Analysis of Rammed Earth Wall Erosion in Traditional Village Dwellings in Zhuhai City

Abstract

(1) Background: this article focuses on the durability decline problem of rammed earth buildings in Paishan Village, Zhuhai City under the influence of complex environments. It aims to reveal the erosion mechanisms of rammed earth walls caused by different environmental factors (acid rain, salt spray, humidity, biological activities, etc.), and provide a scientific basis for formulating targeted remediation strategies. (2) Methods: a technical framework combining macroscopic investigation and microscopic analysis was adopted. Ion chromatography, scanning electron microscopy (SEM), and characterization (XRD) were used to study the damage to buildings in Paishan Village under the influence of different environmental factors. (3) Results: The acid rain and sulfate buildup could cause cracks and peeling on the south wall of the rammed earth wall. Salt spray and high humidity conditions exacerbated surface weathering on the west wall. Vibrant biological activity and high humidity made the north wall’s minerals easily dissolve, leaving the structure loose. The east wall was affected by the changing dynamics of carbonate rocks, which made it more vulnerable to weathering. (4) Conclusion: according to the analysis of different walls, specific steps should be taken during future restoration to improve the durability of rammed-earth buildings.

1. Introduction

1.1. Background and Purpose

As one of the most typical dwellings in China, rammed earth buildings have a long history and are the crystallization of the wisdom of the ancient Chinese working people. Rammed earth construction is a type of wall or structure that is built by filling moist soil (usually a mixture of clay, sand, and gravel) into formwork in layers and compacting it layer by layer [1]. Its core principle is to use mechanical force to tightly combine soil particles to form strong and durable building material. It has the advantages of a simple process, low cost, energy saving, and being warm in winter and cool in summer [2]. Paishan Village in Zhuhai City was first built during the Qianlong period of the Qing Dynasty (1778) and has a history of nearly 250 years. As one of the best-preserved ancient villages in Zhuhai, it was listed in the third batch of China’s traditional villages list in 2014 and was named “Guangdong Provincial Ancient Village” in 2015. It is an important sample for studying Lingnan farming culture and immigration history. There are more than 40 rammed earth buildings in the village dating from the Qing Dynasty to the Republic of China period, with the largest thickness reaching 50 cm [3].

The “earth” of Paishan Village comes from the local sand and soil. They use traditional three-in-one earth (soil, sand, and ash) as the base and combine organic glues like glutinous rice paste and brown sugar paste with Lingnan characteristics to make a building that is both authentic [4]. This ratio not only enhances the density of the wall but also improves the crack resistance and weather resistance through the bonding effect of organic matter so that the building remains structurally stable after more than 200 years. However, Zhuhai’s high temperature and high humidity climate accelerates the weathering of rammed earth walls, and the frequent occurrence of extreme weather also causes the surface erosion of the wall to far exceed the traditional maintenance cycle [5]. The government-led restoration project in 2021 discovered a 30% decrease in the core structural strength of some century-old walls. As time goes by, some old houses are currently facing the risk of collapse due to disrepair. We urgently need to explore and solve problems such as the traditional rammed earth’s resistance to rainwater erosion, earthquake resistance, and the durability of organic materials.

This study takes the traditional rammed earth residential buildings in Paishan Village, Bajia Village Committee, Doumen Town, Doumen District, and Zhuhai City as the research objects. We systematically studied the erosion mechanism and protection mechanism of rammed earth buildings through field investigation, experimental analysis, ion chromatography, and microscopic characterization research. First, a general survey of the rammed earth residential buildings in Paishan Village was conducted, and the types, distribution characteristics, and severity of typical damage such as cracks were recorded in detail. For research into microscopic mechanisms, scanning electron microscopy (SEM) was used to describe the microstructure of rammed earth walls. The main goal was to examine how shifts in elemental composition (e.g., key cations/anions) correlate with degradation mechanisms and erosion resistance over time. Ion chromatography was also used to look at the different anions and cations that were present in the top layer of rammed earth. This showed how the change of law changed at the microscopic level during erosion and how different erosion methods affected the wall [6,7]. We paid particular attention to the impact of acidic substances on the wall. The research results will provide a scientific basis and technical support for the protection and restoration of traditional rammed earth buildings of Paishan Village. The research will analyze the status and causes of damage to the rammed earth buildings in Paishan Village, explore the scientific understanding of the erosion resistance of rammed earth building materials in Paishan Village, and provide innovative solutions for the protection and restoration of traditional rammed earth buildings. Through scientific research and practice, the sustainable development of traditional building technology can be promoted while protecting cultural heritage, and it can provide valuable references and references for future research and protection work.

1.2. Literature Review

Residential villages are important material carriers of regional civilization, and the protection of ancient buildings is also a way to inherit local traditional culture [8,9]. Traditional Chinese villages widely use traditional rammed earth houses, one of the most typical residential buildings in China [10]. As a typical representative of Lingnan traditional villages and the largest existing rammed earth building complex in the Pearl River Delta, the rammed earth building complex in Paishan Village provides a cross-border research sample for architecture and material science and has a high research and protection value. In the process of historical evolution and inheritance, it has continuously adapted to the environment and formed a unique rammed earth culture [11]. However, as time passed and modern humanistic activities intensified in the village, the humidity of the coastal terrain caused inevitable natural and cultural damage to many ancient, rammed-earth houses. In recent years, the academic community has conducted in-depth research on the rammed earth buildings in Paishan Village. Some studies have compared the traditional rammed earth buildings in Zhuhai and Macau and analyzed the similarities and differences between the two places in terms of construction technology, material selection, and cultural background [12]. Some scholars have analyzed the causes of erosion of rammed earth materials and are committed to modifying and upgrading them by exploring the microstructure of rammed earth to protect the ancient houses in traditional villages. Some scholars have analyzed the causes of eroding rammed earth materials from a biological perspective [13]. At the same time, some researchers have noticed that the soluble salts in rammed earth go through repeated deliquescence and crystallization when water meets them. This causes salt damage, which eventually causes the wall to become unstable and fall [14]. In addition, the different chemical and mineral compositions of the soil affect its mechanical properties; the different particle compositions of the soil affect its durability. From the plastic forming of the soil to the hardening of the soil, the process from the combination of soil particles to the evaporation of water molecules will inevitably cause the volume of the material to change to varying degrees. This change may cause cracks, which will seriously affect the bearing capacity of the rammed earth wall [14,15]. In the process of the transformation and upgrading of rammed earth materials, domestic and foreign scholars have begun to improve the erosion resistance of rammed earth materials from different perspectives [16,17]. Some scholars choose traditional cement and lime to reinforce buildings in a modern way [18]. Some researchers have used natural fibers, such as straw and date palm polypropylene, to stabilize the mechanical properties of rammed earth. Due to the large carbon emissions associated with cement production, some researchers have chosen to evaluate Xanthan gum and animal glue as bio-binders [19]. All in all, further research is necessary to analyze the causes of erosion in rammed earth materials and to test material modification [20].

2. Materials and Methods

2.1. Study Area: Rammed Earth Dwellings in Paishan Village, Zhuhai City

The research area is located in Zhuhai City, Guangdong Province, China. It is situated on the west bank of the Pearl River Estuary and has a South subtropical Marine monsoon climate, featuring both tropical and subtropical climate characteristics. The seasonal fluctuations of the climate and extreme weather jointly shape its highly variable characteristics. The region has an average annual temperature of 22.6 °C, 2080.6 mm of rainfall, 80% humidity, 1876.3 h of sunshine, and an accumulated temperature of 8166.6 °C. The accumulated temperature above 10 °C exceeds 8000 °C. The rainy season is from April to September, and the rainfall accounts for 84.7% of the annual total. The average annual number of rainstorm days is 11, with 3 heavy rainstorms, and the maximum daily rainfall is 393.7 mm. Each year, it is directly affected by a tropical cyclone. The peak period of typhoons is from July to September, accounting for 75% of the annual total.

Paishan Village is in the southern part of Doumen Town, Zhuhai City, Guangdong Province, China (Figure 1), about 2 km away from the town government, and is in the hilly area of Huangyang Mountain [21,22]. The village was established in the eighth year of Qianlong (1743). Because the village field follows the slope and goes from high to low, it was named “Xie Pai” and later named “Paishan”. Its geographical environment is characterized by mountains and seas, and the Five Ridges stand majestically to the north. Due to its location in the tropics and subtropics, it has a relatively hot, humid, and sunshine climate. In this natural environment, the ancient village of Paishan was able to emerge and develop and was also deeply influenced and restricted by it, thus forming a unique architectural culture (Figure 2). The yellow silty soil rammed earth houses in Doumen Town, Zhuhai City, are typical representatives of traditional residential buildings in Lingnan [23]. Among them, the most historically and culturally valuable is Paishan Village, which was built during the Qianlong period of the Qing Dynasty. This village is not only the best-preserved traditional rammed earth building complex in Zhuhai, but also the largest existing rammed earth residential settlement in Guangdong Province. In 2012, Paishan Village was selected as one of the “Third Batch of Ancient Villages in Guangdong Province”, further highlighting its important historical and cultural status.

Paishan Village is built on the mountain. The layout of the village is scientific and reasonable. The buildings are arranged in lines horizontally and vertically, showing a high degree of regularity and order. Its drainage system is exquisitely designed, reflecting the wisdom of ancient craftsmen in adapting to local conditions. More than 70 yellow silty soil rammed earth houses are completely preserved in the village. These buildings are mainly made of rammed earth walls and tile and wood structures, with an area of about 70 square meters each. Mud carvings, brick carvings, and wood carvings are dotted in between, with dragons and phoenixes carved, vivid and unique, showing superb traditional craftsmanship and artistic standards. Although most of the ancient residential complexes are vacant, the overall original architectural layout and traditional historical style are still maintained. With their unique building materials and construction technology, these buildings have become important samples for studying the architectural technology, cultural heritage, and geographical adaptability of traditional residential buildings in Lingnan [24,25,26,27]. At the same time, they also provide a valuable physical basis for exploring the formation and development of Paishan Village’s architectural culture.

2.2. Sample Collection

The walls of the yellow rammed earth house are built using traditional rammed earth technology. The materials are mainly a mixture of yellow mud (yellow silty soil), lime, glutinous rice, brown sugar, oyster shells, and other natural substances [28,29]. This unique rammed earth formula not only gives the wall excellent adhesion and structural strength but also shows the extraordinary creativity of ancient craftsmen in the science of building materials. The rammed earth wall is evenly made, delicate, strong in compaction, uniform in color, and comfortable. As the core architectural heritage of Paishan Village, the yellow mud (yellow silty soil) rammed earth house is not only a precious material for studying the traditional architectural technology of Lingnan, but also a window to understand the social environment, economic form, and cultural characteristics of Zhuhai since the Qing Dynasty.

To fully understand the weathering characteristics of the rammed earth walls of residential buildings and their environmental impact, the researchers selected one of the representative and well-preserved traditional residential buildings as the research object (Figure 3) and systematically collected rammed earth samples in the four directions of south, east, north, and west. For studies of cooling system size/cost, a sky matrix derived from the STAT file’s clear sky radiation should be used (Figure 4). They are also useful for evaluating the impact of a building’s Damage to the surface of rammed earth buildings caused by the expansion and contraction due to the heating and cooling from solar radiation. It can be seen from the three pictures that the solar radiation of the building changes significantly in winter and summer. The drastic temperature fluctuations compromise the structural integrity of the building, inducing repeated expansion and contraction in the rammed earth materials. This thermal stress leads to structural cracking or deformation over time.

The selection of sampling points fully considered the differences in factors such as orientation (Figure 5), light, and wind erosion to ensure that the samples can objectively reflect the environmental characteristics of the building in different directions. The south wall of the residential building is exposed to direct sunlight all year round; the wall is dry, and there are some signs of weathering; the east environment is relatively humid, and some walls have turned black due to humidity; the west wall is dark and humid, and most of the walls have turned gray due to humidity. The north also has sufficient sunlight, less wall shade, and is relatively dry. By selecting these sampling points with obvious environmental differences, the researchers hope to fully explore the potential impact of the natural environment on the aging and damage of rammed earth.

We set the specific sampling location at a height of about 1.5 m from the ground. This height can effectively avoid the interference of ground moisture and plant roots on the samples, and it can fully preserve the true state of the rammed earth wall after long-term exposure to the natural environment [29]. The sampling time is 3 pm on 12 January 2025. The temperature during this period is relatively stable, which can effectively reduce the impact of environmental temperature and humidity fluctuations on the samples. During the sampling process, the researchers used a strictly disinfected medical scalpel as the main tool and slowly scraped the samples longitudinally along the surface of the rammed earth wall at a 30-degree angle. The researchers controlled the sampling depth within a range of 3–5 mm. The surface samples were chosen because they are the most exposed to environmental factors like weathering, biological erosion, and rain. This is a good starting point for studying how the chemicals in rammed earth walls change over time. This ensures that the samples can fully reflect the weathering characteristics of the rammed earth surface and minimize the damage to the wall structure. At each sampling point, we collected about 10 g of samples and stored them in 20 mL sterilized centrifuge tubes. We meticulously marked the sampling time, direction, height, and other pertinent information on the centrifuge tubes to guarantee the data’s integrity and traceability. A high-definition digital camera was used to take panoramic and detailed photos of the sampling points to fully preserve the environmental characteristics of the sampling area and the surface state of the wall. After the sampling was completed, the researchers immediately put the samples into a pre-cooled sterile cold box and evenly placed several ice packs in the box for low-temperature storage to ensure that the samples were always maintained at a constant temperature of 2–8 °C during transportation to maximize the original state and chemical stability of the samples. These samples have been quickly sent to professional laboratories equipped with advanced testing equipment.

2.3. Analytical Techniques

This study used three experimental methods—ion chromatography, scanning electron microscopy (SEM), and characterization (XRD)—to look into how ancient, rammed earth homes in Paishan Village keep their structures from eroding. In a macro sense, these methods can look at the structure and make-up of rammed earth walls. In a micro sense, they can show how certain parameters affect the wall’s ability to stop erosion.

2.3.1. Mineralogical Characterization (XRD) Analysis

Systematic sampling was conducted from the rammed earth site in the east, west, north, and south directions, and mineral composition analysis was carried out using the SmartLab high-resolution the X-ray diffractometer produced by Rikaku Co., Ltd of Japan. According to the determination procedure of X-ray Diffraction Analysis Method for Clay Minerals and Common Non-clay Minerals in Sedimentary Rocks (SY/T 5163-2018), non-destructive testing was carried out on each group of block samples with a mass of approximately 10 g and a thickness precisely controlled at 10 mm. Through the fine analysis of XRD characteristic diffraction patterns, the phase composition of clay and non-clay minerals such as quartz, plagioclase, potassium feldspar, calcite, and montmorillonite is identified with focus, and their crystallinity parameters and semi-quantitative relative contents are obtained simultaneously. Mineralogical characteristic analysis indicates that the differences in quartz content in samples from different orientations directly affect the pH value distribution of the soil. The expansibility characteristics of layered silicate minerals (such as montmorillonite) show a significant correlation with the degree of pore development, while the dissolution characteristics of carbonate minerals control the ion exchange capacity of the soil site. This kind of refined research based on the theory of mineral crystal fields provides crucial microstructure data support for revealing the weathering mechanism of rammed earth sites and formulating targeted protection plans.

2.3.2. Ion Chromatography Detection

An ion chromatograph from ZEISS Gemini SEM 360 (Carl Zeiss AG of Germany) and a potentiometric titration robot from Wantong 855 were used to find the main anions (SO₄²⁻ and HCO₃⁻) in the rammed earth samples used in this study. The experimental method was based on the relevant standards of the Chinese Geotechnical Testing Methods (2023 edition). In the sample pretreatment stage, the ultrasonic dispersion water dissolution method was used to treat the rammed earth samples, which aims to improve the extraction efficiency of soluble ions. At the same time, the samples were stirred with the help of high-speed shaking equipment to promote the full release of ions, thereby improving the measurement accuracy. Additionally, we used the solid-phase extraction column to filter impurities, thereby reducing the interference of non-target ions and enhancing the test stability. After following the steps in the Chinese national standard DZ/T 0064.49-2021 [30], the Wantong 855 potentiometric titration robot was used with a pH electrode and a 0.1 M hydrochloric acid titration solution to find SO₄²⁻ and HCO₃⁻ anions in the ion chromatography detection link [31,32]. We weighed 0.3 g–0.4 g of solid sample to the nearest 0.01 mg during specific operations. To improve data reliability, each sample was measured three times, and the average value was calculated. The concentration of each ion (mg/g or %) was then calculated and correlated with the environmental exposure characteristics.

2.3.3. Scanning Electron Microscope (SEM)

In addition, the researchers used scanning electron microscopy (SEM) to conduct a detailed microstructural characterization of the samples in four different directions (east, south, west, and north), and analyzed the differences in particle morphology, particle size distribution (this refers to a qualitative morphological assessment, such as using SEM), and microstructure, and explored the possible causes of these differences [33]. By comparing different directions, the anisotropic characteristics of the samples during synthesis or processing were revealed. In terms of experimental methods, the Zeiss SEM scanning electron microscope was selected as the core equipment [33,34]. During the experiment, the acceleration voltage was set to 10.0 kV, and the magnification was flexibly adjusted according to actual needs, with values of 5.00 KX, 10.00 KX, and 20.00 KX. Imaging was performed in a high vacuum environment. The specific operation steps were as follows: first, mark the east, south, west, and north directions of the sample and firmly fix them on the SEM sample holder; second, place the sample in the SEM sample chamber and perform the vacuum operation; then, reasonably set the acceleration voltage and working distance, and finely adjust the focus and astigmatism; then, scan different areas of the four faces of east, south, west, and north in turn, and simultaneously record the morphology images at different magnifications; finally, use image analysis software to conduct statistical analysis on the morphology and particle size of the particles, to compare the differences between the four directions, respectively.

3. Results

3.1. Ion Detection Results

3.1.1. Anion Content (SO₄²⁻, HCO₃⁻)

In this experiment, the anion content (SO₄²⁻, HCO₃⁻) of the four walls was measured, and the data are as follows (Figure 6):

(1) Sulfate (SO₄²⁻)

The content of the south wall is the highest (1.5266 mg/g), which is related to industrial acid rain deposition or the enrichment of sulfur-oxidizing bacteria metabolites. Its reaction with wall minerals may cause microcracks to expand and reduce structural strength.

The west wall has the lowest content (0.2525 mg/g), which is mainly caused by the dominant wind direction weakening the direct erosion of acid rain.

(2) Bicarbonate (HCO₃⁻)

The highest content is in the south wall (9.717%), aligning with observations that rainfall-driven accumulation of alkaline ions (e.g., Ca²⁺) enhances calcium carbonate precipitation, improving particle cementation [35]. The lowest content is in the west wall (2.192%), which reflects that the limited dissolution of carbonates has led to a decrease in the wall’s cementation ability and poor erosion resistance.

3.1.2. Effect of Ion Distribution on Wall Corrosion Resistance

By analyzing the distribution of anions in the surface soil of the four walls of the ancient rammed earth village in Zhuhai, it was found that after long-term exposure to the natural environment, rammed earth buildings will be affected by various factors, such as weathering, humidity changes, acid rain, salt spray, and biological activities, which will cause the wall structure to change to varying degrees. Since the sampling points are all from the outermost layer of the wall, this part is directly exposed to the environment, so its chemical composition gradually differs over time. These differences determine that some walls can remain relatively stable, while others are more susceptible to erosion and damage.

(1)

South wall (higher SO₄²⁻): sulfation may cause microcracks on the wall surface to expand, and peeling may occur in the long term. High HCO₃⁻ content helps to enhance structural stability in the short term but may cause loose structure due to humidity changes in the long term.

(2)

West wall (lower SO₄²⁻): The sulfate concentration is low, and the wall is relatively less affected by acid rain. However, due to the low HCO₃⁻ content, the cementing capacity of calcium carbonate is insufficient, and the durability of the wall is poor.

(3)

North wall (higher HCO₃⁻): humidity has a greater impact, and HCO₃⁻ enrichment may be due to the dissolution of minerals by microbial activities, resulting in the wall being in a dynamic equilibrium of carbonate chemical reactions for a long time.

(4)

East wall (HCO₃⁻-SO₄²⁻ balance): short-term enhancement of structural density, and long-term changes in environmental factors may lead to calcium carbonate dissolution-precipitation cycles, making the wall structure tends to be loose.

Furthermore, the influencing factors can be summarized into three aspects. (1) Influence of physical factors: The south wall is exposed to the sun for a long time, and the temperature and humidity change greatly, which may accelerate the evaporation of water, cause the soluble salts inside the wall to migrate, and cause the deposition of gypsum (CaSO₄·2H₂O). This situation can make the wall more compact in the short term, but as the gypsum continues to accumulate and absorb water and expand, the wall is prone to fine cracks, which in turn affects the overall strength. Because the west wall faces a distant river, it is affected by wind and salt spray all year round. The salt in the air adheres to the surface of the wall, making it more susceptible to erosion, causing the surface to powder and reduce durability.

(2) Influence of chemical factors: Acid rain is an important factor affecting the chemical changes of the wall, and the south and east walls are most affected. The sulfate (SO₄²⁻) content of the south wall is the highest, indicating that the industrial acid rain settles more significantly in this direction. The acidic substances will react with the wall minerals, resulting in material dissolution and loss, and promote the formation of gypsum crystals. While this process may initially increase wall density, progressive gypsum accumulates and its subsequent expansion effect will ultimately compromise structural integrity. The higher content of bicarbonate (HCO₃⁻) in the east wall indicates that it may be affected by a strong carbonic acid balance, causing carbonates to dissolve and precipitate continuously. In the short term, this helps the cementation of the wall, but in the long term, it may increase porosity and make the wall more susceptible to weathering and damage.

(3) Influence of biological factors: The north wall is suitable for the growth of mosses and microorganisms due to its shady location and high humidity. These microorganisms produce organic acids (such as oxalic acid) during metabolism, which accelerates the dissolution of wall minerals and increases the concentration of bicarbonate (HCO₃⁻). This biological weathering will gradually weaken the structure of the rammed earth wall, making the wall loose, making it more susceptible to erosion and damage than in other directions.

In general, the four walls of rammed earth buildings evolve differently under the influence of a long-term environment. The south wall may become denser first due to acid rain and changes in temperature and humidity, but it is prone to cracking over time; the west wall is severely eroded by salt spray, and the surface powdering problem is prominent; the north wall has high humidity and vigorous microbial activity, which leads to mineral dissolution, loose walls, and reduced stability; the east wall is affected by bicarbonate, which may strengthen the structure in the short term, but in the long run, it may increase the porosity of the wall and make it more susceptible to weathering. These findings not only reveal the different deterioration mechanisms of rammed earth walls but also provide important references for future building protection and restoration.

3.2. Comparison Results of Scanning Electron Microscope (SEM)

3.2.1. Comparative Analysis of SEM Images

Through SEM imaging analysis of samples from the east, south, west, and north, it was found that there were significant differences in their microscopic morphology, particle distribution, and surface characteristics.

The particles of the east samples are regular spherical or polygonal (Figure 7), small in size and evenly distributed, with slight agglomeration, and a relatively smooth overall surface. In contrast, the particles in the south are more irregular in shape, mostly polygonal in structure, with large size variations, uneven distribution, and obvious agglomeration (Figure 8). Its surface is rough and accompanied by cracks, indicating that this orientation may have been affected by greater stress during the formation process.

The particle morphology of the west samples is between the east and south, mainly spherical and ellipsoidal, with relatively consistent size, more concentrated distribution, less agglomeration, and a relatively flat surface, showing good particle uniformity (Figure 9). The particles of the north samples show fragmented or flaky characteristics, significant size differences, a wide distribution range, and more obvious agglomeration, a rough surface, cracks, and pores (Figure 10).

In general, the particles of the east and west samples have more regular morphology, relatively concentrated particle size distribution, and smooth surface, indicating that the growth environment of these two directions is more uniform and stable during the synthesis or processing. The particles of the south and north have a more complex morphology, wider particle size distribution, and surface characteristics showing strong roughness and cracks, which may be due to crystal orientation, local temperature gradient, or stress concentration. These phenomena indicate that the microstructure of the samples in different directions is affected by the synthesis or processing conditions, thus showing obvious anisotropic characteristics.

3.2.2. Particle Size Distribution Analysis

The particle sizes of the four rammed earth walls were statistically analyzed, and the results are as follows (

Table 1. Particle size statistics.

Wall Orientation	Average Particle Size (nm)	Particle Size Standard Deviation (nm)	P10 (nm)	P50 (nm)	P90 (nm)
East	281.149	128.367	153.563	399.528	645.492
South	315.276	142.235	160.345	417.562	683.459
West	276.593	120.498	148.218	388.452	610.315
North	298.462	134.876	162.774	410.389	657.923

Source: statistics by the author.

):

There are significant differences in the morphology and particle size of the samples at different wall orientations, indicating that the samples may have been affected by different external environments (such as temperature, reaction rate, or stirring rate) during the past ramming synthesis or processing, forming anisotropic structural characteristics. The average particle size of the east (

Table 2. Particle size statistics of the east rammed earth wall samples.

Total Particles		186
Particle size information	Average particle size	185.841 nm
	Particle size standard deviation	74.159 nm
	P10	118.378 nm
	P50	276.216 nm
	P90	434.054 nm

Source: Statistics by the author.

, Figure 11) and west (

Table 3. Particle size statistics of the west rammed earth wall samples.

Total Particles		113
Particle size information	Average particle size	0.431 µm
	Particle size standard deviation	0.253 µm
	P10	0.38 µm
	P50	1.039 µm
	P90	1.698 µm

Source: Statistics by the author.

, Figure 12) walls are similar, 281.149 nm and 276.593 nm, respectively, with a narrow distribution range, indicating that the synthetic conditions of rammed earth production were more uniform and stable in these directions in the past. The average particle size of the south (

Table 4. Particle size statistics of the south rammed earth wall samples.

Total Particles		67
Particle size information	Average particle size	1.488 µm
	Particle size standard deviation	1.388 µm
	P10	1.215 µm
	P50	4.048 µm
	P90	6.881 µm

Source: Statistics by the author.

, Figure 13) and north (

Table 5. Particle size statistics of the north rammed earth wall samples.

Total Particles		143
Particle size information	Average particle size	245.012 nm
	Particle size standard deviation	129.568 nm
	P10	144.684 nm
	P50	434.054 nm
	P90	723.424 nm

Source: Statistics by the author.

, Figure 14) walls are larger, 315.276 nm and 298.462 nm, respectively, and the standard deviation is larger, indicating a wider particle size distribution and diverse particle morphology. The P50 (median particle size) of the east and west walls is similar, 399.528 nm and 388.452 nm, respectively, while the P50 of the south and north walls is 417.562 nm and 410.389 nm, with more obvious agglomeration and cracks, which may be caused by crystal orientation or processing stress, and also indicates that particles are more likely to form larger sizes in the south and north walls Figure 15).

3.3. Comparative Analysis of Mineralogical Composition (XRD)

By analyzing the XRD spectra of the surface of rammed earth walls in the four directions of east, south, west, and north, the mineral composition and stability of the surface materials were understood. Four main (Figure 16) minerals were marked in the figure: quartz, mullite, lepidocrocite, and nontronite. Quartz is a very stable mineral with high hardness and durability, which can enhance the overall stability of the material. It can be seen from the figure that the diffraction peaks of quartz are relatively obvious in all samples, indicating that it occupies a certain proportion of the material. Mullite is a high-temperature phase mineral with good thermal stability and chemical stability. Lepidocrocite is easily affected by the environment and changes. Nontronite is a layered silicate mineral with certain water absorption and expansion properties. Its presence can cause volume changes in the material when the humidity changes, thereby affecting the stability of the structure. The composition of the north sample is 5.6% lepidocrocite, 45.6% quartz, and 48.8% mullite. In the north sample, the content of mullite is relatively high, indicating that the material in this direction has good thermal stability and chemical stability. Quartz also occupies a considerable proportion, further enhancing the hardness and durability of the material. The composition of the west sample is 8.1% lepidocrocite, 88.7% quartz, and 3.2% mullite. In the west sample, the content of quartz is significantly higher than that of other minerals, with extremely high hardness and durability, and good overall stability. The content of mullite is relatively low. The composition of the east sample is 72.8% quartz and 27.2% nontronite. The composition of the south sample is 70.4% quartz and 29.6% nontronite. Quartz is still the main mineral, ensuring the hardness and durability of the material. Nontronite is a layered silicate mineral, and its water absorption and expansion properties may cause volume changes in the material when the humidity changes.

4. Discussion: Long-Term Impact on the Wall and Protection Strategy

By analyzing the anion distribution of the surface soil of the four walls of the ancient, rammed earth village in Zhuhai, the effects of different environmental factors on the long-term stability of rammed earth buildings were revealed. The study found that the long-term exposure of the walls to external environments such as acid rain, humidity changes, salt spray, and biological activities led to significant differences in the chemical composition and stability of walls in different directions. These differences determine which walls are more susceptible to damage and which can maintain good stability under certain conditions. Based on this phenomenon, this study summarizes the main influencing factors and their mechanisms of action on rammed earth walls and puts forward corresponding protection and restoration suggestions.

4.1. South Wall: The Most Obvious Impact of Acid Rain

According to the statistics of the changing trend of acid rain frequency and acid rain days of different intensities in Zhuhai from 2000 to 2015, in the past, the number of acid rain days and strong acid rain days in Zhuhai reached a peak in 2008 and then declined, and the number of acid rain days (pH < 5.6) and strong acid rain days (pH < 4.5) showed similar trends [36]. The number and frequency of acid rain and strong acid rain days showed an increasing trend from 2000 to 2008, a decreasing trend from 2008 to 2015, and the acid rain frequency was less than 50% from 2012 to 2015. After 2008, the acid rain situation in Zhuhai improved significantly, and the annual average precipitation pH value also showed an increasing trend. In such an environment, the current situation of rammed earth walls in Zhuhai’s traditional villages has been affected to a certain extent. The south wall is most severely eroded by acid rain and has the highest content of sulfate (SO₄²⁻). Sulfur dioxide (SO₂) emitted by local industries oxidizes in the air, combines with precipitation and deposits on the surface of the wall, and reacts with mineral components to form gypsum (CaSO₄·2H₂O). This process may enhance the wall density in the short term, but as the gypsum gradually accumulates, absorbs water, and expands, the wall is prone to cracks and even peeling. Therefore, in terms of the protection of the south wall, measures should be taken to reduce the impact of acid rain, such as using waterproof lime slurry or calcium carbonate coating to reduce the accumulation of sulfates, and regular inspections and maintenance should be carried out to prevent the wall from being damaged by gypsum expansion [37,38].

4.2. West Wall: Salt Spray Has a Greater Impact

Since the west wall faces the nearby river, it has been affected by river wind and salt spray for a long time. This wall seems to be the most affected according to current observations. The salt in the air will adhere to the surface of the wall and continue to deposit with the dry-wet cycle. These salts may accelerate the weathering of the surface of the wall, making it more prone to powdering and peeling, reducing the durability of the wall. For the protection of the west wall, hydrophobic materials can be used to reduce the retention of salt, while optimizing the drainage system, reducing the humidity inside the wall, reducing the accumulation of salt, and thus delaying the weathering process.

4.3. North Wall: The Influence of Humidity and Biological Activity

Because the north wall is in the shade and has high humidity, moss and microorganisms are easy to grow. These microorganisms release organic acids (such as oxalic acid) during metabolism, which accelerates the dissolution of wall minerals and increases the concentration of bicarbonate (HCO₃⁻), further affecting the stability of the wall. High humidity may also aggravate the softening and loosening of the wall, reducing its durability. Therefore, during the restoration of the north wall, the growth of microorganisms should be controlled, the moss on the wall surface should be cleaned regularly [39], and a breathable but waterproof protective coating should be used to reduce humidity and reduce the impact of biological weathering [40].

4.4. East Wall: Carbonate Balance Has a Greater Impact

The east wall has a higher content of bicarbonate (HCO₃⁻), indicating that it is greatly affected by the dynamic balance of carbonates. This situation may be related to the temperature difference between day and night, the penetration of condensed water, and the carbonate composition of the wall itself. In the short term, the enrichment of HCO₃⁻ helps the precipitation of calcium carbonate, enhances the density of the wall, and improves its resistance to weathering. However, in the long term, the dissolution-precipitation cycle of calcium carbonate may increase the porosity of the wall, making it more susceptible to weathering. Therefore, in the protection process of the east wall, coating with stable carbonates can be used to reduce the changes in calcium carbonate and maintain the long-term stability of the wall.

5. Conclusions

The four walls of rammed earth buildings show different deterioration trends under the influence of different environmental factors. The south wall is most seriously affected by acid rain, and sulfate accumulation may cause cracks and peeling; the west wall is affected by marine salt spray, and the surface weathering is aggravated; the north wall is affected by high humidity and vigorous biological activity, and the wall minerals are easily dissolved, resulting in a loose structure; the east wall is affected by the dynamic changes of carbonates, which strengthens the structure in the short term, but may increase porosity in the long term, making the wall more susceptible to weathering. These research results have important practical significance for the protection and restoration of rammed earth buildings. In the actual restoration process, the characteristics of different walls should be observed with emphasis. For example, considering the corner areas where walls meet, due to the accumulation of rainwater and poor ventilation, a high-humidity micro-environment is prone to form. It is necessary to focus on designing the drainage slope or adding drainage channels to prevent water accumulation and seepage [41]. Overall, targeted measures should be taken according to the characteristics of different walls, such as reducing acid rain erosion, reducing humidity effects, controlling microbial growth, and stabilizing carbonating structures, to improve the durability of rammed earth buildings and extend their service life.

Through SEM analysis, it was found that the rammed earth walls in different orientations (east, south, west, and north) of the samples had significant differences in morphology and particle size distribution, indicating that the samples had anisotropic characteristics during growth and processing. The particles on the east and west sides have more regular morphology and concentrated particle size distribution, which may be since the synthesis conditions (such as temperature, solvent concentration, etc.) are more uniform and stable in these areas. The particles on the south and north sides have irregular morphology, wider particle size distribution, and significant cracks and pores, which may be due to the local temperature gradient or stress concentration during the growth process. The difference in surface agglomeration and roughness may be related to the electrostatic effect or van der Waals force on the surface of the material, which can be improved in the future by regulating the reaction environment (such as solvent polarity, stirring speed, or catalyst concentration). The results show that by optimizing the reaction conditions and cooling process, it is expected to obtain more uniform particle morphology and particle size distribution in different orientations, thereby improving the overall performance and stability of the material.

Prospects for subsequent research: given the exploratory nature of the initial research phase, SEM (Scanning Electron Microscopy) imaging is currently being conducted on samples collected from the east, south, west, and north faces of the wall. In the subsequent analysis, the number of sampling points on each wall will be increased to five, symmetrically distributed across the upper, middle, and lower sections, as well as both sides. This adjustment will expand the total number of samples to 20, enhancing both the representativeness and accuracy of the analysis.