# Recycling or downcycling? The difference between raw and stabilised earth explained

Mathieu Audren, Simon Guihéneuf, Tangi Le Borgne, Damien Rangeard, Arnaud Perrot

► To cite this version:

Mathieu Audren, Simon Guihéneuf, Tangi Le Borgne, Damien Rangeard, Arnaud Perrot. Recycling or downcycling? The difference between raw and stabilised earth explained. Materials and structures, 2025, 58 (6), pp.206. 10.1617/s11527-025-02721-x. hal-05272917v1

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Submitted on 8 Dec 2025 (v1), last revised 8 Dec 2025 (v2)

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# Recycling or downcycling? The difference between raw and stabilised earth explained

Mathieu Audren · Simon Guihéneuf · Tangi Le Borgne · Damien Rangeard · Arnaud Perrot

Received: 24 February 2025 / Accepted: 18 June 2025 / Published online: 24 July 2025
© The Author(s) 2025

**Abstract** The development of innovative earthen materials necessitates a thorough evaluation of their recyclability. Although stabilisation is often regarded as a barrier to the recyclability of earthen materials, this study investigates the feasibility of recycling compressed earth blocks (CEBs) stabilised with 5% CEM III A or NHL 3.5. To evaluate this property, the study involved repeated cycles of material characterization, compaction using the modified Proctor method, compressive strength testing, water durability tests and subsequent crushing. The raw material remained unchanged through the recycling process, retaining its clay activity, manufacturing properties, and compressive strength. In contrast, the stabilised material showed a significant decline in clay activity, rendering it negligible. After three recycling cycles,

the Methylene Blue Value (MBV) of the cement- and lime-stabilised materials decreased from 0.7 to 0.1, while the materials do not show any plastic behaviour whatever their water content. Furthermore, the use of these stabilisers resulted in a decrease in the optimal dry density by 80 kg/m³ for CEM III A and 50 kg/m³ for NHL 3.5. Despite this reduction, there was no corresponding decrease in compressive strength following the first recycling cycle. This indicates the binder's effectiveness may remain similar because of the diminished clay activity. These results also indicate that stabilised materials must be re-stabilised during recycling to prevent a loss of compressive strength compared to the raw material. Considering these experimental results, recycling stabilised earthen materials should rather be defined as a downcycling process or a recycling process that needs additional stabiliser while raw earth displays an infinite recyclability.

**Keywords** Recyclability · Earth construction · Stabilisation · Clay activity · Compressive strength

## 1 Introduction

Earth has been employed as a construction material across the globe, with techniques adapted to suit local conditions, materials and cultural heritage [1–3]. Numerous traditional methods are used in earthen construction, such as rammed earth, adobe,

<footnote>M. Audren · T. Le Borgne · D. Rangeard
Research & Development Department, Laboratoire CBTP,
35530 Noyal-Sur Vilaine, France
e-mail: mathieu.audren@univ-pau.fr</footnote>

<footnote>T. Le Borgne
e-mail: tangi.leborgne@lcbtp.com</footnote>

<footnote>D. Rangeard
e-mail: damien.rangeard@lcbtp.com</footnote>

<footnote>M. Audren · S. Guihéneuf · A. Perrot
IRDL, UMR CNRS 6027, Université de Bretagne Sud,
56100 Lorient, France
e-mail: arnaud.perrot@univ-ubs.fr</footnote>

<footnote>S. Guihéneuf
e-mail: simon.guiheneuf@univ-ubs.fr</footnote>
and compressed earth blocks (CEB) [4–8]. In modern times, earthen construction must adapt not only to its surroundings but also to the demand of contemporary building standards [9]. Traditional construction methods often involved regular maintenance, such as the application of sacrificial plasters [6]. However, these practices are no longer common in contemporary construction and to ease their large-scale development, earthen materials could evolve to provide greater durability and lower maintenance requirements [10–13]. To meet this challenge, earth is sometimes stabilised with hydraulic binders, enhancing its durability and making it better suited to the demand of modern construction [14–18].

The stabilisation of earth implies a significant impact on the environmental characteristics of earthen construction [19–21]. Techniques like CEB and rammed earth require large quantities of material, and the stabilisation process often involves the use of considerable amounts of cementitious products. These additives, which have a high environmental impact [22], undermine the otherwise positive environmental profile of earthen structures.

While the impact on global warming potential (measured as CO₂eq emissions) is often emphasised, stabilisation also negatively affects the nearly infinite recyclability of raw earth [23, 24]. Recyclability is defined as the process of utilising and transforming waste materials to create a new product [25, 26]. Evaluating the recyclability of stabilised earth has become essential for conducting accurate life cycle assessments [27]. A better understanding of this parameter allows for a more accurate evaluation of the benefits and impacts during the product's end-of-life stage. Furthermore, although earth is locally abundant in many regions [28], it remains a non-renewable resource on a human timescale. Recycling this material is vital to reduce transportation distances and support a circular and locally sourced economy. Earth stabilisation using hydraulic binders is well-documented [10–13, 15, 18, 29, 30] but limited research has investigated the impact of stabilisation on the recyclability of earth [23, 24]. Also, hydraulic stabilisation can alter the crystalline structure of clay, reducing its swelling capacity and potentially causing flocculation, which in turn affects the macroscopic properties of the soil [14, 31–33].

In this study, the recyclability potential of stabilised earth blocks is assessed by examining changes

in material properties, processability, and mechanical strength after several cycles of compaction, crushing, and mixing. This study focuses on the recycling of CEB to produce new CEB as the earth waste is subjected to crushing and compaction rather than being reused in its original state [34]. Block compaction is simulated using a modified Proctor procedure. Key parameters, including Atterberg limits, methylene blue value (MBV), particle size distribution (PSD) evolution, optimal water content obtained from the modified Proctor test, and compressive strength are measured. Recyclability is analysed for earth stabilised with cement, hydraulic lime, and in its raw unstabilised state. This study proposes a methodology to qualify the recyclability of compressed earthen construction materials, offering a systematic approach to evaluate their potential for reuse as a recycled earth-based aggregate over multiple cycles.

## 2 Materials

A single soil type is used in this study, and it is a quarry by-product. This material is a granitic silt (G.S.), which refers to weathered granite found in many places around the world. The by-product contains fragile, coarse particles of varying sizes mixed with silty and clayey particles. It is of particular interest in a recyclability study, as it may be especially sensitive to crushing, potentially leading to changes in its particle size distribution (PSD).

The PSD of the material in its natural (non-recycled) state is determined through wet sieving, in accordance with the ISO 17892-4 standard [35]. The PSD of grains smaller than 63 µm is determined using both laser diffraction and sedimentation analysis. These two methods are employed as they provide different results [36–38] and are used here to establish a range for the particle size distribution of this fine fraction. Figure 1 presents the PSD of the material. It contains particles with a maximum diameter of 14 mm, but a significant proportion is composed of sand. Approximately 86% of the material consists of aggregates smaller than 4 mm. The clay fraction is assumed to correspond to the proportion of particles smaller than 2 µm [39, 40] and is found to be 1% when determined by laser diffraction and 6% when measured using sedimentation analysis. The silt content which contributes to the material behaviour is
<figure><img src="image_2.png" /><figcaption>**Table 1** Atterberg limits and MBV of G.S</figcaption></figure>

<table><thead><tr><th>Plastic limit (W<sub>p</sub>)</th><th>Plasticity index (PI)</th><th>Liquid limit (W<sub>L</sub>)</th><th>MBV</th><th>PI/clay fraction</th><th>MBV/clay fraction</th></tr></thead><tbody><tr><td>24.3%</td><td>18.0</td><td>42.3%</td><td>0.71</td><td>59</td><td>10.5</td></tr></tbody></table>

Two hydraulic binders are used to stabilise the earth: a low-carbon cement, CEM III/A 42.5 R, and a hydraulic lime, NHL 3.5. Previous studies have shown how these two hydraulic additives enhance mechanical strength of the material both at dry state and after immersion [14–18]. The materials were stabilised with a 5% proportion of hydraulic binder. The amount of stabiliser depends on implementation practices and the type of soil used [14, 45]. The proportion employed here is comparable to the amount typically used for stabilised CEB made for load-bearing wall applications [15, 46].

equal to 18%. To further complement this characterization, Atterberg limits and the methylene blue value (MBV) were also measured [41].

The Atterberg limits were measured on the fraction of the soil smaller than 400 µm, using a cone penetrometer following the specifications of the ISO 17892-12 standard and Feng (2001) method [42, 43]. The methylene blue value (MBV) was determined following the procedure outlined in the NF P94-068 standard [44]. The results, presented in Table 1, indicate a low but significant clay activity considering the relatively low clay content [41]. These activity indices were determined using two parameters: PI/clay fraction and MBV/clay fraction. The clay fraction refers to the soil fraction smaller than 2 µm, determined by sedimentation.

## 3 Methods to assess the recyclability potential

To study the recyclability of raw and stabilised earth, samples were produced, crushed, and then recompacted. The material is not directly exposed to in-service conditions, such as carbonation, for instance. However, this approach offers an initial assessment of the reuse potential of raw and stabilised soil. The experimental recycling procedure includes the following steps:

- Characterization of the material (Atterberg limits, MBV, and PSD).

- Possible stabilisation of the material.

- Modified Proctor test according to the NF P 94-093 standard [47] (this step also mimics the production of compacted blocks. The obtained samples are then cured for mechanical testing as described thereafter).

- Drying of the samples made during the modified Proctor test:

Raw samples are dried for 2 days at 20 °C/50% RH before being dried in an oven at 40 °C until mass equalisation.

Stabilised samples are placed in an airtight bag in a room at 20 °C/90% RH during 28 days before following the drying protocol used for the raw samples.
- Dry compressive strength test on most of the produced samples.

- For CEM III-stabilised samples, drip tests and wet compressive strength tests were conducted for the following stages: Initial, 1st recycling and 2nd recycling.

- Crushing of the material was performed using a Provitech shear mixer (Cutter), with a bowl rotation speed of 12 rpm and blade rotation speed of 1500 rpm for 15 s. The material was then remixed and crushed again for an additional 15 s, this time with a blade rotation speed of 3000 rpm.

During the compression tests, no system to limit friction between the compression machine and the sample was implemented. Frictional effects may occur and strength values are here considered for comparison purpose in the frame of the study.

The soil was crushed using a shear mixer to obtain a particle size distribution close to the original material and to evaluate the influence of binder setting on recyclability, while avoiding the consideration of energy consumption during crushing [48, 49]. However, in practice, crushing may be performed using tools that produce coarser aggregates or may include a screening step. These scenarios are not considered in the present study. It is reasonable to assume that soil crushed into coarser clumps would be more difficult to recycle while maintaining similar mechanical properties, as this could lead to increased porosity due to a poorly optimised granular skeleton [50–54].

This cycle is repeated twice to evaluate the effect of the recycling process on the material's characteristics, including both its compaction and mechanical properties. The influence on compaction characteristics is assessed by observing changes in behaviour during the modified Proctor test [50, 55–57]. It is assumed that the curing time of 28 days is sufficient for allowing most of the hydraulic stabiliser to react [57]. The samples produced using the Proctor test are cylindrical, approximately 115 mm in height and 100 mm in diameter, and can therefore be considered as having a slenderness ratio of 1. The compressive strength test is conducted at a rate of 1 mm/min to comply with the NF EN 13286-41 standard [58].

The effect of recycling without adding any hydraulic binder after the first life cycle of the product was also examined. In this case, after the first use of the

stabilised material, the material is reused without the addition of any further stabiliser.

For the remainder of this paper, the following terminology is used and the Fig. 2 provides an overview of the mix-design of the studied samples:

The first item of the sample's name refers to the type of stabilisation and on the possible use of binder:

**Raw** Material that has never been treated with a hydraulic binder.

**CEM III A** The material is stabilised with 5% CEM III A.

**NHL 3.5** The material is stabilised with 5% NHL 3.5.

A preliminary study was conducted to determine the appropriate binder content, testing proportions of 1%, 3%, and 5% [59]. This pre-study evaluated both dry compressive strength and the durability of the stabilised formulations. The results indicated that a 1% binder content negatively affected mechanical strength due to interactions between the binder and the clay [60]. A 5% binder content offered only marginal improvements in durability compared to the 3% formulation without exhibiting a decrease in mechanical strength [29, 46, 61]. Nevertheless, the 5% formulation was retained for this recyclability study in order to represent a conservative, less favourable scenario.

The second item informs about the stage of the product's life cycle to which the data refers:

**Initial** First life of the product. This refers to the initial life of the material in its raw or stabilised, non-recycled state.

**1R** First recycling, second life of the product. It characterizes the material after it has been recycled for the first time.

**2R** Second recycling, third life of the product. The material is recycled for the second time.

**3R** Third recycling, fourth life of the product. The material is recycled for the third time.

**No stabilisation, 1R** The material was stabilised during its first life but is not stabilised during the recycling stage for its second life.

To better assess the effect of recyclability of the material performances, additional tests were performed to assess water-durability as a key parameter

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Overview of the terminology of the studied mix designs regarding the recycling stage</figcaption></figure>

to consider their service life properties. A drip test, following the German standard DIN 18945:2018-12 [62] and the French standard XP P13-901 (2022) [63] was conducted on the following formulations: Raw; Initial, CEM IIIA; Initial, CEM III A; No stabilisation; 1R, CEM III A; 1R and CEM III A; 2R. 10 cm of the samples were immersed for 10 min in water and a percentage of mass loss was determined for each formulation by weighting the dried samples. For the formulations that were able to resist to the drip test, a wet compressive strength test was conducted, adapted from the French standard XP P13-901 (2017) [64], with an immersion

duration that was adapted to the size of the samples following the recommendations of Guihéneuf et al. [65, 66] study. Thus, a strength retention ratio after immersion was determined for some of these formulations, dividing the obtained mean wet compressive strength after immersion by the mean dry compressive strength of samples of a given mix-design. As the CEM III A allowed to reach better mechanical performances more easily, this water-durability study was limited to these formulations. It is worth to note that the studied earth presents a low clay activity, that emphasizes the beneficial effect of hydraulic binders on water-durability.
The recyclability of an earthen material is characterised using two key parameters:

The modification of mechanical properties—here mostly limited to compressive strength—is one of the key indicators. It is assumed that maintaining consistent compressive strength ensures a similar product quality, as compressed earth is used for load-bearing structures. However, water-durability tests (drip test, wet compressive tests after immersion) were performed for samples stabilised with the cement to better assess their quality and their service life properties.

The alteration of the resource itself is also considered. Raw earth may possess the capacity to be recycled by reintegrating the material into its original environment, in line with a cradle-to-cradle approach [20, 21].

This definition of recycling is primarily based on the product's life cycle, considering both its functional performance (i.e., compressive strength) and its potential end-of-life pathway, in line with a cradle-to-cradle approach. It is intended to be relevant within life cycle assessment (LCA) frameworks. When evaluating the benefits and burdens beyond the product's end of life, it becomes necessary to quantify changes in product quality [27, 67]. This is what the

present study aims to address through the analysis of two key parameters: mechanical properties and soil characteristics.

However, this study does not consider energy, economic, or societal impacts. It focuses primarily on the material itself and the feasibility of its recycling. This lack of multidisciplinary data constitutes a significant limitation. As such, the study alone cannot fully justify the use of earth materials within a circular economy framework [48, 49].

# 4 Results and discussions

## 4.1 Material characteristics

### 4.1.1 Particle size distribution

The PSD of a granular material significantly influences its mechanical strength. Additionally, in the case of earth materials, it provides the first insights to assess the clay fraction of a given material. For this comparative analysis, in order to rapidly highlight the influence of recycling strategies on the PSD of the earthen material, the PSD of the fraction smaller than 63 µm is measured using the laser diffraction method [36, 38]. Figure 3 shows the granular range of the selected earthen material in its initial state and

<figure><img src="image_5.png" /><figcaption>**Fig. 3** Raw and stabilised materials before and after recycling stage</figcaption></figure>
after recycling, for both stabilised and raw materials. The effect of recycling on the PSD varies between raw and stabilised materials. Raw material generally becomes finer with crushing cycles performed for recycling, which confirms the fragile nature of this granitic sand, whereas stabilised material exhibits the opposite behaviour.

To understand the behaviour of cement-stabilised earthen materials it is relevant to consider how clay particles interact with the cementitious hydrates [68]. The cement stabilisation process can be described in several stages. Initially, cement changes the pH and ionic nature of the suspensions making clay particles agglomerating, strengthening the flocculated network [69, 70]. Additionally, pozzolanic reactions occur between the clays particles and portlandite, altering the crystalline structure of the clays [32, 33]. Finally, hydrates are produced, bonding clay particles into agglomerates. CEM III A incorporates furnace slag as a supplementary cementitious material. This material primarily contains silicates, aluminosilicates, and calcium oxide. In contact with water, these compounds undergo pozzolanic reactions. The interactions between CEM IIIA and clays are similar to those of lime, resulting in clay flocculation and the formation of hydraulic bonds [71–74].

First, Fig. 3 shows how grinding step affect the coarsest particles during recycling. The maximum particle size is reduced from 14 to 8 mm, regardless whether the material is raw or stabilised. It is due to the relative mechanical weakness of the weathered aggregates of the G.S. and cannot be attributed to a bias related to grain agglomerates, as the granulometric analysis is conducted following a wet method. The results also indicate that, in the case of raw material, the sandy fraction becomes finer, whereas for stabilised material, this same fraction shows the opposite behaviour. However, below 8 mm, the grains may be bound by hydraulic bonds, which are not affected by the crushing and washing process.

Then, depending of the earth nature, a reduction in the maximum particle size and a modification of the sandy fraction could be expected during the crushing step. For the same earth, this effect depends on whether the material is raw or stabilised. These phenomena could alter the granular skeleton and may potentially impact the mechanical strength of the recycled materials [50–54]. For some recycled earthen materials, the addition of sand or aggregates

could help to optimise the material compacity by making a broader and more uniform the particle size distribution.

The silty fraction and sandy fraction display a similar behaviour regarding the PSD. In the case of using raw earth, the fraction finer than 63 µm increases by 5%. The recycled material becoming finer, while the stabilised material shows the opposite behaviour, this silty fraction decreasing by 3%. Thus, grinding may increase the silt fraction in the raw material, whereas the hydrates bonds in the stabilised material prevent this phenomenon from happening, even reducing the amount of silt by agglomerating the particles.

In the case of earthen materials, the fraction of elements smaller than 2 µm offers a quantitative measurement of the clay content in the soil which is a first assessment of the suitability of earth for construction purpose [39, 40]. The clay proportion in the raw material remains unchanged after recycling, remaining at 1%. The grinding applied in this study does not seem to be sufficient to break the grains at the clay scale. In contrast, the stabilised material shows a reduction in the clay fraction, decreasing to 0.2% which is probably due to the fact that clay particles are bonded by cementitious hydrates [14, 31, 75]. Also, it is likely that cementitious bonds between the clay particles and other soil particles cannot be broken during crushing [31, 33, 46, 76]. No mineralogical analyses were conducted on the stabilised materials to confirm the presence of hydration products, assess pozzolanic reactions, or distinguish the relative contributions of flocculation versus cement hydration to the effectiveness of recycling [32, 33, 69, 71, 73, 76]. However, during particle size distribution measurements, the materials were washed with water and allowed to sediment in water for 24–48 h. The observed decrease in the proportion of particles below 2 µm with increasing recycling cycles suggests that at least part of the material is bound by water-insensitive bonds, and not only reversible flocculation. To better understand the effect of successive stabilisations on the clay fraction, it seems important to further study the evolution of the clay activity of the earth throughout their life cycles.

### 4.1.2 Clay activity

The clay activity of the studied earthen material after recycling was evaluated by measuring MBV and
Atterberg limits. These tests were conducted to evalu-
ate the effect of earth recyclability on the clay activity
using tools that are commonly available in geotechni-
cal laboratories.

As shown in Fig. 4, the MBV remains constant
throughout the recycling process in the case of the
raw unstabilised material, stabilising at a value of
0.66±0.05. Detailed results are provided in the
appendix. This result aligns with the particle size
distribution analysis, which highlights no variation
in the silt and clay fractions. However, in the case of
stabilisation with CEM III A and NHL 3.5, the MBV
is reduced after each recycling cycle. By the third
recycling cycle (fourth life of the material) the MBV
decreases to 0.1, emphasising a rather negligible
clay activity [77–80]. The accurate definition of clay
activity is the ratio between the MBV and the frac-
tion of soil particles smaller than 2 µm, which can be
considered as clay. This 2 µm fraction decreases over
successive recycling cycles of the stabilised formula-
tions. However, the clay minerals themselves do not
disappear. As with the PSD analysis, this phenom-
enon is likely due to clay agglomeration by hydrates
bonds that cannot be broken, either mechanically or
through water washing [14, 31, 75]. Clay activity
decreases because the MBV decreases, while the clay
content remains constant, as the material is retained
throughout the study. In this case, it seems that stabi-
lisation with 5% hydraulic binder does not completely

inhibit clay activity, although consecutive recycling
stages gradually transform the active clay into an
inert material. The clay particles are either bound by
hydrates or flocculated, but given the curing period of
only 28 days, pozzolanic reactions could be limited
further modifying the specific surface area of these
agglomerated particles [15, 31, 32, 81, 82]. It is there-
fore possible that a longer stabilisation period could
result in stronger bonds. This represents a limitation
of the present study.

Tests conducted on the material stabilised with CEM III A or NHL 3.5 during its first life and unstabilised during its second life display that the MBV remains constant for the two tested binders at a low value of 0.3. This suggests that an initial stabilisation permanently alters clay particles and their activity. Furthermore, it indicates that the 28-day curing period is sufficient for tested binders to reach full hydration.

The effect on the fine fraction of the soil can also
be analysed through its water-related behaviour, as
described by the Atterberg limits [83]. The plasticity
index of the raw material (Fig. 5) remains unchanged
after three recycling cycles, ranging from 15 to 17.9.
Once again, these results align with those of the MBV
and PSD, indicating that recycling the raw material
has no impact on its clay fraction.

However, for the successive stabilisations with
CEM III A and NHL 3.5, Fig. 5 clearly indicates a

<figure><img src="image_6.png" /><figcaption>**Fig. 4** Evolution of MBV with recycling cycles for unstabilised and stabilised earth</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5** Plasticity index of the material over its lifetime</figcaption></figure>

decrease in the plasticity index with an increasing number of recycling cycles. This value approaches zero during the third life of the product (2R) and becomes immeasurable in its fourth life. At this stage, the recycled earthen material is no longer behaving as a clayey material displaying a plastic behaviour in a given range of water content. This reduction in plasticity index is due to an increase in the plastic limit induced by the stabilisation with hydraulic binders [17, 84, 85].

The material stabilised during its first life and unstabilised during its second life shows a preservation of the lower plasticity index that resulted from its first life. This result further demonstrates that once the material is stabilised, it can rapidly and irreversibly lose its cohesive properties.

Preserving the clay properties in the raw material seems to be mandatory to obtain an earthen material with infinite recyclability. The material contributes to the circular economy by providing a local and reliable building material, as it has already been used to build an earth structure [86]. It also avoids the need to extract material from quarries, but requires deconstruction and crushing [87]. The stabilised materials that display a major loss of clayey activity after several recycling processes are still recyclable, however, this process can be defined as downcycling, as it prevents the recovery of the original material properties [86, 88].

## 4.2 Compaction characteristics

The modification of the clay activity in a soil may indicate a change in its optimal compaction characteristics [40, 89–91]. In this study the optimal compaction characteristics for CEB are approximated by using the modified Proctor optimum [50, 92]. The raw material shows no change in these compaction characteristics after recycling, that are plotted in Fig. 6. The compaction curves overlap, indicating the same optimum density and moisture content, 2160 kg/m³ and 9%, respectively. These results also highlight the high and theoretically infinite recyclability of raw earth [23, 24]. These findings are consistent with the previously observed preservation of the clay activity during the recycling process.

Figure 7 shows a modification in the optimal mixing characteristics after the recycling of the stabilised earth. The cement-stabilised material exhibited a decrease in optimal density of 78 kg/m³, while the NHL 3.5-stabilised material displayed a decrease of 51 kg/m³ [93, 94]. The optimum moisture content before recycling was approximately 11% for both stabilised materials. After recycling, it reaches 10.5% for the lime-stabilised material and 10% for the cement-stabilised material. While there are insufficient data to draw definitive conclusions about the effect of a second recycling cycle, the available data suggest an important decrease in dry density. This reduction in
<figure><img src="image_8.png" /><figcaption>**Fig. 7 Modified Proctor compaction curve of recycled stabilised earth**</figcaption></figure>

<figure><img src="image_9.png" /></figure>

moisture content can be explained by the decrease in clay content and the increase in flocculated coarser particles content, which lowers the water demand for compaction [40, 89–91]. To achieve optimal compaction for CEBs, recycled stabilised material should not be compacted following the initial compaction characteristics.

Figures 8 and 9 show the modified Proctor behaviour of stabilised materials recycled without

adding a stabiliser. For both materials, reusing them without stabilisation results in a reduction of dry density by 20 kg/m³–30 kg/m³ for cement and hydraulic lime stabilisation, respectively. This slight decrease may be attributed to changes in the particle size distribution (PSD). This slight decrease may be attributed to changes in the particle size distribution (PSD) resulting from a less broad granular skeleton containing lower mount of small particles
<figure><img src="image_10.png" /><figcaption>**Fig. 9** Modified Proctor compaction curve of recycled NHL 3.5 stabilised earth without the addition of hydraulic binder</figcaption></figure>

<figure><img src="image_11.png" /></figure>

(clay content), [51–53, 95, 96], to hydration-related shrinkage of stabilised fines or possibly due to a hydration reaction of cement and lime particles that were not fully hydrated during the product’s previous life. However, this third hypothesis contradicts the characterization results obtained in this study. During the recycling of a stabilised material, without adding a stabiliser, the mixing characteristics remain unchanged.

## 4.3 Mechanical strength and water-durability

The various elements analysed so far provide insights into the impact of recycling on the clay fraction and the manufacturing characteristics of the blocks. However, as the primary function of CEB is to bear loads, it is essential to examine their mechanical strength. In this study, the dry compressive strength was evaluated by testing the previously compacted samples.
This approach enables the development of a strength-density relationship curve. Figure 10a and b clearly show a linear increase in compressive strength with increasing density for all formulations [89, 97, 98]. In any case, the samples show a brittle behaviour due to their dry state and level of density. The raw material

shows consistent behaviour, with maximum compressive strengths ranging from 2.6 to 3 MPa. This variation may be attributed to imperfect surface conditions of the samples prepared during the modified Proctor tests. Recycling raw earth does not seem to negatively impact its compressive strength. These observations

<figure><img src="image_12.png" /><figcaption>**Fig. 10** a Compressive strength behaviour of raw and CEM III A-stabilised materials before and after recycling stages and b Compressive strength behaviour of raw and NHL 3.5-stabilised materials before and after recycling stages</figcaption></figure>
definitively validate the infinite recyclability of unsta-
bilised earthen materials.

Figure 10 illustrates the effect of stabilisation on the strength-density relationship. Although the slopes remain similar, ranging from 0.12 to 0.3 MPa·m³/kg, the stabilised material exhibits higher compressive strength than the raw material at the same density [29, 30, 84, 99]. Following recycling, the stabilised material does not show a reduction in mechanical strength despite the decrease in density previously highlighted by modified proctor tests. The material stabilised with CEM III A shows a 2.6% decrease in compressive strength, whereas the one stabilised with NHL 3.5 shows a 10% increase. This phenomenon could be attributed to the improved efficiency of the hydraulic binder due to the reduced clay content and clay activity of the recycled material. Indeed, clays can interfere with the setting process of cement or lime by reacting with ions, particularly Ca²⁺[99]. In the case of the stabilised material that is recycled using more binder at each cycle, earthen materials are gradually evolving into cement-based materials, the fine particles being progressively replaced by coarser aggregates. The limited availability of material after the first recycling stage hindered the preparation of a sufficient number of samples to model the material's behaviour in Fig. 10. However, the twice-stabilised and twice-recycled materials appear to align with the evolution of the materials recycled once with NHL 3.5 or CEMIII A, each sort of binder having its own signature on the displayed strength-density trends. These findings suggest that stabilised earthen materials can still display a good recyclability, almost infinite, but this recyclability relies on the use of additional virgin binders at each recycling stage. In the case of this recycling scenario, the earthen material is progressively transformed into ordinary aggregate. Following stabilisation, the clays appear to form aggregates, preventing them from acting as a binder, primarily due to their reduced high specific surface area (MBV). Adding binder thus results in a change of the material nature, causing the soil to behave like an assembly of inert aggregates. According to the literature, it could be considered, over time, as downcycling [86, 88, 100]. Furthermore, the material can no longer be used in a cradle-to-cradle approach, due to the modification of its intrinsic characteristics and the decrease of the mechanical characteristics of CEB [19, 101]. During the recycling of soils stabilised with

hydraulic binders, the setting process evolves due to
pozzolanic reactions or carbonation. These processes
require long-term studies (several months or even
years), which could not be carried out in the context
of this work. It is therefore possible that, under stand-
ard use conditions, the material may exhibit higher
compressive strength [32, 74, 81, 82, 84]. This study
remains relevant, however, as the majority of the
compressive strength is typically achieved within 28
days.

Finally, the behaviour of the initially stabilised material but recycled without the addition of further stabiliser (Fig. 10a and b), is similar to the raw material behaviour. However, the decrease in bulk density due to the first stabilisation results in a reduction in mechanical strength. It seems that recycling a stabilised material to produce a CEB without re-stabilising it does not produce a material with the same mechanical strength as raw earth, which has never been stabilised. This can be attributed to the reduction in clay activity [50, 102], the reduction of plasticity and the increase in coarser aggregate proportion. To be recycled, the soil may need to be stabilised and may no longer be reusable as an earthen building material without further stabilisation. This recycling scenario can thus be considered as a downcycling scenario [86, 88, 100].

Figure 11 is providing an overview of the 3 previ-
ously stated scenarios regarding the recyclability of
unstabilised and stabilised earthen materials.

To better evaluate the recyclability of both raw and stabilised earthen materials, a complementary water-durability study was carried out on various mix designs (unstabilised and stabilised with CEM IIIA). Two tests were conducted: a drip test and a measurement of compressive strength after immersion. The results, shown in Fig. 12, highlight key observations and reveal certain limitations of the study in terms of the chosen functional unit.

While dry compressive strength remains a criti-
cal property for structural earthen materials, water
durability must also be considered—either through
appropriate structural design or by directly evaluating
material performance. This study focuses on the lat-
ter, and it clearly shows that water resistance signifi-
cantly differs between raw and stabilised materials.

As seen at the top of Fig. 12, the drip test results (DRIP value) vary depending on whether cement was added to the mix. For both raw and recycled materials
<figure><img src="image_13.png" /><figcaption>**Fig. 11** Overview of the different recycling scenarios for compressed earth blocks for a structural application: a infinite circular model for unstabilised CEB, b downcycling model for stabilised CEB to avoid further use of hydraulic binder and c circular model for stabilised CEB that requires the addition of binder at each cycle</figcaption></figure>

without cement, the submerged portion of the samples deteriorated completely, leading to identical DRIP values.

In contrast, samples containing cement remained nearly intact, with a mass loss of less than 1%. Interestingly, the DRIP value tends to decrease with an increasing number of recycling cycles, suggesting that the material becomes progressively more resistant to water exposure.

This trend is further supported by the evolution of the strength retention ratio (defined as the ratio of compressive strength in the dry state to that after immersion), which increases with each recycling cycle, as shown in the bottom of the Fig. 12. This confirms that the material behaves more inertly after successive recycling.

It is also worth noting that samples without cement addition—whether from raw material or recycled
<figure><img src="image_14.png" /><figcaption>**Fig. 12** % mass loss after the dip test and strength retention ratios obtained for raw and CEM IIIA stabilised earthen samples for different recycling stages and scenarios</figcaption></figure>

from previously stabilised blocks—were completely destroyed during immersion. This indicates that the residual cement from initial stabilisation no longer contributes to water resistance in recycled mixes.

## 5 Conclusion

It is common practice today to use hydraulic stabilisers in the production of CEBs to meet current construction standards. While the effects of stabilisation on mechanical strength and durability are well-documented, few studies explore the recyclability of these materials. The results of this study show that stabilisation significantly reduces the clay activity of earthen materials, their coarser aggregate fraction increasing. When recycled several times, these stabilised and re-stabilised earthen materials are gradually turning into cement-based materials. In the study, this behaviour is characterized by a very low MBV and a complete loss of plasticity (PI=0). The impact of stabilisation on clay content also influences the manufacturing characteristics. As a result, the reduction in clay activity led to a decrease in the optimal compaction density. However, despite this reduction in density after the first recycling, the compressive strength at the optimal dry density showed little variation. This could be attributed to the improved efficiency

of the binder, resulting from the reduced clay activity [99]. The recyclability of stabilised earthen materials is made possible only with the addition of further hydraulic binders to aim for equivalent mechanical strengths.

The raw material displays no change in its clay activity, manufacturing characteristics, or compressive strength, highlighting the infinite recyclability of unstabilised earthen materials.

Tests on the initially stabilised earthen material recycled once without additional stabiliser highlighted a low remaining clay activity resulted from the initial stabilisation, lower compressive strengths than the raw material and optimal compaction density. To be recycled, the soil may need to be stabilised, and it may no longer be reusable as an earthen building material without further stabilisation. In this case the recycling process is rather a downcycling process.

Further studies should examine other types of soil and binders to assess the influence of mineralogy on the recycling potential of soil, depending on the binder use [23]. Additionally, exploring other techniques, such as rammed earth, plasters, light earth or cob, would be valuable to propose a global approach for earthen construction recycling strategies. It would be relevant to conduct recyclability tests with varying binder contents in order to assess whether the binder dosage leading to near-zero
MBV and plasticity index depends on the amount of binder added per cycle. Finally, recyclability was assessed primarily through mechanical performance. However, it would be relevant to evaluate it while also considering durability-related phenomena. This study was conducted solely at the material scale, representing a compressed earth brick. It would be interesting to conduct tests at the structural scale in order to evaluate the different behaviours of the walls. Additionally, this would allow for consideration of the use of mortar or earth plaster, as these materials are difficult to separate from earth block walls during deconstruction [49].

**Acknowledgements** This research was supported by the ANR (Agence National de la Recherche) within the framework of the Labcom CoLoRe "Construction with Local Ressources" ANR-21-LCV3-0008.

**Author contributions** Conceptualisation: Arnaud Perrot, Damien Rangeard; Formal analysis and investigation: Mathieu Audren, Simon Guihéneuf; Funding acquisition: Damien Rangeard, Arnaud Perrot; Investigation: Mathieu Audren, Tangi Le Borgne, Simon Guihéneuf, Damien Rangeard, Arnaud Perrot; Methodology: Mathieu Audren, Tangi Le

Borgne; Project administration: Damien Rangeard, Arnaud Perrot; Resources: Tangi Le Borgne; Supervision: Simon Guihéneuf, Tangi Le Borgne; Validation: Damien Rangeard, Arnaud Perrot; Visialisation: Mathieu Audren; Writing—original draft preparation: Mathieu Audren; Writing—review and editing: Arnaud Perrot, Simon Guihéneuf.

**Funding** Open access funding provided by Université de Bretagne Sud. Agence Nationale de la Recherche, ANR-21-LCV3-0008.

## Declarations

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

**Data availability** Data will be made available on request.

## Appendix

See Table 2

<table><caption>**Table 2** Geotechnical and mechanical properties of the earth materials throughout the different life cycles of the product</caption><thead><tr><th rowspan="2">Terminology</th><th colspan="3">Atterberg limits</th><th rowspan="2">MBV</th><th colspan="2">Compaction characteristics</th><th rowspan="2">Compres- sive strength (MPa)</th></tr><tr><th>W<sub>P</sub> (%)</th><th>IP</th><th>W<sub>L</sub> (%)</th><th>ρ<sub>d</sub> (kg/m<sup>3</sup>)</th><th>W% (%)</th></tr></thead><tbody><tr><td>Raw; Initial</td><td>24.4</td><td>17.9</td><td>42.3</td><td>0.71</td><td>2150</td><td>9</td><td>2.4</td></tr><tr><td>Raw; 1R</td><td>24.9</td><td>15.2</td><td>40.1</td><td>0.65</td><td>2160</td><td>9</td><td>3.2</td></tr><tr><td>Raw; 2R</td><td>25.6</td><td>15.9</td><td>41.1</td><td>0.61</td><td>2170</td><td>9</td><td>3</td></tr><tr><td>Raw; 3R</td><td>25.9</td><td>15.5</td><td>41.4</td><td>0.62</td><td>Insufficient number of points</td><td></td><td></td></tr><tr><td>CEM III A; 1R</td><td>33.9</td><td>8.26</td><td>42.18</td><td>0.41</td><td>2060</td><td>11.2</td><td>8.3</td></tr><tr><td>CEM III A; 2R</td><td>49.8</td><td>1.4</td><td>51.2</td><td>0.17</td><td>1970</td><td>10.9</td><td>8.1</td></tr><tr><td>CEM III A; 3R</td><td>No plastic behaviour</td><td></td><td>62.3</td><td>0.05</td><td>Insufficient number of points</td><td></td><td></td></tr><tr><td>CEM III A; No stabilisation; 1R</td><td>36.0</td><td>7.9</td><td>43.9</td><td>0.29</td><td>1970</td><td>10.6</td><td>1.6</td></tr><tr><td>NHL 3.5; 1R</td><td>41.1</td><td>4.17</td><td>45.26</td><td>0.31</td><td>2010</td><td>11.1</td><td>6.5</td></tr><tr><td>NHL 3.5; 2R</td><td>46.8</td><td>0.23</td><td>47.1</td><td>0.19</td><td>1960</td><td>10.5</td><td>7.1</td></tr><tr><td>NHL 3.5; 3R</td><td>No plastic behaviour</td><td></td><td>60</td><td>0.1</td><td>Insufficient number of points</td><td></td><td></td></tr><tr><td>NHL 3.5; No Stabilisation; 1R</td><td>37.6</td><td>6</td><td>44.0</td><td>0.28</td><td>1970</td><td>11</td><td>1.7</td></tr></tbody></table>
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