Durability Analysis of Brick-Faced Clay-Core Walls in Traditional Residential Architecture in Quanzhou, China

Abstract

This study analyzes the durability of brick-faced clay-core walls (BCWs) in the traditional residential architecture of Quanzhou—a UNESCO World Heritage City. Taking the northern gable of Ding Gongchen’s former residence as an example, the mechanical properties, microscopic structure, and changes in chemical symbol, oxides and minerals of the red bricks and clay-cores were analyzed using finite element mechanics analysis (FEM), scanning electron microscopy (SEM), X-ray fluorescence (XRF), and X-ray diffraction (XRD). The results indicate a triple mechanism: (1) The collaborative protection and reinforcement mechanism of “brick-wrapped-clay”. (2) The infiltration and destruction mechanism of external pollutants. (3) The material stability mechanism of silicate minerals. Therefore, the key to maintaining the durability of BCWs lies in the synergistic effect of brick and clay materials and the stability of silicate mineral materials, providing theoretical and methodological support for sustainable research into brick and clay constructions.

1. Introduction

1.1. Background and Significance

The brick-faced clay-core wall (BCW, also known as a “cavity wall” or “sealed brickwork”) is a common brick–clay construction form of traditional residential architecture in China that is widely used in Fujian and Taiwan, as well as surrounding areas in Putian, Quanzhou, Xiamen, Zhangzhou, Chaoshan, and Taiwan [1]. The load-bearing strength of a BCW is poorer than a solid brick wall [2]. Therefore, the Code for Design of Masonry Structures, GB50003, has prohibited the use of BCWs since 2001 [3]. However, as a traditional building material, BCWs have profound historical and cultural value. They are widely present in Chinese traditional architecture and difficult to eliminate on a large scale in the short term [4]. Compared with other regions in China, the BCWs in Fujian and Taiwan bear not only the red brick cultural characteristics of Southern Fujian but also the collective wisdom of the people of Quanzhou—a UNESCO World Heritage City. Therefore, studying the internal factors of the BCW’s durability (the ability to maintain one’s own carrying capacity) in Quanzhou, in order to explain its widespread vitality in folk use, is of great significance for the safety of “brick and clay heritage” in subtropical regions.

1.2. Review of Durability Research

Studies on BCWs’ durability have focused mainly on compressive and seismic resistance and reinforcement. Thenmozhi, R.; Saranya, K. et al. compared the compressive strength of 18 manually cast BCW samples, which included block materials such as red bricks, fly ash bricks, and solid bricks in different proportions [5]. Zhu, B.; Wu, M. et al. conducted compression tests on BCWs at a load of 27 kg/cm² and reported that the ultimate load of different masonry methods was approximately 60% to 85% of that of actual masonry walls in the 1980s [6]. Meanwhile, Romina, S. et al. studied ways to improve durability through the addition of accessories for BCWs during the period from 2023 to 2024 [7].

In addition to mechanical studies, researchers have also proposed various filler improvement methods, yet the intrinsic material mechanisms remain insufficiently explored. For example, Zhou, T.; Tian, P.; Deng, M.; Zhao, X.; Liang, Z. proposed the use of engineered cementitious composite (ECC) reinforcement technology in 2018 [8]; Argentina, O. et al. proposed filling the cavity of an empty bucket wall with PCM to enhance its thermal insulation performance in 2021 [9]; and Deng, J. studied the role of microorganisms in filling clays in 2023 [10]. Meanwhile, a small number of studies also began to pay attention to the waterproof performance of BCWs’ skin brick material in 2024, for example, D’Ayala, D. et al. [11].

1.3. Research Objective

Current research mainly focuses on using external intervention to enhance the compressive, seismic, or thermal insulation performance of BCWs by using reinforcement fittings or materials. However, the BCW has been able to be used by the public for a long time without technological improvements. The inherent characteristics of its structure and materials inevitably have advantages in terms of durability. Therefore, it is feasible and necessary to thoroughly and comprehensively analyze the internal factors of BCWs’ durability from the perspectives of construction and material properties.

2. Materials and Methods

2.1. BCW Construction and Sampling

A BCW is a wall structure constructed of overlapping “mortise and tenon” joints made of a mixture of red bricks and clay cores [12]. Among them, the “cavity” built with red bricks is filled with clay cores (rammed earth, pebbles, or brick fragments) inside, providing effective insulation and sound insulation functions for residential spaces [13,14]. BCWs often use “Xiaoxiu bricks (150 mm × 100 mm × 20 mm)” or “salt bricks (210 mm × 150 mm × 20 mm)” as horizontal ties (Mian bricks) and “Shixing bricks (210 mm × 170 mm × 20 mm)” as vertical ties (Li bricks), forming a wall with a width of 210 mm using “Mian-and-Li” mortise and tenon overlap. The masonry methods can be divided into two main types: horizontal bonding and vertical bonding. The horizontal bonding is composed of a “shun” (stretcher) and a “ding” (header), including patterns of ding-and-shun, ding-and-double-shun, or ding-and-triple-shun (Figure 1a–c); the vertical bonding method is composed of a “mian” (solid layer) and a “dou” (hollow layer), i.e., including patterns of mian-and-dou, mian-and-double-dou, or mian-and-triple-dou (Figure 1d–f) [15].

Figure 1. The masonry method and size of BCWs. (a) Ding-and-shun. (b) Ding-and-double-shun. (c) Ding-and-triple-shun. (d) Mian-one-dou. (e) Mian-and-double-dou. (f) Mian-and-triple-dou. (g) Size specifications of red bricks (chart source: Revit 2022. Produced by Autodesk, San Francisco, CA, USA).

Based on the above construction features, the following text evaluates its durability performance in real environments through on-site sampling and experimental analysis. The research team collected and extracted four types of red brick and clay-core samples, categorized by a lower and higher degree of weathering, from sampling points A and B on the northern gable of Ding Gongchen’s former residence.

After field sampling, the samples were promptly transferred to the laboratory, where they were packaged, initially ground, and finely sieved (with a powder size less than 0.25 mm [16]). The samples were then compressed into tablets using a Ruishenbao tablet press set at 220 kN to provide experimental samples. They were coded as red brick (Z), clay-core (T), with a low degree of weathering (01), and with a high degree of weathering (02). Thus, Z-01 (red brick with a low degree of weathering), Z-02 (red brick with a high degree of weathering), T-01 (clay-core with a low degree of weathering), and T-02 (clay-core with a high degree of weathering) were used to ensure the systematic and traceable analysis of the subsequent experimental results (Table A1).

2.2. Basis for Sample Selection

The former residence of Ding Gongchen (No. 107 Guanyin Lane, Anduo Village, Chendai Town), located near the mouth of Quanzhou Bay in the Jinjiang River Basin, was selected as the research sample (Figure 2) to reveal the internal factors affecting the durability of BCWs in terms of the structure and materials of red bricks and clay cores, based on the following:

Figure 2. Location of study area (chart source: http://www.bigemap.com/source/tree/satel-1.html (accessed on 8 June 2025) and http://bzdt.ch.mnr.gov.cn/ (accessed on 8 June 2025)).

(1)

Quanzhou—as a world marine commerce center and a World Heritage City in China during the Song and Yuan dynasties—can be considered a model for the construction of traditional residential architecture in Southern China [17]. The research object of this study is the historic “three-bay double-courtyard” residence in Quanzhou built by Ding Gongchen, a scientist in the Qing dynasty, during the Jiaqing era (1800–1875). This is a typical case for the study of BCWs (Figure 3).

Figure 3. Sampling point identification diagram of Ding Gongchen’s former residence. Note that, due to the fact that the area is a no-fly zone, it was not possible to use drones for aerial photography (chart source: we obtained point cloud data by the XGRIDS Lixel L 3D laser scanner (Produced by Shenzhen Qiyu Innovation Technology Co., Ltd., Shenzhen City, China) and then imported the point cloud data into Revit 2022 for layout and annotation).

(2)

The sample collection area was located on the south bank of Jinjiang River in Quanzhou City, belonging to a subtropical humid climate zone. Due to its proximity to the sea, the salinity in both the air and water is high. In addition, the region has experienced severe environmental pollution in the past 30 years [18]. Therefore, the weathering of BCW materials in the region makes it an ideal case for durability research.

(3)

The north mountain wall of the sample building shows three different states of BCW: (A) fully preserved wall, (B) wall damaged due to years of disrepair, and (C) a temporary reinforced wall completed in 1991. According to the principle of “recognizability” in the protection and restoration of ancient architecture, contemporary crafts and materials can be compared, distinguished, and excluded based on the current situation of point (C), in order to accurately identify the crafts and materials of points (A) and (B), which is a traditional practice in the Qing Dynasty of China [19] (Figure 3, Figure 4 and Figure 5).

Figure 4. Current condition of sampling points: (a) current condition of sampling point A; (b) current condition of sampling point B; and (c) current condition of sampling point C (chart source: photography during on-site inspections).

Figure 5. Field survey and sampling site: (a) on-site sample collection; (b) on-site sample classification; and (c) sealing samples in transparent plastic containers sterilized with anhydrous ethanol (chart source: photography during on-site inspections).

2.3. Experimental Methods

Finite element mechanical analysis (FEM) was used to analyze the structural relationship between red bricks and clay cores in the BCW. Then, scanning electron microscopy (SEM), X-ray fluorescence (XRF), and X-ray diffraction (XRD) were used to determine the microstructure, chemical symbols, and changes in oxide and mineral composition of the two materials from the perspective of material chemistry, in order to demonstrate the durability advantage and internal factors of BCWs’ “brick-wrapped soil” structure. The specific method is as follows:

2.3.1. FEM Analysis

Using SOLIDWORKS 2024 (SW. software is produced by Dassault Systemes company, Concord, MA, USA) software, finite element mechanics (FEM) simulation analysis was conducted on three structural samples of the BCW (Figure 6, Figure 7 and Figure 8). The analysis utilized triangular meshes with an element size of 35 mm, as set within the software. The sample models were created using Revit 2022, with boundary conditions ranging from approximately 1 m³ to 1.5 m³ (710 mm × 970 mm × 210 mm), and the Revit model data were then converted to the IFC format for importation into SW.

Figure 6. Mechanical FEM analysis of Construction I. (Chart source: Revit 2022 modeling, exporting IFC format model, analyzed using SOLIDWORKS software.)

Figure 7. Mechanical FEM analysis of Construction II. (Chart source: Revit 2022 modeling, exporting IFC format model, analyzed using SOLIDWORKS software.)

Figure 8. Mechanical FEM analysis of Construction III. (Chart source: Revit 2022 modeling, exporting IFC format model, analyzed using SOLIDWORKS software.)

Considering the nonlinear relationship between the brick and clay materials, special attention was given to the interfaces between these different materials during the modeling process. The material properties for both the brick and clay were defined according to the software’s material library and relevant building standards (Table 1). Fixed clamps were placed at the bottom of each experimental sample, and a uniform pressure of 27 kg/cm² was applied at the top [10]. The analysis focused on the von Mises stress, static strain, and static displacement of the samples in order to evaluate the mechanical interactions between the red brick and clay materials and their impact on the durability of the BCW.

Table 1. Material properties settings for SW. Experimental samples.

Material	Elastic Modulus	Poisson’s Ratio	Shear Modulus	Mass Density	Yield Strength
Unit	MPa	(μ)	N/m2	kg/m3	N/m2
Red brick	1300	0.2	5.15 × 108	2100	107
Clay core	100	0.3	3.81 × 107	1600	105

Chart source: the property settings of the material library in SOLIDWORKS 2024 and References [3,5].

2.3.2. SEM Analysis

A trace amount of powder was affixed on the conductive adhesive, and the powder was plated using a Quorum SC7620 sputtering coater (Produced by Quorum Technologies, Laughton, East Sussex, UK) or 45 s with the current set to 10 mA. SEM was performed using a ZEISS Sigma 360 (Produced by ZEISS, Jena City, Germany) The magnification was set from 5000× to 500×, the WD was 6.7 mm to 9.4 mm, and the microstructure topography of the sample was determined. During imaging, the accelerating voltage was set to 5 kV, and the detector was an SE2 secondary electron detector (Produced by ZEISS, Jena City, Germany) to observe the microstructural changes during weathering [16]. Then, based on the principle that the more pores, cracks, and crystal expansion in the SEM images, the higher the average gray level of the images, the density changes in the microstructure of the four samples were analyzed using ImageJ software (version: 1.50d, Produced by National Institutes of Health, Bethesda, MD, USA) combined with Origin 2022 (Produced by Electronic Arts Inc., EA, Redwood City, CA, USA) [20].

2.3.3. XRF Analysis

A ZSX Primus III+ X-ray fluorescence spectrometer (Produced by Rigaku, Akishima City, Kyoto, Japan) was employed, with a total scan time of 10 min, to semi-quantitatively detect the changes in the content of various chemical symbols in the samples, and the migration and enrichment patterns of the elements during the weathering process were analyzed by Rigaku (Produced by Rigaku, Akishima City, Kyoto, Japan) to reveal the main internal factors influencing their durability [21]. Meanwhile, the C content of the experimental samples did not exceed 10% and the carbon element content was relatively low, which did not affect the accuracy of the test; therefore, the LOI treatment was not carried out.

2.3.4. XRD Analysis

A D/MAX-2600 X-ray diffractometer (Produced by Rigaku, Akishima City, Kyoto, Japan) was used to identify the mineral composition and changes in the amorphous phase of the samples and to semi-quantitatively analyze their structural deterioration and weathering mechanisms. The experimental parameters were as follows: scan mode, 2 theta; scan type, continuous scan; X-ray voltage, 40 kV; current, 150 mA; DivSlit, 1°; DivH.L Slit, 10 mm; SctSlit, 1°; and RecSlit, 0.3 mm. The total scan time was 21 min, the testing range was 3–64 degrees, and the testing rate was 3 degrees per minute.

JADE 9 (Produced by Materials Data Inc., Livermore, CA, USA) software was used for mineral composition analysis, while Origin software was used to generate diffraction peak patterns. For qualitative phase analysis, a powder diffraction database was utilized for both manual and computer retrieval, enabling the identification of matching phases from the database (PDF cards compiled and published by the International Centre for Diffraction Data, which contain standard powder diffraction spectra for various phases) [22].

During the analysis, it is important to consider factors such as overlapping peaks and preferred orientation, which can lead to spectral anomalies. Additionally, the presence of amorphous phases should be assessed. For phases that are difficult to identify, a comprehensive evaluation can be made using sample information or alternative methods.

3. Results

3.1. FEM Simulation Analysis

FEM analysis experiments were conducted for clay-filled masonry (Construction I), cavity-filled earth masonry (Construction II), and masonry with weathered or peeled surface bricks (Construction III). The results are as follows (Figure 6, Figure 7 and Figure 8).

3.1.1. Construction I

The maximum value of stress is 3.88 × 10⁸ N/m², which is far greater than the design strength of conventional building materials, indicating that the material in some areas has already undergone plastic deformation, and that the structure is at risk of local yielding under this load. Moreover, the stress distribution is uneven; the stress is concentrated in some locations in the upper and middle parts of the wall, whereas the lower part is relatively small, and the minimum is 7.52 × 10² N/m².

The maximum value of static strain reaches 1.32 × 10⁵, indicating that under the action of load, the deformation of the wall is relatively large, the microstructure inside the material underwent prominent changes, and there has been a tendency for cracking or local failure [10]. The minimum value is 4.92 × 10⁻¹. The strain distribution echoes the stress distribution, which further highlights the nonuniform deformation characteristics of the unfilled soil masonry under loading.

The maximum static displacement reaches 2.89 × 10⁸ mm, which means that the overall displacement of the wall is relatively large, and that the structural stability is poor. Under this load, the wall can tilt significantly or displacement can accumulate, and the long-term stability and safety of the structure cannot be guaranteed. The minimum displacement is 1.00 × 10⁻³⁰ mm, which can be disregarded.

This uneven stress distribution is due to the lack of clay filling in the masonry, which leads to significant stress concentrations and exacerbates the nonlinear behavior of material strain and displacement [10].

3.1.2. Construction II

The maximum value of stress is 5.11 × 10⁷ N/m², which is within a reasonable range, indicating that the overall stress level of the wall is reasonable; the fill soil effectively shares part of the load, making the stress distribution more uniform; the stress coordination of each part of the wall is good, and there is no obvious stress concentration area; and the minimum value is 8.08 × 10⁴ N/m². This shows that the fill soil can significantly improve the load-bearing performance of the wall and enhance its bearing capacity [12].

The maximum static strain is 6.41 × 10⁴, which is within a reasonable range and far lower than the maximum strain value of masonry without filled soil. The above results show that with the filled soil, the internal damage of the wall material is light, the deformation is controllable, and the structural integrity is good. The minimum value is 9.96. The strain distribution is uniform, indicating that the filling soil could coordinate the deformation of the wall under loading and reduce the risk of local damage [12].

The maximum value of the static displacement is 6.02 × 10⁷ mm, which is significantly lower than that of masonry without filled soil, indicating that the filled soil has a positive effect on the overall stability of the wall. The wall has a lower displacement under load and can maintain a better geometric shape and position. The minimum value is 1.00 × 10⁻³⁰ mm.

There is a certain frictional force between red bricks and clay, which helps to disperse stress and reduce the nonlinear response of material strain and displacement.

3.1.3. Construction III

The maximum stress value is 4.08 × 10⁷ N/m²; Although it does not exceed the ultimate strength of traditional building structural materials, it is close to the upper limit [23]. The results indicate that after the surface bricks fall off, the local stress of the wall increases, and the stress distribution becomes uneven to a certain extent. The peripheral stress of the separation position is relatively concentrated, while the internal stress is relatively small. The minimum value is 1.28 × 10⁵ N/m². Its load-bearing capacity is between Construction I and Construction II.

The maximum value of static strain is 6.95 × 10³, which is less than the maximum strain value of the masonry without filled soil, indicating that although the detachment of the surface bricks caused some damage to the wall, the global deformation is still acceptable under the action of the filled soil, the wall still has a certain bearing capacity [12], and the minimum value is 7.35. The strain distribution is uneven, reflecting that the wall is affected by the shedding of surface bricks, and that the local deformation capacity of the wall is reduced.

The maximum value of static displacement is 6.01 × 10⁶ mm, which is between Constructions I and II, indicating that the detachment of surface bricks has a certain impact on the wall displacement, but the fill soil can still have a partial confinement effect, ensuring that the wall displacement within a certain range is a minimum of 1.00 × 10⁻³⁰ mm.

The detachment of surface bricks weakens the friction and interaction between red bricks and fill soil, resulting in poor stress transmission in certain areas, increased local stress, an uneven stress distribution, and an appropriate increase in strain and displacement, reflecting the nonlinear characteristics at the material interface.

3.1.4. Summary: The Mechanical Relationship Between Bricks and Clay

Experiments have shown that Construction II (filled soil masonry) has the best mechanical properties; The maximum von Mises stress is 5.11 × 10⁷ N/m², the maximum static strain is 6.41 × 10⁴, and the maximum static displacement is 6.02 × 10⁷ mm, which is lower than the values of structures I and III. This indicates that filling the cavity with soil can make the deformation of the wall more uniform and controllable and improve the stability of the wall.

In addition, the mechanical properties of structure III (masonry with stripped-skin bricks) are between structures I and II, with a maximum von Mises stress of 4.08 × 10⁷ N/m², a maximum static strain of 6.95 × 10³, and a maximum static displacement of 6.01 × 10⁶ mm. Both are superior to structure I but not as good as structure II. The detachment of surface bricks has a certain negative impact on the performance of the wall. But the fill in the cavity can still barely support the force distribution and overall performance of the wall [8].

In summary, BCWs’ “brick-wrapped soil” mortise and tenon structure improves the overall durability of the wall. At the same time, even if the surface bricks become hollow and peel off, the clays in the inner core can still bear weight before high weathering.

3.2. SEM Microstructure Analysis

3.2.1. SEM-Red Brick

A comparison of the SEM images of the Z-01 and Z-02 samples (Table A2, Table 2, Figure 9) reveals that the grain boundaries of the Z-01 sample are clear, the grains are small and dense, and the average gray value of the SEM image is low, which is attributed to the manual brick-making process involving clay kneading, low heat dehydration, and high-temperature firing [23]. However, there are local areas with marked gray value fluctuations. For example, when the Mag (KX) value is 0.2, the gray value of Z-01 drops from 127,889 to 96,865, revealing local loose structures that are due to over-firing or under-firing. However, these issues generally do not impact normal use [24]. In contrast, the Z-02 sample has a loose structure with blurred grain boundaries and a higher (average value: 143,459) overall average gray value, reflecting severe chemical element loss and mineral transformation from weathering, as well as crystal expansion and physical damage caused by external material absorption [25].

Table 2. SEM image average mean grayscale value.

No. of Observations	Mag (KX)	Z-01	Z-02	T-01	T-02
1	5	109,124	159,747	149,542	127,539
2	1	131,942	158,001	137,266	148,312
3	0.5	114,391	154,923	122,237	136,525
4	0.2	96,865	119,649	109,963	121,846
5	5	129,237	152,001	148,924	155,796
6	1	115,287	145,134	149,112	142,807
7	0.5	123,102	145,394	140,929	130,344
8	0.2	127,889	112,821	111,434	108,938
Average value		118,480	143,459	133,676	134,013

Chart source: statistics based on Table A2 data. Grayscale value unit: dimensionless.

Figure 9. Curve graph variation in SEM image average mean grayscale value (chart source: drawing by Origin 2022).

3.2.2. SEM-Clay Core

A comparison of the SEM images and gray values of the T-01 and T-02 samples (Table A2 and Table 2, Figure 9) shows that the T-01 sample has clearer grain boundaries, a smoother surface, and a more uniform particle distribution than the T-02 sample, but it also has local crack areas. This is due to material mixing, the introduction of masonry cavities, and uneven compaction during the artificial clay-core manufacturing process [26]. The average grayscale value of the T-02 sample image is almost the same as that of T-01 but slightly higher. For instance, at a Mag (KX) value of 5, the gray value of the T-01 sample is 148,924, while that of the T-02 sample is 155,796. This indicates that clay, protected within the masonry cavity, experiences less weathering but still suffers from natural aging and environmental impacts, leading to crystal expansion, structural damage, and crack growth [27].

3.2.3. Summary: Microscopic Images of Materials

Both the red bricks and clay cores are handmade, making regions with large pores and cracks inevitable. Natural weathering and the ingress of external pollutants cause the loss of chemical symbols, mineral transformation, and crystal expansion in both materials (including a small amount of dark mineral enrichment). These factors result in structural damage and a negative impact on their durability.

Meanwhile, comparing the four samples, it was found that the average gray value of the bricks has increased significantly (118,480 to 143,459), while the parameters of clay have not changed significantly under the protection of the bricks (Table 2). Therefore, we can conclude that clay cores generally suffer less damage than red bricks.

3.3. XRF Chemical Composition Analysis

3.3.1. XRF-Red Brick

X-ray fluorescence (XRF) spectroscopy analysis revealed significant changes in the major chemical composition of the red bricks due to the combined effects of long-term disrepair and external environmental erosion (Figure 10, Table A3 and Table A4). From low weathering to high weathering, the SiO₂ content decreased from 60.906% to 54.211%, a reduction of 10.99%; Al₂O₃ decreased from 20.672% to 19.212%, a reduction of 7.06%; and Fe₂O₃ dropped from 8.398% to 7.816%, a decrease of 6.93%. MnO, K₂O, and P₂O₅ showed slight losses. The Na₂O content surged from 1.21% to 5.88%, demonstrating an enrichment of 386.35%; the Cl content also increased from 0.32% to 3.84%, indicating an enrichment of 1096%. Trace elements such as Co and Pb also showed some degree of enrichment. These changes suggest that the increase in microcracks within the red bricks made it easier for external salts to penetrate, accelerating the hydrolysis and dissolution of silicate minerals. At the same time, the dry and wet deposition of sea salt aerosols further accelerated the weathering of the red bricks, and secondary oxides such as NaCl’s formation weakened the structural stability of the red bricks further [26].

Figure 10. X-ray fluorescence (XRF) diffraction to red brick patterns: (a) Z-01 XRF diffraction pattern; (b) Z-02 XRF diffraction pattern. (Chart source: XRF chemical composition detection experiment.)

3.3.2. XRF-Clay Core

Similarly, the clay core was also influenced by external environmental factors, but the changes in the major elemental oxides were relatively smaller (Figure 11, Table A5 and Table A6). The SiO₂ content decreased from 60.080% to 58.06%, a reduction of 3.36%; Al₂O₃ decreased from 22.46% to 21.98%, a reduction of 2.14%; and Fe₂O₃ dropped from 5.53% to 4.10%, a decrease of 25.86%. The CaO content increased from 1.14% to 1.39%, indicating an enrichment of 21.90%; the Na₂O content increased from 2.34% to 3.47%, demonstrating an enrichment of 48.44%. Trace elements such as Zn, Ba, and Pb also showed some degree of loss. Although the clay core exhibited a relatively lower weathering, secondary Ca oxides and trace elements resulting from rainwater or industrial pollution negatively impacted its structural stability [27]. The SO₃ content increased from 0.55% to 0.92%, a 65.50% increase, reflecting the infiltration of external pollutants from the pores of red bricks into clay and their impact on its durability.

Figure 11. X-ray fluorescence (XRF) diffraction to clay-core patterns: (a) T-01 XRF diffraction pattern; (b) T-02 XRF diffraction pattern. (Chart source: XRF chemical composition detection experiment.)

3.3.3. Summary: Changes in Chemical Composition

In summary, both the red bricks and the clay cores exhibited increased weathering, with the red bricks showing a more significant degree of weathering. This was primarily reflected in the loss of SiO₂ and Al₂O₃, which have a “skeleton effect,” making it easier for external NaCl and SO₃ to penetrate and exacerbate the hydrolysis and dissolution of silicate minerals [26]. In contrast, due to the protective effect of the outer layer of red bricks, the clay core has undergone a relatively low weathering, resulting in less damage to its “skeleton effect”. However, external pollution leads to the infiltration of secondary calcium oxides and trace elements, which also has a negative impact on the durability of clay.

3.4. XRD Mineral Analysis

3.4.1. XRD-Red Brick

The mullite content in the red bricks decreased from 6.5% to 0%. This is due to the breakage of the Al–O bond of mullite and the limited migration ability of Al³⁺, which almost completely eliminates the “skeleton protection effect” of red bricks and significantly reduces the structural stability of the material, which gradually becomes powdery [27]. The hematite content decreased from 3.1% to 2.9%, a loss of 0.2%, reflecting the dissolution of hematite during the weathering process [28]. The content of the amorphous phase decreased significantly from 51.6% to 45.5%, a loss of 6.1%, which was mainly due to mineral substance dissolution and transformation and physical damage during the weathering process [27,28]. Moreover, the illite content was enriched by 1.3%, increasing from 1.7% to 3.0%; the quartz content was enriched by 5.2%, increasing from 27.6% to 32.8%; and the potassium feldspar content was enriched by 5.5%, increasing from 9.0% to 14.5%. These findings indicate that minerals such as illite and quartz are relatively enriched in the weathering process because of their stable structure [27]. In addition, the halite content was enriched by 0.8%, increasing from 0.5% to 1.3%, indicating that external salt affects red brick through rainwater or air and accelerates damage to the material [26] (Figure 12, Table 3).

Figure 12. X-ray diffraction (XRD) to red brick; (a) Z-01 XRD; (b) Z-02 XRD. (Chart source: XRD mineral analysis detection experiment).

Table 3. Statistical analysis of the mineral content of the samples.

No.	Kaolinite	Chlorite	Illite	Amphibole	Gypsum	Quartz	K-Feldspar	Plagioclase	Mullite	Halite	Hematite	A.ph.
Z-01	/	/	1.7	/	/	27.6	9.0	/	6.5	0.5	3.1	51.6
Z-02	/	/	3.0	/	/	32.8	14.5	/	/	1.3	2.9	45.5
T-01	32.8	3.7	11.2	/	/	46.5	2.2	3.6	/	/	/	/
T-02	32.0	3.6	10.9	0.5	0.5	44.9	2.2	5.4	/	/	/	/

Unit: mass%; To ensure a reliable and comparable analysis results, the K value method was used, adding 10% corundum (α-Al₂O₃) as an internal standard to correct diffraction intensity and standardize the results. Although XRD was used for semi-quantitative analysis in this study, JADE software was used to effectively control the relative deviation for different mineral content ranges. Specifically, when the mineral content is >40% (20%–40%, 5%–20%, ≤5%), the relative deviation is <10% (<20%,<30%,<40%) [29]. Chart source: Author 6 used instrument detection, and team members jointly analyzed and verified it.

3.4.2. XRD-Clay Core

The chemically stable quartz content in the clay cores decreased from 46.5% to 44.9%. The illite content decreased from 11.2% to 10.9%, representing a loss of 0.3%; the kaolinite content decreased from 32.8% to 32.0%, indicating a loss of 0.8%; and the chlorite content decreased from 3.7% to 3.6%, demonstrating a loss of 0.1%. Although the content of all of the above minerals decreased, the loss of minerals that affected the compression resistance and viscosity of the soil was lower due to the protection of the external red bricks, and only a small part was converted and lost in the weathering process. Moreover, the augite content ranged from 0 to 0.5, which was the result of the weathering transformation of chlorite [30]. The plagioclase content increased from 3.6% to 5.4%, with an enrichment of 1.8%. This is because plagioclase is relatively stable during the weathering process, and its proportion is relatively enriched due to the decrease in other minerals [31]. In addition, the gypsum content ranged from 0 to 0.5%, which was due to the fusion of Ca dust and clay core to form gypsum during environmental pollution [32] (Figure 13, Table 3).

Figure 13. X-ray diffraction (XRD) to clay core: (a) T-01 XRD; (b) T-02 XRD. (Chart source: XRD mineral analysis detection experiment).

3.4.3. Summary: Changes in Minerals

Under the influence of weathering mechanisms, the mineral composition of the red bricks and clay cores has undergone significant changes. In the red brick samples, the content of mullite, hematite, and amorphous materials decreased, whereas illite and quartz were relatively enriched, and the halite content increased under the influence of external salinity. Meanwhile, the clay core retained a good compression resistance and bond strength under the protection of red bricks. However, there were still some losses due to the influence of illite, kaolinite, etc., forming new minerals such as ordinary pyroxene and gypsum, which reduces the viscosity of the clay ores. As a result, the viscosity between the red brick and Clay decreases and it falls off.

4. Discussion

Based on the above findings, under the influence of natural weathering and external environmental factors, three mechanisms can be summarized that affect the durability of BCWs: (1) The protection and reinforcement mechanism of “brick-wrapped clay” joint and tenons’ coordination. (2) The infiltration and destruction mechanism of external pollutants. (3) The material stability mechanism of silicate minerals. The details are as follows:

4.1. The Protection and Reinforcement Mechanism

The mortise and tenon overlap between the red bricks and the clay cores significantly enhances the stability of the structure through the complementarity of the materials and the reasonable distribution of the load. FEM and material analysis jointly verified its advantages: brick–clay masonry (Construction II) exhibited superior integrity and load-sharing capacity. At the same time, the soil fill provides “internal support,” and its plastic deformation ability can absorb local stress and avoid stress concentration in the red brick. In addition, the following points can also illustrate the improvement effect of red bricks on the durability of clay.

The SEM results revealed that the changes in the mean grayscale values of both materials indicate that the brick has undergone significant damage, while the change in clay is not significant. Clay is not easily affected by external environmental factors and did not weather significantly under the protection of bricks.

XRF also revealed that the loss rate of elements in soil is generally lower than that in red bricks (e.g., the loss rate of Al is 6.98%, but the clay core performs better, because it is protected by red bricks), which confirms that structural design has an important role in delaying materials’ weathering. Meanwhile, it is evident from Figure 14 that Z-02 is closer to “K₂O-Al₂O₃-CaO-Na₂O” than T-02, indicating that bricks are more prone to weathering than clays.

Figure 14. SiO₂ and Na₂O-Ka₂O on a K₂O-Al₂O₃-CaO-Na₂O ternary diagram. (Chart source: Drawing based on XRD mineral analysis results.)

Moreover, XRD analysis confirmed that mineral loss in the clay core was relatively slight (kaolinite only decreased by 0.8%), which was mainly ascribed to the effective blocking of external salt by the red bricks (in XRF, the Na enrichment in the clay veneer was 149%, which was far lower than the 387% of red brick) and acid precipitation.

Combining the mechanical FEM analysis, the soil filled in the cavity exhibited a uniform stress distribution in Construction II, whereas the loss of amorphous red brick material (XRD showed a decrease of 6.1%) led to embrittlement. These two structures form a sharp contrast and highlight the significant effect of the “combination of rigidity and flexibility” in improving durability.

Therefore, through red bricks for weathering resistance, clay core for load bearing, and the mechanical synergy of the two types materials, the design of “brick-encased soil” can extend the life of the clay core far beyond its independent use limit, reflecting the outstanding wisdom of traditional construction technology.

4.2. The Infiltration and Destruction Mechanism

External environmental factors such as sea wind erosion and rising groundwater lead to mineral decomposition, increasing halite; improper industrial waste treatment raises gypsum content; and heavy metal pollutants in industrial waste cause dual damage to BCWs’ materials, including physical expansion and chemical degradation. The specific analysis is as follows:

SEM analysis revealed the consequences of pollution: the crystal expansion of red brick (Z-02) was caused by the infiltration of rock salt, resulting in an increase in the mean gray value. Meanwhile, compared with T-01, the average grayscale value of clay (T-02) remained relatively unchanged. However, crack propagation and pore enlargement were clearly observed in the microstructure, fully confirming the physical damage of pollutants.

XRF element migration was used to quantify the pollution paths. In areas with a frequent dry–wet alternation and severe industrial pollution, chlorine was enriched by 1096% (Na↑387%), and the capillary adsorption of salt exacerbated the propagation of microcracks in red bricks; sulfur was enriched by 68.38% (Ca↑23.75%), which provided the material base for the formation of gypsum in the clay core.

XRD analysis of the mineral transformation confirmed the chemical metamorphic mechanism. The halite content in the red bricks increased by 0.8%, which accelerated the destruction of the dissolution–crystallization cycle. The newly formed gypsum (0.5%) in the clay core squeezed the silicate mineral structure, resulting in the transformation of chlorite into gypsum. Mineral modification directly reduces the bond strength [27].

FEM demonstrated that the performance of masonry with peeling bricks (Construction III) is more degraded than Construction II, and the stress is reduced by 1.03 × 10⁷ N/m², reflecting the gradual loss of material viscosity and structural bearing capacity after the accumulation of pollution.

In summary, the intervention of pollutants formed a chain reaction of “salt infiltration → element migration → mineral deterioration → structural loosening,” eventually leading to a decrease in compressive strength and durability.

4.3. The Material Stability Mechanism

Silicate minerals (such as mullite, illite, quartz, and chlorite) play critical supporting roles in the compressive strength of red brick and clay core.

From the perspective of the SEM microstructure, compared with Z-02, due to the uniform distribution of silicate minerals in the raw materials, Z-01 has the characteristic of small particles and pores (low mean grayscale value); however, Z-02 has a loose structure with expanded pores (high mean grayscale value) due to mineral transformation, which fully confirms the important influence of mineral stability on microscopic integrity.

XRF elemental analysis showed that Al₂O₃ in Z-01 decreased from 20.67% to 19.21% in Z-02, and SiO₂ decreased from 60.91% to 54.21%. The increase of Na₂O + K₂O from 5.37% to 9.95% indicates that with the increase in weathering, the loss of Al₂O₃ and SiO₂ and mineral degradation weaken the strength and stability of the material. The weathering trend from 01 to 02 indicates (Figure 14) that the mass fraction of K₂O-Al₂O₃-CaO + Na₂O decreased for brick and clay materials, while Na₂O + K₂O and SiO₂ increased, once again indicating accelerated salt acid synergistic destruction, the decomposition of silicate minerals, and the release of alkaline earth minerals, which weaken the durability of the material (Table 4). Therefore, the degradation of silicate minerals during weathering is the main cause of reduced strength in brick and clay.

Table 4. The ternary analysis diagram of the main oxide conversion relationship.

No.	Oxidate	Z-01	Z-02	T-01	T-02
1	SiO2	60.91	54.21	60.08	58.06
2	Na2O + K2O	5.37	9.95	5.99	7.14
3	K2O-Al2O3-CaO + Na2O	30.42	3430	36.69	37.70
Note	Al2O3	20.67	19.21	22.46	21.98

The changes in the main oxide content in the brick and clay samples were determined according to Table A4 and Table A6.

XRD analysis further confirms the importance of silicate minerals in BCW materials. Firstly, the complete disappearance of mullite resulted in the complete pulverization of red bricks. At the same time, quartz (enriched 5.2%) and illite (1.3%) in red bricks were relatively enriched during weathering, indicating that silicate minerals also play a role in slowing down weathering efficiency. At the same time, there was slight loss of chlorite and kaolinite in the soil, but due to the protection of the outer layer of red bricks, the degree of mineral change only slightly decreased [27].

Therefore, the stability of materials formed by silicate minerals through micro compactness, crystal stability, and chemical inertness is the core element for maintaining the durability of BCW materials.

5. Conclusions and Expectation

5.1. Conclusions

Focusing on building construction and material analysis, the internal factors of BCWs’ durability were analyzed, and the effects of minerals, external pollutants, and “brick-wrapped clay” structures in raw materials on overall building durability were elucidated. The results show that

(1)

The synergy of “brick-encased soil” mortise and tenon construction greatly enhances structural stability. The protective function of the red bricks and the internal support function of the soil wall complement each other, optimize the load dispersion effect, and delay the process of clay cores’ weathering and deterioration, thereby extending their service life.

(2)

External pollutants (such as moisture, halite, gypsum, and acidic substances) destroy the material structure through physical expansion and chemical deterioration, resulting in a reduction in compressive strength and durability.

(3)

Silicate minerals (such as illite, quartz, mullite, et al.) improve the compressive strength and durability of the red brick and clay core through their compact microstructure, crystal stability, and chemical inertness.

Therefore, the key to maintaining the durability of BCWs lies in the reinforcement and protection provided by the “brick-wrapped soil” structure. Although a BCW is often subjected to infiltration and erosion by external pollutants, the stability of silicate minerals slows down the weathering rate of the BCW and enhances the durability of the structure.

5.2. Expectation

BCWs are also distributed in Zhejiang, Anhui, Guangdong, and other places in China, as well as in countries such as the United Kingdom, Italy, and Canada. The chemical composition and minerals of BCW brick soil materials in these regions vary, but their mortise and tenon structures are highly similar [7,11,25]. However, existing research often overlooks material properties, and there are significant differences in climate and weathering mechanisms between different regions. In modern engineering, similar structures are only used as non-load bearing infill walls, and their performance urgently needs systematic evaluation.

This study only focuses on the aspects of structure and material properties, and four subsequent works can be carried out:

(1)

Multiple factors can be considered to affect the durability of BCWs and expand research, such as “tie brick reinforcement”, “wood column wall interlocking”, etc.

(2)

Cross-regional comparisons should be made to elucidate the adaptability of BCWs to different climates.

(3)

By combining wind load testing, fatigue resistance testing, or interface detachment nonlinear contact simulation, the durability mechanism under multiple degradation conditions will be revealed.

(4)

Based on the results of this study and the emphasis on the principle of “minimal intervention” in the ICOMOS Authenticity Guidelines, further research will be conducted on protection and restoration strategies for BCWs [33].

In summary, the durability of a BCW refers to its ability not only to withstand compressive and seismic forces but also to maintain its own load and extend its service life. From ancient times to the present, the BCW has been mostly used as a maintenance structure and less as a load-bearing structure. In most cases, it only needs to maintain its own stability and durability. Therefore, the GB50003 standard in China prohibits it without conducting comprehensive and in-depth research on it, and the comparison with the bearing capacity of solid load-bearing walls may not be appropriate [3,34]. Therefore, the established “FEM-SEM-XRF-XRD” quadruple analysis method, which is a comprehensive evaluation method composed of mechanical properties, microstructure, chemical composition, and mineral changes, provides theoretical and methodological support for the sustainable development of BCWs and brick and soil buildings.