Scientists Test Ground Concrete Rubble as a Carbon-Capturing Ocean Additive

In the small harbour community of Taliarte on the east coast of Gran Canaria, an unusual experiment is unfolding on the water. Twelve giant floating enclosures, sealed off from the surrounding Atlantic, have been transformed into an open-air laboratory where an international research team led by the GEOMAR Helmholtz Centre for Ocean Research Kiel is asking a question that sounds almost paradoxical: could the rubble of demolished buildings help the ocean fight climate change? Over the next seven weeks, the team is testing, for the first time under such realistic conditions, whether finely ground concrete demolition waste can be used to raise the alkalinity of seawater and thereby boost the ocean’s capacity to absorb carbon dioxide from the atmosphere. The approach belongs to a family of proposed techniques known as Ocean Alkalinity Enhancement, or OAE, which deliberately mimics the slow natural weathering of rocks that has buffered the planet’s carbon cycle for geological time.

The scientific rationale behind the experiment is rooted in basic ocean chemistry. Since the beginning of industrialisation, atmospheric carbon dioxide concentrations have risen sharply, and a substantial fraction of that gas has dissolved into the sea. This influx has shifted seawater chemistry toward progressively lower pH, a process known as ocean acidification, which places particular stress on organisms that build calcareous structures such as mussels, corals and many planktonic species. At the same time, the added carbon dioxide is a principal driver of global warming. Increasing the alkalinity of seawater raises its buffering capacity, which counteracts acidification and allows the ocean to take up additional CO2 and convert it into more stable dissolved forms. In principle, the chemistry is straightforward. What remains uncertain is how marine ecosystems respond to different alkalinity sources, concentrations and modes of delivery, and where the ecological tolerance limits lie.

That uncertainty is precisely what the Taliarte experiment is designed to address. According to Emeritus Professor Ulf Riebesell, a marine biologist at GEOMAR and co-lead of the study, this year’s trial is about comparing a liquid source of alkalinity with ground concrete rubble and assessing how well both substances are tolerated by the marine environment. The work forms part of the international research project OceanAlkAlign, which aims to standardise measurement and assessment methods for Ocean Alkalinity Enhancement and thereby create a robust scientific foundation for future decisions about whether, where and how such techniques might ever be deployed. The choice of location is no accident: the Canary Islands Marine Research Institute PLOCAN, a long-standing research partner of GEOMAR, sits directly beside the Taliarte harbour, from which the mesocosms are deployed, filled and monitored throughout the entire duration of the experiment.

The tools of the trade are the KOSMOS mesocosms, short for Kiel Off-Shore Mesocosms for Future Ocean Simulations, which GEOMAR has been developing and deploying since 2006. These enclosures function like enormous test tubes suspended in the sea, each enclosing a natural slice of the coastal ecosystem, including planktonic food webs, microbial communities and the biogeochemical processes that connect them. Because the water inside each mesocosm is isolated from the surrounding ocean, researchers can manipulate conditions in individual units and compare outcomes against untreated controls with a precision that would be impossible in the open sea. Across 23 experiments to date, the KOSMOS platform has been used to investigate ocean acidification, warming, nutrient dynamics and potential countermeasures including artificial upwelling and various alkalinity enhancement approaches, making it one of the most experienced experimental infrastructures of its kind in the world.

The experimental design in Gran Canaria follows a comparative logic. Natural plankton communities will be observed over several weeks inside the twelve mesocosms. Some of the systems receive increasing quantities of finely ground concrete rubble, while others are treated with liquid sodium hydroxide as a reference representing pure dissolved alkalinity. A further set of enclosures remains untouched as controls. Throughout the seven-week run, the team will track changes in the seawater carbon dioxide system, including pH and total alkalinity, alongside the degree of CO2 uptake achieved. In parallel, an extensive suite of biological measurements will capture the composition and productivity of the phytoplankton, the responses of zooplankton, microbial process rates, and any indications of shifts rippling through the food web. Together, these data streams allow the researchers to evaluate both the effectiveness of each alkalinity source and any potential side effects along the chain from microbes to grazers.

The choice of concrete as a candidate material is driven by a striking waste-management reality. Concrete rubble is one of the largest waste streams on the planet, with an estimated five billion tonnes generated every year, and only a fraction of that material has so far been reused. Because concrete contains cement, which is inherently alkaline, finely ground demolition waste could in principle serve as a cheap and abundant source of alkalinity for the ocean. Model estimates are already exploring whether large quantities of carbon dioxide could be sequestered in this way over the long term, potentially turning a costly disposal problem into a component of climate mitigation. The appeal is obvious: if a material that already exists in staggering quantities can safely enhance the ocean’s carbon uptake, the economics and logistics of carbon dioxide removal could look very different from approaches that require manufacturing alkalinity from scratch.

Yet the researchers are emphatic that apparent availability does not equal environmental safety. A material that seems unproblematic on land can behave very differently at sea. Ground concrete introduces not only alkalinity but also solid particles, which can increase turbidity in the water column and potentially harm microorganisms, with consequences that could propagate through the food web. Associate Professor Kai Schulz of Southern Cross University in Australia, co-leader of the experiment, stresses that a waste product does not automatically become a sustainable solution simply because it is available. In his view, the field needs data showing under what conditions Ocean Alkalinity Enhancement could be ecologically acceptable and where its limits lie. That caution reflects a broader tension in the emerging carbon removal landscape, where the urgency of climate targets creates pressure to scale technologies faster than the ecological evidence base can comfortably support.

The wider context makes the stakes clear. According to many current scenarios, emission reductions alone will not be sufficient to meet the goals of the Paris Agreement, which has pushed methods for active carbon dioxide removal to the forefront of climate policy discussions. Among the portfolio of candidate approaches, Ocean Alkalinity Enhancement is regarded as an option with high potential, provided it can be implemented effectively and in an environmentally responsible manner. The ocean already absorbs a substantial share of humanity’s carbon emissions each year, and even modest enhancements to that capacity, applied across vast areas, could translate into gigatonne-scale removal. But the same vastness that makes the ocean attractive as a carbon sink also makes mistakes difficult to reverse, which is why controlled, ecosystem-level experiments like the one in Taliarte are considered essential prerequisites before any larger deployment could be responsibly considered.

The ultimate objective of the Gran Canaria study is to help define what the researchers describe as a safe operating space for alkalinity enhancement. The results are intended to inform threshold values: which dosages alter the water chemistry in the desired way, and at what point ecological effects become apparent? What differences emerge between solid particulate sources and dissolved alkalinity? And how can findings from laboratory studies, mesocosm experiments and open-ocean field trials be combined into a body of evidence robust enough to guide real-world decisions? Schulz distils the philosophy of the project into a simple maxim: understanding before scaling up. If Ocean Alkalinity Enhancement is ever to be discussed at a meaningful scale, he argues, it must rest solely on transparent data regarding both benefits and risks.

For now, the twelve floating enclosures in Taliarte harbour will continue their seven-week vigil, quietly recording how plankton communities, microbes and water chemistry respond to each dose of ground concrete or sodium hydroxide. Whatever the outcome, the experiment marks a rare moment when two enormous problems, the mounting mountains of construction demolition waste and the urgent need to remove carbon dioxide from the atmosphere, are tested against each other in the same seawater. If the rubble proves both effective and benign, it could open a genuinely circular pathway from demolition site to ocean carbon sink. If it reveals unexpected harms, the results will have done exactly what careful science is meant to do: draw a clear line around what is safe before anyone is tempted to cross it.

Subject of Research: Ocean alkalinity enhancement using ground concrete demolition waste tested in mesocosm experiments

Article Title: Construction waste for climate protection in the ocean?

Article References: Construction waste for climate protection in the ocean?. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: ocean alkalinity enhancement, carbon dioxide removal, concrete rubble, mesocosm experiment, ocean acidification, GEOMAR, KOSMOS, Gran Canaria, PLOCAN, OceanAlkAlign, marine ecosystems, climate mitigation