Research on the BEM Reinforcement Mechanism of the POSF Method for Ocean Stone Construction

Abstract

The Planting Oysters to Strengthen the Foundation (POSF) method, as a construction technique for coastal stone structures in the Northern Song Dynasty of China (1059), has been preserved to this day. Exploring its long-term reinforcement mechanism can provide theoretical support and practical guidance for the protection and sustainable development of world marine cultural heritage. This article uses Crustacean Ash Triad Clay (CATC) from Shihu Ancient Wharf in Quanzhou as a case study and conducts a systematic investigation using XRD, Raman, SEM-EDS, FTIR, and 16S rRNA high-throughput sequencing. The results show that CATC has a core skeleton of 94.6% quartz, with potassium feldspar, dolomite, and metal compounds as auxiliary components; that its 19.04% porosity provides enrichment space for positively charged ions and tide-borne microorganisms; that electrostatic adsorption between barnacle adhesive and the material achieves physical reinforcement; and that microbial metabolism promotes dolomite formation, producing chemical reinforcement. Thus, the ternary coupling of Biology–Environment–Materials forms a BEM long-term reinforcement mechanism suitable for low-carbon construction in the ocean.

1. Introduction

1.1. Background and Significance

Ocean constructions are the material carriers through which humans explore the ocean and expand living and development spaces. However, their structural durability faces long-term threats from the unique marine environment. These threats include prolonged tidal scouring, salt spray erosion, and biological attachment [1,2,3]. Therefore, research and development in long-term reinforcement technologies for coastal constructions, as well as the analysis of their principles, have become a hot topic today.

In China’s Northern Song Dynasty, the Planting Oysters to Strengthen the Foundation (POSF) method created by Cai Xiang became a reinforcement technique for coastal stone structures along the southeast coast of China. It embodies unique wisdom in adapting to marine ecology. World cultural heritage sites such as Quanzhou’s Anping Bridge, Wan’an Bridge, and Shihu Ancient Wharf have maintained structural stability to this day after being reinforced by the POSF method [4,5]. Its long-term effectiveness makes it an important model for analyzing the reinforcement principles of coastal constructions.

Hence, the significance of this study is as follows. At the theoretical level, the composition and reinforcement mechanism of CATC are clarified. The technical principles of the POSF method are analyzed in depth. These principles are refined into a long-term reinforcement theory. At the practical level, scientific support is provided for the protection and sustainable reuse of global marine cultural heritage. It provides an application path for the research and development of low-carbon construction materials in contemporary ocean environments, encompassing coastal land and seawater contact environments.

1.2. Literature Review

As early as 8400 years ago, humans used stone structures to build coastal shelters, with shell-made daily necessities and decorations found inside. This reflects coastal residents’ wisdom in utilizing stone structures and marine shellfish [6]. Along China’s southeast coast, the Northern Song-era POSF method—embodying this wisdom—became a classic coastal stone structure reinforcement technology [4].

The POSF method enhances the durability of building structures through biophysical attachment and biochemical reactions between crustaceans (such as barnacles and oysters) and construction materials [4]. However, some studies argue that crustacean attachment may increase structural load, and their metabolic products might corrode the substrate, reducing structural durability [7,8]. The contradiction lies in the fact that existing research has failed to clarify the logic of multi-factor synergy.

Regarding research on the CATC, academic circles worldwide have focused on applying calcined products of crustacean and mussel shells to marine construction materials. When shell ash is calcined, it produces calcium oxide, which reacts with seawater to form calcium hydroxide; this calcium hydroxide combines with carbon dioxide to form calcium carbonate, reducing the demand for hydrated lime in traditional cement mortar. It aligns with the concept of low-carbon sustainable development [9]. However, most relevant studies have focused on modified modern construction concrete materials, including ultra-high-performance concrete (UHPC) and low-carbon concrete (LC3) [8,10]. In contrast, research on CATC materials with no concrete incorporation or a low concrete mixing ratio remains extremely scarce. Only a limited number of scholars have conducted preliminary investigations on such materials, such as CATC materials mixed with glass powder and glass slag [11].

Research related to barnacles mainly focuses on their attachment to shipwrecks. It is known that barnacle adhesive is composed of peptide chains made of proteins and amino acids; the carboxyl (-COOH) groups in these peptides tend to lose H⁺ to form COO⁻, which can bind to positively charged metal ions in the marine environment through electrostatic adsorption, enabling firm attachment to substrates [7]. However, studies on the impact of the marine environment indicate that barnacle growth is closely related to tides and temperature. Gentle tidal flushing (which avoids larval detachment due to strong impacts) and moderate temperatures (approximately 15–25 °C, with growth inhibited below 10 °C or above 30 °C) favor barnacle attachment and reproduction, whereas intense tidal action or extreme temperatures reduce survival and attachment capability [12,13]. But research on the impact of barnacles on coastal stone structures is rarely seen.

In studies of microbial and organic modification, CATC has been combined with modifiers such as cactus juice and coconut shell ash to introduce lignin and cellulose, thereby improving bonding performance and enhancing carbon cycle efficiency. Research has found that such organic substances can promote microbial reproduction. They accelerate the carbon cycle and generate a large number of carbonate ions. These ions combine with calcium hydroxide in the material to produce more calcium carbonate. This achieves early strength reinforcement and continuous reinforcement of building materials [14,15].

In summary, existing research has obvious limitations. Firstly, the research dimension is single. Most studies focus on isolated links such as material composition, biological damage, microbial metabolism, or environmental factors. They do not form an analysis of the interaction laws among “Biology–Environment–Materials”. Secondly, most studies focus on modern concrete modification. They ignore the unique application of the POSF method in traditional coastal stone construction. Consequently, it is difficult to fully explain the long-term reinforcement mechanism of the POSF method.

1.3. Research Objectives

Based on the limitations of existing research, this study proposes four objectives: to clarify CATC mineral composition and amorphous component distribution; explore CATC pore characteristics and interactions with the marine environment; analyze coordinated reactions among crustaceans, microorganisms, and CATC; and reveal the long-term BEM (Biology–Environment–Materials) ternary coupling reinforcement mechanism to improve POSF theory.

2. Materials and Methods

2.1. Research Object: POSF and CATC

Since the Northern Song Dynasty of China, ocean stone construction using the POSF method has exhibited excellent durability. The CATC material used in the masonry blocks of these structures differs from modern binders in that it contains no concrete—it takes calcium oxide from the ash of calcined marine crustacean shells, seashells, and other crustacean hard shells as the core raw material and then mixes it with marine sand, a small amount of clay, and seawater to form a masonry block binder. This technology is unique and representative, with wide application along the coastal continental shelves in East Asia, and the Shihu Ancient Wharf in Quanzhou serves as a typical example (Figure 1).

Located on the Shihu Peninsula of Shishi City along Quanzhou Bay in East Asia, this ancient wharf is built on a natural reef flat. It has long been exposed to the natural forces of the marine environment. It not only retains intact traces of the POSF method’s application but also offers advantages like convenient sample collection and suitable experimental conditions. It can provide authentic and effective environmental and material data support for research.

To obtain high-quality research samples, three candidate sampling points (A, B, and C) were selected based on the wharf’s environmental characteristics and material distribution. Among them, the following is true:

(1)

Sampling point A faces the eastern sea directly. It is significantly affected by strong tidal scouring and sea breeze erosion. Barnacles here are easily stripped off by external forces [13], leading to a small number of attachments. So, it does not meet the sample collection requirements.

(2)

Sampling point B is located in the crevices of the revetment reefs. It is in a state of long-term seawater immersion, which is not conducive to the retention and accumulation of Crustacean Ash Triad Clay (CATC) materials [13]. The material reserves are relatively scarce.

(3)

Sampling point C, located on the embankment side with gentle tidal action and mild sea breeze impact, provided abundant CATC in stone crevices and was selected as the core sampling area (Figure 2).

2.2. Sample Processing

Samples from sampling point C were divided into chemical analysis samples (S-C-CATC) and biological analysis samples (S-M-CATC) and were pretreated separately before experiments (Figure 3). The specific steps are as follows:

2.2.1. Processing of Chemical Analysis Samples (S-C-CATC)

(1)

Non-destructive testing: keep the original surface of the sample. Remove dust with a sterile soft brush for Raman testing [16].

(2)

Sample cutting: cut into 1 cm × 1 cm × 1 cm bean-sized test blocks for SEM-EDS and XRD testing, respectively.

(3)

XRD pretreatment: take particles from the core area of the sample. Grind them in an agate mortar and pass through a 200-mesh sieve. Add an internal standard substance (corundum). Dry at room temperature in a desiccator for 24 h, and prepare the sample on a glass slide [17].

2.2.2. Processing of S-M-CATC

(1)

FTIR sample pretreatment: in a dry environment, take visible samples and an appropriate amount of dry potassium bromide powder (ratio about 1:100) into a mortar. Grind thoroughly multiple times, then place in a tablet press to form transparent thin slices for functional group testing.

(2)

16S rRNA sample pretreatment: use a sterile knife to scrape CATC from the barnacle attachment area (about 0.5 g). Quickly transfer to a sterile packaging bag. Store in an ultra-low-temperature refrigerator at −80 °C within 45 min to avoid microbial nucleic acid degradation and ensure the accuracy of subsequent high-throughput sequencing.

2.3. Experimental Methods

2.3.1. XRD Analysis

A Rigaku D/MAX-2600 X-ray diffractometer (Nippon Science Corporation, Tokyo, Japan) was used for testing. The tube voltage was 40 kV, and the tube current was 30 mA. The test range was 5–90°, the scanning speed was 2°/min, and the step size was 0.02°. The light source was Cu-Kα radiation with a wavelength of 1.540562 Å. The pretreated powder sample was evenly spread in the sample slot for scanning. XRD qualitative analysis was performed by comparing measured patterns with the JCPDS standard mineral database using MDI Jade 6 software (Materials Data Inc., Livermore, CA, USA), and semi-quantitative mineral contents were calculated by the K-value method [17].

2.3.2. Raman Analysis

An Omnic RM4100 Raman spectrometer (Wellrun Technology Co., Ltd., Wuxi, China) was used. The excitation source was a 532 nm helium–neon laser with a power of 5 mW. The spectral collection range was 100–2000 cm⁻¹, and the resolution was 1 cm⁻¹. The integration time was 20 s. Area scanning was performed on the material. Raman area scanning was performed, and characteristic peaks were identified using the RRUFF database; graphs were plotted using Python 3.12.0 (CWI, Amsterdam, The Netherlands) [18].

2.3.3. SEM Analysis

A ZEISS Sigma 360 SEM (Carl ZEISS AG, Oberkochen, Baden-Württemberg, Germany) was used to examine the microstructure of CATC. Equipment parameters were set as EHT = 20.0 kV, WD = 5 mm–6 mm, and Mag = 1000×. Three SEM images with uniformly distributed micropores (pore size < 10 μm) were selected. ImageJ 1.8.0.345 software (NIH, New York, NY, USA) was employed to process the images: the Area Threshold function captured voids and calculated average porosity.

2.3.4. EDS Analysis

A BURKER F6100 SEM coupled with an XFlash 7600 EDS system (utilizing the built-in database of the Wellrun light SEM V1.0 software platform) (BURKERT and WELLRUN Technology Co., Ltd., Wuxi, China) was used. The energy range for energy spectrum acquisition is 0~16 keV. Conduct microscopic morphology observation and semi-quantitative analysis of chemical element composition for the porous and non-porous areas in turn [17].

2.3.5. FTIR Functional Group Detection

A Nicolet iS20 Fourier transform infrared spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was used for testing. First, the background was collected, then the infrared spectrum of the sample was collected. The resolution was 4 cm⁻¹, the number of scans was 32, and the test wavenumber range was 300–4000 cm⁻¹. FTIR spectra were collected with an ATR accessory; characteristic functional groups were identified by comparison with Omnic 8.2 software and standard database, and relative contents were estimated by peak area normalization [19].

2.3.6. The 16S rRNA Sequencing

The NovaSeq 6000 16S rRNA sequencing was performed on the Illumina MiSeq PE300 platform (Illumina, San Diego, CA, USA); raw reads were quality-controlled, denoised, and clustered into ASVs, and species annotation was performed against the Silva database using QIIME 2 (International open source community, West Hollywood, CA, USA) [20].

3. Results and Discussion

3.1. Combined XRD and Raman Analysis: Material Composition Characteristics

The semi-quantitative XRD analysis and Raman spectroscopy testing jointly reveal the core composition characteristics of CATC, providing a solid material foundation for long-term reinforcement.

XRD results indicate CATC is dominated by quartz (SiO₂) at 94.6% by mass, with potassium feldspar (KAlSi₃O₈) at 4.3% and dolomite (CaMg(CO₃)₂) at 1.1%, providing a rigid skeleton [17,19] and interface bonding, and ion (Ca²⁺, Mg²⁺) supply, respectively, reserving core substances for subsequent chemical reinforcement reactions [7,21] (

Table 1. Mineral composition and functional positioning.

No.	Mineral	Chemical Formula	Mass
1	Quartz	SiO2	94.6
2	Potassium feldspar	KAlSi3O8	4.3
3	Dolomite	CaMg(CO3)2	1.1
Total			100

Unit is the mass fraction%; XRD adopts semi quantitative analysis, and the relative deviation of different mineral content ranges is controlled by JADE software. The relative deviation is less than 10% when the mineral content is greater than 40%, less than 20% when it is between 20% and 40%, less than 30% when it is between 5% and 20%, and less than 40% when it is ≤5%; chart source: the research team used the Rigaku D/MAX-2600 X-ray diffractometer for detection and acquisition.

, Figure 4).

Raman spectra corroborate XRD findings, showing characteristic peaks for quartz, potassium feldspar, and dolomite, and also detect metal compounds such as hematite and chalcopyrite that supplement the material composition (

Table 2. Raman spectral characteristics and compound functions.

No.	Chemical Compound	Chemical Formula	Main Characteristic Peaks
1	Quartz	SiO2	128.0, 207.1, 467.4
2	Potassium feldspar	KAlSi3O8	154.3, 279.6, 511.0
3	Dolomite	CaMg (CO3)2	297.9, 1094.3, 1325.7
4	Chalcopyrite	CuFeS2	293.1, 410.8, 611.3
5	Hematite	Fe2O3	151.3, 288.2

The main characteristic peak unit is cm ⁻¹. Chart source: the team used the Omnic RM4100 Raman spectrometer for testing to obtain characteristic peak data.

, Figure 5). The rigid support of quartz, the bonding connection of potassium feldspar, and the ion supply of dolomite and metal compounds together form the crystalline mineral skeleton characteristics of CATC [22,23,24]. Therefore, the basic load-bearing reinforcement mechanism of CATC in the POSF method is clarified.

3.2. SEM-EDS Analysis: Element Distribution and Pore Characteristics

SEM-EDS analysis shows an average porosity of 19.04% and detects marine elements (Na, Cl, S) around pores, indicating pores act as enrichment spaces for ion migration, barnacle attachment, and microbial colonization (

Table 3. SEM porosity calculation table.

Sampling Area	Gray Value	Mean Gray Value	Percentage of Pore Area
1	139,369	83.354	19.56
2	122,771	66.571	17.30
3	144,605	64.726	20.28
Mean porosity			19.04

The units of porosity Mean, Min, and Max are mass fraction%; chart source: the Threshold function in ImageJ is used to automatically adjust the image Area Threshold to capture gaps and calculate their porosity.

, Figure 6).

EDS reveals higher Ca (10.51%) and Mg (6.90%) in pore areas compared with non-pore areas (Ca 0.65%, Mg 5.84%), creating local enrichment favorable for chemical reinforcement (Figure 7 and Figure 8,

Table A1. Chemical element content and regional distribution characteristics.

No.	Element	Pore Area	Non-Porous Area
1	C	20.95	38.49
2	N	1.80	2.83
3	O	39.00	29.73
4	Na	3.11	4.09
5	Mg	6.90	5.84
6	Al	1.21	2.04
7	Si	9.23	5.69
8	S	0.28	1.01
9	Cl	4.61	5.55
10	K	0.13	1.21
11	Ca	10.51	0.65
12	Fe	1.54	1.23
13	Zr	0.48	-
14	Pt	2.81	2.88
	Total	102.57	101.26

Unit is the mass fraction%; the stable distribution of marine characteristic elements (Na, Cl, S, etc.) in the pore area confirms their correlation with the marine environment; trace metal elements such as Ca, Mg, and Fe provide conditions for charge adsorption and chemical reactions; chart source: own statistics and plotting.

).

Marine characteristic elements optimize the biological metabolism and chemical reaction environment by regulating the internal ionic strength of the material [25,26,27]. The positively charged ions such as Ca²⁺, Mg²⁺, and Fe²⁺ enriched around the pores create favorable conditions for the electrostatic adsorption of barnacle glue [8]. Consequently, the “interaction space” provided by porosity and the “material supply” formed by regional element enrichment act synergistically. They form an integrated “space–material” support system [27,28].

3.3. FTIR Analysis: Physical Reinforcement Mechanism

FTIR identifies O-H, C-O, Si-O-Si, and other characteristic peaks and detects carboxyl-related peaks consistent with barnacle adhesive organics, supporting a physical reinforcement mechanism via electrostatic adsorption [8]. At the same time, characteristic peaks related to carboxyl (-COOH) are detected, confirming the attachment and distribution of barnacle glue-derived organic matter in the material (

Table 4. Characteristics and functions of FTIR functional groups.

No.	Chemical Functional Groups	Main Characteristic Peaks
1	O-H	3360
2	C-O	1449
3	Si-O-Si	1086
4	Si-O-Si	798, 779
5	Si-O	695
6	Si-O	462

The main characteristic peak unit is cm ⁻¹; chart source: detection using Nicolet is20 Fourier transform infrared spectrometer.

, Figure 9).

Barnacle adhesive carboxyl groups (-COOH) deprotonate to COO⁻ and electrostatically adsorb to positively charged ions (Ca²⁺, Mg²⁺, and Fe²⁺) enriched around CATC pores, filling microvoids and forming a biological waterproof layer that enhances erosion resistance [8]. However, coastal stone structures themselves have natural pores, and the adsorption of barnacles on the building components increases the load, reflecting the duality of barnacle attachment [7,8,21]. Thus, the physical reinforcement mechanism is delineated. Barnacle adhesive fills pores via electrostatic adsorption.

3.4. The 16S rRNA Analysis: Chemical Reinforcement Mechanism

The 16S rRNA sequencing detected 110 bacterial species; the top 10 most abundant taxa include aerobic, microaerophilic, and facultative anaerobic bacteria that collectively drive carbonate production and sulfur transformations contributing to dolomite deposition and iron sulfide formation (Figure 10 and Figure 11,

Table A2. Top 20 bacterial species statistics.

No.	Species	Abundance	Metabolic Type
1	Nitriliruptoraceae	0.042	Heterotrophic
2	Muricauda	0.039	Heterotrophic
3	Cyanobacteriales	0.036	Photosynthetic aerobic
4	S0134_terrestrial_group	0.034	Heterotrophic
5	Limimaricola	0.026	Photosynthetic aerobic
6	Euzebyella	0.022	Heterotrophic
7	Pleurocapsa PCC-7319	0.021	Photosynthetic aerobic
8	Ilumatobacter	0.021	microaerophilic
9	Phormidesmis ANT.LACV5.1	0.020	Photosynthetic aerobic
10	Erythrobacter	0.011	Heterotrophic

Chart source: own statistics and plotting.

) [25,26].

Photosynthetic aerobic bacteria (Cyanobacteriales. et al.) fix CO₂ and release O₂ [25], while heterotrophic (Erythrobacter. et al.) and microaerophilic bacteria (Ilumatobacter. et al.) produce CO₂ that is converted to carbonate ions. It was found that elements such as Ca, Mg, C, and O are enriched around the pores through SEM-EDS analysis, which may be due to dolomite sedimentation [26,27,28]. Dolomite is dynamically deposited in the material pores and interface areas, filling microvoids and improving structural compactness [29,30]. Accordingly, the differential metabolism of the top 10 abundant microorganisms clarifies the chemical reinforcement mechanism.

In addition, it is worth noting that facultative anaerobes metabolize sulfur to produce H2S and sulfate species, which may react with iron oxides to precipitate iron sulfides, but they do not exert a significant corrosive effect on stone structures [8,31].

4. Conclusions

Taking CATC from Quanzhou Shihu Ancient Wharf as the research object, a multi-technique combination is adopted to explore the long-term reinforcement mechanism of the POSF method for ocean stone construction.

The BEM ternary coupling mechanism of POSF is a dynamic closed-loop system: tides provide organisms (such as barnacles and microorganisms) and ions (Ca²⁺, Mg²⁺, Fe²⁺, etc.). CATC offers a skeleton and pore spaces for interaction (porosity 19.04%). Organisms achieve long-term structural stability through physical and chemical reinforcement.

The innovation clarifies the BEM ternary coupling reinforcement mechanism of the POSF method for coastal stone structures. Unlike marine concrete structures such as UHPC and LC3, it achieves long-term reinforcement without metal components [32] and addresses the academic controversy over barnacle attachment: barnacle impact is position-dependent—attachment to the CATC surface forms a protective layer via biomineralization synergy, while non-contact attachment increases structural load, allowing targeted cleaning in practice.

5. Prospects

This study is only a preliminary exploration into the Planting Oysters to Strengthen the Foundation (POSF) method. Future research can be advanced in multiple directions: first, break the limitation of the single sample in this study and expand the research scope to enhance the robustness of the conclusions; second, supplement direct quantitative data on the formation rate of microbial dolomite; third, investigate the laws governing the impacts of barnacles in different marine environments and on different attachments. All these research efforts are aimed at further exploring and improving the BEM ternary coupling mechanism of the POSF method and providing new insights for in-depth research on the protection and sustainable utilization of global ocean cultural heritage.