separate, they could watch exactly how the chemical reactions unfolded in the fine dust versus the coarser grains. They measured the acidity of the water, tracked the release of minerals, and examined the physical changes in the sand grains using powerful microscopes and chemical analysis tools.
The results showed a sharp divide at a specific size threshold of 1.18 millimeters. For the larger grains, the process was a success. Because these particles are bigger, they have less surface area relative to their volume, which meant the chemical reaction happened more slowly. This slower pace allowed the carbon dioxide to penetrate deep into the pores of the sand before a solid crust could form on the outside. As a result, the new mineral formed inside the holes, plugging them up and making the grains denser and stronger. The water absorption of these larger grains dropped, and their ability to resist crushing improved significantly.
In contrast, the finest grains, those smaller than 1.18 millimeters, suffered a different fate. These tiny particles have a massive surface area, which caused them to release their internal minerals into the water very quickly. This rapid release triggered an immediate reaction on the surface of the grains, creating a thick, hard layer of new mineral almost instantly. While this sounds beneficial, it actually created a barrier. This surface crust blocked the carbon dioxide from reaching the interior of the grains. Meanwhile, the rapid chemical reaction stripped away some of the binding material inside the grains, leaving them with more holes and a weaker structure. The total volume of pores in these fine grains increased by nearly 19 percent, making them more fragile and absorbent than before.
Despite this deterioration in the finest fraction, the overall story ends on a high note. The researchers recombined all the treated grains, mixing the strengthened large ones with the weakened small ones to create a complete, graded sand suitable for construction. When they used this mixed, treated sand to make mortar, the final product was significantly stronger than mortar made with untreated recycled sand. At a replacement rate of 100 percent, the new mortar was 25.1 percent stronger after 28 days. The researchers suggest that the gains from the strengthened large grains were substantial enough to outweigh the losses from the fine grains. Furthermore, the chemical changes in the fine grains, while damaging to their individual structure, produced a silica-rich material that may help bind the cement together in the final mix, offering a secondary benefit.
This study highlights that recycling construction waste is not a one-size-fits-all solution. The effectiveness of upgrading recycled aggregate depends heavily on the specific size of the particles being treated. The process works best when the reaction is allowed to proceed slowly enough to penetrate the material rather than just coating the surface. For the construction industry, this means that future recycling plants might need to sort their waste more carefully, perhaps treating different size fractions with different timing or methods to maximize the strength of the final building material. By understanding these microscopic interactions, engineers can better transform the mountains of urban rubble into the foundation of a more sustainable world.
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