# Stabilising compressed earth materials with untreated and thermally treated recycled concrete: A multi-scale investigation

Vincent Rozzi, Agostino Walter Bruno, Antonin Fabbri, Antonio Barbucci, Elisabetta Finocchio, Alberto Lagazzo, Antonio Brencich, Domenico Gallipoli

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Vincent Rozzi, Agostino Walter Bruno, Antonin Fabbri, Antonio Barbucci, Elisabetta Finocchio, et al. Stabilising compressed earth materials with untreated and thermally treated recycled concrete: A multi-scale investigation. Journal of Cleaner Production, 2023, 423, pp.138614. 10.1016/j.jclepro.2023.138614. hal-05052677

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# Stabilising compressed earth materials with untreated and thermally treated recycled concrete: A multi-scale investigation

Vincent Rozzi<sup>a</sup>, Agostino Walter Bruno<sup>b,*</sup>, Antonin Fabbri<sup>a</sup>, Antonio Barbucci<sup>b</sup>, Elisabetta Finocchio<sup>b</sup>, Alberto Lagazzo<sup>b</sup>, Antonio Brencich<sup>b</sup>, Domenico Gallipoli<sup>b</sup>

<figure><img src="image_2.png" /></figure>

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## ARTICLE INFO

Handling Editor: Mingzhou Jin

### Keywords:

- Compressed earth materials

- Recycled concrete stabilisation

- Thermal treatment

- Microstructural testing

- Mechanical properties

## ABSTRACT

This paper presents a novel process to recycle the fine fraction of ground concrete waste as a construction binder. The fine fraction of ground concrete is rich of partly hydrated cementitious products, whose binding capacity is further enhanced by treatment at relatively moderate temperatures. The viability of the recycled binder has been demonstrated through the stabilisation of two earth building materials with kaolinitic and illitic clay fractions, respectively. At the microstructural level, X-ray diffractometry tests, mid-infrared spectroscopy tests and thermogravimetric tests have been performed to investigate the mineralogical composition and chemical bonds of the stabilising phase. At the macrostructural level, unconfined compression tests have been performed to evaluate the influence of stabilisation on the elastic modulus, peak strength and energy at failure. Results show that the addition of untreated recycled concrete to both earth types induces an average increase of 4% in apparent elastic modulus, 52% in compressive strength and 108% in failure energy relative to the unstabilised material. The addition of thermally treated recycled concrete induces an even larger increase of 30% in apparent elastic modulus, 93% in compressive strength and 187% in failure energy relative to the unstabilised earth. This is due to the higher hydration capacity of the thermally treated recycled concrete and the consequent formation of stronger inter-particle bonds. These preliminary results indicate a promising way to recycle concrete waste into earth stabilisers, which would ease difficulties associated to the disposal of demolition waste while, at the same time, improving the sustainability of current building practices.

## 1. Introduction

The building sector is responsible for 40% of global greenhouse gas emissions with over one-third of this amount being caused by the manufacture of construction materials (GlobalABC, 2021). Concrete is the most used construction material worldwide, with a yearly production well above 10 billion tons (Garside, 2023), largely contributing to greenhouse gas emissions and depletion of mineral aggregates (IPCC, 2022; UNEP, 2022). Demolished concrete also makes up 35% of all European municipal waste (Eurostat, 2014) and is commonly disposed of in landfill sites. Addressing the environmental impacts of the building sector is not a trivial issue, and it requires the implementation of multiple complementary strategies (Ahmad et al., 2022, 2023a, 2023b). One of them, for which extensive research has been conducted, is repurposing construction and demolition waster into either coarse aggregates

for use in fresh concrete (Attri et al., 2022; Revilla-Cuesta et al., 2022) or fine aggregates for constructing pavement sub-bases or surfaces (Magar et al., 2021; Salehi et al., 2021). These recycling processes can decrease carbon emissions up to 15% for fresh concrete and 50% for bituminous concrete (Jain and Singh, 2022; Xiao et al., 2022).

To date, the residual binding capacity of the cementitious matrix of concrete has not been sufficiently investigated and possibly exploited. Only a handful of studies have proposed a protocol for the thermal treatment of hardened cement pastes at temperatures between 450 °C and 800 °C to reactivate the binder hydration capacity (Wang et al., 2018; Carriço et al., 2020; Bogas et al., 2022). Nevertheless, the application of this protocol to concrete waste requires the previous separation of the cement matrix from mineral aggregates via thermo-mechanical processes, which are however more suited for recovering the mineral aggregates than the cement matrix (Tam et al., 2007; Kwon et al., 2015;

<footnote><sup>a</sup> École Nationale des Travaux Publics d'État, Lyon, France</footnote>

<footnote><sup>b</sup> Dipartimento di Ingegneria Civile, Chimica e Ambientale, Università degli Studi di Genova, Genoa, Italy</footnote>

<footnote>* Corresponding author. Università degli studi di Genova Dipartimento di Ingegneria Civile, Chimica e Ambientale Via Montallegro 1 Ufficio T-58 16145, Genova, Italy.</footnote>

<footnote>E-mail address: agostinowalter.bruno@unige.it (A.W. Bruno).</footnote>
Al-Bayati et al., 2016). To overcome these limitations, the present study proposes a novel protocol for recycling concrete waste into construction binders, whose viability is then demonstrated with reference to the stabilisation of two distinct earth building materials.

Earth materials have recently attracted the interest of civil engineers worldwide due to their low embodied carbon (Morel et al., 2001; Minke, 2013; Fernandes et al., 2019), advantageous hygrothermal properties (Soebarto, 2009; Bruno, 2016; McGregor et al., 2016; Allam et al., 2018; Fabbri et al., 2022) and good mechanical characteristics (Yamin et al., 2004; Röhlen and Ziegert, 2011; Ávila et al., 2021; Fabbri et al., 2022). Unfortunately, besides these valuable attributes, earth materials exhibit a high vulnerability to water infiltration (Bui et al., 2014), which is typically counteracted by stabilisation with conventional binders such as cement or lime (Ojo et al., 2015; Ávila et al., 2022). This solution is however unattractive because the large environmental footprint of conventional binders undermines the green credentials of the earth material (Reddy and Kumar, 2010; Arrigoni et al., 2017).

The mineralogical and mechanical tests presented in this work demonstrate, for the first time, that concrete waste can be effectively recycled into a viable and sustainable soil stabiliser, thus increasing the technical and environmental acceptance of earth building practices. The results from the present work may also pave the way for future research on the application of the proposed protocol to the production of recycled binders for fresh concrete.

## 2. Materials and methods

This section describes the physical characteristics of the two earth building materials together with the manufacture of unstabilised/stabilised samples for the subsequent performance of mineralogical and mechanical tests.

### 2.1. Sample preparation

The two earth materials were respectively provided by the Nagen and Bouisset brick factories in the region of Toulouse (France). The grain size distribution of both materials was measured by wet sieving and sedimentation tests in agreement with the norms XP P94-041 (AFNOR, 1995a) and NF EN ISO 17892-4 (AFNOR, 2018a). Plasticity tests were also performed on the fine fraction passing the 400 µm sieve to define the Atterberg limits as prescribed by the norm NF EN ISO 17892-12 (AFNOR, 2018b). The clay activity A (i.e. the ratio between the plasticity index and the soil fraction smaller than 2 µm) was calculated as 0.79 for the Nagen earth and 0.60 for the Bouisset earth, which classifies the former material as normally active and the latter one as inactive (Skempton, 1953). The specific gravity Gs (i.e. the ratio between particles density and water density) was instead measured as the average of three pycnometer tests according to the norm NF EN ISO 17892-3 (AFNOR, 2015) and is equal to 2.66 for the Nagen earth and 2.65 for the Bouisset earth. Table 1 summarises the main properties of both materials.

Broken concrete blocks were obtained from an ongoing experimental campaign at the structural laboratory of the University of Genoa (Italy). The blocks were smashed into two granulometric fractions, respectively larger and smaller than 5 mm, using an electrical jaw-crusher machine commercialised by the Italian company Crunchy. The larger grains were then reinserted into the crushing machine to produce the same two granulometric fractions. The fine fractions from both passages were then blended and the resulting grain size distribution was measured according to the norm NF EN ISO 17892-4 (AFNOR, 2018a) as shown in Fig. 1.

To maximise mortar content, only the fraction smaller than 300 µm was retained for further work (Hansen, 1992; Padmini et al., 2009; Florea and Brouwers, 2013; Wang et al., 2018) while the excess material was repurposed as construction aggregate (Etxeberria et al., 2007). Hereafter, the term “untreated recycled concrete” (or URC) designates the fraction smaller than 300 µm dried at 105 °C for at least 24 h

(AFNOR, 1995b) while the term “treated recycled concrete” (or TRC) designates the same fraction subjected to thermal treatment for reactivating the latent hydration potential.

The thermal reactivation process consisted in exposing the recycled concrete inside a furnace to a temperature ramp of 7 °C per minute from room conditions to 450 °C (Wang et al., 2018). The final temperature was maintained for a period of 8 h before the heating resistance was switched off and the material was left to cool inside the furnace back to room temperature. A temperature of 450 °C is high enough to convert the calcium hydroxide (Ca(OH)₂) of the cement matrix into calcium oxide (CaO), which reacts with water (H₂O) to form calcium silicate hydrates (C-S-H) and with the atmospheric carbon dioxide (CO₂) to form calcium carbonate (CaCO₃) as demonstrated by Song et al. (2018). A temperature of 450 °C is also small enough to avoid the decomposition of existing CaCO₃ into CaO and CO₂, thus limiting carbon emissions. The specific gravities of both untreated and treated recycled concrete were respectively measured as 2.67 and 2.69 by means of pycnometer tests in compliance with the norm NF EN ISO 17892-3 (AFNOR, 2015). Fig. 2 shows samples of the two earth materials, i.e. Nagen (Fig. 2a) and Bouisset (Fig. 2b), as well as the untreated recycled concrete after sieving to 300 µm.

The minimum embodied energy increase per unit mass ΔE<sub>e,min</sub> of recycled concrete due to thermal reactivation is here calculated as the integral of the temperature-dependent specific heat of the material, as determined by Cerny et al. (2000), between the room temperature of 25 °C and the target temperature of 450 °C. This calculation leads to a value of ΔE<sub>e,min</sub> of about 0.4 MJ kg<sup>-1</sup>, which is at least 2.5 times lower than that consumed by the thermal treatments proposed by Kwon et al. (2015), who fired recycled concrete at temperatures ranging from 900 °C to 1450 °C. Note, however, that ΔE<sub>e,min</sub> is a lower bound of the actual embodied energy as it assumes an efficiency of the heating system equal to one. A more realistic estimate would imply consideration of the thermal inertia of the firing system as well as possible heat losses, which is outside the scope of the present work and constitutes matter for future research.

The Nagen earth (N) and Bouisset earth (B) were either left unstabilised (U) or dry mixed with a variable percentage (i.e. 10% or 20%) of recycled concrete (i.e. URC or TRC) as shown in Table 2. The materials were then moistened at different water contents (Table 2) before being statically compacted as discussed later. For the unstabilised samples, the compaction water content w<sub>soil</sub> coincided with the Proctor optimum as determined by Bruno et al. (2017a) for Nagen earth and by Cuccurullo et al. (2021) for Bouisset earth. Instead, for the URC and TRC stabilised samples, the compaction water content w<sub>stab</sub> coincided with the weighted average of the water content of the unstabilised earth w<sub>soil</sub> (i.e. the Proctor optimum) and the water content of the recycled concrete w<sub>cem</sub> (i.e. the water/cement ratio) as:

$$w_{stab} = \frac{w_{soil} \times m_{soil} + w_{cem} \times c \times m_{stab}}{m_{soil} + m_{stab}} \quad (2)$$

where m<sub>soil</sub> is the earth mass and c is the cement fraction of the recycled concrete mass m<sub>stab</sub>. In Equation (2), a cement water content w<sub>cem</sub> = 55% was chosen according to Wang et al. (2018) while a cement fraction c = 59% was selected following the work of Chekroun (2008).

All samples were statically compacted inside a cylindrical mould having an inner diameter of 54 mm to a target dry density of 1900 kg/m³ with a slenderness ratio (i.e. the ratio between sample height and diameter) of two. Compaction took place immediately after mixing with water to avoid hydration of the binder and the consequent formation of intergranular bonds prior to moulding. After extrusion, the sample mass was measured via a balance with a resolution of 0.1 g while the sample volume was calculated from the average of three diameter and height measurements. The three diameter measurements were taken at different sample heights while three height measurements were taken at different axial angles via a calliper with a resolution of 0.01 mm.
<table><caption>**Table 1**
Main properties of Nagen and Bouisset earth.</caption><thead><tr><th>Grain size distribution</th><th>NAGEN</th><th>BOUISSET</th></tr></thead><tbody><tr><td>Gravel</td><td>0.4%</td><td>0.0%</td></tr><tr><td>Sand</td><td>0.063–2 mm</td><td>26.6%</td></tr><tr><td>Silt</td><td>0.002–0.063 mm</td><td>41.9%</td></tr><tr><td>Clay</td><td>&lt;0.002 mm</td><td>31.5%</td></tr><tr><td><strong>Plasticity properties</strong></td><td></td><td></td></tr><tr><td>Liquid limit, w<sub>l</sub></td><td>33.0%</td><td>35.5%</td></tr><tr><td>Plastic limit, w<sub>p</sub></td><td>20.1%</td><td>16.7%</td></tr><tr><td>Plasticity index, I<sub>p</sub></td><td>12.9%</td><td>18.8%</td></tr><tr><td>Clay activity, A (–)</td><td>0.79</td><td>0.60</td></tr><tr><td><strong>Specific gravity of soil grains</strong></td><td></td><td></td></tr><tr><td>Specific gravity, G<sub>s</sub> (–)</td><td>2.66</td><td>2.65</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Grain size distribution of finely crushed concrete.</figcaption></figure>

Table 3 summarises the measured average post-compaction properties of two samples for each material type together with the corresponding standard deviations in square brackets, thus confirming that the average dry density of all samples is about 1900 kg/m³. In Table 3, the particle density $\rho_s$ was determined as the mass-weighted average of the particle densities of Nagen or Bouisset earth and the untreated or treated recycled concrete. The bulk density $\rho_b$ was instead calculated as the ratio between the measured sample mass and volume. Finally, the dry density $\rho_d$, the void ratio $e$ and the porosity $n$ were calculated according to the following relationships:

$$ \rho_d = \frac{\rho_b}{1 + w} \qquad (3) $$

$$ e = \frac{\rho_s}{\rho_d} - 1 \qquad (4) $$

$$ n = \frac{e}{1 + e} \qquad (5) $$

Note that the water content $w$ in Equation (3) coincides with either $w_{soil}$ or $w_{stab}$ depending on the value shown in Table 2. After compaction,

<table><caption>**Table 2**
Dry masses and compaction water contents of all samples.</caption><thead><tr><th>Sample Name</th><th>Nagen soil (g)</th><th>Bouisset soil (g)</th><th>URC (g)</th><th>TRC (g)</th><th>w<sub>soil</sub> (%)</th><th>w<sub>stab</sub> (%)</th></tr></thead><tbody><tr><td>N–U</td><td>470.0</td><td>–</td><td>–</td><td>–</td><td>11.7</td><td>–</td></tr><tr><td>N-10% TRC</td><td>427.3</td><td>–</td><td>–</td><td>42.7</td><td>–</td><td>13.6</td></tr><tr><td>N-20% TRC</td><td>391.7</td><td>–</td><td>–</td><td>78.3</td><td>–</td><td>15.2</td></tr><tr><td>N-20% URC</td><td>391.7</td><td>–</td><td>78.3</td><td>–</td><td>–</td><td>15.2</td></tr><tr><td>B–U</td><td>–</td><td>470.0</td><td>–</td><td>–</td><td>12.0</td><td>–</td></tr><tr><td>B-10%TRC</td><td>–</td><td>427.3</td><td>–</td><td>42.7</td><td>–</td><td>13.9</td></tr><tr><td>B-20%TRC</td><td>–</td><td>391.7</td><td>–</td><td>78.3</td><td>–</td><td>15.4</td></tr><tr><td>B-20% URC</td><td>–</td><td>391.7</td><td>78.3</td><td>–</td><td>–</td><td>15.4</td></tr></tbody></table>

all samples were left to cure for seven days in the laboratory atmosphere at a temperature $T = 26 \,^{\circ}\text{C} \pm 2 \,^{\circ}\text{C}$ and a relative humidity $RH = 70\% \pm 10\%$ to ensure hydration of the binder (Al Haffar et al., 2021). Fig. 3 shows that the samples mass decreased over time and attained a constant level after 2–5 days, which indicates equalisation with the surrounding environment. Regardless of the earth type, the TRC stabilised samples exhibited a faster mass decrease compared with the URC stabilised samples. This may be due to differences of pore structure and possibly indicates a better moisture buffering capacity of TRC stabilised earth compared to URC stabilised one, though this conclusion should be supported by further investigation.

Table 4 summarises the average post-curing properties of two samples for each material type together with the corresponding standard deviations in square brackets. It is here assumed that the decrease of water content during curing coincides entirely with the amount of evaporation measured in Fig. 3 and that the water consumed by hydration of the binder is comparatively negligible. Unstabilised samples exhibit slightly lower water contents than stabilised ones, which may be partly due to the smaller water content at compaction. This phenomenon is more evident for Bouisset earth because of the predominantly kaolinitic clay fraction, which is less hydrophilic than the illitic clay fraction of Nagen earth as shown by Bruno et al. (2017b) and Cuccurullo et al. (2020). Similarly, the TRC stabilised samples exhibit slightly lower water contents than the URC stabilised ones, which may suggest the presence of an additional class of nanopores in the former material. Inspection of Table 4 also indicates that samples underwent different amount of shrinkage depending on the material type. The average dry density of all samples slightly increased from about 1900.0 kg/m³ after compaction to about 1950.0 kg/m³ after curing.

## 2.2. X-ray diffraction tests

To investigate material mineralogy, X-ray Diffraction (XRD) tests were performed on specimens of both untreated and treated recycled concrete as well as on fragments of the samples listed in Table 2. XRD tests were conducted by means of a Cu X-ray source emitting radiation at 1.54 Å wavelength and a generator operating at 30 kV and 10 mA. The

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Nagen earth (a), Bouisset earth (b) and untreated recycled concrete URC (c).</figcaption></figure>
<caption>**Table 3**</caption>

Samples properties after compaction ($\rho_s$: particle density; $\rho_b$: bulk density; $\rho_d$: dry density; $e$: void ratio; $n$: porosity) with corresponding standard deviations is square brackets.

<table><thead><tr><th>Samples</th><th>ρ<sub>s</sub> (kg/m<sup>3</sup>)</th><th>ρ<sub>b</sub> (kg/m<sup>3</sup>)</th><th>ρ<sub>d</sub> (kg/m<sup>3</sup>)</th><th>e (-)</th><th>n (-)</th></tr></thead><tbody><tr><td>N-U</td><td>2664.0</td><td>2135.8<br>[8.1]</td><td>1910.2<br>[7.3]</td><td>0.395<br>[0.005]</td><td>0.283<br>[0.003]</td></tr><tr><td>N-10%</td><td>2666.1</td><td>2159.4<br>[12.5]</td><td>1906.7<br>[9.2]</td><td>0.399<br>[0.007]</td><td>0.285<br>[0.004]</td></tr><tr><td>TRC</td><td>2667.8</td><td>2165.4<br>[8.1]</td><td>1900.7<br>[4.3]</td><td>0.402<br>[0.003]</td><td>0.287<br>[0.002]</td></tr><tr><td>N-20%</td><td>2665.0</td><td>2146.4<br>[8.0]</td><td>1876.7<br>[6.4]</td><td>0.421<br>[0.005]</td><td>0.296<br>[0.002]</td></tr><tr><td>URC</td><td>2650.0</td><td>2136.3<br>[11.5]</td><td>1912.5<br>[10.6]</td><td>0.386<br>[0.008]</td><td>0.278<br>[0.004]</td></tr><tr><td>B-U</td><td>2653.4</td><td>2180.0<br>[3.3]</td><td>1922.6<br>[1.6]</td><td>0.382<br>[0.002]</td><td>0.276<br>[0.001]</td></tr><tr><td>B-10%</td><td>2656.2</td><td>2168.6<br>[4.6]</td><td>1894.9<br>[2.9]</td><td>0.400<br>[0.002]</td><td>0.286<br>[0.001]</td></tr><tr><td>TRC</td><td>2653.3</td><td>2161.5<br>[0.2]</td><td>1876.9<br>[1.6]</td><td>0.414<br>[0.001]</td><td>0.293<br>[0.001]</td></tr></tbody></table>

crystalline phases of the materials were detected by simultaneously rotating both the X-ray source and receptor with a total angle to the horizontal of $2\theta$, where $\theta$ is the angle between the X-ray source (or receptor) and the horizontal. During XRD tests, the first portion of each spectrum was measured by varying $2\theta$ from 3° to 16° and by placing a knife between the source and the receptor, thus avoiding that the low rotation angle would fictitiously increase the measured intensity. The second portion of each spectrum was then measured by varying $2\theta$ from 16° to 90° without placing the knife between the source and the receptor. For each sample, the two portions of the spectrum were then collated by adopting the same baseline, i.e. by shifting the whole second portion of the spectrum to impose the coincidence of the intensities measured at $2\theta = 16°$ from both testing configurations (i.e. with the knife for $3° \le 2\theta \le 16°$ and without the knife for $16° \le 2\theta \le 90°$). Results from XRD tests are presented in the following section.

## 2.3. Mid-infrared spectroscopy tests

To investigate the nature of the chemical bonds within the crystalline structure, mid-infrared spectroscopy tests were performed on specimens of both untreated and treated recycled concrete as well as on fragments of the samples listed in Table 2. Specimens of about 8 mg were carefully mixed with about 700 mg of potassium bromide (KBr) in an agate mortar and then compacted into a circular tablet with a diameter of 20 mm. The material spectra were subsequently recorded by means of a FT-IR ThermoNicolet Nexus instrument (100 scans, DTGS detector, background in air, OMINC™ software). Results from the mid-infrared spectroscopy tests (i.e. 4000-400 cm⁻¹) are presented in the following section.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Mass variation of stabilised and unstabilised Nagen (a) and Bouisset (b) earth samples during curing.</figcaption></figure>

<caption>**Table 4**</caption>

Samples properties after curing ($\rho_s$: particle density; $\rho_b$: bulk density; $\rho_d$: dry density; $e$: void ratio; $n$: porosity) with corresponding standard deviations is square brackets.

<table><thead><tr><th>Samples</th><th>ρ<sub>s</sub> (kg/m<sup>3</sup>)</th><th>ρ<sub>b</sub> (kg/m<sup>3</sup>)</th><th>ρ<sub>d</sub> (kg/m<sup>3</sup>)</th><th>w (%)</th><th>e (-)</th><th>n (-)</th></tr></thead><tbody><tr><td>N-U</td><td>2664.00</td><td>2037.50<br>[1.35]</td><td>1964.90<br>[1.96]</td><td>3.7<br>[0.04]</td><td>0.356<br>[0.001]</td><td>0.262<br>[0.001]</td></tr><tr><td>N-10%</td><td>2666.09</td><td>2031.47<br>[3.14]</td><td>1961.64<br>[2.09]</td><td>3.6<br>[0.05]</td><td>0.388<br>[0.014]</td><td>0.279<br>[0.007]</td></tr><tr><td>TRC</td><td>2667.83</td><td>2011.79<br>[3.27]</td><td>1937.40<br>[3.90]</td><td>3.8<br>[0.04]</td><td>0.360<br>[0.001]</td><td>0.265<br>[0.001]</td></tr><tr><td>N-20%</td><td>2665.00</td><td>1996.35<br>[19.79]</td><td>1921.45<br>[18.68]</td><td>3.9<br>[0.02]</td><td>0.376<br>[0.003]</td><td>0.273<br>[0.002]</td></tr><tr><td>URC</td><td>2650.00</td><td>2028.11<br>[10.70]</td><td>1990.63<br>[10.61]</td><td>1.9<br>[0.01]</td><td>0.331<br>[0.007]</td><td>0.249<br>[0.004]</td></tr><tr><td>B-U</td><td>2653.36</td><td>1999.26<br>[11.35]</td><td>1956.85<br>[9.77]</td><td>2.2<br>[0.07]</td><td>0.380<br>[0.006]</td><td>0.276<br>[0.003]</td></tr><tr><td>B-10%</td><td>2656.17</td><td>1991.84<br>[9.05]</td><td>1944.82<br>[8.45]</td><td>2.4<br>[0.02]</td><td>0.357<br>[0.007]</td><td>0.263<br>[0.004]</td></tr><tr><td>TRC</td><td>2653.33</td><td>1970.99<br>[9.92]</td><td>1922.49<br>[8.83]</td><td>2.5<br>[0.05]</td><td>0.364<br>[0.006]</td><td>0.267<br>[0.003]</td></tr><tr><td>B-20%</td><td>2653.33</td><td>1970.99<br>[9.92]</td><td>1922.49<br>[8.83]</td><td>2.5<br>[0.05]</td><td>0.364<br>[0.006]</td><td>0.267<br>[0.003]</td></tr><tr><td>URC</td><td>2653.33</td><td>1970.99<br>[9.92]</td><td>1922.49<br>[8.83]</td><td>2.5<br>[0.05]</td><td>0.364<br>[0.006]</td><td>0.267<br>[0.003]</td></tr></tbody></table>

## 2.4. Thermogravimetric tests

Thermogravimetric (TG) tests were carried out to determine the presence and concentration of hydrated and dehydrated components inside the TRC and URC stabilised earth. This measure allows a better understanding of the hydration mechanisms inside the recycled concrete and their correlation with the thermal treatment described earlier. All TG tests were conducted by means of a TG-DSC Netzsch Gerätebau STA 409, produced by the German company Netzsch, and equipped with a balance with a sensitivity of 0.1 mg and a Netzsch 410 furnace temperature controller system. Specimens of approximately 200 mg were

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Stress-strain curve measured on N-20%TRC sample and graphical determination of peak compressive strength, apparent Young's modulus and compression energy at failure.</figcaption></figure>

<figure><img src="image_7.png" /></figure>
statically compacted to a pressure of 125 MPa before being placed inside an alumina crucible with a volume of 100 mm³ and an inner diameter of 6 mm. The crucible was slightly larger than the diameter of the compacted specimen, which avoided spurious boundary effects caused by possible contact. The crucible was then positioned on the holder of the TG apparatus in pair with another crucible containing a mass of 110 mg of kaolin that served as reference. The TG apparatus was then sealed and temperature was raised in static air from about 25 °C to 1000 °C with a nominal rate of 10 °C per minute while the sample mass was simultaneously monitored. Results from the thermogravimetric tests are presented in the following section.

## 2.5. *Mechanical tests*

Unconfined compression tests were performed to assess the influence of stabilisation on the stiffness, strength and compression energy at failure of the compacted earth. Tests were performed by means of a MetroCom Engineering S.P.A. press with a constant displacement rate of 0.2 mm/s to measure the post-peak region of the stress-strain curve. A ball joint ensured that the upper press plate could rotate and adjust to the top sample face, thus minimising spurious misalignment effects. The compressive stress was calculated as the ratio between the applied force and the initial cross-sectional area of the sample while the axial strain was determined as the variation of sample height with respect to the initial value.

The compressive strength was determined as the peak of the stress-

<figure><img src="image_8.png" /><figcaption>Fig. 5. N–U sample after compaction with mould and pistons (a), during unconfined compression (b) and after failure (c).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. X-ray diffractometry spectra of URC and TRC (a). Isolated normalised peaks of calcium hydroxide (b) and calcium carbonate (c).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7. X-ray diffractometry spectra of unstabilised and stabilised Nagen earth samples (a). Isolated normalised peaks of silica (b) and calcium carbonate (c).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. X-ray diffractometry spectra of unstabilised and stabilised Bouisset earth samples (a). Isolated normalised peaks of silica (b) and calcium carbonate (c).</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** FT-IR spectra of URC and TRC (a) and changes of peak absorbance of TRC relative to URC (b).</figcaption></figure>

strain curve whereas the apparent Young's modulus was measured as the slope of the best-fit line over the stress range between 20% and 30% of peak strength. The lower limit of 20% was chosen to eliminate bedding-in effects due to the imperfect contact between the sample ends and the press plates while the upper limit of 30% was chosen to prevent occurrence of relatively large plastic strains as recommended by Kouakou and Morel (2009). The failure energy was determined as the area under the stress-strain curve up to peak strength. As an example, Fig. 4 reports the stress-strain curve of one of the two N-20%TRC samples together with the adopted graphical constructions to measure peak strength, apparent Young's modulus and failure energy. Fig. 5 shows the unstabilised sample N-U after compaction with mould and pistons (Fig. 5a), during unconfined compression (Fig. 5b) and after failure (Fig. 5c).

# 3. Results and discussion

## 3.1. X-ray diffraction analyses

X-ray diffraction tests were performed on both TRC and URC specimens to assess the effect of thermal reactivation on the mineralogical composition of the recycled concrete. Similar tests were also performed on unstabilised, URC stabilised and TRC stabilised earth samples to investigate the influence of the binder content/type on mineralogy.

Fig. 6a shows the raw spectral data of TRC and URC specimens while Fig. 6b and c shows the corresponding peak-normalised values of calcium hydroxide and calcium carbonate, respectively. Inspection of Fig. 6b confirms that thermal treatment at 450 °C induces a significant decomposition of Ca(OH)₂ into CaO and H₂O. This renders the TRC more prone to hydration than the URC, thus inducing stronger calcium carbonate bonds and better mechanical properties as confirmed by mechanical tests. Fig. 6c illustrates negligible changes of CaCO₃ after thermal treatment, thus indicating that the moderate firing temperature adopted in this work avoids decomposition of existing CaCO₃, and the

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** FT-IR spectra of unstabilised and stabilised Nagen earth (a) and changes of peak absorbance of stabilised samples relative to unstabilised ones (b).</figcaption></figure>

consequent undesirable emission of CO₂. This is further confirmed by mid-infrared spectroscopy and thermogravimetric tests (see Sections 3.2 and 3.3, respectively) in agreement with existing literature on the thermal treatment of recycled cement paste (Alarcon-Ruiz et al., 2005; Chromá et al., 2011; Bogas et al., 2019; Mostazid et al., 2022).

Figs. 7 and 8 present the results from similar tests on Nagen and Bouisset earth samples, respectively. In the Nagen case, both TRC and URC stabilisation increase the peaks of silica (Fig. 7b) and calcium carbonate (Fig. 7c), which is the first product of C-S-H carbonation, as also observed by Black et al. (2007). Instead, in the Bouisset case, no notable variations are observed for the silica peaks (Fig. 8b) while most (but not all) calcium carbonate peaks increase after stabilisation (Fig. 8c). This difference between Nagen and Bouisset samples could be explained by the larger presence of quartz crystals and limestone in the latter material, which interfere with the intensity of silica and calcium carbonate peaks. Figs. 7a and 8a also show the formation of hydrated products with irregular and complex crystalline structures within the stabilised samples. These products, which could not be identified via existing X-ray spectra databases, exhibit intensity peaks for rotation angles $2\theta$ between 3° and 18°.

## 3.2. Mid-infrared spectroscopy analyses

Fig. 9a shows the mid-infrared spectra of both TRC and URC indicating that the main bands are due to the vibrational modes of the hydration water (broad band in the range 3600–3300 cm⁻¹ and shoulder at
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** FT-IR spectra of unstabilised and stabilised Bouisset earth (a) and changes of peak absorbance of stabilised samples relative to unstabilised ones (b).</figcaption></figure>

1650 cm⁻¹), partially superimposed to adsorbed water vapour on the hygroscopic KBr matrix, and calcium carbonate (1450 and 874 cm⁻¹). The assignation of other significant features, such as the vibrational modes of OH and Si–O, is also reported in Fig. 9a and discussed by Farcas and Touzé (2001). Fig. 9b presents instead the changes of peak absorbance of TRC relative to URC, which confirms that the treatment at 450 °C decomposes a significant part of Ca(OH)₂ while slightly increasing the amount of CaCO₃ in agreement with the X-ray diffrac-tometry data of Fig. 6 and the results of Song et al. (2018). Assuming a constant amount of adsorbed water on KBr, the TRC exhibits a smaller

content of hydration water and therefore a stronger hydration capacity compared to the URC.

Figs. 10 and 11 show the results from similar tests on Nagen and Bouisset earth samples, respectively, including the changes of peak absorbance of the stabilised specimens relative to the unstabilised ones. The results indicate that both TRC and URC stabilisations induce the formation of various hydrated and carbonated products (i.e. Ca(OH)₂, CaCO₃, C–S–H and Ettringite) as also detected from XRD tests. In particular, regardless of the earth type, the TRC stabilised samples feature more hydrated products (e.g. C–S–H) compared with the URC stabilised ones at the same binder content. This is because the thermal treatment decomposes the Ca(OH)₂ inside the recycled concrete as discussed in relation to the X-ray diffractograms and, therefore, increases the hydration potential of the binder. For TRC stabilised samples, the absorbance related to hydrated and carbonated products grows with increasing stabiliser content.

Inspection of Figs. 10 and 11 confirms that, in general, the stabilised samples contain more hydration water than unstabilised ones. However, the hydrophilic nature of the illitic clay fraction of Nagen earth (Table 4) complicates the comparison of hydration water and Ca(OH)₂ due to the overlap of bands in the OH stretching region groups. Moreover, the larger presence of both CaCO₃ and SiO₂ inside Nagen earth compared with Bouisset earth leads to lower relative absorbance peaks of CaCO₃ and C–S–H, whose diagnostic bands are the Si–O vibrational modes in the wavenumber range 1200–850 cm⁻¹.

### 3.3. Thermogravimetric analyses

Fig. 12 shows the mass loss during thermogravimetric tests on both TRC and URC samples (Fig. 12a) together with the corresponding differential curve (Fig. 12b). Inspection of Fig. 12a indicates that the URC mass decreases about 5% when the temperature increases from room conditions to 450 °C whereas the TRC mass decreases considerably less, i.e. about 1.5%, over the same temperature range. This is due to the evaporation of adsorbed water, which occurred during the TG test in the former case and during the preliminary thermal treatment in the latter. Interestingly, the URC exhibits a peak of the differential curve at 450 °C, which is attributed to the decomposition of Ca(OH)₂. A similar peak is not visible for the TRC because, in this case, the thermal treatment has already decomposed most of the available Ca(OH)₂. The largest mass loss for both materials occurs at temperatures between 700 °C and 800 °C because of the decomposition of CaCO₃. This is more evident for TRC because the previous decomposition of Ca(OH)₂ leads to the formation of CaCO₃ as described by Wang et al. (2018) and Song et al. (2018).

Fig. 13 shows results from thermogravimetric analyses performed on unstabilised, TRC stabilised and URC stabilised samples of both Nagen (Fig. 13a and c) and Bouisset (Fig. 13b and d) earth. The unstabilised samples of both earth materials exhibit significant mass losses between

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Mass loss of URC and TRC samples (a) and corresponding differential curve (b).</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Thermogravimetric analyses of unstabilised/stabilised Nagen samples (a) with corresponding differential mass loss (c) and unstabilised/stabilised Bouisset samples (b) with corresponding differential mass loss (d).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Fig. 14.** Stress-strain curves from unconfined compression tests on Nagen (a) and Bouisset (b) earth samples (two test repeats for each material type).</figcaption></figure>

room temperature and 100 °C, which is caused by the evaporation of free water. This is less evident in the stabilised samples due to the additional presence of hydration water, which evaporates at higher temperatures between 250 °C and 300 °C, as confirmed by the corresponding differential mass loss peaks in Fig. 13c and d. A peak of differential mass loss is registered starting around 450 °C corresponding to the decomposition of Ca(OH)₂ as well as to the further evaporation of both adsorbed water in clay minerals and hydration water in C–S–H gel. Another peak of differential mass loss, attributed to the decomposition of CaCO₃ and to further loss of hydration water in C–S–H gel, can be observed only for the stabilised samples at temperatures between 700 °C and 800 °C.

### 3.4. Mechanical analyses

Fig. 14 shows the stress-strain curves measured during unconfined compression tests on both Nagen (Fig. 14a) and Bouisset (Fig. 14b) earth samples. All samples exhibit a brittle failure with the stabilised ones

showing a sharper post-peak decrease of strength than the unstabilised ones. The more abrupt failure of the stabilised samples may be caused by the relatively large presence of hydration products that increase strength but also render the material more fragile.

Fig. 15 presents the mean and standard deviation of the apparent Young's modulus, peak compressive strength and compression energy at failure calculated from two test repeats for each material type. It also includes, in square brackets, the percentage variation of each mean value of the stabilised material respect to the unstabilised one. Inspection of Fig. 15 indicates that unstabilised Nagen earth samples exhibit better mechanical properties than unstabilised Bouisset ones. This is due to the stronger capillary bonds generated by the higher specific surface of the illitic clay in Nagen earth compared to the kaolinitic clay in Bouisset earth. As for the stabilised samples, the addition of either TRC or URC increases the strength and energy at failure without significantly affecting the apparent Young's modulus, probably because of the relatively flexible inter-particle cementation by C–S–H gel bridges. Fig. 15 also shows that TRC stabilisation induces better mechanical properties
<figure><img src="image_18.png" /><figcaption>Fig. 15. Results from mechanical tests on Nagen/Bouisset earth samples: apparent Young's modulus (a, b), compression energy at failure (c, d) and peak compressive strength (e, f).</figcaption></figure>

than URC stabilisation because of the larger presence of hydrated products as demonstrated by mineralogical tests and that the mechanical properties improve as the stabiliser content grows. Finally, the values of Young's modulus exhibit a larger dispersion compared with those of peak strength and failure energy, which may be partly explained by the adopted measuring procedure. Overall, the relatively low values of standard deviation indicate a good repeatability of test results.

## 4. Conclusions

This paper has presented an innovative procedure for the valorisation of concrete waste by exploiting the residual hydration capacity of the cement matrix to produce construction binders. The recycled binder was obtained from the fine fraction of crushed concrete, which was either left untreated or thermally treated to enhance hydration capacity. The untreated and treated powders were mineralogically characterised to assess their respective residual hydration potential before being used

for the stabilisation of kaolinitic and illitic earth materials. Mineralogical and mechanical tests were performed on both unstabilised and stabilised earth samples to compare the measured values of stiffness, strength and ductility. The key results of the investigation are summarised as follows.

* The fraction of crushed concrete smaller than 300 µm can be recycled as a construction binder due to its residual hydration potential.

* Thermal treatment of the recycled binder at 450 °C decomposes the Ca(OH)₂ into CaO, which increases hydration capacity. This moderate thermal treatment does not affect existing CaCO₃ as shown by both X-Ray diffraction and mid-infrared spectroscopy tests.

* Thermally treated material embodies significantly less energy than that consumed by other thermal treatments proposed in the literature.

* Carbonation of the CaO generated from thermal treatment promotes the capture of atmospheric CO₂ via mineralisation of CaCO₃.
* The application of the recycled binder to the stabilisation of earth building materials produces a significant improvement of mechanical performance.

* The improvement of mechanical properties is larger for the thermally treated recycled binder compared to the untreated one and for the kaolinitic earth compared to the illitic one.

Further investigation should be directed to assessing the durability and environmental performance of the proposed earth stabilisation methodology.

## CRediT authorship contribution statement

**Vincent Rozzi:** Conceptualization, Methodology, Investigation, Data curation, Visualization, Writing – original draft, Writing – review & editing.
**Agostino Walter Bruno:** Conceptualization, Methodology, Investigation, Data curation, Supervision, Writing – original draft, Writing – review & editing.
**Antonin Fabbri:** Methodology, Investigation, Supervision, Writing – review & editing.
**Antonio Barbucci:** Investigation, Resources, Writing – review & editing.
**Elisabetta Finocchio:** Investigation, Resources, Writing – review & editing.
**Alberto Lagazzo:** Investigation, Resources, Writing – review & editing.
**Antonio Brencich:** Investigation, Resources, Writing – review & editing.
**Domenico Gallipoli:** Methodology, Investigation, Supervision, Writing – review & editing.

## Declaration of competing interest

I wish to confirm that there are no conflicts of interest associated with this publication and there has been no financial support for this work that could have influenced its outcome.

## Data availability

No data was used for the research described in the article.

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