Mineralogical and Mechanical Characterization of Concrete Blocks for Artificial Reefs: A Comparative Study with Natural Coral Skeletons

Abstract

Coral reefs are very important ecosystems for the planet, offering ecological and socio-economic benefits. However, they are under threat due to anthropogenic factors and environmental changes. This study assesses the feasibility of weathered Portland cement concrete as a material for marine artificial reefs by comparing its physicochemical and mechanical properties with those of natural coral skeletons from the coast of Paraíba, Brazil. Analyses included microstructural and physical characterization, compressive strength and ultrasonic pulse velocity tests, as well as pH monitoring. The results indicated that weathered concrete exhibits mineralogical similarity to corals, with the presence of carbonate phases and portlandite absent due to advanced carbonation. The compressive strength of the concrete (27.6 MPa) was significantly higher than that of the coral samples (1–6 MPa), while the porosity of the corals (34–41%) exceeded that of the concrete (14%). The alkaline nature of the concrete (pH 9.7) remained stable. Although differences in physical and mechanical properties are evident, the values are within the ranges reported for cementitious materials in marine applications. Mineralogical similarities between coral skeletons and concrete support its potential as a functional analog in artificial reefs, while adjustments in geometry and porosity are suggested to enhance ecological compatibility.

1. Introduction

Coral reefs are critically important marine ecosystems due to their rich biodiversity and the support they provide to a wide range of marine life [1]. Nevertheless, these natural habitats are increasingly threatened by environmental impacts associated with rising sea temperatures and sea levels, shifts in storm intensity, ocean acidification, and excess sediment and nutrient loads [2]. One of the most severe consequences is coral bleaching, which can trigger a cascade effect, impacting associated fauna and entire reef ecosystems [3,4]. Understanding bleaching physiology is crucial for enhancing the resilience of both wild corals and those used in restoration programs [5,6].

In response to the global decline of natural reefs, many studies have explored strategies for marine ecosystem restoration [7,8], including coral nurseries [9,10], coral transplantation [11,12], and the development of artificial reefs. Artificial reefs consist of underwater structures designed to replicate natural reef habitats, offering shelter, feeding grounds, and settlement surfaces for marine species. Paxton et al. [13] assessed the effectiveness of artificial coral reefs in enhancing fish diversity and abundance compared to natural reefs, reporting comparable ecological performance in several cases.

However, the success of artificial reef projects depends on multiple factors, including site selection, structural design, long-term management, and, critically, the choice of construction materials [14]. A wide range of materials has been used, such as plastics, tires, ship hulls, steel, and concrete [15]. A recent global survey by Vivier et al. [16] analyzing over 100 artificial reef projects found that approximately 60% were built using concrete as the primary material. The widespread use of concrete has been linked to its ability to form stable, non-toxic substrates with surface textures, similar to natural rocky habitats, that support the establishment and growth of diverse benthic organisms [17,18].

The main drawbacks associated with the use of concrete as artificial reefs are the significant environmental impact of its production, as Portland cement manufacturing is a major contributor to global CO₂ emissions [19], and the inherently high surface pH of the material, which hinders initial biological colonization. This elevated alkalinity requires several months of submersion before it approaches the pH of seawater, which, during this period, limits the colonization of a wide range of marine species due to both the high pH and the leaching of calcium ions [20,21].

Recent research on concrete for artificial reefs has largely focused on the development of new mix designs with optimized properties [17,22,23]. Regarding durability, many studies assess the performance of such concretes under controlled or accelerated conditions, with carbonation tests [24,25] and chloride penetration tests [26,27] commonly used to predict long-term performance. However, there is still a lack of research on the use of aged concrete that has naturally weathered in the marine environment over long periods of time as artificial reefs [28].

In this context, the Seixas coral reefs [29], which form part of the coastal reef systems of the state of Paraíba, Brazil, stand out for their ecological, economic, and social importance. In recent decades, studies have reported on geobiology, geomorphology [30], marine biodiversity, and bleaching in scleractinian, hydrocoral, and octocoral species during thermal stress events and under anthropogenic pressures [31,32]. However, the physical, chemical, geological, and biological characteristics of the Seixas coral reefs are still not sufficiently discussed. Further investigation is required to support the design and selection of materials for artificial reefs, ensuring compatibility with the local environment and enhancing success in larval recruitment, benthic colonization, biofilm formation, and eventual ecological integration.

The history of artificial reef deployment in Brazil includes early initiatives in the 1980s, such as triangular concrete structures installed in Sepetiba Bay, Rio de Janeiro [33]. An additional attempt to deploy 400 concrete blocks in the northeast of Brazil was interrupted, leaving the blocks stored for more than 20 years without a defined purpose [34]. Currently, the Strategic Program for Marine Artificial Structures of Paraíba (PREAMAR), supported by funding from the State Government of Paraíba, Brazil, is planning to install 1000 Portland cement concrete blocks as artificial reefs in the coming years.

This scenario presents an opportunity for PREAMAR to repurpose the concrete blocks currently stored, transforming these unused resources into functional components for artificial reef projects. This approach not only promotes sustainable resource utilization but also provides a cost-effective and practical pathway to support marine habitat restoration. However, it remains uncertain whether concrete blocks exposed to 20 years of tropical weathering have indeed developed physical, chemical, and mineralogical properties that are compatible with those of natural Seixas coral reefs.

In this way, the present study aims to apply characterization techniques to compare the physical, chemical, and mineralogical properties of concrete blocks weathered in a tropical environment, with a view to their potential use as artificial reefs, highlighting the functional similarities between the selected blocks and natural corals found along the coast of Paraíba, Brazil.

The characteristics found in the weathered concrete blocks will also be compared to the literature on the use of this material in artificial reefs in order to validate them for future studies on launch and for in situ application.

2. Materials and Methods

2.1. Materials

Three types of samples were used for comparative purposes: (i) natural coral samples of three different species, collected in consolidated reef areas; (ii) cores extracted from reef fragments; and (iii) concrete blocks weathered in a tropical environment.

The fragments were obtained from a shallow reef area (Figure 1a), located in northeastern Brazil (7°9′21″ S, 34°47′10″ W), on the coastal reefs of Seixas beach in the city of João Pessoa, Paraíba, where the predominant climate is coastal tropical, and the average sea surface temperature is 28 °C [30]. The samples were collected manually during snorkeling by biologists who are members of the PREAMAR project. The samples were prepared for testing by washing in running water to remove salts and organic materials.

The biological samples of natural coral (Figure 1b–d) are made up of coral skeletons of the species Millepora alcicornis (“sample C1”), Mussismilia harttii (“sample C2”), and Siderastrea stellata (“sample C3”). In addition, five samples of reef fragments (Figure 1e,f) were extracted using a 5 cm diameter hole saw machine, with the samples named “T”. The names given to the samples are summarized in

Table 1. Sample identification.

Sample	Code
Millepora alcicornis	C1
Mussismilia harttii	C2
Siderastrea stellata	C3
Core samples of reef fragments	T

.

The concrete samples were extracted from a block stored at the fishing terminal in the city of Cabedelo—metropolitan region of João Pessoa (Figure 2), manufactured to be used as an artificial marine structure [34].

It should be noted that the concrete blocks were produced two decades ago by an independent company, and therefore, detailed information regarding their preparation and mix design is not available. It is known that they are composed of Portland cement type CP IV-RS, in accordance with ABNT NBR 16697 [35], which is comparable to ASTM C595 Type IV Pozzolanic Cement [36]. The concrete mix design included sand, fine aggregate, and coarse aggregate in a volumetric ratio of 1:7.5:3.5:7.2, along with the addition of microsilica and a superplasticizer admixture. All the concrete blocks were manufactured simultaneously using identical molds and mix designs. The quantities of materials used for the production of one concrete block are detailed in

Table 2. Material construction per concrete block.

Sample	Code
Portland Cement (CP IV-RS)	350 kg
Coarse Sand	0.27 m3
Small Aggregate	0.125 m3
Medium Aggregate	0.26 m3
Silica Fume (microsilica)	12% of cement weight
Superplasticizer	2% of cement weight

.

Figure 2. Concrete samples: (a) Located in Cabedelo, on the coast of the state of Paraíba, Brazil (adapted from [37]); (b) Fishing terminal; (c) Block storage, arrow indicating block stacking arrangement; (d) Extraction of concrete core samples; (e) Concrete core samples.

Figure 2. Concrete samples: (a) Located in Cabedelo, on the coast of the state of Paraíba, Brazil (adapted from [37]); (b) Fishing terminal; (c) Block storage, arrow indicating block stacking arrangement; (d) Extraction of concrete core samples; (e) Concrete core samples.

After extracting the concrete core, the carbonation test was carried out by spraying phenolphthalein into the hole. An 8 mm layer of carbonated front was observed around the blocks. This superficial part of the concrete was used to carry out the microstructural and PH tests.

This value of 8 mm of carbonated front for twenty-year-old concrete is in line with many models for predicting the progress of carbonation in concretes [38].

2.2. Microstructural Characterization

Mineralogical characterization was performed by X-ray diffraction (XRD), using a BRUKER D8 diffractometer with CuKα radiation (λ = 1.5406 Å), operated in continuous scan mode from 5° to 70° (2θ), with a step size of 0.02°/s. Phase identification was conducted by comparison with the PDF-2 database (ICDD), and quantitative analysis was carried out using the TOPAS software (Bruker, version 4).

Functional group identification and chemical bonding analysis were performed using Fourier-transform infrared spectroscopy (FTIR) on KBr pellets. The spectra were acquired with a Bruker Alpha II spectrometer, using 32 scans and a resolution of 4 cm⁻¹ over the range of 4000–400 cm⁻¹.

The mass loss and energy variation of the samples were evaluated by Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC), using a Discovery SDT 650 instrument (TA Instruments, New Castle, DE, USA). The tests were conducted from 30 °C to 1300 °C at a heating rate of 15 °C/min, under a nitrogen atmosphere with a flow rate of 50 mL/min. Experimental data were processed using the TRIOS software.

All the XRD, FTIR, and TGA samples were tested in the form of ground powder within 30 days of their extraction, following the recommendations for preparation in practical guides [39].

The elemental composition was also assessed by Energy-Dispersive X-ray Spectroscopy (EDS), using a 30 mm² Bruker detector coupled to a FEI Quanta 450 Scanning Electron Microscope (SEM). The analyses were performed at an accelerating voltage of 15 kV, under high vacuum conditions, and the results were reported as average atomic percentages. For these tests, the samples were prepared in the form of small pieces of polished sections, impregnated with low-viscosity epoxy resins, polished, and coated with gold, following the recommendations for preparation in practical guides [39].

2.3. Physical and Mechanical Characterization

The bulk density, open porosity, and water absorption of the samples were determined by gravimetric methods, in accordance with ASTM C642—Standard Test Method for Density, Absorption, and Voids in Hardened Concrete [40]. Samples were oven-dried at 105 °C for 24 h, weighed in dry condition, saturated in water, and subsequently weighed while immersed. Total porosity was expressed as a percentage relative to the dry mass.

Ultrasonic Pulse Velocity (UPV) measurements were conducted to evaluate the structural integrity of the concrete core samples and reef fragments (T1–T5). The tests were performed in accordance with ASTM C597—Standard Test Method for Ultrasonic Pulse Velocity Through Concrete [41], using a Pundit Proceq device operating at a frequency of 54 kHz.

Additionally, compressive strength tests were conducted in accordance with ASTM C39—Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens [42].

2.4. pH Test

The pH evaluation of the solid samples was carried out through controlled immersion in deionized water, aiming to investigate the chemical interaction between the materials and the aqueous medium over time. A fixed mass of 7 g was used for all samples, which were individually placed in hermetically sealed containers containing 70 mL of deionized water, maintaining a mass-to-volume ratio of 1:1, in accordance with procedures adapted from the literature [43,44].

The solutions were kept at room temperature (25 °C), without agitation, and protected from direct light exposure. The pH measurements were performed after 1, 3, and 7 days of immersion, using a properly calibrated pH meter with standard buffer solutions (pH 4.0, 7.0, and 9.0). The electrode was inserted directly into the supernatant, avoiding contact with sedimented solids, in order to reflect the effective pH of the medium in equilibrium with the material surface.

Although it does not fully replicate environmental or oceanic conditions, this protocol allows for the identification of the initial pH relevant to chemical interactions, such as hydroxide leaching, prior to exposure to field phenomena. Future studies may complement this work with in situ tests or in buffered media, aiming to address applicability in real-world environments.

3. Results and Discussions

3.1. X-Ray Diffraction (XRD)

Analysis of the diffractograms (Figure 3) allowed for the identification of crystalline phases present in the samples. The diffraction patterns of the biological materials (Figure 3a) revealed the presence of crystalline phases characteristic of calcium carbonate, with peaks corresponding to both aragonite (orthorhombic phase) and magnesian calcite (trigonal phase).

Aragonite was identified by peaks around 26.2°, 27.2°, and 45.9°, whereas magnesian calcite exhibited peaks near 29.8°, 48.2°, and 49.10°, indicating a slight angular shift due to the partial substitution of Ca²⁺ by Mg²⁺ compared to the pure calcite structure. The presence of magnesian calcite is typical of tropical marine environments, where Mg bioaccumulation frequently occurs in the skeletons of calcifying organisms [45,46].

The simultaneous occurrence of these phases directly influences the mechanical and chemical properties of the material [47,48] and may be associated with seasonal, physiological, or taxonomic variations, commonly observed in organisms biomineralizing calcium carbonate in tropical environments [49].

For the concrete sample (Figure 3b), the diffractogram revealed a complex polycrystalline system, with a predominance of quartz (SiO₂), identified by intense peaks at 20.8°, 26.6°, and 50.1°, corresponding to 43% of the total crystalline phase. The significant presence of calcite (21.2%) and magnesian calcite (14.3%), along with the absence of characteristic portlandite peaks typically observed around 18° and 34°, indicates carbonation reactions occurring in the cementitious matrix. These reactions involve ionic substitutions that favor the stabilization of Mg²⁺ within the carbonate lattice.

Carbonation in concrete occurs when its portlandite reacts with CO₂ dissolved in water, leading to the formation of calcite, as described in the reactions below:

$$\mathrm{C}{\mathrm{O}}_2 + {\mathrm{H}}_2\mathrm{O} \to {\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3$$

(1)

$${\mathrm{C}\mathrm{a}\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3 \to \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3+2{\mathrm{H}}_2\mathrm{O}$$

(2)

Carbonation not only stabilizes the matrix pH but also promotes the formation of carbonate surfaces more compatible with biological adhesion [50,51], which may be advantageous in the context of artificial reefs. The conversion of portlandite into carbonate phases brings the concrete mineralogy closer to that of coral skeletons, reinforcing the material’s potential for larval recruitment [52]. This finding is particularly relevant, as it demonstrates that concrete, under certain conditions, can partially resemble the mineral composition of coral skeletons, thereby reducing the ecological rejection of the material.

3.2. Fourier-Transform Infrared Spectroscopy (FTIR)

The obtained FTIR spectra (Figure 4) were analyzed to identify the functional groups present. All samples exhibited similar peaks, particularly in the regions associated with carbonate groups (C–O), with subtle variations between the spectra of the biological samples (T; C1; C2; C3) and the concrete sample.

The biological sample spectra displayed characteristic carbonate bands at 712, 875, and 1415 cm⁻¹, typical of calcite, along with additional bands in the ranges 700–730 cm⁻¹ and 1080–1130 cm⁻¹, indicative of aragonite. These findings are consistent with previous studies [53,54]. Such vibrational signatures align with the literature reports for scleractinian corals [55,56,57]. However, no bands related to organic compounds typical of biogenic tissues, such as amines, polysaccharides, or proteins, were detected [58,59]. This organic matrix plays a critical role in the nucleation and orientation of aragonite crystals during biomineralization [60,61]. The absence of these bands may be attributed to degradation or removal of organic material during sample preparation or to their low relative concentration compared to the dominant carbonate mineral matrix [62,63].

The FTIR spectra of the concrete samples showed clear evidence of carbonation, as indicated by the absence of the band at 3640 cm⁻¹, typical of portlandite. This observation corroborates the XRD results, where portlandite was also not detected. The reaction of calcium hydroxide leading to the formation of calcium carbonate, mainly in the form of calcite, was confirmed by the intense bands near 875 cm⁻¹ and 1415 cm⁻¹, corresponding to the carbonate phases identified by XRD [64,65].

From a spectroscopic standpoint, both the coral and concrete samples exhibit vibrational signatures dominated by bands assigned to carbonate groups, reflecting the predominant presence of mineral phases such as aragonite and calcite, respectively. This similarity suggests that, despite their distinct origins—biogenic in corals and geochemical in carbonated concrete—both materials share structural aspects in terms of calcium carbonate mineral composition.

3.3. Thermogravimetric Analysis (TG)

Differential thermal analysis combined with derivative thermogravimetry was employed to describe the physical and chemical reactions occurring within the samples as the temperature increases (Figure 5).

The thermogravimetric curves of the biological samples (Figure 5a–c) exhibit similar behavior, consistent with the thermal decomposition of biogenic carbonate materials. The main thermal event occurred between 600 °C and 800 °C, indicating the decomposition of carbonate (~50% mass loss), present as aragonite and/or calcite–crystalline forms predominant in coral skeletons [66,67], as confirmed by the XRD results.

The absence of significant mass loss below 200 °C indicates a low content of physically adsorbed water or volatile organic compounds. Additionally, the region between 200 °C and 400 °C—which could indicate the degradation of organic matter—shows only minor enthalpy fluctuations or mass loss, corroborating the FTIR observations, where no amide I or II bands (proteins) or characteristic polysaccharide bands were detected [61].

The concrete sample (Figure 5d) exhibits a distinct thermogravimetric profile. Mass losses occurred mainly in two stages: up to 150 °C (water) and between 600 and 800 °C (carbonates). The initial mass loss (~5%) between 50 °C and 150 °C is attributed to dehydration of free and adsorbed water, as well as hydrated phases such as C–S–H, as reported by Bui et al. [64]. The main mass loss (~10%) between 600 °C and 800 °C is due to the decomposition of CaCO₃ into CO₂ and CaO. This finding is also supported by the carbonate bands identified in the FTIR spectra (875 and 1415 cm⁻¹). The intense endothermic peak in the DSC curve within this range confirms the thermal transition, and the lower mass loss compared to coral samples suggests a lower relative CaCO₃ content in the concrete—consistent with the phase quantification from XRD analysis. The thermal overlap with coral decomposition suggests partial compatibility in thermal behavior, despite differences in composition.

A low-intensity thermal event with a mass loss below 1% occurs between 400 and 500 °C, likely corresponding to residual dehydration of portlandite or secondary hydrated gels. This behavior is consistent with the pozzolanic additions present in the concrete formulation and further supports the advanced carbonation observed, corroborating XRD and FTIR data.

Thermal analysis reveals that, although exhibiting distinct behaviors, both biological material and concrete share key thermal zones associated with carbonate decomposition. This convergence suggests a degree of mineralogical similarity, even though the relative content and nature of the involved phases differ. The thermal profile of the biological samples confirms their predominantly carbonate nature, with mass losses exceeding 50%, whereas the concrete exhibits more modest losses (~10%), consistent with a partially carbonated matrix. Thus, thermal analysis reinforces the potential of carbonated concrete as a mineralogical analogue to coral-derived matrices, contributing to the assessment of its applicability in artificial reef systems.

3.4. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS)

Figure 6 presents the morphological analysis and Figure 7 the compositional analysis of a biological sample of Millepora alcicornis, performed using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS).

The SEM images at different magnifications (Figure 6) reveal a highly porous skeletal structure, with both intrinsic and interconnected porosity. At 100× magnification (Figure 6a), the macrostructure of the hydrozoan coral is evident, characterized by a branched architecture with large interconnected cavities and well-defined internal channels. At higher magnifications (500× and 1000×, Figure 6b,c), finely textured microstructures become visible, including interlinked channels and microporous surfaces, which are consistent with regions of calcium carbonate crystal growth and deposition [63]. This morphology is typical of marine calcifying organisms and is associated with several important biological and ecological functions [68,69,70].

From a compositional perspective (Figure 7), the elemental mapping by EDS confirms the predominance of calcium (Figure 7b), which is continuously and homogeneously distributed throughout the mineral matrix. This supports the conclusion that the skeleton is primarily composed of calcium carbonate, more specifically, magnesian calcite or aragonite, as identified by X-ray diffraction (XRD).

The presence of magnesium (Figure 7c) suggests partial isomorphic substitution of Ca by Mg within the crystal lattice, which can alter structural properties and solubility—a common feature in shallow marine environments [48]. According to the literature, the Mg/Ca ratio in coral skeletons may vary with water temperature and environmental conditions [55,71].

Sulfur (Figure 7d), detected in low concentrations and with a faint distribution pattern, is likely associated with the residual organic matrix, which includes compounds such as sulfated polysaccharides and acidic glycoproteins [45,69]. This organic matrix plays a key role in regulating nucleation and crystal orientation during the biomineralization process and also contributes to the mechanical properties and biocompatibility of the skeleton [72,73].

Figure 8 and Figure 9 shows the morphological and compositional analysis, respectively, of a weathered concrete block sample using SEM coupled with EDS.

SEM micrographs (Figure 8) reveal the heterogeneous surface of the concrete, characterized by cracks, roughness, and interconnected micropores, likely resulting from weathering and hydration/carbonation processes. Although the porosity is lower than that observed in natural corals, this microtexture still contributes to the formation of microhabitats favorable to the settlement of marine organisms [50,74].

BSE images and EDS mapping (Figure 9) complement these observations by highlighting compositional contrasts: silicon (Si) is associated with aggregates, whereas calcium (Ca) is predominantly distributed in the cement paste matrix. It should be noted that EDS alone cannot differentiate Ca from hydrated paste and Ca from carbonation. However, this limitation is overcome by XRD/FTIR results, which confirm the presence of calcium carbonate, thus clarifying the mineralogical nature of the material.

The heterogeneous surface microtexture, combined with the presence of calcium carbonate, jointly creates favorable microhabitats that may support marine organism settlement and larval recruitment [17,50]. Therefore, the composition and microstructure of the concrete surface reinforce its potential as a suitable substrate for artificial reefs [23].

From the perspective of artificial reef engineering, the natural porous architecture of coral and its calcium carbonate-rich composition serve as critical references for the design of bioinspired substrates. Recent studies have shown that materials exhibiting similar morphology, high porosity, and CaCO₃-rich composition can enhance larval settlement, biofilm formation, and the recruitment of reef organisms [23,51,75]. Therefore, the SEM/EDS data reinforce the relevance of biological corals as a benchmark for the development and evaluation of synthetic materials aimed at marine restoration.

3.5. Physical and Mechanical Characterization

Figure 10a shows that the specific densities of the biological samples ranged from 1.5 g/cm³ (C2) to 2.46 g/cm³ (T2). In comparison, the concrete exhibited a specific density of 2.45 g/cm³, a value consistent with dense matrices composed of Portland cement and silica fume [76,77]. Porosity varied widely among the biological samples, from 26.51% (C1) to 41.32% (T3), contrasting with the lower value observed for concrete (14.11%), indicative of a compact and homogeneous structure.

These findings are corroborated by the low water absorption of the concrete (6.72%) (Figure 10b), significantly lower than that of the biological samples, which reached up to 31.7% (C2). This discrepancy reflects the highly porous nature of coral skeletons, whose biological function involves intercellular flow and exchange with the aquatic environment. Some studies suggest that water absorption values above 20% may be associated with coral structures exhibiting a higher proportion of open pores, possibly resulting from long-term bioerosion or dissolution processes [78,79].

Ultrasonic pulse velocity (UPV) is employed to assess the density, structural integrity, degree of compaction, and presence of microdefects or porosity in both cementitious and biomineralized materials. Figure 8c shows that biological samples exhibited UPV values ranging from 1600 to 3600 m/s, strongly influenced by their high porosity. Concrete, on the other hand, presented a higher velocity of approximately 4100 m/s, reflecting greater internal cohesion and a lower volume of interconnected pores in the dry state. An inverse correlation between porosity and UPV is observed: the higher the porosity, the lower the wave velocity. For example, sample T3, with 41.32% porosity and 30.28% water absorption, exhibited a UPV of 1684 m/s, whereas sample T1, with lower porosity (36.3%), showed substantially higher values. This is consistent with the results reported by Ma et al. [80], who observed porosity ranging from 41.08% to 54.86% and ultrasonic pulse velocity values between 2200 and 2700 m/s for coral skeletons from shallow reef environments.

In concrete, the high UPV values reflect its low porosity and absorption, associated with good particle packing and the potential formation of secondary products such as densified C–S–H due to silica fume incorporation [81]. FTIR and XRD analyses confirm the presence of carbonates, indicating carbonation, which contributes to local matrix densification and increased ultrasonic velocity, a phenomenon also reported by [82,83].

Xu et al. [84] reported velocities ranging from 2500 to 3100 m/s for both porous coral skeletons, values consistent with those observed in this study. In contrast, cementitious materials with low w/c ratios and optimized curing conditions may exhibit UPV values exceeding 4000 m/s [85].

The biological samples exhibited variable UPV responses, reflecting differences in morphology, internal porosity, and possible structural and mineralogical alterations. In contrast, concrete displayed more predictable behavior, consistent with its compact microstructure.

Figure 11 presents the compressive strength results obtained from concrete cores and natural reef fragments, clearly showing that the concrete samples exhibit significantly higher mechanical strength compared to the biological specimens.

The average compressive strength of the concrete was found to be 27.60 MPa, indicating satisfactory mechanical performance and suggesting enhanced durability of the artificial reef structures. In contrast, the natural coral samples displayed considerably lower strength values, typically ranging between 1 and 6 MPa, which is consistent with previous findings reported by Chamberlain [86] and Rodgers et al. [87], who demonstrated that tropical corals possess inherently low compressive strength.

In this context, the mechanical properties of the reef core samples are widely accepted as representative, given that, according to Rodgers et al. [87], there is a general consensus that the low organic content in coral skeletons results in comparable mechanical behavior among dry, saturated, and rehydrated specimens. This aspect is particularly relevant to the present study, as it ensures that the mechanical characterization and long-term analysis of dry coral skeletons can reliably reflect the performance of living coral structures, allowing a valid comparison with the engineered concrete formulations designed for artificial reef applications.

This pronounced strength disparity highlights the importance of utilizing robust artificial reef materials such as carbonated concrete, which can withstand marine environmental loads over extended periods. While significantly stronger than natural coral skeletons, the engineered material remains functionally designed to mimic the habitat required to support marine life colonization and ecological restoration.

3.6. pH Test

The pH curve as a function of time (Figure 12) reveals contrasting patterns between the biological samples and the concrete, reflecting structural, chemical, and mineralogical differences.

The pH curves for the biological samples remain within a relatively stable range of 7.5 to 8.5, close to the natural pH of seawater (approximately 8.0, according to [88,89]). This overall behavior suggests chemical equilibrium, indicating that coral skeletons do not induce significant changes in the alkalinity of the surrounding medium. Such stability is consistent with the fact that these samples are predominantly composed of calcium carbonate (aragonite or magnesian calcite), minerals characterized by low solubility and high chemical compatibility with marine environments. This behavior also aligns with water absorption and porosity data, which indicate high permeability and fluid exchange with the surrounding environment, without significantly altering the pH.

Studies such as Hofmann et al. [90] report similar behavior for coral-based substrates, which tend to buffer pH and maintain a biochemically stable microenvironment—an essential condition for the viability of ecological processes such as larval adhesion, biofilm formation, and biomineralization.

In contrast, the concrete samples exhibited stable pH values ranging from 9.2 to 9.7, with a slight upward trend over time. This intermediate pH is more alkaline than that of seawater but significantly lower than the typical pH of freshly cured concrete (generally between 11 and 13, according to Bui et al. [64]), suggesting that the material underwent a carbonation process. This process involves the consumption of hydroxide ions (OH⁻), which decreases the matrix alkalinity and consequently reduces the pH of the leachate. The presence of carbonates detected through FTIR and XRD analyses directly supports this interpretation.

The dense structure and low porosity of the concrete—characterized by 14.11% total porosity and only 6.72% water absorption—indicate that carbonation likely occurred in the superficial layers of the material. Nevertheless, this surface alteration is sufficient to affect the pH of the surrounding environment, which is particularly relevant in marine applications.

Furthermore, the findings align with studies by Shi et al. [91] and Azizi & Samimi [82], which report that the incorporation of mineral admixtures such as silica fume accelerates the pozzolanic reaction, reducing the content of Ca(OH)₂ and contributing to a decrease in free alkalinity. As a result, the pH tends to stabilize at lower values when compared to conventional concretes, especially following natural carbonation processes.

From an ecological perspective, the reduction of pH to values close to 9 is advantageous. While the concrete still retains sufficient alkalinity to ensure its long-term durability, it no longer represents a highly aggressive environment for marine organisms sensitive to elevated pH, such as larvae and microorganisms. In environments with pH values above 11—typical of early-ages concrete—biological colonization is often inhibited [20,21]. In contrast, a moderately alkaline pH and carbonated surfaces favors the formation of biofilms, larval settlement, and the initiation of biomineralization processes [92].

However, it should be noted that after months of seawater reducing the pH of concrete, even in situations with short curing times and little carbonation, Portland cement concrete can become a suitable substrate for coral larvae [93]. Other factors besides the high initial pH of concretes are more relevant to coral performance and reef restoration, such as topographical conditions, substrate orientation and durability, habitat structure, sedimentation rates, and coral predation [50,92,94].

Therefore, the chemical condition observed in the concrete analyzed in this study appears promising for artificial reef applications. It reinforces both the technical and ecological feasibility of carbonated concrete as a functional alternative for the construction of artificial substrates in marine environments.

4. Conclusions

This study compared natural coral skeletons with Portland concrete blocks exposed to weathering, confirming the initial hypothesis that, despite physical and mechanical differences, the concrete shares important mineralogical and chemical properties with coral skeletons.

Analyses via XRD, TG, SEM, and FTIR revealed carbonated phases in both materials, with the concrete showing carbonation after 20 years of exposure. This mineralogical similarity is crucial, as carbonated surfaces are known to favor larval settlement [17,50]. Furthermore, the concrete maintained a stable pH (9.7), a level that does not inhibit marine colonization, aligning with the literature on cement-based substrates conditioned in seawater [93].

Physically and mechanically, the differences found between the concrete and the natural coral skeletons were expected, given that they are materials of distinct origins and formation processes. The concrete showed higher apparent density (2.45 g/cm³), lower porosity (14.11%), and lower water absorption (6.72%) compared to the biological samples, which varied from 1.50 to 2.46 g/cm³ in density, 26.51% to 41.32% in porosity, and up to 31.7% in water absorption. The ultrasonic pulse velocity (UPV) was also higher in the concrete (4100 m/s), reflecting greater internal compactness. Compressive strength varied from 1 to 6 MPa in the natural fragments, while the concrete averaged 27.6 MPa. These properties are consistent with studies reporting higher durability and structural stability of concrete modules in artificial reefs, even under intense hydrodynamic action [23,93].

Although some of the concrete’s physical properties might suggest potential limitations for its ecological performance, the observed values, such as low porosity and high internal compactness, are within the ranges reported in the literature for cementitious materials in marine applications. The obtained porosity aligns with the ranges established by previous studies, which demonstrate variations between 8.9% and 16.2% for similar concretes [95]. Likewise, the ultrasonic pulse velocity (UPV) values are consistent with typical parameters for conventional concrete, which generally range from 3000 to 4500 m/s [23].

Research on concretes implanted in marine environments shows that these physical properties do not inhibit biological colonization. Knoester et al. [50] confirmed the successful attachment of corals, calcifying algae, and filter-feeding invertebrates to cementitious materials, even when they exhibit high mechanical strength. These results indicate that factors such as surface composition, microtopography, and chemical conditioning may be more determinant for ecological success than the absolute physical properties of the material.

Therefore, it is concluded that the analyzed concrete, when compared to natural coral skeletons, shows significant similarities in mineralogical and chemical characteristics, especially regarding the predominance of calcium carbonate and the identified thermal behavior. Combined with other physical and mechanical properties considered satisfactory by the literature references, these findings reinforce the viability of weathered concrete blocks as artificial reef structures, legitimizing their application and monitoring within the PREAMAR program.

However, for the fabrication of new concrete blocks, it is recommended to optimize porosity, pH, and surface microtexture, aiming to enhance the formation of microhabitats and, consequently, expand their ecological effectiveness. Additionally, studies involving the analysis of the dynamics of colonization and development of organisms in situ can be valuable for the practical validation of the project.