Waste valorization of coconut shell char as a partial cement replacement in sustainable mortar: fresh-state, setting, and mechanical performance
This study demonstrates that incorporating 5–10% coconut shell char, produced via pyrolysis, as a partial cement replacement optimizes the balance between fresh-state properties and mechanical performance in sustainable mortar, with the 5% dosage yielding superior compressive and flexural strengths compared to the control.
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
Concrete is the most widely used building material on Earth, forming the skeleton of our cities, bridges, and homes. Yet, the primary ingredient in concrete, Portland cement, carries a heavy environmental cost. The process of making it releases vast amounts of carbon dioxide and consumes significant mineral resources, prompting engineers to look for ways to reduce its use without compromising the strength of the final structure. One promising avenue involves replacing a portion of the cement with waste materials that have been transformed into a fine, carbon-rich powder. This approach aims to turn agricultural leftovers into useful building components, potentially lowering the carbon footprint of construction while managing waste. The challenge lies in finding the right amount to add; too little might not make a difference, while too much could weaken the material or make it impossible to pour and shape before it hardens.
In a recent study, researchers investigated whether coconut shells, a common waste product in tropical regions, could serve as such a replacement. They took discarded coconut shells and heated them in a low-oxygen environment at a high temperature of 750 degrees Celsius for two hours. This process, known as pyrolysis, turned the hard shells into a black, lightweight powder called coconut shell char. The resulting material was a fine, porous substance with a high carbon content, distinct from the heavy, mineral-based cement it was meant to replace. The team mixed this char into standard mortar—a mixture of cement, sand, and water—at various levels, ranging from a small amount up to nearly a third of the solid material in the mix. Their goal was to see how this addition affected the mixture's behavior when wet, how long it took to set, and how strong it became after a month of curing.
The results showed that adding the coconut shell char changed the mortar in predictable ways. Because the char is lighter and full of tiny holes, the fresh mixture became less dense as more was added. However, this porosity also made the mixture harder to work with. The rough, sponge-like surface of the char particles absorbed some of the mixing water and created more friction between particles, causing the flow of the mortar to decrease significantly as the amount of char increased. The mixture also took longer to harden; the time it took for the mortar to begin setting and to fully harden both increased as more char was included, likely because there was less cement available to trigger the chemical reactions that bind the material together.
Despite these changes in the fresh state, the mechanical strength of the hardened mortar followed a surprising pattern. When the researchers tested the strength of the samples after 28 days, they found that adding a small amount of the char actually made the mortar stronger. The mixture containing 5 percent coconut shell char by volume achieved a compressive strength of 38.1 megapascals, which was notably higher than the 31.6 megapascals recorded for the control mixture with no char. This same 5 percent mix also showed improved flexibility, resisting bending forces better than the standard mortar. However, this benefit did not continue indefinitely. As the researchers increased the amount of char beyond 10 percent, the strength began to drop. At higher levels, the reduction in cement content and the introduction of too many porous, non-reactive particles overwhelmed the benefits, leading to a weaker structure.
The study concludes that there is a specific window where coconut shell char is most effective. A replacement level between 5 and 10 percent offered the best balance, allowing for a reduction in cement use while maintaining or even improving the structural performance of the mortar. This range reduced the amount of cement needed by roughly 3.6 to 7.2 percent by mass without sacrificing the ability of the material to bear weight or resist cracking. While the char did not act as a chemical reactant to strengthen the cement in the way some other additives do, its physical properties helped fill gaps and improve the packing of the mixture at low doses. The researchers emphasize that while this approach shows promise for turning waste into a useful resource, it is not a complete solution on its own. The environmental benefits depend on the energy used to create the char and the long-term durability of the material, areas that require further study. For now, the work demonstrates that a modest amount of processed coconut shell can be a viable ingredient in sustainable mortar, offering a practical step toward reducing the reliance on traditional cement.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.