In 2023, researchers crushed discarded coconut shells into aggregate substitutes, replacing 10% of coarse aggregate produced 34.8 N/mm² concrete strength

Every year, the global coconut industry leaves behind huge piles of tough, woody shells with often nowhere useful to go. The majority of these shells are either burned or thrown into landfills, causing greenhouse gas emissions. However, recent developments in the world of civil engineering suggest these shells could have a second life as a partial replacement for gravel in concrete. A 2023 study titled ‘Alteration of Structure and Characteristics of Concrete with Coconut Shell as a Substitution of a Part of Coarse Aggregate’ published in Materials by engineers from the Don State Technical University in Russia found that replacing 10 percent of coarse aggregate in concrete with crushed coconut shells barely affected its strength.

Minimal strength loss at 10% replacement

The Don State team tested concrete with 0% to 30% of the coarse aggregate replaced by coconut shell in 5% increments. However, the findings turned out to be better than expected. Even at a 5% replacement, there was an improvement in compressive strength, which was recorded as 38.0 N/mm², a 4.1% increase over normal concrete strength (about 36.6 N/mm² for the control mix). In the case of 10% replacement, there was a slight reduction in strength to only 34.8 N/mm², a decrease of 4.7%. The study noted that once replacement reached 15%, the losses were steep, and when coconut shell constituted 30% of the coarse aggregate, the losses approached 40%. The study suggested that 10% coconut shell content performed best in these tests, cutting down on the need for gravel without significantly weakening the concrete.

A similar pattern from Malaysia

It is not an isolated finding because an engineering group in Sarawak, Malaysia, tested similar samples using M30-grade concrete. They found that concrete containing 10% coconut shell achieved a compressive strength of 44.7 N/mm² within 28 days of curing, which was almost as much as the 45.4 N/mm² achieved by ordinary concrete, according to a 2021 study published in the journal BioResources. What's even more interesting is that the same research team found that the coconut shell concrete was 3.75% lighter than the ordinary concrete. Although the weight reduction is modest, a building's self-weight can still place added demand on its foundations and columns.

Why lighter concrete is a genuine advantage

Coarse aggregate, mostly made of crushed stones and gravels, forms the bulk of an ordinary concrete mixture, and its use has increased consistently over decades along with construction activities across the world. Rising demand can put pressure on local gravel and stone supplies, which is one reason researchers have long tested agricultural waste materials such as coconut shells as substitutes. Earlier research from 2017, published in the journal Construction and Building Materials, found that the concrete mixture made for a target strength of 20 N/mm² did not require any additional use of cement in case of 5% coconut shell incorporation, and just 3.6% additional use in case of 10% incorporation. That study also found that concrete with 40% coconut shell content was 7.47% lighter than conventional concrete. Lighter concrete can reduce loads on a building's foundation and may offer savings in some projects.

Not quite ready to replace gravel everywhere

This doesn't mean coconut shell concrete will show up on every construction site. Coconut shells are better at absorbing moisture compared to rock, and because of their smooth texture, coconut shells cannot bond to cement paste as well as gravel can, which ends up being one of the reasons why the strength falls at high replacement levels. Most research so far has been on lab-scale cube tests rather than whole buildings, and long-term durability, especially in wet or humid climates, still needs more study. At the roughly 10% replacement level that multiple studies point to, the strength results support structural use; it's at higher replacement levels, where weight savings matter more than peak strength, that applications shift toward lightweight, non-structural work like pavements, boundary walls, and precast panels.

A small fix with a bigger point

The appeal of this small fix is clear. Using coconut shells as an alternative to burning or disposing of them solves two problems: firstly, there is less waste, and secondly, less stress is placed on natural sources of gravel. For coconut-growing countries, that combination could matter more than any single strength figure.