# Assessment of bio-colonisation, durability and microstructure of concrete exposed to intertidal and subtidal marine environments

Mohammed Zelloufi, Violeta Ramos, Badreddine El Haddaji, Amel Bourguiba, Sam Broom-Fendley, Gavyn Rollinson, Alexandra Guedes, Jens Andersen, Mohamed Boutouil

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Mohammed Zelloufi, Violeta Ramos, Badreddine El Haddaji, Amel Bourguiba, Sam Broom-Fendley, et al.. Assessment of bio-colonisation, durability and microstructure of concrete exposed to intertidal and subtidal marine environments. Case Studies in Construction Materials, 2025, 23, pp.e05119. 10.1016/j.cscm.2025.e05119. hal-05329932

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# Assessment of bio-colonisation, durability and microstructure of concrete exposed to intertidal and subtidal marine environments

Mohammed Zelloufi<sup>a</sup>, Violeta Ramos<sup>b,*</sup>, Badreddine El Haddaji<sup>a,c,d</sup>,

Amel Bourguiba<sup>a,e</sup>, Sam Broom-Fendley<sup>b</sup>, Gavyn K. Rollinson<sup>b</sup>, Alexandra Guedes<sup>f</sup>,

Jens Andersen<sup>b</sup>, Nassim Sebaibi<sup>a</sup>, Mohamed Boutouil<sup>a</sup>

## ARTICLE INFO

Keywords:

Concrete
Intertidal zone
Subtidal zone
Biofouling
Microstructure
Durability

## ABSTRACT

This study aims to investigate the impact of intertidal and subtidal marine environments on the bio-colonisation, durability, and microstructural evolution of concrete designed for marine infrastructure, such as dyke modules. Concrete samples were deployed in the Port of Cherbourg, France, for a 12-month of exposure time, a critical period during which the material is subjected to various physical, chemical, and biological stresses. Biological assessments revealed a greater biomass accumulation in the intertidal zone, dominated by green algae, while barnacles were more prevalent in the subtidal environment. Intertidal exposure led to surface roughening and partial binder loss due to physical deterioration, whereas subtidal samples retained smoother surfaces. Despite the more aggressive conditions, the intertidal zone showed a slight increase in compressive strength, likely due to matrix densification from carbonation. Subtidal samples showed stable mechanical performance, limited carbonation, and early signs of magnesium-induced decalcification without significant strength loss. The results suggest that biological colonisation and environmental exposure jointly influence early-stage concrete performance in marine settings. These findings support the potential of bio-receptive concrete for use in eco-designed marine structures, though further long-term evaluation is recommended.

## 1. Introduction

Concrete is the principal material used for maritime infrastructure, such as dykes, jetties, quays and groynes [1]. Such structures protect coastlines from erosion, flooding, and the effects of other natural phenomena [2,3]. These structures are exposed to both intertidal (area between high and low tides) and subtidal (submerged below the low tide level) marine environments which impose distinct constraints on concrete and influence its durability in different ways [4]. The intertidal zone is alternately exposed to air and

<footnote><sup>a</sup> BUILDERS Ecole d'Ingénieurs, Unité de Recherche "Builders Lab", ComUE NU, Campus Caen, Epron, France</footnote>

<footnote><sup>b</sup> Camborne School of Mines, University of Exeter, Penryn Campus, Treliever Road, Penryn, Cornwall TR10 9FE, UK</footnote>

<footnote><sup>c</sup> Institut de Recherche de la Construction, ESTP, 28 Avenue du Président Wilson, Cachan F-94230, France</footnote>

<footnote><sup>d</sup> ESITC Paris, Arcueil 94110, France</footnote>

<footnote><sup>e</sup> Master Builders Solutions, 10 Rue des Cévennes, Lisses 91090, France</footnote>

<footnote><sup>f</sup> Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências da Universidade do Porto and Instituto de Ciências da Terra, Pólo Porto, Portugal, Rua do Campo Alegre, Porto 4169-007, Portugal</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: v.i.monteiro-ramos@exeter.ac.uk (V. Ramos).</footnote>
water and can be subject to rapid variations in temperature and humidity, wetting-drying cycles and fluctuating oxygen concentrations [5]. These changing conditions cause mechanical and chemical stress, accelerating degradation processes such as erosion, corrosion of reinforcement, and crack formation [6–8]. Contrarily, the subtidal zone remains constantly submerged, limiting harsh exposure conditions. However, this zone can be subjected to salinity and biological activities [9–11].

In marine settings, the colonisation of concrete surfaces by microorganisms and larger marine species present in seawater initiate various biological processes [12]. This colonisation, known as bio-colonisation, typically progresses through two sequential phases: microfouling and then macrofouling [13–15]. Microfouling refers to the swift development of a microbial biofilm, beginning within minutes of submersion. These biofilms consist of one or more bacterial species that adhere to the surface and to each other, embedded within a matrix of extracellular polymeric substances (EPS) [16–18]. Macrofouling refers to the attachment of micro-organisms, including microalgae, to the substrate [19]. These algae contribute to the biofilm's expansion by increasing the surface area for colonisation and producing chemical compounds that attract additional species such as barnacles, which attach themselves to the biofilm and reinforce the biological structure of the submerged concrete [20,21]. Bio-colonisation illustrates how bio-receptive surfaces can evolve to become micro-habitats favorable to biodiversity.

Bio-receptivity is defined as a material's inherent ability to attract, support, and promote the settlement and growth of living organisms on its surface [22]. Materials with high bio-receptivity are well-suited to host and support various biological activities over time [23]. In concrete, bio-receptivity can be influenced by various factors, like the type and the nature of the binder or the aggregates, the incorporation of specific additives, as well as key properties such as pH, porosity, and surface roughness [24–26]. The formation of marine biofilms and the accumulation of biomass can have both beneficial and detrimental impacts on the durability of concrete in marine environments, depending on various elements such as the type of micro-organisms present, the concrete formulation and the environmental exposure conditions [7,23,24]. Marine biofilm can act as a protective barrier, reducing concrete exposure to aggressive agents and limiting the penetration of corrosive substances in the same way as conventional surface treatment [21,27]. Bio-receptive

<table><caption>**Table 1**

Research on bio-colonisation and durability of concrete in marine environments, including short and long exposure duration.</caption><thead><tr><td>Main Objective</td><td>Environment</td><td>Duration</td><td>Key Findings</td><td>Limitations</td><td>Reference</td></tr></thead><tbody><tr><td>Evaluation of bio-colonisation and durability of concrete containing oyster shell aggregates.</td><td>Subtidal and artificial seawater</td><td>360 days</td><td>Biofilm development acts as a protective barrier, reducing chloride ion penetration and thereby enhancing concrete durability.</td><td>Limited to subtidal exposure. Mineralogical evaluation was not performed.</td><td>[28]</td></tr><tr><td>Study of bio-colonisation, durability, and microstructural analysis of concrete incorporating magnetite and oyster shell waste aggregates.</td><td>Subtidal and artificial seawater</td><td>24 months</td><td>Biomass development depends more on surface roughness and binder type, while the nature and properties of aggregates influence concrete durability.</td><td>Limited to subtidal exposure.</td><td>[29]</td></tr><tr><td>Investigation of the effect of biofilm and barnacles on the surface of concrete</td><td>Intertidal</td><td>40 days</td><td>Barnacles and biofilm enhance concrete durability by adhering to the surface and sealing underlying microcracks.</td><td>Limited to intertidal exposure, with a short exposure duration of 40 days. Durability was assessed using chloride migration tests and EPMA (SEM-WDX).</td><td>[21]</td></tr><tr><td>Enhance the durability of concrete at a marine site by using ground palm oil fuel ash (GPOFA)</td><td>Intertidal</td><td>7 years</td><td>Optimal durability was achieved with 15–35 % GPOFA replacement by binder weight.</td><td>Biomass development on concrete specimens and its effect was not examined.</td><td>[30]</td></tr><tr><td>Improve the durability of concrete in a marine environment by using ground rice husk–bark ash (GRBA).</td><td>Intertidal</td><td>5 years</td><td>The durability of concrete considerably improved by utilizing GRBA as high as 35 %.</td><td>Biomass growth and its effect was not discussed.</td><td>[31]</td></tr><tr><td>Durability study of concrete with blended cement in harsh marine environments.</td><td>Intertidal and subtidal</td><td>18 years</td><td>Type V mixtures experienced a high degree of corrosion.</td><td>The bio-receptivity of concrete and the development of biomass were not studied. Mineralogical evaluation was not investigated.</td><td>[32]</td></tr><tr><td>Identify the short-term interactions between cementitious materials, biofilm developing at their surface, and seawater.</td><td>Subtidal</td><td>90 days</td><td>CEM III and CEM V showed higher chloride binding and better durability than CEM I, with no notable effect of cement type on early biofilm development. Diverse microbial communities developed over time, with biomineral-forming serpulid polychaetes identified among the macrofouling species.</td><td>Limited to subtidal exposure, with a short exposure duration of 90 days.</td><td>[33]</td></tr><tr><td>Determine the effect of time and surface type on the composition of biofilm communities on concrete exposed to seawater.</td><td>Mesocosm</td><td>455 days</td><td>Time and concrete surface type (roughness) and steel fiber reinforcement influence biofilm microbial community composition.</td><td>Limited to mesocosm environment for the development of biofilm. Concrete durability was not investigated.</td><td>[34]</td></tr></tbody></table>
concrete can contribute to the ecological restoration of coastal ecosystems by providing habitats for various marine species [23].

Several studies have investigated the bio-colonisation and the durability of concrete in marine environments (Table 1). Most of them focused on a single type of exposure (intertidal or subtidal) or were limited to short-term laboratory simulations. Very few studies have assessed both biological colonisation and material durability in real marine conditions, and even fewer have directly compared these phenomena across different marine exposure zones. Additionally, most prior research lacks mineralogical and microstructural analysis, as well as long-term observations of early-stage degradation mechanisms.

Protecting the ecological status of coastal waters has become imperative with climate change, along with the impacts of fishing, recreational activities, and coastal development on coastal dynamics and marine biodiversity. Biomimicry aims to create artificial structures capable of interacting positively with the marine environment, promoting local biodiversity and minimizing environmental impacts [35–38]. In this context, the European project MARINEFF (2018–2023), funded by the European Regional Development Fund, implemented a multidisciplinary eco-design approach focused on the development and evaluation of biomimetic infrastructures. These included oyster prism modules, dyke modules, boat moorings, and rockpools, designed to improve the ecological quality of marine environments [39].

As part of the MARINEFF project, this study presents a comprehensive field evaluation of a concrete formulation based on CEM II/A-LL cement, siliceous sand, and quartzite aggregates, exposed for 12 months to real intertidal and subtidal marine conditions. The investigation integrates bio-colonisation, mineralogical transformations, microstructural changes, and mechanical performance. This short-term assessment provides valuable insights into how different marine environments influence concrete durability, and supports the evaluation of its potential for eco-designed marine dyke applications.

## 2. Materials and methods

### 2.1. Materials

The concrete formulation used in this study was designed in accordance with the specifications for marine dyke structures outlined in the NF EN 206+A2 standard [40]. The mix design meets the requirements of a water-to-cement (W/C) ratio of 0.5 and a compressive strength class of C35/45, ensuring adequate mechanical performance in harsh environments. The concrete is classified under exposure class XS3, which corresponds to structures in tidal, splash, and spray zones — areas subjected to frequent wetting and drying cycles, direct contact with seawater, and exposure to atmospheric CO2. This classification guarantees the suitability of the material for aggressive marine conditions, particularly where chloride ingress and carbonation are major concerns. Additionally, a workability class of S4, with a slump range of 160–210 mm, was selected to ensure high workability. This facilitates the casting of complex formwork and ensures proper compaction and placements in precast modular components such as marine dyke structure.

The concrete formulation used alluvial siliceous sand 0/2 mm as fine aggregate, and two crushed coarse aggregate fractions (4/10 mm and 10/20 mm) of identical lithological origin. The raw materials used in the mix design are shown in Fig. 1, and their main physical properties are summarised in Table 2. The absolute density and the 24-hour water absorption coefficient (WA24) of the aggregates were determined in accordance with NF EN 1097–6 [41]. The granular compactness was assessed using the method specified in NF EN 932–2 (1999) [42]. These properties were used to optimise the granular skeleton and ensure the durability and mechanical performance required for marine exposure.

The fine aggregate consists of a well-rounded and moderately well-calibrated quartz-rich sand. Petrographic characterisation confirmed that quartz (92.01 vol%) is the main constituent of the sand, occurring as monocrystalline or polycrystalline grains. Altered K-feldspar and plagioclase, chlorite, biotite and distinct rock fragments are also present in trace proportions (Fig. 2). The coarse aggregate corresponds to a quartzite (quartz 93.70 vol%) presenting an orange color due to Fe-oxide staining on some of its surfaces (Fig. 1). The microscopic aspects of both aggregates are illustrated in Fig. 2. The modal analyses of the aggregates are presented in the Supplementary Materials.

Before concrete casting, the aggregates were dried at 105°C for 24 h in an oven. This is recommended to obtain completely dry

<figure><img src="image_3.png" /><figcaption>Fig. 1. Aggregates selected for formulation used in dyke construction. A) Coarse aggregate G4/10. B) Coarse aggregate G10/20. C) Fine aggregate 0/2 (siliceous rolled sand).</figcaption></figure>
<table><caption>**Table 2**

Characteristics of the aggregates used in the concrete formulation.</caption><thead><tr><th>Components</th><th>Grain size (mm)</th><th>Nature</th><th>Absolute density (kg/m³)</th><th>Water absorption coefficient WA24 (%)</th><th>Granular compactness (%)</th></tr></thead><tbody><tr><td>Sand</td><td>0/2</td><td rowspan="3">Siliceous</td><td>2740</td><td>0.54</td><td>66</td></tr><tr><td rowspan="2">Gravel</td><td>4/10</td><td>2700</td><td>0.62</td><td>58</td></tr><tr><td>10/20</td><td>2704</td><td>0.79</td><td>58</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Optical microphotographs of the used aggregates. Photos in plane-polarized light. A) General aspect of the sand 0/2; B) General aspect of the quartzite (4/10) used as coarse aggregate. Qz – quartz; Kfs – K-feldspar; Bt – biotite; Rock Frag. – rock fragment.</figcaption></figure>

aggregates and to avoid variations due to residual humidity in the aggregates. The cement employed in this study was CEM II/A-LL, compliant with the standard NF EN 197-1 [43], and belongs to the 42.5 R strength class. It is a Portland-limestone cement, ensuring suitability for marine applications, following NF P 15-317 [44]. The cement chemical composition is shown in Table 3, and the principal clinker components are presented in Table 4. A high-water reducer admixture was included in this formulation to respect the specification. The optimised dyke concrete formulation is presented in Table 5.

Following the standard NF P18-459 [45], the water-accessible porosity and density were measured using cylindrical samples (11 × 5 cm) after 28 days of curing in water at 20°C. Additionally, cylindrical samples (11 × 22 cm) were prepared to evaluate the compressive strength following NF EN 12390-3 [46], with measurements taken after 1, 3, and 28 days of curing under the same conditions. The diffusion coefficient *D* (m²/s) was also determined in compliance with NF EN 12390-11 [47]. The results are presented in Table 6. The dyke concrete formulation attained the specified requirements of C35/45 compressive strength after 28 days of water curing.

## 2.2. Methods

### 2.2.1. Deployment

The study area is situated in the English Channel, with two experimental zones selected within the “Rade de Cherbourg” (Manche), located at the southern tip of the Cotentin Peninsula in Normandy, northwestern France. This site offers a range of distinct marine conditions, making it particularly suitable for investigating concrete performance under real intertidal and subtidal exposures. This semi-enclosed area features a sandy seabed, interspersed with pebbles and hard substrates, where the hydrodynamics are influenced by the presence of the harbour dyke [48], [49]. The first experimental area was designated for intertidal exposure. At the same time, the second area was used for subtidal exposure (Fig. 3). The subtidal zone is in the eastern part of the Grande Rade, south of Fort de l'Île Pelée. The site has an average bathymetry of 4 m at low tide, with a fine sand substrate.

According to regional monitoring data from Ifremer (French Research Institute for Exploitation of the Sea) and local environmental agencies, the typical seawater characteristics in the Rade de Cherbourg (English Channel) include pH values generally ranging from 7.8 to 8.2, and temperatures fluctuating between approximately 8 °C in Winter and 18 °C in Summer. The average salinity is around 35 g/L. In terms of chemical composition, the seawater contains approximately 19,300 mg/L of chloride (Cl⁻), 2700 mg/L of sulfate (SO₄²⁻), and 140 mg/L of bicarbonate (HCO₃⁻).

After optimising the dyke concrete formulation to meet the required specifications, several cylindrical samples (11 × 22 cm) were prepared at *Builders Ecole d'Ingénieurs* for deployment in the two selected experimental areas. After 28 days of water curing at 20 °C, the

<table><caption>**Table 3**

Chemical composition of Calcia© used cement as reported in the supplier's technical data sheet.</caption><thead><tr><th>Elemental composition</th><th>LOI</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>TiO<sub>2</sub></th><th>MnO</th><th>CaO</th><th>MgO</th><th>SO<sub>3</sub></th><th>K<sub>2</sub>O</th><th>Na<sub>2</sub>O</th><th>P<sub>2</sub>O<sub>5</sub></th><th>S<sup>2-</sup></th><th>Cl<sup>-</sup></th></tr></thead><tbody><tr><th>%</th><td>3.90</td><td>19.40</td><td>4.50</td><td>3.80</td><td>0.30</td><td>0.00</td><td>63.70</td><td>1.00</td><td>2.60</td><td>0.86</td><td>0.030</td><td>0.30</td><td>0.10</td><td>0.03</td></tr></tbody></table>
<table><caption>**Table 4**
Principal clinker components in Calcia© used cement.</caption><thead><tr><th>Component</th><th>C<sub>3</sub>S</th><th>C<sub>2</sub>S</th><th>C<sub>3</sub>A</th><th>C<sub>4</sub>AF</th><th>MgO</th></tr></thead><tbody><tr><th>Value by mass (%)</th><td>68.0</td><td>8.9</td><td>6.9</td><td>12.0</td><td>1.1</td></tr></tbody></table>

<table><caption>**Table 5**
Dyke concrete mix design formulation.</caption><thead><tr><th>Components</th><th>Quantity (kg/m³)</th></tr></thead><tbody><tr><td>Cement</td><td>350</td></tr><tr><td>Sand (0/2)</td><td>810</td></tr><tr><td>Gravel (4/10)</td><td>600</td></tr><tr><td>Gravel (10/20)</td><td>500</td></tr><tr><td>High water reducer admixture</td><td>7</td></tr></tbody></table>

<table><caption>**Table 6**
Dyke concrete formulation properties.</caption><thead><tr><th>Properties</th><th>Time after casting (days)</th><th>Value</th></tr></thead><tbody><tr><td>Water-accessible porosity (%)</td><td>28</td><td>11 ± 0.47</td></tr><tr><td>Density (kg/m³)</td><td></td><td>2315</td></tr><tr><td>Diffusion coefficient D (m²/s)</td><td></td><td>1.2 × 10<sup>-11</sup></td></tr><tr><td rowspan="3">Compressive strength (MPa)</td><td>1</td><td>14.3 ± 0.97</td></tr><tr><td>3</td><td>32 ± 0.88</td></tr><tr><td>28</td><td>43 ± 0.6</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Google maps screenshot of deployment zones selected within Cherbourg Harbour (Manche), France. Location: 49.666075, -1.580734. Google Maps screenshot date: 22/11/2024.</figcaption></figure>

samples were secured in steel cages to ensure their protection and facilitate deployment. The *University of Caen*, a partner in the MARINEFF project, deployed the concrete samples in intertidal and subtidal zones, as illustrated in Fig. 4.

### 2.2.2. Bio-receptivity evaluation

The biomass (mass of living organisms) developed on the surface of a material serves as an indicator of its bio-receptivity [24,25]. In this work, the bio-receptivity was evaluated by quantifying the biomass of organisms that colonised the concrete surface over time
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Deployment of concrete samples in the intertidal zone (A) and subtidal zone (B). Pictures taken by the marine station of the University of Caen.</figcaption></figure>

[29,50]. After retrieving the samples from the experimental zones, the sample surfaces were carefully scrapped with a steel scraper to collect the attached organisms. The harvested biomass was then weighed using a precision scale with an accuracy of ±1 g. Measurements were taken at 6 and 12 months after immersion, and the average of five individual readings was used as the final value. This methodology was implemented to monitor how the concrete formulation supported biological colonisation over time. The resulting data also provide insight into how bio-receptivity influences the concrete's microstructural integrity and long-term performance under both subtidal and intertidal conditions. Photographs of the samples were taken using a standard digital camera to visually document the colonisation patterns and species distribution.

### 2.2.3. Carbonation test

As a preliminary assessment method, the carbonation test was performed following BS EN 14630 (2006) [51]. This test determined the thickness of the carbonated layer on the surfaces of slabs cut from cylindrical samples. Carbonation depth was measured at three separate locations along a cross-section-oriented perpendicular to the slab surface, with the mean of these values taken as the representative depth. The assessment was carried out after 12 months of exposure in both intertidal and subtidal environments.

### 2.2.4. Mineralogical and microstructural characterisation

Microstructural and mineralogical analysis of the samples is a key aspect in this work, offering precise insight into the effect of the deployment zones on the structural and mineralogical properties of the concrete blocks during the early stages of exposure (before significant biomass development occurs). This assessment was carried out on samples exposed for 12 months in both intertidal and subtidal zones, with vertical zoning applied to reflect the primary exposure-related gradients acting along the specimen height.

This investigation was mainly conducted at the laboratories of the *Camborne School of Mines, University of Exeter, UK*. To assess the internal microstructural evolution of concrete under marine exposure, the cylindrical specimens (11 × 22 cm) were longitudinally bisected after the deployment. Thin sections (30 µm thick) were prepared from both marginal and central regions of the cross-section, targeting three distinct vertical zones: the top (T), middle (M), and bottom (B) portions, as shown in (Fig. 5). This vertical segmentation

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Slice of the concrete block (11x22 cm) immersed in the subtidal zone. The yellow rectangles indicate the sites where polished thin sections were prepared for the top (T), middle (M) and bottom (B) sections.</figcaption></figure>
was designed to capture potential gradients in microstructural changes resulting from environmental exposure, particularly under intertidal conditions where wetting-drying cycles, surface carbonation, and biofilm accumulation are likely to affect the upper portion more significantly.

A combination of automated scanning electron microscopy (QEMSCAN®), optical petrography, and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) was conducted to characterise the composition and spatial distribution of mineral phases within the samples. Additionally, Raman spectroscopy analysis was performed at the laboratories of the Faculty of Sciences, University of Porto, Portugal, to identify calcium carbonate polymorphs.

Modal mineral abundances and false-color mineral maps of polished thin sections from the concrete blocks were obtained using a QEMSCAN® 4300 [52–55]. This system comprises an automated Zeiss® EVO 50 SEM equipped with four Bruker® SDD EDS detectors for sample analysis. The analytical process involved two stages: an automated measurement phase followed by manual data processing. Data collection was performed using iMeasure version 4.2SR1, with measurements acquired under a default setting of 1000 X-ray counts per point at 25 kV. Data processing was conducted using iDiscover versions 4.2SR1 and 4.3 software. Samples were analysed using Fieldscan Mode at a 10 µm X-ray pixel resolution, a 1500 microns square field size and x44 magnification (for more details see [54]). The resin media is ignored as its brightness falls below the predefined back-scattered electron (BSE) threshold. After data processing, the resulting fields were stitched together to create an overall image. The data underwent meticulous refinement using a modified version of the standard LCU5 SIP database, where all mineral entries were verified and adjusted as necessary to correspond with the sample types. The database was specifically tailored for concrete phases, categorizing minerals based on both previous research samples and the current dataset to enhance its robustness. Additionally, its accuracy was validated through complementary techniques, including optical microscopy, X-ray diffraction (XRD) analysis, and SEM-EDS (for more details see [56]). In some samples, the Fieldscan measurement excluded the last few mm of the top and bottom edges of the thin section to prevent interference from the metal sample holder. Certain thin sections were prepared very close to the edge of the glass slide, requiring adjustments to the measurement area to avoid capturing the holder's edge, which could otherwise impact data processing. Considering the 1500 µm field size, these adjustments occasionally resulted in the exclusion of the extreme edges of the sample.

To further complement the mineralogical data obtained from QEMSCAN® analysis and characterise micro-textures, polished thin sections were examined under transmitted light using a Nikon Eclipse E600 Pol optical microscope, equipped with a Nikon® Digital Sight 5MP camera. Image acquisition was performed using NIS Elements F3.0 software. For additional confirmation of mineral phases and micro-texture analysis, a TESCAN® Vega 3 GMU scanning electron microscope (SEM) was utilised in high vacuum mode with an accelerating voltage of 20 kV. The SEM was paired with an Oxford® EDS system (XMax 80 nm EDS) and operated using Aztec® software (version 3.3 SP1). Polished thin sections were analysed using X-ray analysis and BSE imaging, with samples coated in a 25 nm carbon thin film using an Agar automatic carbon coater.

Elemental distribution mapping was conducted using a JEOL® JXA-8200 Superprobe electron-probe micro-analyser (EPMA) to analyse C, Si, Mg, S, Cl, Al, Ca, and Fe within the outer edges of the top and bottom polished thin sections of the investigated blocks. Elemental maps were generated using a 15 kV accelerating voltage and an 80 nA focused electron beam. The detection of elements in all concrete blocks was carried out using a wavelength-dispersive X-ray spectroscopy (WDS) detector. These maps provide elemental distributions rather than absolute, calibrated concentrations.

X-ray diffraction (XRD) analysis was performed on powdered samples using a Siemens® D5000 diffractometer, equipped with a 1.5 kW Cu K-alpha radiation tube, set at 40 kV and 30 mA. Data were collected over a 2θ range of 2–70 degrees, with a step size of 0.02 degrees and a scan time of 1 s per step. The resulting diffraction patterns were processed using EVA software (v. 18.0.0.0), referenced against the JCPDS PDF-2 (2004) database. Raman analysis was performed using a Horiba XploRA™ Microscope Raman spectrometer, equipped with a 532 nm excitation wavelength from a Nd:YAG laser operating at 25 mW, and diffraction gratings with 1800 lines mm⁻¹. The Olympus optical microscope’s × 50 objective lens was used to collect the dispersed radiation and concentrate the laser

<figure><img src="image_8.png" /><figcaption>Fig. 6. Steel cages containing the concrete samples after a 12-month exposure in intertidal zone (A) and subtidal zone (B).</figcaption></figure>
beam on the sample.

### 2.2.5. Compressive strength

Compressive strength was conducted by NF EN 12390-3 [46], using a 3 R© compression press with a 4000 kN capacity and a loading rate of 0.6 MPa/s. Measurements were conducted on a minimum of three cylindrical samples (11 x 22 cm) for each exposure condition, with testing performed at 6 and 12 months after deployment. This test is designed to monitor the mechanical performance of the dyke concrete formulation under the investigated exposure conditions.

## 3. Results and discussion

### 3.1. Bio-receptivity evaluation

After 12 months of exposure in steel cages in intertidal and subtidal environments, the cylindrical dyke concrete samples were recovered and subjected to bio-receptivity assessment. Fig. 6 shows two steel cages containing the concrete samples after their 12-month exposure in intertidal and subtidal marine zones. Fig. 6A shows the steel cage recovered from the intertidal zone. Although the cage limits visibility, the concrete samples are covered with green algae biomass. The rocks around them also show some colonisation, indicating that the concrete substrate is colonised with apparently the same organisms as the rocky substrate in an intertidal environment, where algae principally grow on rocky surfaces. Rocky substrates or large blocks are typically capable of withstanding hydrodynamic forces and provide ideal conditions for algae attachment. The texture and type of rock are also important, as they determine the microhabitats (hollows, asperities, porosities) in which algae can attach themselves [57]. Fig. 6. B shows the steel cage recovered from the subtidal zone (fully submerged throughout the exposure period). Red algae have colonised the cage, while the concrete samples are covered with red algae and other marine organisms.

Fig. 7 illustrates a comparison of biological colonisation on dyke concrete samples after 12 months of exposure in intertidal (A) and subtidal (B) environments. The biomass recovered from the intertidal zone consists primarily of green algae. Algae differ from higher plants because they have no roots, stems, leaves or flowers. Algae need light to carry out photosynthesis to ensure their growth in the environment [58]. In practice, they are often classified according to their size (macroalgae and microalgae), although this criterion does not reflect a phylogenetic classification. Macroscopic algae living in coastal areas are known as benthic macroalgae. The main categories of benthic macroalgae include red algae, green algae, brown algae, filamentous yellow-green algae, and blue-green algae [59]. Their distribution depends on several factors, principally the duration of immersion and the alternation of emersion-immersion cycles (wetting-drying cycle), which cause significant fluctuations in physicochemical parameters [60]. Added to this are the strength of the swell and currents, light availability (therefore depth), substrate type (rocky, sandy or muddy), and salinity (coastline, estuary) [61–64]. The distribution of algal populations is also affected by latitude and the quality of coastal waters [57]. Changes in macroalgal communities can also be attributed to a variety of anthropogenic pressures. The main ones include eutrophication, chemical and thermal pollution of coastal waters, activities leading to physical disturbance (such as fishing on foot or professional exploitation), and the introduction of exogenous marine species linked to shellfish farming, maritime transport and port activities. These pressures can disrupt or even destroy the coastal habitats that are essential to the development of macroalgae [65,66]. Macroalgae are a source of food for various animal species such as molluscs and herbivorous fish. Through the action of bacteria, the mineral elements making up the macroalgal biomass are recycled and once again available for subsequent generations of plants. Macro-algae can provide ideal support for epiphytic algae (an organism that grows using other plants as a support) and invertebrate larvae, as well as offering various

<figure><img src="image_9.png" /><figcaption>Fig. 7. Biological colonisation on concrete samples after 12 months of exposure in intertidal (A) and subtidal (B) environments.</figcaption></figure>
types of organism protection against predation during the crucial stages of their development [67]. Macro-algal communities also contribute to the creation of complex and heterogeneous habitats, providing shelter for the recruitment and/or development of a wide variety of benthic invertebrates (shellfish, crustaceans and marine worms) and juvenile fish [68].

Barnacles dominate the colonisation in the subtidal zone (Fig. 7B). Following the initial formation of a bacterial biofilm on the substrate surface, cypris barnacle larvae actively swim in search of an appropriate attachment substrate. They preferentially settle on surfaces covered in biofilms, guided by biological cues [69]. The biofilm gradually disappears when cypris larvae transition into their juvenile stage, and barnacle footprints eventually take its place [70]. These footprints, which differ from species to species, consist of protein-based tissues and between 30.8 % and 48.8 % calcareous chemicals [71]. Barnacles start secreting stronger adhesive compounds at 40 days after settlement, which increases their resistance to removal [72].

Fig. 8 illustrates the variation in recovered biomass from concrete samples exposed in intertidal and subtidal zones over immersion periods of 6 and 12 months. At 6 months, both zones exhibit relatively low biomass levels with small differences. At 12 months, both zones experience increased biomass with exposure time, demonstrating progressive biological colonisation. However, the biomass accumulation is notably higher in the intertidal zone. Regular exposure (immersion and emersion) to air encourages the growth of photosynthetic algae (green algae) and creates a unique colonisation gradient [73]. The intertidal zone is subject to wetting and drying cycles, temperature, desiccation, wave forces, or rapid fluctuations in salinity, which could limit the diversity of colonising organisms compared to continuous immersion [74]. The subtidal zone, which is continuously submerged, offers more stable conditions in terms of temperature, humidity and availability of nutrients [75]. These favourable conditions encourage continuous biological colonisation, particularly by calcifying organisms such as barnacles.

Evidence suggests that the bio-colonisation of coastal concrete structures can have detrimental impact, by modifying the surface topography of the concrete and accelerating its abiotic deterioration. However, studies have also shown that common intertidal species can play a protective role by forming a biological layer that limits the concrete's direct exposure to microclimatic variations, such as repeated wetting and drying cycles, as well as to chemical and mechanical aggression from waves or suspended particles. Intertidal organisms can reduce the rate of alteration of coastal substrates and offer significant bioprotective benefits compared to a bare substrate exposed directly to environmental forces [6]. The colonisation of barnacles can enhance the resistance to water absorption and the chloride ion penetration of the concrete. As barnacle coverage increases, it further improves the concrete's durability in the marine environment [76,77].

It is important to note that the method used to evaluate bio-receptivity, which involves scraping and weighing the biomass, has limitations. This approach primarily quantifies visible biofouling and may not fully capture microbial or microscopic colonization, which is crucial in the early stages of biofouling. Microbial biofilms form rapidly upon immersion and can significantly influence the subsequent colonization of larger organisms. However, these early microbial communities are not accounted for in the biomass measurements, as the scraping method may not efficiently recover all attached microorganisms, particularly those that are smaller or more firmly adhered to the substrate. Consequently, while the biomass quantification provides valuable insights into the overall bio-colonisation, it may underestimate the full extent of biofouling, especially during its initial phases. Therefore, the results should be interpreted with these limitations in mind, and future evaluations could benefit from more sensitive techniques, such as microscopic analysis or molecular methods, to capture the complete spectrum of biofouling from microbial to macroscopic stages.

## 3.2. Carbonation test

The samples exposed to the intertidal zone were subject to drying and wetting cycles and atmospheric exposure, carbonation was allowed to take place more readily. A maximum carbonation depth of 1 mm was observed on one of the lateral edges of the cylinder slice, while a depth of 0.5 mm was measured at the top edge (Fig. 9B and C). No carbonation was detected at the remaining lateral and bottom edges, where the depth was recorded as 0 mm. Notably, the highest carbonation rates typically occur when relative humidity is

<figure><img src="image_10.png" /><figcaption>Fig. 8. Recovered biomass as a function of exposure time in intertidal and subtidal zones.</figcaption></figure>
maintained between 50 % and 75 % [78,79]. In contrast, a carbonation depth of 0 mm was observed on all the sides of the concrete slice from the cylinder in the subtidal zone, which exhibited a reddish-purple colour after being sprayed with phenolphthalein (Fig. 9F). The carbonation test result obtained for this sample is in concordance with what is expected for concrete that is always kept immersed as for high humidity conditions (>75 %) the moisture present in pores limits CO₂ penetration [78–80]. Notably, the observed reddish colour indicates a sufficient presence of Ca(OH)₂, though it may have carbonated to a lesser extent ([79,81]), which must be confirmed through petrographic examination.

Although the measured carbonation depths in the intertidal samples were relatively shallow (0.5–1 mm), even limited carbonation can influence the microstructural integrity of the concrete surface and near-surface zones. Carbonation leads to the formation of calcium carbonate from the reaction of CO₂ with portlandite (Ca(OH)₂), which can initially cause a densification of the pore structure. However, this reaction also reduces the pH of the concrete, potentially compromising the passivation layer around steel reinforcement (in reinforced concrete), and initiating localized degradation. In the context of unreinforced concrete exposed to marine conditions, carbonation, combined with repeated wetting-drying cycles and salt crystallization, can lead to surface microcracking and weakening of the cement matrix. Over time, these surface degradations can propagate inward and contribute to a reduction in compressive strength, particularly in concrete subjected to cyclic environmental stressors such as those in the intertidal zone. Therefore, even small carbonation depths may signify the early stages of more extensive deterioration processes that affect mechanical performance. Furthermore, the development of biomass on concrete surfaces may modify micro-environmental conditions, potentially influencing carbonation rates by altering moisture retention and gas exchange. Previous studies have similarly reported that biofilms and macro-organisms can either hinder or facilitate carbonation depending on their density and coverage [82].

### 3.3. Mineralogical and microstructural characterisation

Quartz, the primary component of both coarse and fine aggregates used, was identified as the main mineral phase in the investigated samples, with average values of 70.30 vol% and 72.78 vol% for samples in intertidal and subtidal zones, respectively (Fig. 10 and Supplementary Material - Tables 1 and 2 and Fig. 1). In addition to quartz, plagioclase is another significant component of the

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Carbonation test results on the concrete slabs (11 × 22 cm) from the investigated blocks. A and D) Without phenolphthalein; B, C and F) With phenolphthalein. A, B and C) Intertidal zone. C corresponds to a zoomed-in image of one of the lateral edges (yellow square) of the concrete slab where a carbonation depth of 1 mm was calculated (yellow arrow); D and F) Subtidal zone.</figcaption></figure>
aggregates. Both samples also contain calcium silicates, primarily in the form of calcium silicate hydrate (C-S-H), the principal constituent of the binder, and is characterised as a poorly crystalline material, as indicated by a noisy background in the X-ray diffractograms (Fig. 12). The most abundant minor mineral phases (<5 vol%) in both blocks are calcium-iron-aluminium silicates, followed by K-feldspar and calcium-magnesium-(iron-aluminium) silicates (Fig. 11 and Supplementary Material - Table 2). Portlandite, an important hydration product of Portland cement, is categorised as calcite in the QEMSCAN® mineral list (Supplementary Material - Table 1). XRD analysis further confirmed its presence as a minor phase across all sections of the blocks (Fig. 12).

As shown by the false-colour mineral maps of the investigated concrete blocks (Fig. 13) and under the polarising microscope (Fig. 14A), the sample from the intertidal zone exhibits rough outer edges due to the loss of cement paste and exposure of aggregates. In contrast, the outer edges of the sample from the subtidal zone are smooth (Fig. 13, Fig. 14B). This difference can be attributed to wave splashing, which causes physical deterioration, through abrasion and erosion, as well as drying cycles that lead to salt crystallisation in the tidal zone. Additionally, processes such as diffusion, sorption, wick action, and permeation can result in extremely high concentrations of aggressive chemical species in this zone. Concrete blocks that remain fully immersed are more prone to chemical degradation rather than physical deterioration [4,83]. Nonetheless, fully submerged concrete is relatively protected, as the ingress of chemical ions occurs primarily due to the concentration gradient between the internal concrete matrix and the surrounding seawater [84]. In examining the mineralogy and microstructures on the outer edges of concrete blocks exposed to seawater, a surface skin (Fig. 14B and C), up to 30 µm thick, composed of calcite as determined by Raman spectroscopy (Supplementary Material - Fig. 2), was observed partially covering the B edge of the sample in the subtidal zone. This thin crust shows signs of Fe staining (Fig. 14B), suggesting a nearby rust source, likely from the steel reinforcement cage used in the block. Minor scaling appears at the edges of this sample but is notably absent in areas covered by the calcite layer, underscoring its protective role as suggested by several authors [4, 85,86].

Under the polarising microscope, the greatest carbonation depth at the block edges was observed in the sample in the intertidal zone (2000 µm), while in the sample in the subtidal zone, it reached a depth of 1600 µm. The phenolphthalein method (BS EN 14630) [51], had already indicated visible carbonation in the intertidal sample. However, it should be noted that this method is best considered as a preliminary assessment. The carbonation depths determined under the optical microscope align with expected values for concrete that is both fully immersed and subjected to drying-wetting cycles and atmospheric exposure, as occurs in the intertidal zone [78,79].

In addition to the edges, carbonation of the cement paste was also observed in both blocks, particularly within the interface zone between the aggregates and the cement paste (Fig. 14E), as well as around air voids (Fig. 14F). These areas are particularly susceptible to carbonation due to their porosity [87,88]. While carbonation in these areas is frequently observed in the intertidal sample, it is notably less common in the subtidal sample. The higher extent of carbonation in the intertidal sample compared to the subtidal sample may contribute to increased compressive strength in the block exposed to the intertidal zone, as suggested by previous studies [89,90].

Minor micro-scaling is present in both edges of the block immersed in the subtidal zone, being completely absent in the areas covered by the surface skin in the outer edge of the B section. Although some very small areas characterized by a decalcified cement paste and enriched in Mg are present at the outer edges of the block immersed in the subtidal zone (Fig. 14D), no evident elemental zonation, i.e. a Mg-rich zone followed by an S-rich and a Cl-rich zones progressing toward the core of the blocks, as described by several authors ([86,91]), was observed in any of the samples through elemental distribution mapping (Supplementary Material - Figs. 3 to 6). The conversion of C-S-H to magnesium silicate hydrate (M-S-H) leads to a very mechanically weak material with non-cementing properties that is easily eroded (e.g. [86,87]).

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Average modal mineral abundances for the concrete samples exposed to the subtidal and the intertidal zones (minerals <0.01 vol% are not represented).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Graphic representations of the average modal abundances of the minor minerals (<5 vol%) for the concrete samples exposed to the subtidal and the intertidal zones (minerals <0.01 vol% are not represented). STZ – subtidal zone; ITZ – intertidal zone.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** XRD diffractogram of the concrete samples in the intertidal zone (ITZ) and in the subtidal zone (STZ). T – top section; M – middle section; B – bottom section.</figcaption></figure>

Air voids in the samples were free of any infillings. Extended exposure time, beyond 12 months, would likely be necessary to observe the effects of a more pronounced seawater chemical attack on the samples. It must also be highlighted, particularly in the sample immersed in the subtidal zone, that it is evident, both at a macroscopic and microscopic scales, the presence of abundant drying shrinkage cracks at the edges, especially at the edge of the B edge section. Furthermore, a period of 12 months appears too short to accurately evaluate the impact of biomass on the investigated concrete samples, as low biomass levels were found for both the investigated environments.

### 3.4. Compressive strength

Fig. 15 illustrates the evolution of compressive strength in concrete samples exposed to the intertidal and subtidal zones over 6 and 12 months of exposure. The results reveal distinct trends depending on the exposure conditions. In the intertidal zone, a gradual increase in strength, from 43 MPa at T0, to 49 MPa at 12 months, suggests a beneficial short-term effect associated with microstructural densification processes. Microscopic analyses (Fig. 14E and F) and carbonation depth measurements confirm the presence of extended carbonation in the intertidal samples (up to 2 mm), particularly at aggregate interfaces and around air voids, where porosity facilitates CO₂ ingress. The presence of Ca-Fe-Al silicates (Fig. 11), associated with the carbonation of portlandite and C-S-H, contribute to matrix densification, which can refine the pore structure, potentially increasing strength [89,90]. The minor mineral
<figure><img src="image_15.png" /><figcaption>**Fig. 13.** QEMSCAN false color mineral maps of the top (T) and bottom (B) edges of the blocks in the intertidal and subtidal zones. T – top; B – bottom. Length approximately 20 mm.</figcaption></figure>

phases detected in intertidal samples (Fig. 11), such as Ca-Mg-(Fe-Al) silicates, may point to early alteration processes that could compromise durability over time. However, this increase in strength must be interpreted alongside surface deterioration phenomena. Mineralogical maps (Fig. 13) and optical images (Fig. 14A) indicate cement paste loss and aggregate exposure at the outer edges of intertidal samples, suggesting ongoing physical degradation due to wetting-drying cycles, abrasion, and salt crystallisation. These effects are not yet fully reflected in compressive strength results, as the degradation is predominantly superficial, while carbonation-induced densification occurs slightly deeper within the material. This illustrates the coexistence of degradation and apparent strengthening, a transitional phase which may precede a long-term decline in mechanical performance, as observed in extended marine exposure studies [92,93].

In the subtidal zone, compressive strength remains stable (around 40 MPa), despite lower carbonation (max. 1.6 mm) and the presence of a thin calcite crust on some surfaces (Fig. 14B and C). The absence of drying cycles limits both carbonation and mechanical degradation, but early microstructural signs of Mg-enriched decalcified zones (Fig. 14D), supported by the identification of Ca-Mg-Fe silicates in the subtidal zone samples (Fig. 11), could indicate the onset of chemical deterioration through Mg intrusion or sulphate attack. These changes have not yet significantly impacted compressive strength but should be monitored in longer-term exposures. Indeed, chloride penetration and sulphate attack can gradually infiltrate the concrete matrix, causing microstructural changes and concrete deterioration [9,10]. These microstructural and mineralogical insights reinforce the importance of monitoring long-term evolution to capture delayed chemical degradation mechanisms in marine-exposed concretes.

The role of bio-colonisation must also be considered. The intertidal samples showed macro-algae development, which may provide a temporary barrier against aggressive agents and potentially slowing surface degradation, thus indirectly supporting mechanical retention. In contrast, barnacle colonisation in the subtidal zone appears to offer less mechanical benefit but may reduce long-term ingress of chlorides or sulphates. Several studies have shown that as barnacles grow and adhere firmly to the concrete surface, they form a biological barrier that can hinder direct contact between the concrete and seawater, thereby mitigating chemical attacks and slowing down deterioration processes [76,77]. Therefore, bio-colonisation presents a dual effect, offering potential short-term benefits through protective coverage while posing long-term uncertainties that depend on species, colonisation density, and interaction with environmental stressors. Further longitudinal studies are essential to evaluate whether these biological layers contribute to enhanced durability or mask underlying degradation processes. This study contributes to that discussion by highlighting the initial phases of biological interaction with concrete surfaces under realistic marine exposures.

## 4. Conclusions

This study evaluated the interaction of a concrete formulation designed for dyke infrastructures under real marine exposure conditions in intertidal and subtidal zones over 12 months. The findings offer valuable insights into the combined effects of biological colonisation, mineralogical evolution, microstructural alteration, and mechanical performance under contrasting marine environments.
<figure><img src="image_16.png" /><figcaption>**Fig. 14.** Microphotographs showcasing the key microtextural and mineralogical features observed in the investigated samples. Photo A in plane polarised light. Photos B, D, E and F in cross-polarized light. C corresponds to an SEM image. A) Aggregates protruding from the cement paste associated with the loss of the binder in the T section (ITZ); B and C) Calcium carbonate crust covering the bottom edge of B section (STZ); D) The arrows point to areas in where the cement paste is decalcified and enriched in Mg (top edge of T section – STZ); E) Carbonated cement paste in the interface coarse/fine aggregates (ITZ); F) Carbonation of the cement paste around spherical air voids (ITZ). Qz – quartz; STZ – subtidal zone; ITZ – intertidal zone.</figcaption></figure>

- **Bio-receptivity and colonisation:** the concrete samples exhibited clear bio-receptivity, with distinct biological communities depending on exposure zone. Green macroalgae dominated the intertidal zone, likely stimulated by light exposure and wetting-drying cycles, while red algae and barnacles colonised the subtidal samples under continuous immersion. Biomass was
<figure><img src="image_17.png" /><figcaption>Fig. 15. Compressive strength of concrete blocks as a function of exposure time in intertidal and subtidal zones over 12 months.</figcaption></figure>

significantly higher in the intertidal zone after 12 months, confirming a dynamic biofouling gradient shaped by hydrodynamic and environmental conditions.

- **Carbonation:** carbonation was detected only in the intertidal samples, with depths up to 2 mm, primarily around porous zones such as aggregate interfaces and air voids. These findings highlight the role of intermittent drying and atmospheric exposure in promoting CO₂ ingress. No visible carbonation was observed in subtidal samples, in line with their permanently saturated state.

- **Microstructural and mineralogical changes:** despite their similar initial composition, intertidal and subtidal samples evolved differently. The intertidal concrete showed signs of surface degradation, loss of cement paste and aggregate exposure, while also presenting internal densification due to carbonation. In contrast, the subtidal samples developed a protective calcite crust and showed minor Mg-enriched decalcification zones, suggesting the early stages of chemical degradation. These mineralogical variations reflect different degradation pathways: physical in the intertidal zone, and chemical in the subtidal zone.

- **Mechanical performance:** compressive strength increased in the intertidal samples, from 43 MPa to 49 MPa over 12 months, likely due to matrix densification from carbonation. However, this apparent strengthening must be interpreted with caution, as surface degradation processes are already underway. In the subtidal zone, compressive strength remained stable (~40 MPa), but early signs of chemical attack were detected, indicating potential durability risks over longer exposures.

- **Role of bio-colonisation:** the presence of macro-organisms may influence concrete durability. Algae in the intertidal zone could provide partial protection by limiting direct exposure, while barnacles in the subtidal zone may form a barrier against chloride or sulfate ingress. However, the long-term effects of bio-colonisation remain uncertain and require further monitoring.

## 5. Limitations and recommendations

While this study focused on evaluating the mechanical, microstructural, and mineralogical characteristics of concrete under marine exposure, it is acknowledged that analysing the evolution of the pore solution composition over time could provide valuable insights into the internal chemical processes affecting durability [94]. The pore solution chemistry plays a crucial role in mediating ion transport, carbonation, and biological interactions. Monitoring parameters such as pH, ionic concentration (e.g., Cl⁻, SO₄²⁻, Na⁺, K⁺, Ca²⁺), and alkalinity would help clarify the mechanisms behind observed physical and chemical transformations, particularly under intertidal versus subtidal conditions. Future studies should incorporate the monitoring of pore solution chemistry to better understand the internal chemical dynamics of concrete exposed to marine environments. Time-resolved analysis of pore water could reveal how marine ions interact with cementitious phases, and how such changes relate to durability performance and bio-colonisation effects. Such data would significantly strengthen interpretations of degradation mechanisms and enhance predictive modelling of long-term performance.

Biomass quantification through physical sampling provides valuable insight into bio-receptivity and its influence on material durability. To further enhance bio-receptivity assessment, incorporating image analysis techniques would be beneficial. Image analysis offers a non-destructive, detailed method to evaluate the spatial distribution, coverage, and morphology of biological colonization on material surfaces. Utilizing such techniques could improve understanding of bio-receptivity patterns and their effects on material performance, allowing for more comprehensive monitoring over time.

The 12-month exposure period provided meaningful insight into the early-stage biological colonisation and durability evolution of the concrete in both intertidal and subtidal environments. However, several indicators, such as the limited depth of carbonation, minimal chemical degradation, and early-stage biomass development, suggest that many long-term effects had yet to fully manifest. Prolonged exposure studies, extending over several years, would be essential to capture the cumulative impact of marine aggressors such as chloride ingress, sulphate attack, and sustained biological activity development on the concrete's microstructure and
mechanical performance. Proliferation of algae in intertidal and subtidal environment and its effect on marine infrastructure is yet another aspect of study [95,96]. Future research should extend the exposure duration beyond 12 months to assess the long-term performance of bio-receptive concrete in marine environments. This will help capture time-dependent deterioration processes [97], quantify the durability impact of persistent bio-colonisation, and validate whether the observed early protective effects from biomass are sustained or diminish over time. Incorporating circular economy materials [98,99] and longitudinal studies will be crucial to support the implementation of such eco-designed concrete formulations in permanent marine infrastructure.

Although this study evaluated bio-receptivity based on visible biomass accumulation, it is important to acknowledge that early microbial attachment, particularly biofilm formation in the initial days and weeks, plays a critical role in determining subsequent macro-organism colonisation. Microbial communities influence surface chemistry, nutrient availability, and the biological attractiveness of concrete. Our biomass quantification method, which relied on scraping and weighing, primarily captured macrofouling and did not account for early-stage microbial films. This represents a limitation in fully understanding the biological colonisation process. Techniques such as microscopy, DNA-based microbial profiling, or EPS (extracellular polymeric substances) analysis would provide a more accurate and comprehensive view of microbial attachment dynamics in future studies [100,101].

Finally, another limitation of this study is the reliance on conventionally cast concrete blocks, which may not fully capture the potential of innovative fabrication techniques. Future research could explore the application of 3D printing technologies for producing biomimetic concrete structures with enhanced geometric complexity and tailored surface textures that promote targeted biological colonisation [102]. Additive manufacturing allows for precise control over porosity, microtopography, and material placement, which could significantly improve ecological performance and durability. Integrating 3D printing in marine eco-concrete design presents a promising direction for optimizing both structural and ecological functionality in a reproducible and scalable manner.

## CRediT authorship contribution statement

**Mohammed Zelloufi:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Violeta Ramos:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Badreddine El Haddaji:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Amel Bourguiba:** Investigation. **Sam Broom-Fendley:** Writing – review & editing, Investigation. **Gavyn K. Rollinson:** Writing – review & editing, Investigation. **Alexandra Guedes:** Writing – review & editing, Investigation. **Jens Andersen:** Writing – review & editing, Methodology, Conceptualization. **Nassim Sebaibi:** Writing – review & editing, Methodology, Conceptualization. **Mohamed Boutouil:** Writing – review & editing, Methodology, Conceptualization.

## Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Violeta Ramos reports article publishing charges was provided by University of Exeter. Violeta Ramos reports travel was provided by Horizon Europe. Violeta Ramos reports financial support was provided by Interreg Europe. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The authors express their gratitude to the cooperation programme INTERREG VA France (Channel) – England, co-funded by the European Regional Development Fund, for financially supporting the MARINEFF project (Project N° 5067). Additionally, this project has been supported by funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No. 101131765 (EXCITE2) for Transnational Access, carried out at the Faculty of Sciences, University of Porto. We would like to express our sincere thanks to the marine station of the University of Caen for installing and recovering the specimens during the diving operations. We also wish to thank Maxime Navon for capturing some photographs used in this work and express our appreciation to all MARINEFF project partners for their valuable contributions. BUILDERS École d'Ingénieurs is committed to advancing sustainable development and societal responsibility (DD&RS), particularly those outlined in the 17 Sustainable Development Goals (SDGs). This scientific work specifically aligns with SDG 13 (Climate Action), reinforcing efforts to address climate-related challenges.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.cscm.2025.e05119.

## Data availability

Data will be made available on request.
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