# Analysis of adobes from vernacular raw earth buildings in the Champagne region (France)

Guillaume Polidori, Adrien Aras-Gaudry, Céline Rousse, Fabien Beaumont, Fabien Bogard, Sébastien Murer, Tala Moussa, Christophe Bliard, Gilles Fronteau, Erwan Hamard

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Guillaume Polidori, Adrien Aras-Gaudry, Céline Rousse, Fabien Beaumont, Fabien Bogard, et al. Analysis of adobes from vernacular raw earth buildings in the Champagne region (France). Construction and Building Materials, 2025, 470, pp.140582. 10.1016/j.conbuildmat.2025.140582. hal-04972374

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# Analysis of adobes from vernacular raw earth buildings in the Champagne region (France)

Guillaume Polidori<sup>a</sup>, Adrien Aras-Gaudry<sup>b, c</sup>, Céline Rousse<sup>a, c</sup>, Fabien Beaumont<sup>a, c</sup>, Fabien Bogard<sup>a</sup>, Sébastien Murer<sup>a, *, d</sup>, Tala Moussa<sup>a</sup>, ChristopheBIard<sup>c</sup>, Gilles Fronteau<sup>b, d</sup>, Erwan Hamard<sup>d</sup>

## ARTICLE INFO

Keywords:

Adobe bricks

Building materials

Earth construction

Compressive strength

Hygrothermal performance

## ABSTRACT

This study contributes to the understanding of the vernacular raw-earth heritage of the Champagne region in France, where such structures are currently being documented. The research investigates the mineral composition, grain size distribution, and physico-chemical, mechanical, thermal, and hydric properties of seven adobe types derived from soils with varying compositions (predominantly silicate or limestone-based soils). In particular, the influence of calcite content, which spans a wide range from 0 % to 84.9 %, was examined. The results indicate a strong dependency of peak compressive strength on calcite content: higher CaCO₃ levels correspond to lower peak compressive strength. Additionally, the study reveals that the metal oxide content of soils is a critical factor directly associated with mechanical performance. Interestingly, it was observed that historical builders often used weaker adobes for load-bearing purposes and stronger ones for filling, likely without adherence to formal construction standards. Rather than compressive strength, wall design appears to have played a more critical role in structural stability. Regarding thermal properties, calcite content showed minimal influence on diffusivity, specific heat capacity, and thermal conductivity across all adobe samples. Furthermore, all adobes demonstrated excellent to very good moisture regulation performance, with corresponding Moisture Buffer Values varying from 1.65 to 3.09 g/(m².%RH). The findings of this study underscore the potential of traditional raw-earth techniques in rediscovering and evaluating earthen architecture, with implications for promoting sustainable and environmentally friendly contemporary earthen construction and renovation practices.

## 1. Introduction

Raw earth represents a traditional vernacular architectural typology, with its earliest constructions dating back to 10,000 BC [1,2]. This ancient building material is abundant, environmentally friendly, and cost-effective, readily available beneath the surface in most regions of the world. Currently, it is suggested that approximately 8–10 % of the global population and 20–25 % in developing countries reside in raw earth buildings [3]. Although it fell out of favor in the early 20th century in both developed and developing nations due to the rise of concrete, earthen architecture remains a niche market. However, it possesses numerous characteristics of a sustainable construction material, making

it a promising solution to address the ongoing climate, economic, and societal crises [4]. Despite its limited structural mechanical performance compared to concrete and fired masonry bricks—two significant contributors to greenhouse gas emissions—it appears that this ancient material is undergoing a notable revival in response to growing environmental pressures [5]. The resurgence can be attributed to increasing demands for circular economy practices and decarbonization, supported by the implementation of government policies [6]. For example, in France, the Ministry of Ecological Transition has approved the Projet National Terre (National Earth Project) for 2021, to deploy raw earth construction on a large scale. In another example, France's new environmental regulation (RE2020), which came into force in

<footnote><sup>a</sup> Université de Reims Champagne-Ardenne, ITheMM, Reims F-51100, France</footnote>

<footnote><sup>b</sup> Université de Reims Champagne-Ardenne, GEGENA, Reims, F-51100, France</footnote>

<footnote><sup>c</sup> Université de Reims Champagne-Ardenne, ICMR, UMR 7312 CNRS, Reims F-51100, France</footnote>

<footnote><sup>d</sup> Université Gustave Eiffel, MAST-GPEM, Bouguenais F-44340, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: sebastien.murer@univ-reims.fr (S. Murer).</footnote>
2022, promotes a mix of biobased and geobased materials with a low environmental footprint for green growth including raw earth among the eco-friendly materials it takes into account.

The three most widely used earth-based techniques for load-bearing masonry are adobe, rammed earth, and cob. Adobe is a straightforward earth-building technique, which is why many ancient structures were constructed using this method [7]. The term "adobe" originates from the Arabic word "attob," meaning sun-dried brick [8]. These bricks were either shaped by hand or produced using rectangular timber molds [9]. During the Neolithic period, hand-molding was the predominant method for adobe production, but this was later replaced by the practice of casting adobes in uniform rectangular shapes using timber molds. This latter technique has persisted to the present day due to its advantages, including consistent brick dimensions, which facilitate faster and more efficient construction [10]. Typical adobe dimensions range from 5 to 10 cm in thickness, 5–20 cm in width, and 20–40 cm in length [11]. The manufacturing process generally involves mixing the soil, optionally adding stabilizers (such as lime, cement, or natural fibers), and shaping the mixture through molding or compaction. This process ensures a homogeneous distribution of the components and allows for control over the material's porosity, which is a critical factor influencing the resulting mechanical and thermal properties [12].

In France, the construction sector is the largest consumer of natural resources and the primary producer of waste, generating an annual volume of 46 million metric tons [13]. French law n°2020-105 (February 10, 2020), known as the AGEC law on combating waste and promoting a circular economy, introduces several measures to encourage the recycling and reuse of materials in the construction industry while also implicitly aiming to bring eco-friendly materials back into focus. Despite its limitations, raw earth could be an excellent candidate for this policy in architectural contexts that do not require high structural capacity, as it demands minimal processing and transportation, resulting in a material with a very low carbon footprint.

Although raw earth construction is present in many regions of France (see Fig. 1), there is no exhaustive comprehensive inventory of this vernacular heritage. Earth architecture accounts for approximately 15% of the national total, according to estimates from the 1980s [14], although this average conceals significant regional disparities. Earthen construction, relying on locally available materials, has given rise to various techniques adapted to geographic, cultural, and historical contexts. The Rhône-Alpes and Auvergne regions are particularly rich in rammed earth structures [15], while timber frame and wattle-and-daub techniques dominate in the North and East, cob in Brittany [16], and adobe in the Southwest [17]. These traditions reflect architectural responses to rural landscapes: wooded areas and bocages favor timber and

wattle-and-daub, while open fields turn to adobe and rammed earth.

The Champagne-Ardenne region is one of the last unexamined territories for raw earth heritage in France [18]. This study aims to contribute to filling this knowledge gap by investigating seven types of unstabilized adobes collected from deconstructed old buildings in the region. Similar research has been conducted in Portugal [19], the Auvergne-Rhône-Alpes region in France [12], and the Midi-Pyrénées region in France [20], among others. However, drawing comparative conclusions from these studies remains challenging, as the regionalization of construction techniques may depend on factors such as available resources, climate, and workforce at the time of construction, rather than solely on objective parameters like the geotechnical properties of the soil [21].

This study focuses on the detailed analysis of the mineral compositions, grain size distributions, and physico-chemical, mechanical, thermal, and hydric properties of seven types of adobes sourced from various soils in the Champagne region. Particular attention is given to the influence of calcium carbonate (CaCO₃) content on the properties of these adobes. Understanding the characteristics and behavior of ancient adobes in this vernacular heritage is fundamental for guiding conservation and restoration initiatives in the Champagne region, which is predominantly characterized by chalky soils.

## 2. Materials and methods

Due to the variety of characterizations presented in this article, Fig. 2 presents a flowchart that summarizes the process applied to each adobe.

### 2.1. Survey location

Adobe samples were extracted from the walls of seven earthen buildings located in the Champagne-Ardenne region of France (see Fig. 3). Situated in northeastern France, Champagne-Ardenne serves as a transitional zone on the eastern edge of the Paris Basin. This region spans approximately 300 km from north to south and 200 km from east to west, covering a total area of 25,600 km². Its geology is characterized by Cenozoic plateaus in the west and Jurassic and Cretaceous plateaus in the east. The alluvial valleys, largely formed during the Quaternary period, are bordered to the north by remnants of the ancient Ardenne Paleozoic Mountain, resulting in a wide variety of soil types. The central part of this region, known as Chalky Champagne, is dominated by an extensive chalk plain [22]. For this study, seven adobe sampling sites were selected within a relatively compact area (150 km from north to south and 70 km from east to west). The soils in these areas range from calcareous soils—often clayey and rich in carbonates in the western part—to silty or clayey silt soils in the eastern areas, derived from Jurassic and lower Cretaceous marls.

### 2.2. Origin of adobes

The samples were collected from demolition and renovation sites encompassing various rural construction typologies, including one-story (51-AT, 51-BU) or two-story (08-CO, 08-DO, 08-EC) old dwellings and barns (10-CH, 51-BR), as illustrated in Fig. 4. In most cases, there was an attic above the one or two-story living area. Written records indicate that this construction method was used in Champagne-Ardenne until the early 20th century [18]. Although the exact construction dates of these buildings are unknown, it can reasonably be assumed that they were erected prior to this period.

Adobes served different purposes depending on the structure: they were used as load-bearing elements in exterior and interior walls (51-AT, 51-BR) or as infill material within load-bearing wooden frameworks in the other cases. Notably, one type of adobe (10-CH) exhibited a single vertical side indentation (see Fig. 4), a molding feature also observed in Portugal [23], though typically in a bilateral form.

The Portuguese survey suggests that these indentations, visible on

<figure><img src="image_2.png" /><figcaption>Fig. 1. Map of earthen patrimony in France. Source: Terra 2016, CRAterre.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2. Flowchart of the multiphysics characterization process for each adobe type.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 3. Sampling sites in Champagne-Ardenne, France. In the adopted 00-XX denomination, the two digits correspond to the number of the French departments listed in alphabetical order (08 = Ardennes; 10 = Aube; 51 = Marne) and the two letters correspond to the first two letters of the town or village from which the adobes originate (AT=Athis; BR=Brugny; BU=Bussy; CH=Chaudrey; CO=Condé; DO=Dommery; EC=Ecordal).</figcaption></figure>

both the exterior and interior surfaces of the masonry, were likely designed to enhance the adhesion of plaster and render to the adobe masonry. In our case, the single vertical indentation improves the bond with the timber supporting structures, to which small timber cleats are nailed (see Fig. 5). This design is particularly relevant in cases involving large infill surfaces, such as in 10-CH, which may be subjected to wind-induced forces. Additionally, an alternative explanation for this vertical cleat connection has been proposed: it provides stable lateral support while allowing for relative movement between the adobe bricks and the timber frame [24].

According to the local literature [25], adobes were manually shaped on-site using rectangular molds with dimensions that, while varying slightly, were generally consistent. An example of old traditional mold used in the Champagne region, on display at the Reims (Economic capital of the Champagne region) museum, is shown in Fig. 6. These are

metal molds with a rectangular double-compartment, a bottom pierced with holes (to facilitate removal from the mold) and inverted handles for easy turning.

Samples were taken on-site, using simple tools: a sledgehammer and a chisel. The first step was to scrape into the mortar, to expose the brick, and to weaken it as little as possible. The bricks were then transported in the boot of the vehicle, protected by bubble wrap and a tarpaulin. The aim was to protect them as much as possible from road shock and vibration. The bricks were then stored (see Fig. 7) in a weatherproof room in the laboratory.

Measurements conducted on representative samples revealed the dimensions listed in Table 1. A notable variation in adobe geometry was observed across different locations (height: 54.7–101.0 mm; width: 94.2–141.0 mm; length: 203.0–296.3 mm), which appears to be related to the brick’s intended function and the construction’s overall dimensions. For instance, adobes used as load-bearing elements (51-BR; 51-AT) exhibited the largest widths (approximately 140 mm) and the largest load-bearing surfaces, measuring 417.8 cm² and 368.2 cm², respectively. This demonstrates how construction techniques were adapted to meet specific mechanical requirements.

## 2.3. Particle size distribution

The granulometric analysis was performed using wet sieving. Random samples were selected for each soil type, and their particle size distributions (PSDs) were determined using nine sieves with mesh sizes ranging from 10 mm to 25 µm in descending order. Each experiment was repeated three times to ensure reproducibility. For particle sizes below 25 µm (corresponding to the finest sieve pass), laser diffraction was employed using a Malvern Mastersizer 2000 particle size analyzer (Malvern Instruments Ltd, Worcestershire, UK). The laser diffraction method was preferred over sedimentation due to its faster processing time and reduced sensitivity to sample preparation. Additionally, laser granulometry is widely regarded as an effective technique for estimating clay content, typically corresponding to particle sizes below 2 µm or 5 µm in common soils, depending on the classification used [27,28], without considering the mineral nature of the particles. The connection between mass-based sieving and volume-based diffraction was established by ensuring that the absolute densities of the corresponding samples were consistent. For chalky soils, this technology provides an upper limit for clay content (defined here as particle sizes below 2 µm), as fine particle sizes may also contain small amounts of coccoliths and limestone fragments.

## 2.4. Morphology and elemental composition

The morphology and chemical composition of the samples were analyzed using a Hitachi TM3030Plus tabletop scanning electron microscope (SEM) (Hitachi High-Tech Corp, Tokyo, Japan) equipped with
<figure><img src="image_5.png" /><figcaption>Fig. 4. Vernacular earthen buildings from which the adobes were collected and detail of the vertical side indentation for adobe 10-CH (lower right).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. Different functionalities of the indented configurations: (a) Enhanced bonding with timber bearing structure (case 10-CH); (b) Higher grip with plaster and render (extrapolated from Parracha et al. [23]).</figcaption></figure>

a Swift ES3000 energy-dispersive X-ray spectroscopy (EDXS) system. X-ray spectra were recorded at an acceleration voltage of 15 kV with an acquisition time of 300 seconds. For SEM observation, the specimens were coated with a conductive Au-Pd layer using a sputter coater to prevent charging effects. Soil samples were compressed into pellets for EDXS analysis. Each experiment was conducted at least three times to ensure the reproducibility of the results.

## 2.5. $CaCO_3$ content and moisture content measurements

Decarbonization was performed using a custom-built single-unit Scheibler apparatus in accordance with the NF EN ISO 10693:1995 standard [29], providing an estimation of the carbonate content (% $CaCO_3$) in the samples. Moisture content was determined using a
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Examples of adobe molds in the Champagne region, on display at the Reims Museum, early 1980s, from H. Guillaud [26].</figcaption></figure>

Sartorius MA100 high-precision moisture analyzer (Sartorius AG, Göttingen, Germany).

## 2.6. Methylene blue value (MBV₁)

The Methylene Blue Value (MBV₁, distinguishable from the Moisture Buffer Value, denoted as MBV₂) is widely employed for the detection of clay minerals within soil matrices. This study facilitates the assessment of clay content in the soil from which the analyzed adobes are sourced, alongside their water sensitivity, an essential factor for construction applications. Numerous methodological variations are available for executing the methylene blue test [30]; however, our analysis adhered strictly to the protocols outlined in the French standard NF P 94–068 [31]. The procedure entails the gradual addition of methylene blue to a soil suspension while maintaining agitation. At predetermined intervals, a drop of the suspension is extracted and placed on filter paper. The

completion of methylene blue adsorption on clay particles is signified by the emergence of a bluish halo surrounding the original blue spot, indicating an excess of methylene blue in that region. MBV₁ measurements, quantified in grams of methylene blue per 100 g of soil, were performed on a selection of randomly chosen samples from various adobes. This approach allows for a reliable characterization of the clay content.

## 2.7. Mechanical analysis

Prior to experimentation, samples were preserved for a duration exceeding one month within a controlled environment maintained at 22°C and 55 % relative humidity (RH) to mitigate excessive moisture absorption in certain adobes that may have been subjected to precipitation during the deconstruction of the buildings. Prismatic samples were systematically sectioned (dimensions: original adobe width * 10 cm * original adobe height); surfaces that interfaced directly with the steel plates of a universal testing machine underwent manual rectification to assure congruence of flatness and parallelism relevant to the compression faces. Aside from this minor surface adjustment, the thickness of the adobes remained unaltered, intentionally reflecting their original in-situ application. In the context of architectural design, tensile stresses inherent to earth materials are typically disregarded, thereby positioning compressive strength as the primary mechanical parameter of interest. Compression tests were executed utilizing a ZwickRoell Z050 (ZwickRoell, Ulm, Germany) testing apparatus equipped with a 50 kN load cell. These assessments were conducted under controlled indoor conditions, specifically at a temperature of 23°C and 50 % RH. The compression rate was calibrated to 8 mm/min in accordance with NF XP P 13–901 [11] specifications, yielding specimen failure within a timeframe of 1–2 minutes. For the purposes of statistical

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Example of adobe transport (left) and laboratory storage (right) for 51-BR adobes.</figcaption></figure>

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

Geometry and masonry function of adobes – Mean (SD).</caption><thead><tr><th>Origin of adobes</th><th>08-CO<br>Humid Champagne area</th><th>08-DO<br>Chalky Champagne plateau</th><th>08-EC</th><th>10-CH</th><th>51-AT</th><th>51-BR</th><th>51-BU</th></tr></thead><tbody><tr><th scope="row">Masonry function</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th scope="row">Building function</th><td>Filling</td><td>Filling</td><td>Filling</td><td>Filling</td><td>Load bearing</td><td>Load bearing</td><td>Filling</td></tr><tr><th scope="row">Geometry of adobes</th><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><th scope="row">Height (mm)</th><td>57.8<br>(1.6)</td><td>57.4<br>(1.1)</td><td>68.0<br>(3.6)</td><td>78.4<br>(1.7)</td><td>101.0<br>(4.0)</td><td>85.1<br>(6.2)</td><td>73.8<br>(1.0)</td></tr><tr><th scope="row">Width (mm)</th><td>98.8<br>(1.8)</td><td>94.2<br>(1.9)</td><td>102.5<br>(3.1)</td><td>124.0<br>(4.2)</td><td>141.0<br>(3.6)</td><td>139.2<br>(8.4)</td><td>104.5<br>(5.3)</td></tr><tr><th scope="row">Length (mm)</th><td>210.6<br>(2.3)</td><td>203.0<br>(2.1)</td><td>217.0<br>(4.1)</td><td>277.2<br>(2.6)</td><td>296.3<br>(8.5)</td><td>264.5 (12.6)</td><td>240.0<br>(4.4)</td></tr><tr><th scope="row">Mean bearing area (cm²)</th><td>208.1</td><td>191.2</td><td>222.4</td><td>343.7</td><td>417.8</td><td>368.2</td><td>250.8</td></tr></tbody></table>
validity, compressive strength analyses were derived from an average of 5–10 samples [32].

## 2.8. Thermal analysis

The thermal performance of earth-building adobes was evaluated through measurements of thermal conductivity, specific heat capacity, and diffusivity. These assessments utilized an ISOMET 2114 analyzer

(Applied Precision, Ltd., Bratislava, Slovakia) which applies heat flux pulses at the material's surface. For each type of adobe, five specimens were subjected to analysis, ensuring a thorough examination of their thermal properties.

## 2.9. Hydric analysis - moisture buffer value (MBV₂)

The moisture-buffering value quantifies a material's ability to

<figure><img src="image_9.png" /><figcaption>Fig. 8. SEM micrographs of the different adobe compositions (x5000).</figcaption></figure>
attenuate fluctuations in relative humidity within a confined environment. Specifically, the MBV2 reflects the efficacy of a material in moisture regulation, impacting factors such as user comfort and the longevity of building components. The assessment protocol adheres to the standards established by the NORDTEST project [33], categorizing moisture-buffering values from negligible to excellent. Initially, samples were equilibrated at a temperature of 23°C and relative humidity (RH) of 50 % for a duration of 14 days. Subsequently, they were subjected to diurnal RH cycles comprising 8 hours at elevated RH (75 %) followed by 16 hours at reduced RH (33 %) within a Binder MKF 720 climatic chamber (Binder GmbH, Tuttlingen, Germany). The cycling continued until the mass variations measured over the final three cycles fell below 5 %. The MBV2 was calculated using the following equation:

$$MBV_2 = \frac{\Delta m}{A(RH_{sup} - RH_{inf})} \quad (1)$$

where MBV₂ represents the moisture buffer value in units of grams per square meter per percent relative humidity (g/(m².%RH)), Δm denotes the mass variation during absorption or desorption in grams, A denotes the sample surface area in contact with air in square meters (m²), and RHsup and RHinf denote the upper (75 %) and lower (33 %) relative humidity levels, respectively. To ensure the reproducibility of measurements and for each type of soil, four parallelepipedal samples measuring 10 × 10 × 4 cm³ were utilized as test specimens and sealed on the side and bottom surfaces with waterproof adhesive tape.

## 2.10. Statistical analysis

For each of the seven soil types studied and for each parameter assessed, between 3 and 6 experiments were carried out to enhance statistical significance. We also attempted to establish correlations between compressive strength and metal oxide or calcite content, using the Pearson correlation coefficient r.

# 3. Results and discussion

## 3.1. Morphology analysis and elemental mapping

Fig. 8 presents the microstructural characteristics of seven raw earthen block materials as observed using a scanning electron microscope (SEM) at a magnification of x5000. A qualitative analysis reveals two distinct microstructural families: the first group, comprised of soils from the Ile de France cuesta and the Chalky Champagne plateau (samples 51-BR, 51-AT, 51-BU, and 10-CH), and the second group from the Humid Champagne region, located to the northeast (samples 08-CO, 08-DO, and 08-EC). The first microstructural family is characterized by a high abundance of calcite micrograins, indicative of chalky soils and subsoils. Notably, coccoliths—structural remnants of fossil planktonic algae (Coccolithophycea)—are detectable in both whole and fragmented states. Furthermore, it is observed that the clay-sized fraction in samples 51-BR, 51-AT, 51-BU, and 10-CH predominantly consists of crystalline calcite particles measuring below 2 µm, consistent with findings in chalky soils [34]. The highly carbonated geomaterial demonstrates porous aggregations formed by calcite nano and micro-grains, where contacts are predominantly punctiform or exhibit recrystallized aggregates interspersed with larger pores resembling cavities or tubular channels. Additionally, larger crystals, including calcite (sparite), clays, and organo-mineral entities, are present in samples 51-AT and 51-BR, alongside small silt quartz in sample 51-BU. Occasionally, micro-needle fibrous calcite [35] is observed in sample 10-CH, potentially linking to specific pedological attributes.

In soil science, clay minerals are classified as fine-grained constituents predominantly comprising hydrous aluminosilicates (phyllosilicates), characterized by a predominantly flattened lamellar morphology and laminated structure in natural environments [36]. Scanning

electron microscopy (SEM) analyses of the secondary sample set, specifically samples 08-CO, 08-DO, and 08-EC, reveal distinct morphological configurations. These samples exhibit a relatively low to very low abundance of calcite grains, contrasted by the presence of numerous sizable aggregates exhibiting clay coatings. Additionally, fine siliceous silts, primarily constituted by quartz grains alongside minor constituents such as feldspar and rutile, potentially embedded within a clay matrix, have been identified (samples 08-DO, 08-EC). Observations also indicate the existence of recrystallized clusters (08-CO, 08-EC), likely interconnected by organo-mineral bridges and coatings. To corroborate these morphological trends, an elemental composition analysis was conducted using an energy-dispersive X-ray spectrometer (EDXS) for elemental mapping and localized chemical analysis, enabling the assessment of different soil sample compositions. Table 2 presents a summary of the corresponding measurements expressed in % atomic, focusing on the principal elements, with trace components excluded.

Calcium (Ca) is the predominant mineral element in the soils of the Champagne-Ardenne chalky plateau, specifically in the regions designated as (10-CH, 51-AT, 51-BR, 51-BU). In contrast, silicon (Si) is predominantly present in the soils of the eastern region, particularly identified as (08-CO, 08-DO, 08-EC), where it is accompanied by significant concentrations of aluminum (Al) and iron (Fe). The Si/Ca ratio serves as a distinguishing criterion for the classification of these soil families; a ratio below 1 indicates limestone soils (10-CH, 51-AT, 51-BR, 51-BU), whereas a ratio above 1 is indicative of siliceous soils (08-CO, 08-DO, 08-EC). Silica (SiO₂) and alumina (Al₂O₃) constitute the principal components of clays, which function as binders and stabilizers in adobe manufacturing. As a result, adobes sourced from the Humid Champagne region (08-CO, 08-DO, 08-EC) are anticipated to exhibit higher clay content and enhanced cohesion among their constituent materials. The ensuing effects on the mechanical, hygric, and thermal properties of these adobes will be examined in the forthcoming sections.

## 3.2. General physical characteristics of adobes

Intact samples sourced from vernacular structures were subjected to laboratory analysis to assess essential physical properties, including absolute density, dry density, porosity, and moisture content. Concurrently, the primary chemical characteristics were evaluated, specifically pH, % CaCO₃, % organic matter, % insoluble content, and MBV₁ value. The average results for five samples per adobe are presented in Table 2. According to established guidelines, adobe bricks achieve moisture equilibrium with ambient air when the mass variation between two weighings, conducted 24 hours apart, is below 0.1 % [37]. In this study, the moisture content of the adobes was recorded within the range of 1.6–2.0 % (mean: 1.76; standard deviation: 0.15), indicating a satisfactory degree of drying in the materials.

The dry density measurements of the examined adobes range from 1435 kg/m³ (51-BR) to 1935 kg/m³ (51-AT), with a calculated mean of 1679 kg/m³ and a standard deviation of 146. The average dry density of 1679 kg/m³ is consistent with the established typical density range for adobe masonry, which various authors and guidelines suggest falls

<table><caption>**Table 2**
Main element composition of adobe samples (Atomic %).</caption><thead><tr><th></th><th>08-Co</th><th>08-DO</th><th>08-Ec</th><th>10-Ch</th><th>51-At</th><th>51-Br</th><th>51-Bu</th></tr></thead><tbody><tr><td>O</td><td>66.2</td><td>66.7</td><td>70.9</td><td>75.6</td><td>71.9</td><td>76.3</td><td>77.9</td></tr><tr><td>Ca</td><td>6.0</td><td>0.5</td><td>7.3</td><td>21.3</td><td>21.5</td><td>19.3</td><td>16.7</td></tr><tr><td>Si</td><td>20.1</td><td>24.4</td><td>13.4</td><td>2.0</td><td>3.2</td><td>2.7</td><td>5.1</td></tr><tr><td>Al</td><td>2.0</td><td>3.8</td><td>3.9</td><td>1.1</td><td>0.7</td><td>1.0</td><td>1.1</td></tr><tr><td>Fe</td><td>1.75</td><td>2.42</td><td>1.62</td><td>0.18</td><td>0.30</td><td>0.24</td><td>0.5</td></tr><tr><td>K</td><td>0.77</td><td>1.15</td><td>1.15</td><td>0.16</td><td>0.26</td><td>0.16</td><td>0.3</td></tr><tr><td>Mg</td><td>0.42</td><td>0.62</td><td>0.62</td><td>0.24</td><td>0.32</td><td>0.2</td><td>0.24</td></tr><tr><td>Si/Ca</td><td>3.3</td><td>48.8</td><td>1.8</td><td>0.09</td><td>0.1</td><td>0.1</td><td>0.3</td></tr><tr><td colspan="8">Predominantly siliceous soils (Humid Champagne area)</td></tr><tr><td colspan="8">Predominantly limestone soils (Chalky Champagne plateau)</td></tr></tbody></table>
between 14.13 and 25.07 kN/m³ [38] or 1400–2200 kg/m³ [24]. Variations in dry density among the different adobes are primarily attributable to differences in mineral composition and porosity, which result from manufacturing techniques that involve varying degrees of manual compaction and water content in the adobe mixture. Notably, for a comparable elemental composition (refer to Table 2), adobe sample 51-AT exhibits a dry density of 1935 kg/m³ with a porosity of 20.2 %, while sample 51-BR demonstrates a lower dry density of 1435 kg/m³ with a higher porosity of 34.6 %.

From a chemical perspective, the soils utilized in the production of these adobes were characterized by a near-neutral pH, exhibiting a calcareous tendency, specifically pH = 7.64 (standard deviation 0.61). The proportion of soluble organic matter, obtained through evaporation of the supernatant following wet sieving and centrifugation, averaged approximately 0.5 % across all analyzed adobes, with the exception of 51-AT, which presented a concentration of 1.5 %. Varghese et al. [39] contended that the influence of organic matter content on geotechnical behavior is negligible when the level is below 4–5 %. Moreover, Houben and Guillaud [40] indicated that organic matter does not significantly affect the mechanical performance of adobes at concentrations as low as 2 %. Nonetheless, these assertions have faced scrutiny, as literature includes cases of historical structures exhibiting organic matter content exceeding 5 % [41].

The carbonate content results obtained from the study align with the geological and pedological analyses of the Champagne-Ardenne region in France, the source of the adobes discussed herein. The carbonate levels observed in the examined adobes exhibited considerable variability, with a complete absence of carbonate (0 % CaCO₃) noted in sample 08-DO, while an exceedingly high carbonate content of 84.9 % was measured in sample 51-BR. The implications of these varying carbonate levels for the cohesion and mechanical properties of the material will be addressed in subsequent sections. Notably, recent findings by Polidori et al. [42] indicate that a high carbonate content of 71 % CaCO₃ in adobes is conducive to meeting the structural requirements for raw earth constructions.

Within the domain of mechanical performance, the emphasis continues to be placed predominantly on clay content, regarded as a critical parameter influencing the structural viability of raw earth as a construction material. The optimal clay content acts as a binding agent within the adobe matrix; it is imperative that this content achieves a balance that provides both adequate mechanical strength and minimizes the risks of cracking and shrinkage in earthen masonry with high clay content, while low clay content weakens material cohesion and might lead to structural collapse during the building stage. Current literature supports the notion that clay content ranging from 5 % to 29 % is generally acceptable in adobe formulations [32,43]. However, it is pertinent to note that a significant portion of earth utilized by historical masons deviated from these suggested parameters, thereby challenging their validity [12,44]. For example, recent investigations have documented clay proportions as low as 4 % [45] and even 1 % [46].

<table><caption>**Table 3**
Mean physical and chemical properties.</caption><thead><tr><th></th><th>08-CO</th><th>08-DO</th><th>08-EC</th><th>10-CH</th><th>51-AT</th><th>51-BR</th><th>51-BU</th></tr></thead><tbody><tr><td><strong>Mean physical properties</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>ρ<sub>Abs</sub> (kg/m³)</td><td>2650</td><td>2612</td><td>2338</td><td>2246</td><td>2469</td><td>2240</td><td>2507</td></tr><tr><td>ρ<sub>Dry</sub> (kg/m³)</td><td>1655</td><td>1695</td><td>1697</td><td>1693</td><td>1935</td><td>1435</td><td>1646</td></tr><tr><td>Porosity</td><td>36.5 %</td><td>33.8 %</td><td>25.5 %</td><td>23.4 %</td><td>20.2 %</td><td>34.6 %</td><td>33.1 %</td></tr><tr><td>Moisture content</td><td>1.6 %</td><td>1.9 %</td><td>1.6 %</td><td>1.6 %</td><td>1.8 %</td><td>2.0 %</td><td>1.8 %</td></tr><tr><td>Clay content</td><td>5.3 %</td><td>6.2 %</td><td>7.7 %</td><td>9.5 %</td><td>10.3 %</td><td>14.2 %</td><td>10.9 %</td></tr><tr><td><strong>Mean chemical properties</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>pH</td><td>7.1</td><td>6.7</td><td>7.6</td><td>8.2</td><td>7.7</td><td>8.5</td><td>7.7</td></tr><tr><td>CaCO<sub>3</sub> Content</td><td>33.2 %</td><td>0 %</td><td>34.7 %</td><td>68.3 %</td><td>77.0 %</td><td>84.9 %</td><td>66.5 %</td></tr><tr><td>Organic content</td><td>0.6 %</td><td>0.4 %</td><td>0.6 %</td><td>0.7 %</td><td>1.5 %</td><td>0.5 %</td><td>0.5 %</td></tr><tr><td>MBV<sub>1</sub> (g/100 g)</td><td>1.18</td><td>1.64</td><td>2.03</td><td>0.71</td><td>0.47</td><td>0.97</td><td>1.26</td></tr></tbody></table>

The quantification of clay content, derived through laser granulometry analysis across seven distinctive types of adobe, is presented in Table 3, with values spanning from 5.3 % (08-CO) to 14.2 % (51-BR), indicative of a loamy sand soil texture [47]. The three adobes sourced from the predominantly siliceous soils of Humid Champagne (08-CO, 08-DO, 08-EC) exhibited the lowest clay content, registering below 8 %. Conversely, the adobes derived from the limestone-rich soils of the Chalky Champagne Plateau exhibited clay contents ranging from 9.5 % to 14.2 %. Although these values may seem low when assessed against literature recommendations, which generally advocate for a clay content within the 10–30 % range, these figures necessitate further contextualization and analysis.

Firstly, it is essential to assert that absolute clay content may not be the paramount focus in this investigation; rather, it is the relative proportion of clay in conjunction with other volumetric constituents (including silt, sand, and gravel) that warrants consideration. The inherent complexity of soil composition complicates any formulation of a universal recommendation, given the extensive variability inherent within soils and the diverse range of clay minerals. Secondly, it is crucial to examine the specific mineralogical composition of the clay, as this aspect can exert a significant influence on the engineering properties of the soil [48].

Despite the limited availability of data pertaining to the mineralogical characteristics of Champagne clays, research conducted by Durand [49], Flogeac et al. [50], and Loisy [51] has identified that these clays are predominantly characterized by smectite and vermiculite-smectite interlayers. It has been demonstrated that smectite clay minerals, specifically montmorillonite, generally exhibit superior strength properties in comparison to clays comprised of kaolinite and illite mineral constituents [52]. Indeed, specific surfaces strongly vary between these two types of clay, ranging from 20 m²·g⁻¹ for kaolinite to 850 m²·g⁻¹ for montmorillonite [53]. Consequently, two distinct earth samples with equivalent clay content may exhibit markedly different behavior if their clay fractions differ in mineralogical composition. This underscores the insufficiency of relying solely on clay percentage for comparative analysis against established standards, thereby affirming that raw earth constructions can feasibly incorporate minimal clay quantities, contingent upon an appropriate clay mineral composition, predominantly featuring smectite.

Interestingly, a substantial positive correlation (Pearson correlation coefficient r = 0.86) has been observed between CaCO₃ content and clay content, suggesting that as the carbonate content increases, so too does the clay content. This hypothesis necessitates further validation through the examination of additional limestone soil samples, as the current dataset is statistically limited. It is also noteworthy that this correlation appears to be significantly influenced by the geographical provenance of the soils involved [54,55].
### 3.3. Particle size distribution and soil classification

The particle size distribution (PSD) curves for the various samples are illustrated in Fig. 9a, offering an additional perspective on the diversity previously noted in the soil compositions from which the adobes are derived. The two categories identified through scanning electron microscopy (SEM) micromorphological analysis are evident within the analyzed granulometric range: (08-CO; 08-DO; 08-EC) associated with the Humid Champagne region and (10-CH; 51-AT; 51-BR; 51-BU) pertaining to the Chalky Champagne plateau. Typically, PSD curves exhibit either a bell-shaped or a sigmoidal distribution pattern.

Gravels and stones (size > 2 mm), along with coarse sand (0.2–2 mm), are predominantly present in the limestone soils of the Chalky Champagne plateau (10-CH; 51-AT; 51-BR; 51-BU). In contrast, samples obtained from the Humid Champagne region (08-CO; 08-DO; 08-EC) exhibit minimal to negligible gravel content. It is critical to note that this classification is predicated solely upon particle size analysis rather than on the mineralogical composition. For instance, particles designated as sand may derive from the erosion of siliceous rocks or the disintegration of calcareous rocks, such as limestone-derived sand. Furthermore, at smaller grain sizes, which fall under the classifications of silts and fine sands, the differentiation between these two categories of adobe becomes ambiguous, resulting in overlapping particle size distribution curves.

In a preceding investigation, Rojat et al. [12] proposed a straightforward, discriminative, and cost-effective decision-making tool for local authorities to evaluate the potential for material reuse in earth-based construction. This tool is grounded in the "Guide des Terrassements Routiers" (GTR) classification published by the French Ministry of Transport [56]. The methodology requires two uncomplicated geotechnical assessments: the percentage of particles passing 80 µm and the methylene blue value (MBV₁). While initially applied to cob and rammed earth, it is suggested that the framework could be extrapolated to additional earth construction techniques, including adobes, thereby facilitating the development of an international statistical database. Fig. 9b illustrates the positioning of the adobes in the current study relative to the GTR classification. The MBV₁ values, indicative of clay activity across all adobes, range from 0.47 (0.02) for 51-AT to 2.03 (0.11) for 08-EC. The majority of adobes are classified under GTR A1, signifying that the earth materials consist of fine soil with low plasticity, possessing the capacity for rapid consistency transformations with minor variations in moisture content. The 51-AT adobe is classified as gravel soil with fines (B5 classification) and exhibits sensitivity to moisture changes.

### 3.4. Compression tests

Comprehensive mechanical assessments were conducted on original adobe specimens derived from seven distinct soil sources, varying in thickness (refer to Table 1), which may constrain the scope of this comparative experimental investigation. From a mechanical standpoint, to facilitate an in-depth analysis of the stress-strain characteristics of the seven adobe types, we initially employed a normalized representation, as illustrated in Fig. 10. The y-axis represents the dimensionless compressive stress, denoted as $\sigma/f_c$, where $f_c$ is the maximum compressive stress, while the x-axis depicts the dimensionless strain, $\varepsilon/\varepsilon_u$, with $\varepsilon_u$ being the strain corresponding to $f_c$. The analysis indicates that all non-dimensional stress-strain curves exhibit a consistent behavioral pattern characterized by a linear elastic phase, succeeded by a non-linear hardening phase until the peak load is reached, which is subsequently followed by a softening response and ultimate failure. Notably, a significant variability in material behavior is observed for $\varepsilon/\varepsilon_u > 1$ (post-maximum stress), a phenomenon corroborated by Illopapas et al. [57], highlighting profound differences in ductile behavior (capacity for deformation without fracture) across various adobe types [58]. In terms of ductility, the least favorable outcome is associated with adobe 51-AT, which possesses the highest dry density at 1935 kg/m³. However, no definitive conclusions can be drawn across all samples due to a weak to moderate negative correlation ($r = -0.30$) between ductility and density.

Dimensional data concerning the mechanical characteristics of peak stress ($f_c$), the corresponding strain ($\varepsilon_u$), and Young's modulus (initial

<figure><img src="image_10.png" /><figcaption>Fig. 10. Stress-strain normalized curves.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 9. (a) Particle Size Distribution (PSD); (b) Soil classification (the right part of the original chart, referring to other soil subclasses, has been omitted for clarity).</figcaption></figure>
tangent modulus, E) are presented in Table 4. Peak stress constitutes a critical parameter in the selection of materials for construction applications. In the present experimental investigation, peak stress values range from 0.74 MPa (51-AT) to 4.80 MPa (08-DO), with a mean of 2.22 MPa and a standard deviation of 1.31. These findings are corroborated by existing literature, which documents a combined range of peak stress values between 0.6 MPa and 8.3 MPa, as indicated by various authors [57,59–63]. Notably, the peak stress values for earthen materials remain relatively constrained and do not approach those attributed to concrete. Analysis reveals that the earthen materials utilized as load-bearing elements in situ, specifically samples 51-AT and 51-BR, exhibited the lowest mechanical strength. However, this mechanical disadvantage was mitigated by a larger load-bearing surface area, facilitating improved load distribution (refer to Table 1).

Examination of the stress-strain curve up to 0.8f<sub>c</sub> allows for the estimation of tangent Young's modulus, which varies from E = 19.3 MPa (sample 51-AT) to 50.1 MPa (51-BU), yielding a mean value of 41.6 MPa and a standard deviation of 14.9. The reported initial tangent modulus values demonstrate considerable variability within the literature, attributed to the heterogeneity of soil composition in adobes and the absence of standardized manufacturing processes. Nonetheless, the mean value of 41.6 MPa is consistent with findings from Illampas et al. [57] and Fratini et al. [64].

The observed mechanical discrepancies in adobes from a geographically limited area (radius ≈ 70–150 km) prompt an investigation into the underlying causes, which likely pertain to the diverse implementations and characteristics of the local soils. This section addresses the factors influenced by these soil properties. One hypothesis posits a correlation between the variable carbonate content present in these adobes and their mechanical properties. To elucidate this relationship, Fig. 11 illustrates the correlation between peak compressive stress and the percentage of CaCO<sub>3</sub>, thereby providing a visual representation of the data supporting this hypothesis. Only the mean values were graphed for each type of adobe (corresponding to 7 different soils). However, Pearson correlation coefficients were established on the basis of 3–6 pairs of data per adobe sample. A minimum of 21 test results were therefore used to define each Pearson correlation coefficient presented in this article (the Pearson coefficient has no statistical value when the cloud is made up of only 7 points).

Although definitive statistical conclusions cannot be derived from the presented graph due to the limited sample size of adobes analyzed, preliminary observations suggest a potential causal relationship between the calcium carbonate (CaCO<sub>3</sub>) content and peak compressive stress. Linear regression analysis demonstrates a significant negative correlation between these two variables (Pearson coefficient r = −0.87). Notably, an increased presence of chalk gravel in the adobe samples (specifically 51-AT and 51-BR) correlates with a reduction in load-bearing capacity. This preliminary finding necessitates further validation through an expanded study involving additional samples from the Champagne region and/or other locations characterized by varying CaCO<sub>3</sub> content. However, it must be noted that this observation alone does not account for the discrepancies in compressive strength among different adobe soil types.

Recent research conducted by Walter et al. [65] emphasizes that high metal oxide concentrations in soils represent a critical factor in their

<figure><img src="image_12.png" /><figcaption>Fig. 11. Chalk content influence on peak stresses.</figcaption></figure>

suitability for earthen construction, as this directly influences mechanical strength. The data presented in Table 2, detailing the elemental composition of soils in atomic percentages, effectively illustrates the abundance of key metal oxides, specifically aluminum (Al), iron (Fe), and magnesium (Mg). Fig. 12 illustrates the relationship between maximum compressive strength and the cumulative atomic percentage of these metallic elements.

A substantial positive linear correlation is observed between the two parameters, as indicated by Pearson's coefficient, which approaches 0.8. It is evident that an increase in the concentration of metallic elements, and by extension, metallic oxides, correlates with enhanced compressive strength. This phenomenon can be attributed to the propensity of metallic elements to aggregate and bond with individual soil particles within the matrix, thereby augmenting soil cohesiveness. The current investigation corroborates the findings reported by Walter et al. [65] concerning soils in the Amazon region. The graphical representation in Fig. 12 illustrates that the three adobe samples from the Humid

<figure><img src="image_13.png" /><figcaption>Fig. 12. Peak compressive stress versus Atomic% of cumulated metallic elements.</figcaption></figure>

<table><caption>**Table 4**

Mechanical properties of adobes – Mean (SD).</caption><thead><tr><th></th><th>08-CO</th><th>08-DO</th><th>08-EC</th><th>10-CH</th><th>51-AT</th><th>51-BR</th><th>51-BU</th></tr></thead><tbody><tr><td>Peak stress</td><td>2.25</td><td>4.80</td><td>2.17</td><td>2.32</td><td>0.74</td><td>1.03 (0.17)</td><td>2.26</td></tr><tr><td>f<sub>c</sub>(MPa)</td><td>(0.18)</td><td>(0.29)</td><td>(0.27)</td><td>(0.33)</td><td>(0.21)</td><td></td><td>(0.44)</td></tr><tr><td>Peak strain</td><td>10,47</td><td>11.27</td><td>7.20</td><td>7.10</td><td>5.75</td><td>4.95</td><td>6.77</td></tr><tr><td>ε<sub>u</sub> (%)</td><td>(3.25)</td><td>(1.97)</td><td>(1.57)</td><td>(1.42)</td><td>(0.51)</td><td>(0.91)</td><td>(0.53)</td></tr><tr><td>Mean tangent modulus</td><td>32.2</td><td>63.9</td><td>45.2</td><td>49.0</td><td>19.3</td><td>31.2</td><td>50.1</td></tr><tr><td>E (MPa)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
Champagne region, characterized by elevated levels of metallic elements, exhibit superior mechanical properties. In summary, the analyses suggest that both the percentage of CaCO₃ and the cumulative atomic percentage of metallic elements may serve as critical determinants in elucidating the mechanical strength of adobe. The expertise of builders has evolved to incorporate these mechanical properties, facilitating the construction of durable residences and edifices that endure over time.

### 3.5. Hygrothermal analysis

The results pertaining to the determination of thermal conductivity, diffusivity, and specific heat capacity are summarized in Table 5. The thermal conductivity of the analyzed samples demonstrates a notable range, spanning from 0.536 W/(mK) (08-EC) to 0.799 W/(mK) (10-CH) with a mean value of 0.701 W/(mK) and a standard deviation of 0.090. These values suggest proficient insulating characteristics, exceeding those of conventional geo-sourced load-bearing materials such as fired brick and concrete. Nonetheless, it is imperative to acknowledge that these earthen materials do not meet the criteria for categorization as insulating materials; specifically, materials are typically classified as excellent insulators when their thermal conductivity (λ) is below 0.03 W/(m·K), in accordance with standard DIN 4108 [66]. The thermal conductivity range identified is consistent with previous literature findings for adobe structures, which indicate typical values of 0.5–1.2 W/(mK) as reported by Rempel and Rempel [67], or 0.46–0.95 W/(mK) as documented by Cuitiño-Rosales et al. [68].

A critical parameter influencing the thermal conductivity of earthen materials is dry density, which incorporates both mineral composition and porosity. Fig. 13 illustrates the correlation between the thermal conductivity values of the analyzed adobes and their respective dry densities. Additionally, the graph includes a range of lambda values summarized by Cuitiño-Rosales et al. [68], alongside the empirical mathematical regression model $λ = 0.0559 * \exp(0.0014ρ_{Dry})$ as proposed by Kadir et al. [69] and cited in Cagnon et al. [70]. Within the density range studied (approximately 1450–1950 kg/m³), it was observed that density does not significantly influence the conductivity values of adobes characterized by varying limestone content, as evidenced by a correlation coefficient (R²) of 0.14. Furthermore, the mean deviation between the present experimental results and those derived from the empirical model of Kadir et al. [69] is approximately 25.6 %. Notably, the measured values align with the range established by Cuitiño-Rosales et al. [68]. The thermal parameters pertinent to thermal comfort, specifically diffusivity (a, in m²/s) and specific heat capacity ($C_p$, in kJ/(kg·K)), are graphically represented in Fig. 14.

The thermal diffusivity of a material quantitatively reflects its thermal inertia and characterizes the rate of heat transfer across its thickness. While thermal diffusivity does not contribute to enhanced thermal comfort during winter months, it plays a significant role in mitigating indoor temperatures during summer, particularly by delaying the penetration of heat after sunset. A lower thermal diffusivity indicates an extended duration for the thermal front to traverse the material's thickness, thereby improving thermal comfort. Within the current analysis, thermal diffusivity values for the studied adobes range from 0.368 (08-EC) to 0.503 (51-AT), with a mean of 0.446 m²/s and a

<figure><img src="image_14.png" /><figcaption>Fig. 13. Conductivity of investigated adobes.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 14. Diffusivity and heat capacity of investigated adobes.</figcaption></figure>

standard deviation of 0.046. The thermal diffusivity of adobe suggests that structures constructed from this material can attenuate external temperature fluctuations more effectively than concrete, but not as effectively as wood, considering their mean diffusivity values are approximately 0.83 m²/s for wood and 0.15 m²/s for concrete [42].

The heat capacity of adobe denotes its ability to absorb and release thermal energy, while specific heat capacity quantifies a substance's heat retention capability. Materials with elevated thermal capacity, when integrated into an insulating envelope, can augment the thermal inertia of a dwelling, thereby regulating and equalizing internal temperatures throughout the year and improving occupant comfort. In this analysis, specific heat capacity values for the examined adobes vary from 0.787 (51-AT) to 1.109 (51-BR), yielding a mean value of 0.961 kJ/(kg·K) with a standard deviation of 0.089. This mean value aligns closely with those reported by Yan et al. [71] at 0.902 kJ/(kg·K) and Saleh [72] at 1.0 kJ/(kg·K) for rammed earth and adobe, respectively. Moreover, it was observed that the content of CaCO₃ does not

<table><caption>**Table 5**
Hygrothermal properties of adobes – Mean (SD).</caption><thead><tr><th></th><th>08-CO</th><th>08-DO</th><th>08-EC</th><th>10-CH</th><th>51-AT</th><th>51-BR</th><th>51-BU</th></tr></thead><tbody><tr><td>Thermal Conductivity</td><td>0.679<br>(0.080)</td><td>0.761<br>(0.048)</td><td>0.536<br>(0.065)</td><td>0.799<br>(0.054)</td><td>0.783<br>(0.021)</td><td>0.669<br>(0.033)</td><td>0.680<br>(0.069)</td></tr><tr><td>λ(W/mK)</td><td>0.459<br>(0.052)</td><td>0.475<br>(0.035)</td><td>0.368<br>(0.040)</td><td>0.479<br>(0.029)</td><td>0.503<br>(0.030)</td><td>0.414<br>(0.019)</td><td>0.427<br>(0.044)</td></tr><tr><td>a (10<sup>-6</sup> m²/s)</td><td>0.979<br>(0.080)</td><td>0.927<br>(0.011)</td><td>0.835<br>(0.014)</td><td>0.969<br>(0.008)</td><td>0.787<br>(0.028)</td><td>1.109<br>(0.009)</td><td>0.949<br>(0.011)</td></tr><tr><td>Specific Heat Capacity<br>C<sub>p</sub> (kJ/(kg·K))</td><td>3.51<br>(0.32)</td><td>3.09<br>(0.26)</td><td>2.49<br>(0.25)</td><td>2.21<br>(0.65)</td><td>1.65<br>(0.18)</td><td>2.54<br>(0.40)</td><td>2.44<br>(0.36)</td></tr><tr><td>Moisture Buffer Value<br>MBV<sub>2</sub> (g/m²RH)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
significantly influence the measured diffusivity, specific heat capacity, or conductivity across the entire set of adobe bricks. The Pearson correlation coefficients were found to be 0.22 for specific heat capacity, 0.15 for conductivity, and 0.03 for diffusivity. Additionally, materials exhibiting a high Moisture Buffering Value (MBV₂) are capable of alleviating indoor humidity fluctuations, which enhances air quality, reduces the proliferation of microbial colonies, and potentially decreases the risk of respiratory health issues. The samples were subjected to five measurements during the absorption phase and two during the desorption phase. The weight variations of the samples throughout these phases are illustrated in Fig. 15. Mass changes stabilized over the final three cycles, facilitating the computation of the MBV₂ value as outlined in Eq. (1). The pertinent measured MBV₂ values are documented in Table 5.

To assess a material's capacity for moisture regulation, the NORDTEST project [33] established a categorization of moisture buffer values, spanning from negligible to excellent. Fig. 16 delineates this classification, positioning the current values for various adobes accordingly. Except for adobe 51-AT (MBV2 = 1.65 g/(m².%RH)), all other examined samples are classified as excellent moisture regulators, with adobe 08-DO demonstrating the highest efficacy at MBV2 = 3.09 g/(m².%RH). In the case of raw earth, the moisture-buffering performance does not depend on t2he processing technique. For example, with Compressed Earth Bricks (CEB), Mc Gregor et al. [73] found MBV2 values in the range of those of the present study, between 1.3 and 3.7 g/(m².%RH) and for unstabilized rammed earth, Arrigoni et al. [74] found 2.0 g/(m².%RH). In comparison, other usual building materials such as fired masonry bricks, concrete, and cellular concrete exhibit MBV values of 0.49, 0.38, and 1.0 g/(m².%RH), respectively, thereby categorizing them as limited-effect moisture regulators [75-77].

Additionally, a robust negative correlation is evident between calcium carbonate (CaCO₃) content and moisture buffer value (r = -0.69), indicating that increased CaCO₃ concentration is associated with diminished moisture regulation capacity. The deduced linear trend equation that can act as predictive model is MBV₂ = -0.0135 * (CaCO₃%) + 3.2671. Nonetheless, it is noteworthy that all samples in this study are collectively considered to exhibit very good to excellent moisture regulation capabilities.

## 4. Limitations

This experimental investigation of seven adobe types sourced from distinct villages within the Champagne region of France is accompanied by several methodological limitations that warrant clarification. Primarily, the analyses are executed in a controlled laboratory environment that is uniform across all tested adobes; this presents a significant

<figure><img src="image_16.png" /><figcaption>**Fig. 15.** Average moisture absorption–desorption cycles of adobes (red line denotes the daily RH cycles).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 16.** Radar chart of Moisture Buffer Values.</figcaption></figure>

limitation as it may not accurately represent the in-situ conditions experienced by these materials, including variable climatic and temporal factors.

Secondly, the sampling methodology is restricted to seven adobe types, which, while allowing for the identification of certain trends, inhibits the capacity for conducting a statistically rigorous analysis at this juncture. Expansion of the sampling size is imperative for a comprehensive investigation.

Furthermore, a pertinent concern raised by Fabbri et al. [78] relates to the adequacy of evaluating adobes in isolation, particularly regarding their mechanical properties. Conducting a study on an expanded scale that incorporates several adobes integrated with mortar in masonry applications may yield more relevant insights. This consideration is currently under review as a prospective direction for future research endeavors. In addition, there are other promising avenues of access to the moisture-regulating capacity of these earth materials, which are less time-consuming to implement. The study of Moisture Buffering Value (MBV) can be further extended to sorptivity analysis, as investigated by Kabir and Garg [79] in cementitious materials. Surface wettability, characterized by the ease with which water spreads on a surface has been identified as an indirect measure of sorptivity. This relationship enables the prediction of the absorption behavior of porous materials without the need for extensive and complex experimental procedures, providing a valuable approach to assessing their durability and performance. Additionally, an experimental study on water interaction at the surface level of three natural porous stones by Lee et al. [80] highlights the presence of an air layer that influences moisture absorption, retention, and release—key factors governing the MBV. The findings suggest that MBV can be effectively linked to droplet impact studies, as materials exhibiting limited absorption upon droplet impact tend to have a lower MBV, making them less effective in regulating indoor humidity. Further research in this area would appear to be relevant.

Another point that merits further analysis concerns the interaction between metal oxides (SiO₂, Al₂O₃, Fe₂O₃...) and the mechanical and thermal behavior of these adobes. Quantifying these oxides using X-ray diffraction analysis would provide an answer to this question.

## 5. Conclusion

This research contributes to the comprehension of the vernacular raw-earth heritage prevalent in the Champagne region of France, where systematic documentation is presently underway. In order to integrate ancient construction techniques from this region into modern local architecture or renovation practices, seven distinct types of ancient adobes, exhibiting significant variation in calcite content ranging from 0 % to 84.5 %, were systematically analyzed. The experimental program yielded several pertinent observations.

The foremost conclusion derived from this study is that historical
masons utilized the least robust adobes for load-bearing applications, while the more resilient variants were employed for infill. This approach was likely not founded on an awareness of differential material strength, as the mechanical properties of the adobes were not a critical consideration at the time. Notably, the selection of appropriate techniques for local soil was determined by the practicality of onsite application rather than mechanical strength alone. It became evident that wall design, characterized by greater thickness in load-bearing structures, was a determinant of strength, rather than advancements in the formulation of the adobes themselves.

A more nuanced examination reveals that, within a constrained geographical zone, two distinct categories of soils—predominantly silicate and predominantly limestone—are present, each giving rise to unique morphologies, granulometric profiles, and elemental compositions.

From a mechanical perspective, the measured maximum compressive stresses exhibit considerable variability, spanning from 0.74 MPa to 4.80 MPa, contingent upon soil composition. The peak compressive stress is inversely related to calcite content; specifically, increased calcium carbonate (CaCO₃) concentration correlates with diminished peak compressive stress, as reflected in a Pearson correlation coefficient of r = -0.87. Additionally, this analysis indicates that the concentration of metallic oxides within the soils constitutes a crucial determinant of their mechanical strength; hence, a higher atomic percentage of accumulated metallic elements (Fe, Al, Mg) is associated with increased peak compressive stress (Pearson coefficient r = 0.76).

Thermal analysis revealed that the CaCO₃ content did not exert a significant influence on diffusivity, specific heat capacity, or thermal conductivity across the studied adobes. The ranges of these parameters correspond with extant literature regarding adobe characteristics.

All examined samples demonstrated commendable to excellent moisture regulation properties. However, a pronounced negative correlation was noted between CaCO₃ content and moisture buffer value (Pearson coefficient r = -0.69); thus, higher levels of CaCO₃ correlated with reduced moisture regulation capacity.

The findings of this study underscore the potential of raw earth techniques in both the rediscovery and evaluation of earthen architecture, thereby advocating for the promotion of modern, environmentally sustainable construction and renovation methodologies.

## CRediT authorship contribution statement

**Bliard Christophe:** Resources, Investigation. **Moussa Tala:** Resources, Investigation. **Murer Sébastien:** Writing – review & editing, Resources, Data curation. **Bogard Fabien:** Supervision, Resources. **Beaumont Fabien:** Methodology, Investigation. **Rousse Céline:** Investigation. **Aras-Gaudry Adrien:** Methodology. **Polidori Guillaume:** Writing – original draft, Supervision, Conceptualization. **Hamard Erwan:** Writing – review & editing, Supervision. **Fronteau Gilles:** Writing – review & editing, Validation.

## Declaration of Generative AI and AI-assisted technologies in the writing process

The authors acknowledge the use of ChatGPT 4o (OpenAI, https://chat.openai.com, accessed 4 December 2024) for language improvement purposes only. The prompt used was “Rephrase to scientific English”.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

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