Sargassum Concrete: A New Initiative to Put Invasive Seaweed to Good Use

If the trial of concrete made with sargassum proves successful, it could be used for road construction projects in Saint Barthélemy.

How can sargassum be put to good use? Since the proliferation of this seaweed in the Atlantic, this question has been on the minds of many communities. Has a solution been found in Saint Barth? That, at least, is what Nicolas Vernoux-Thélot, a native of the island, hopes. The architect is spearheading a project for a new process that uses Sargassum in concrete for road construction. An experimental phase of the project—for which In Situ Lab has filed a patent—was even carried out in Saint Barthélemy on July 17 on a plot of land owned by the Collectivité in Saint-Jean. With a strong commitment to ecology and biodiversity, the founder of the architectural firm In Situ quickly became concerned about the invasion of Sargassum along the coast. For years, he has been working on ways to repurpose this seaweed as a construction material: bricks, insulation, and now, concrete. “It turns out that in Scandinavian countries, seaweed is used as insulation for homes,” explains Nicolas Vernoux-Thélot. “We tried using Sargassum as insulation but ran into several problems. Unfortunately, we couldn’t encapsulate the heavy metals under extreme conditions.”

“Superior mechanical strength”
Committed to research, the government-certified architect is pursuing another idea: making concrete from seaweed. The proportion of Sargassum is lower—less than 10%—which makes it possible to “meet all health standards,” particularly the encapsulation of heavy metals. Thanks to a collaboration with the École normale supérieure Paris-Saclay (ENS Paris-Saclay), a formulation was finalized to ensure the durability of the seaweed concrete, which consists of a mixture of sargassum with cement, sand, aggregate, and natural ingredients. The seaweed used comes “from the Saint-Jean storage site.” Crush tests were conducted in June 2026 by ENS Paris-Saclay to verify “the durability, consistency, and longevity” of this fiber-reinforced concrete. “This sargassum concrete has greater mechanical strength than standard concrete,” says Nicolas Vernoux-Thélot. “The addition of sargassum provides added value both mechanically and economically, since it would allow us to use less concrete.”

A Real-World Experiment
A real-world test was conducted in Saint-Jean on July 17, on the local authority’s site where the sargassum is stored. Two types of concrete were poured side by side: the type typically used for roadways, and the sargassum concrete. “This will allow us to compare how they age under the weight of trucks driving over them,” explains the architect. Samples will also be taken at various intervals to test the process’s durability. “We’ve already conducted these tests in the lab, but this time we’re testing under real-world conditions with various variables such as humidity, sunlight, and wind,” adds Nicolas Vernoux-Thélot. If the one-year testing period yields satisfactory results, sargassum concrete could be incorporated into road construction projects on the island. The sargassum that washes ashore in Saint Barthélemy could thus be put to good use. The architect declined to provide further details about the process, particularly regarding water consumption, but he assures that it is “an extremely low-tech process using simple methods, which adapts to the collection system and storage capacity” available on the island.