Revive marine ecosystems with bioactive concrete

Location of the initiative: Germany

Coastal development, fueled by population growth and urbanization, has led to the destruction of vital marine habitats like mangroves, seagrass beds, and coral reefs. These ecosystems serve as nurseries for countless species, protect shorelines from erosion, and absorb carbon dioxide. The loss of marine biodiversity is weakening the ocean ecosystem and its ability to withstand disturbances, to adapt to climate change.

The German city of Kiel is home to the Kiel Canal – the world's busiest man-made water- way navigable by seagoing ships. Located in the Baltic sea the water exchange to the Atlantic Ocean is very low, so that the oxygen content in the water is not really good for several species. Until recently, the area at the city’s Friedrichsort pier was a barren post-war marine munition dumpsite, with little marine life. Local authorities recently cleaned away the ammunition and launched a project to create a new pedestrian pier. It was also of paramount importance to restore marine life in the area.

Holcim created and donated an artificial reef to this area of the pier, showcasing its commitment to innovation, sustainability and building a nature-positive future with solutions that help biodiversity thrive. The reef is revitalizing the local marine ecosystem by using several of Holcim’s innovative marine solutions: CO₂-reduced Xstones, bioactive concrete elements, and 3D-printed components.

Bioactive concrete has a surface texture and mineral composition that is designed to stimulate natural colonization by a great diversity of local marine animals and plants. This enhances marine biodiversity and supports ecosystem regeneration, transforming hard infrastructure into living reefs.

Xstone is a coastal armor block designed to enhance coastal defense systems, including river groynes, banks, offshore wind farms, and various other applications. It enables significant reductions in material usage for equivalent structures, as its unique shape and high porosity allow for more efficient construction with less material. At the Holcim Innovation Center, biologists are developing advanced techniques to enhance the ecological value of the bioactive concrete used in Xstone blocks. This innovation transforms traditional hard infrastructure—such as protective structures against waves and rising water levels—into living, natural reefs that support algae and other foundational organisms within the marine food chain. The block’s distinctive geometry creates natural habitats, featuring internal corridors and sheltered zones that foster the growth of marine flora and fauna, thereby contributing to the restoration of biodiversity in coastal ecosystems.

Used to make the Xstones that comprise the artificial reef donated to the city of Kiel, these materials promote algae growth, home for shells and provide critical habitat for small marine life and fish species such as cod, with the specially developed and 3D-printed “fish hotels” offering additional shelter and breeding spaces. Holcim also used iECOPlanet low-carbon cement and recycled aggregates in Xstone production to make it a sustainable building solution for coastal infrastructure and marine restoration.

Sustainability impact

Climate

Holcim also created a special Net-Zero* Xstone - Xstone is a pioneering coastal protection block that combines biochar and bio-active concrete to address both climate change and biodiversity loss. Designed as a coastal armor block, it not only safeguards shorelines from waves but also functions as a natural reef, optimizing material use through its smart and sustainable design. By incorporating biochar—a carbon-rich material derived from organic waste—Xstone enables the permanent sequestration of CO₂, capturing up to 3 tons of CO₂ per ton of biochar. As a result, Xstone achieves a net-zero* carbon footprint over its life cycle. (*Based on the scope of concrete production phases [A1–A3 cradle-to-gate] in the Life Cycle Assessment, assuming average transportation distances of 175 km for cement and filler, and 165 km for aggregates.)

Holcim uses ECOPlanet low-carbon cement and recycled aggregates in production. A LCA confirms the product's carbon neutrality in its production stage (A1-A3). Net-Zero* Xstone is a milestone in sustainable marine engineering—combining coastal resilience, carbon removal, and ecological regeneration in one modular solution.

Nature

Innovative concrete solutions enable artificial reefs that can help rehabilitate damaged coastal ecosystems. The concrete mix-design and placing process patented by Holcim allows the formation of a dense bottom layer topped with a layer of porous concrete. The structural and chemical characteristics of this innovative concrete, as well as the reef’s design, were specifically defined to allow rapid colonization by a great diversity of local marine animals and plants.

In the case of the donated artificial reef in the city of Kiel, a multi-year monitoring program is in place with the aim to gain valuable insights for marine conservation by comparing the performance of the different materials. The bio-active concrete that is used in Xstone has up to 141% benefit in photosynthetic biomass compared to natural limestone.

Social

Xstone artificial reefs function not only as a reef but can also be used to protect coastal cities from flooding, strengthen shoreline resilience, and expand marine habitats. By combining safety, ecological value, and societal benefits, Xstone reefs contribute to vibrant, resilient, and thriving coastal communities.

Business impact

Benefits

Building artificial reefs with Xstones, fish hotels using bio-active concrete opens opportunities in the coastal protection and marine infrastructure market. It strengthens a company’s positioning as innovator in climate adaption solutions. It results in the following additional benefits:

• New revenue streams from climate adaptation, marine habitat restoration and nature-inclusive design

• Opportunities for collaboration with research institutions and governments

• Employee pride and stronger employer branding through contribution to coastal resilience

• Improved project acceptance due to strong societal and ecological benefits

Costs

• Operating costs: If used in equal structures (volume wise) operating costs are slightly higher than conventional stone, but in structures where dimensions can be optimized (steeper slopes, more narrow profile) the artificial reef structure is potentially more cost effective than conventional stone.

• Dependencies: Costs vary based on quantities, used installation method and installation location.

• Optimization: Xstones are standardized units where several types of concrete or decking layers are available at different sizes to allow combination of sizes in structures and the blocks to be optimized in layer thickness/size for project specific design parameters.

Typical business profile

• Engineering and construction companies active in water infrastructure

• Coastal cities and governments seeking climate adaptation measures

• Offshore energy developers looking for cable protection/offshore wind monopile scour protection

• Sectors with assets exposed to either sea level rise or any river works

Approach

1. Site selection & baseline study (i.e. local ecology, hydrodynamic conditions, etc.)

2. Co-design with stakeholders (i.e. authorities, scientists, communities). Hydraulic model tests are available in this step as well.

3. Production of Xstones (standardized molds, low-carbon concrete).

4. Transport and installation.

5. Optional: Monitoring of structural and ecological performance.

Stakeholders involved

• Municipality of Kiel (Tiefbauamt Kiel, Daniel Lenz) allowed the installation and collected the permissions of all involved authorities

• Ostseedienst (Company for the installation of the reef)

• Unterwasserkrause Mutzeck GmbH (Diving company for the installation)

• GoCircular

• Holcim Schweiz (timeline, organization of events)

• Holcim Germany (On Scene Coordinator and delivery of natural stones)

• Holcim Netherlands (Design of the reef and delivery of the Xstone)

• Holcim Innovation Center (Design of Fishhotels, preparation of testing specimen)

• PERI (printing of Fishhotels)

Key parameters to consider

The Kiel artificial reef took approximately 1 year from ideation to implementation. Initiatives using Holcims’ bio-active concrete have a lifetime of 50-100 years with minimal maintenance. Pre-requisites to implement this solution are known data regarding the environment to install it, as it allows for optimization in terms of design and mix formulation. Further constraints are local bathymetry, permitting and logistics.

This solution is suitable for coastal regions or regions with rivers/lakes. Almost any location where natural stone is currently used, an innovative Xstone solution can be developed as well.

Implementation and operations tips

• Challenges: Early buy-in from authorities and financiers due to perceived novelty.

• Solutions: Showcase pilot results, emphasize lifecycle savings, and highlight co-benefits

• Success factors: Clear monitoring framework, strong communication of societal benefits, and community involvement.

• Long-term management: Engage local stakeholders in monitoring, ensure flexibility in adaptive reef design, and integrate data into climate resilience reporting.