# Reactivity of Recycled Aggregates Used for Pavement Base: From Field to Laboratory

Jean David Lau Hiu Hoong, Yunlu Hou, Philippe Turcry, Pierre-Yves Mahieux, Hakim Hamdoun, Ouali Amiri, Jérôme Lux, Abdelkarim Aït-Mokhtar

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## ► To cite this version:

Jean David Lau Hiu Hoong, Yunlu Hou, Philippe Turcry, Pierre-Yves Mahieux, Hakim Hamdoun, et al.. Reactivity of Recycled Aggregates Used for Pavement Base: From Field to Laboratory. Journal of Materials in Civil Engineering, 2021, 33 (6), 10.1061/(ASCE)MT.1943-5533.0003661. hal-03797121

HAL Id: hal-03797121

https://hal.science/hal-03797121v1

Submitted on 22 Nov 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Reactivity of Recycled Aggregates Used for Pavement Base: From Field to Laboratory

Jean David Lau Hiu Hoong¹; Yunlu Hou²; Philippe Turcry³; Pierre-Yves Mahieux⁴; Hakim Hamdoun⁵; Ouali Amiri⁶; Jérôme Lux⁷; and Abdelkarim Aït-Mokhtar⁸

**Abstract:** The aim of this study is to investigate the cause of the self cementing properties of recycled aggregates used for pavement base. Clay bricks were added to the recycled aggregates to study the pozzolanic reactions. The evolution of the mechanical performances of the materials was investigated in the laboratory (California Bearing Ratio) and in the field (plate test). Both tests showed that initially, the mechanical performances of recycled aggregates are equivalent or even lower than those of natural aggregates. However, recycled aggregates become much better than natural aggregates with time. Therefore, setting tests, microcalorimetric analysis, and two pozzolanicity tests (pH evolution and consumption of Ca²⁺ ions) were carried out in a laboratory in order to explain the observations made on the test beds. The microcalorimetric analysis showed that there is only a slight hydraulicity due to residual anhydrous cement content in recycled aggregates. However, the other tests highlight that the presence of clay bricks particles in recycled aggregates results in pozzolanic reactions that can explain the time evolution of mechanical performances. This study suggests that recycled aggregates have a high added value when they are used as pavement base.

**Author keywords:** Recycled aggregates; Natural aggregates; Chemical and mechanical properties; Compaction; Pozzolanicity.

## Introduction

In the context of sustainable development, Europe is encouraging its member states to recycle 70% of their construction and demo lition waste by 2020 through the Horizon 2020 program (Official Journal of the European Union 2008). This measure is significant:

in France, for example, the building industry is the largest consumer of natural materials of mineral origin, and each year 400 million tons of natural aggregates (NAs) are extracted from the Earth's crust for construction purposes (buildings, public infrastructures, and other civil engineering projects). Meanwhile, 260 million tons of waste are produced per annum, accounting for 70% of total waste production (Ministry of Environment, Energy and Sea 2017). Although waste recovery and recycling are now possible through the reuse of road materials directly on site (38% of construction and demolition waste), the refurbishment of quarries at the end of production (19% of construction and demolition waste), and the recycling as recycled aggregates on specialized platforms (8% of construction and demolition waste), too much material still ends up in specialized storage facilities.

Recycled aggregates (RAs) are complex materials obtained by crushing mainly demolition concrete (Lau Hiu Hoong et al. 2020) that have been produced for many years. However, as underlined by Denot and Crosnier (2009), contracting authorities are sometimes reluctant to use them. There is not much feedback on a national scale on the use of alternative materials for roadworks. François et al. (2009) have published the results of a survey based on 17 French experimental sites. It appears that roads that integrate RAs are generally low traffic, which limits their representativeness. Further more, the monitoring period after construction is too short for deriving relevant insights. However, long term monitoring is a relevant tool to better understand the behavior of roads built with RAs, as shown for instance by Liu et al. (2020a, b).

In 2011, Hornych completed a state of knowledge on recycling practices in various European countries (Hornych 2011). Among the studied countries, the case of Sweden is of special interest: RAs can be intended for Swedish road foundations, depending on the strength of the concrete from which they were made. They are assigned a modulus of 450 MPa for design. In the Netherlands, in situ studies on test sites were carried out showing that the modulus was found to be between 600 and 800 MPa in the short term (depending on the proportion of concrete in the RAs) but can reach

<footnote>1Ph.D. Candidate, Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR CNRS 7356, La Rochelle Université, Ave. Michel Crépeau, La Rochelle, Nouvelle-Aquitaine 17000, France. ORCID: https://orcid.org/0000-0001-9086-7297</footnote>

<footnote>2Ph.D. Candidate, Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR CNRS 7356, La Rochelle Université, Ave. Michel Crépeau, La Rochelle, Nouvelle-Aquitaine 17000, France. ORCID: https://orcid.org/0000-0003-1796-5143</footnote>

<footnote>3Associate Professor, Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR CNRS 7356, La Rochelle Université, Ave. Michel Crépeau, La Rochelle, Nouvelle-Aquitaine 17000, France.</footnote>

<footnote>4Associate Professor, Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR CNRS 7356, La Rochelle Université, Ave. Michel Crépeau, La Rochelle, Nouvelle-Aquitaine 17000, France (corresponding author). Email: pierre-yves.mahieux@univ-lr.fr</footnote>

<footnote>5Postdoctoral Student, Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR CNRS 7356, La Rochelle Université, Ave. Michel Crépeau, La Rochelle, Nouvelle-Aquitaine 17000, France.</footnote>

<footnote>6Professor, Institut de Recherche en Génie Civil et Mécanique (GeM), UMR CNRS 6183, Université de Nantes, 2 Rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, Pays de La Loire, France.</footnote>

<footnote>7Associate Professor, Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR CNRS 7356, La Rochelle Université, Ave. Michel Crépeau, La Rochelle, Nouvelle-Aquitaine 17000, France.</footnote>

<footnote>8Professor, Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR CNRS 7356, La Rochelle Université, Ave. Michel Crépeau, La Rochelle, Nouvelle-Aquitaine 17000, France.</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Planète Recyclage platform of crushing and screening process of inert waste to produce recycled aggregate: (1) loading of raw material; (2) crushing; (3) screening; and (4) loading recycled aggregate for storage. (Image by Philippe Turcry.)</figcaption></figure>

more than 1,000 MPa in the long term. This shows that the latent hydraulicity of the RAs can give them significant performance.

It should be noted that the existing literature about the mechanical properties of RAs for geotechnical applications is relatively poor. On the other hand, a lot of research has been carried out on the complete recycling of concrete (i.e., the use of crushed concrete as aggregates in the composition of concrete) since the early 1980s, e.g., Hansen and Narud (1983).

Arm (2001) has studied the long term behavior of compacted demolition waste (containing at least 95% of concrete) by means of triaxial tests. The author shows an increase with time in the stiffness of the RA based material, while in the case of quarry aggregates, the modulus stays constant. In the course of this research, in situ experimentations were also carried out with deflection measurements on experimental roads. The outcome was found to be identical to that of laboratory testing, whereby RA stiffness in creases over time. However, this increase was much greater in situ: after two years, the modulus was twice as high as what is was one month after construction. The same patterns were also observed in Vegas et al. (2011).

Vegas et al. (2011) carried out California bearing ratio (CBR) tests and showed an increase in the midterm bearing strength, an increase that the authors attributed to the hydraulicity of the materials. The authors also showed that the presence of materials other than concrete in RAs was not necessarily a disadvantage. For example, a small amount of ceramic sand (fragments of clay tiles, for example) admittedly decreases the resistance to fragmentation, as evaluated by Los Angeles tests, but improves the midterm or long term stiffness because these fines can produce pozzolanic reactions.

Furthermore, Bennert et al. (2000), Lancieri et al. (2006), da Conceição Leite et al. (2011), Yueqin Hou et al. (2015), and Yunlu Hou et al. (2019) also observed similar or enhanced mechanical characteristics with road structures that include RAs rather than NAs. The RAs used by Lancieri et al. (2006), Yueqin Hou et al. (2015), and Yunlu Hou et al. (2019) contained significant amounts of clay brick and Yueqin Hou et al. (2015, 2019) suspect that pozzolanic reactions involving this brick are responsible for the increase in mechanical performance.

Researchers were interested in the reactivity of fines obtained by the grinding of concrete. The intended application is the general reuse of fines in concrete mixes, as a binder or mineral addition. For this type of application, the reactivity is fairly low; the strengths of concretes formulated with fines of crushed concrete are relatively weak (Oksri Nelfia 2015; Oksri Nelfia et al. 2016).

The preceding studies suggest that RAs may still be reactive what is responsible for increase in mechanical performances over time. However, the cause of the reactivity of RA is not clearly identified. The aim of the present study is to provide explanation for the

self cementing property of RAs, and especially for the effect of ceramic material. To this end, studies have been carried out on two scales: in situ and in the laboratory. Experiments were carried out in situ with four large scale test beds. The pozzolanic character of RAs was investigated using both RAs containing small amounts of clay brick and RAs mixed with ceramic sand obtained by crushing terracotta tiles. All studied materials were produced industrially on a real recycling platform. To complement in situ testing, laboratory investigations were also carried out to assess the residual hydraulicity and pozzolanicity of RAs and ceramic sand.

## Experimental In Situ Platform Investigations

### *Studied Materials*

Four materials were studied. The first one is a 0/31.5 mm RA produced by PLANETE RECYCLAGE (recycling platform based in La Rochelle, France) using a Kleemann Mobirex impact crusher (Göppingen, Baden Württemberg, Germany). The crushing process and the operating principle of the crusher are shown on Fig. 1.

With reference to the NF EN 933 11 standard (BSI 2009), the size fraction of the studied RA above 4 mm shows a composition of 87.9% demolition concrete aggregates, 10.7% natural aggregates, and crushed terracotta tiles for the rest (Table 1).

The second aggregate (denoted RA·CS) is a mix of the first recycled aggregate and a 0/4 mm ceramic sand with a weight per centage of 10%. This ceramic sand (CS) is produced by crushing/screening terracotta tiles in a separated process before mixing with RA.

The last two aggregates are natural aggregates, one being a 0/31.5 mm limestone aggregate (NA A) and the other a 0/20 mm

<table><caption>**Table 1.** Constituents of 4/31.5 mm RA in accordance with European Standard NF EN 933 11 (BSI 2009)</caption><thead><tr><td>Description of constituents</td><td>Weight percentage (%)</td></tr></thead><tbody><tr><td>Rc (concrete, concrete products, mortar, and concrete masonry units)</td><td>87.9</td></tr><tr><td>Ru (unbound aggregate, natural stone, and hydraulically bound aggregate)</td><td>10.7</td></tr><tr><td>Ra (bituminous materials)</td><td>0.0</td></tr><tr><td>Rb (clay masonry units, calcium silicate masonry units, and aerated nonfloating concrete)</td><td>1.4</td></tr><tr><td>Rg (glass)</td><td>0.0</td></tr><tr><td>X (other: cohesive materials, metals, nonfloating wood, plastics, gypsum plaster, etc.)</td><td>0.0</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) Example of punching shear resistance curves of RAs, where the stress exerted by the piston on the sample is expressed as a function of the displacement; and (b) California Bearing Ratio calculation, where P (2.5 mm) and P (5 mm) are the stress values (MPa) measured for vertical displacements of 2.5 and 5 mm, respectively, and 7 and 10.5 MPa correspond to the stress of a reference soil.</figcaption></figure>

aggregate of amphibolitic nature (NA B). Both of them are used as low and high references respectively.

## Pretesting in the Laboratory

The absolute density was measured according to European standard NF EN 1097 6 (BSI 2014b), using a pycnometer for the particles between 0 and 4 mm, and a cylindrical container for those above 4 mm. Los Angeles (LA) and micro Deval (MDE) tests were carried out according to European standards NF EN 1097 2 (BSI 2010a), and NF EN 1097 1 (BSI 2011), respectively. The water soluble sulfate content was determined by the recycled aggregate manufacturer according to standard NF EN 1744 1 (BSI 2014a). In short, this determination is done with a sample placed in water for 15 min at 60°C. The concentration of soluble sulfate in the solution after filtration is then determined through a colorimetric test. For both natural aggregates, NA A and NA B, all these properties were provided by the manufacturers.

The standard Proctor compaction test was carried out according to European standard EN 13286 2 (BSI 2010b) to obtain the maximum dry bulk density ($\gamma_d$) and the critical water content ($w_{OPN}$). Following this test, the punching shear resistance of RA and NA A & B at $w_{OPN}$ under endogenous and submerged conditions was determined by the California Bearing Ratio (CBR) test according to European standard NF EN 13286 47 (BSI 2012), where a cylin drical piston with a cross section of 19.3 cm² punches out the compacted sample at a constant rate (1.27 mm · min⁻¹), as shown in Fig. 2(a). Fig. 2(b) illustrates the curves obtained on a RA based material, loaded by the monitoring system. These curves are then used to evaluate the stress exerted by the piston on the sample as a function of displacement (piston sinking). The punching shear resistance is expressed using the CBR calculated with the equation given in Fig. 2(b). Three replicates were used for each material.

## Design and Construction of the In Situ Experimental Platform

Four test beds of 50 m² each (10 × 5 m) were planned, with a thickness of around 0.60 m before compacting. The materials of the study were successively laid on the upper part of the earthwork

before being simultaneously compacted by a tandem vibratory roller (Fig. 3). An extra mixing step was considered during the addition of ceramic sand for the fourth test bed (steps 5 and 6, Fig. 3).

## Tests on the In Situ Experimental Platform

The compaction was verified for each test bed with a gamma densimeter after eight passes of the tandem vibratory roller. Simultaneously, the apparatus evaluated the water content $i_n$ and the dry bulk density $\rho_d$ over the 30 cm of the test beds. All tests were completed in accordance with French standard NF P98 241 1 (BSI 1993).

The Plate test static deformation modulus (EV2) of the four test beds was measured over time at 0, 3, and 8 months in accordance with French standard NF P94 117 1 (BSI 2000). Three plate tests per beds at each time were carried out. The test consists in successively applying a stress of 0.25 then 0.20 MPa ($p2$) on a 60 cm diameter plate ($d$) and measuring the settlement ($z2$) under the plate. The modulus EV2 was determined using Boussinesq's equation [Eq. (1)] as follows:

$$EV2 = \frac{\pi}{4}(1 - \nu^2) \times \frac{p_2 \cdot d}{z_2} \quad (1)$$

In an approximation of Poisson's ratio, $(1 - \nu^2)$ can be considered to be close to 1.

## Experimental Laboratory Investigation

The main explanation in the existing literature for the reactivity of RA is the presence of residual anhydrous cement in demolition concrete. This residual cement could provide a certain hydraulicity to RA. The addition of ceramic sand could also promote the reactivity of RA through pozzolanic reactions. To confirm these hypotheses, the laboratory tests described hereafter were carried out on the size fraction of RA and CS under 100 µm, which is believed to be the most reactive part of the aggregates. In the following, the fines obtained by sieving RA, RA-CS, and CS are denoted F RA, F RA-CS, and F CS, respectively.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (1) 4) Synopsis of the setup from the creation of the support platform to the control of aggregate compaction (RA, RA-CS, NA A, NA B); and (5) and (6) making of the RA-CS. (Images by Philippe Turcry.)</figcaption></figure>

## Setting Test

In order to observe the macroscopic effects of hydration and poz zolanic reactions on RA fines, setting kinetics were determined using a test similar to the Vicat needle test, which is described in the NF EN 196 3 European Standard (BSI 2017). This testing was carried out on two slurries made with F RA and F RA-CS. The slurries were prepared by mixing fines with a certain amount of water. The water/fines mass ratios were determined beforehand

following the protocol set out by (Sedran 1999). That method con sists in determining the quantity of water that needs to be added to a mixture to induce the change from solid to plastic state. After mix ing, the slurry samples were placed outside in metal containers, 4 cm in diameter and 4 cm in height, and kept in endogenous con ditions at 20°C ± 2°C. The penetration of a needle under a total weight of 100 g was measured at different timepoints. As shown in Fig. 4, the *R* ratio is defined as the ratio between the needle pen etration and the total height of the material. For *R* = 0, the needle is fully penetrated into the sample; for *R* = 1, the material is fully solidified.

## Isothermal Microcalorimetry Analyses

Isothermal microcalorimetry tests were carried out on F RA and on an ordinary portland cement (CEM I 52.5 type) used as a reference hydraulic material. The tests were carried out at a temperature of 20°C using a TAM Air microcalorimeter (TA Instruments, New Castle, Delaware) on samples of slurry with a water to powder mass ratio of 0.26. The mass of the tested samples was 4 g. Samples were placed in the apparatus immediately after mixing.

## Pozzolanicity Testing Using Chemical Investigations

Pozzolanic reactions lead to the consumption of lime in the material. In order to assess the pozzolanicity of the ceramic sand added in RA, we used the tests proposed by Sánchez de Rojas et al. (2006). 10g of material were placed in a solution of Ca(OH)$_2$ at 17.68 · 10$^{-3}$ mol · l$^{-1}$ with a liquid to solid mass ratio of 10. The suspension was agitated inside a hermetic glass reactor with a mag netic bar rotating at 200 rpm. At different time points, the concen tration of Ca$^{2+}$ ions was determined with ionic chromatography. A Metrohm apparatus including a Metrosep A Supp 5 100/4.0 col umn and an automatic sample changer (Ionenstrasse, Herisau, Switzerland) was used. The tests were carried out for F RA and F CS.

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Schematic representation of the setting test and definition of the *R* ratio.</figcaption></figure>
<table><caption>**Table 2.** Physical and mechanical properties of the four studied materials</caption><thead><tr><th>Materials</th><th>Absolute density (kN·m<sup>-3</sup>)</th><th>LA (%)</th><th>MDE (%)</th><th>w<sub>OPN</sub> (%)</th><th>γ<sub>d</sub> (kN·m<sup>-3</sup>)</th><th>Compactness (%)</th></tr></thead><tbody><tr><td>RA</td><td>24.1</td><td>34</td><td>35</td><td>8</td><td>18.8</td><td>78</td></tr><tr><td>RA·CS</td><td>23.4</td><td>N/A</td><td>N/A</td><td>10</td><td>20.0</td><td>85</td></tr><tr><td>NA A</td><td>26.0</td><td>45</td><td>67</td><td>10</td><td>20.6</td><td>79</td></tr><tr><td>NA B</td><td>28.6</td><td>&lt;40</td><td>&lt;35</td><td>6</td><td>23.2</td><td>81</td></tr></tbody></table>

Note: N/A = not available.

Preliminary tests with monitoring of pH over a period of 35 days were carried out with two sands, i.e., CS and sand obtained by screening RA at 4 mm, placed in the lime solution.

# Results and Discussion

## Properties of the Studied Materials

Table 2 shows the physical and mechanical properties of the four materials studied (RA, RA·CS, NA A, and NA B). Compactness is the ratio between the optimal dry bulk density ($\gamma_d$) and absolute density for the water content $w_{OPN}$.

The absolute density of both RA's is lower than that of both NA's because the former are mainly composed of demolition concrete aggregates (87.9%). This difference is more salient in the case of RA·CS due to the low absolute density of ceramic sand (17.3 kN·m⁻³).

The resistance to fragmentation and attrition of RA and RA·CS, characterized by LA and MDE indexes, respectively, appears lower than that of NA A&B but suitable to be used in pavement subbase and base.

RA is a material exhibiting a low sensitivity to water because the dry bulk densities show only slight variations over a wide range of water content (a minimum $\gamma_d$ of 16.9 kN·m⁻³ was measured for a water content of 12%). RA has a $w_{OPN}$ of 8% and a $\gamma_d$ of 18.7 kN/m³. An addition of 10% by mass of CS shifts the $w_{OPN}$ to 10%, with only a slight increase in $\gamma_d$. This result could be explained by the fact that CS particles are approximately two times more porous than those of concrete (porosities of around 30% and 15%, respectively). Therefore, water is partly absorbed by CS and less water participates in the arrangement of the solid grains during compaction. Regardless of the materials studied, the

compactness is close to 80% and complies with the densification objectives intended for pavements sub base and base.

The sulfate soluble contents were found equal to 0.27% and 0.17% for RA and CS, respectively. Because these contents are lower than 0.7%, the studied materials are suitable for road construction according to European standards NF EN 13242 and NF EN 13285 (BSI 2008, 2018).

On mechanical aspects, Figs. 5(a and b) show the evolution of the CBR of RA, NA A, and NA B after compaction at $w_{OPN}$ and for two different types of cures, i.e., endogenous conditions and immersion under water, respectively. Note that the evolution over time of the bearing index of the RA·CS was not determined.

Just after compaction, the CBR was higher for the natural aggregates than for the recycled ones. The CBR of NA B is almost constant, while that of NA A, which is not very reactive at first glance, increases a little. However, this increase is not significant when compared to the standard variations. On the other hand, there is a major increase in the bearing index of RA under both endogenous and water immersion conditions.

These results confirm those mentioned in literature. The whole physical and mechanical study shows that both RA and RA·CS can be used for the subbase or base of pavements with regard to the technical specifications proposed in France for alternative materials (CEREMA 2014).

## Compaction of the Four In Situ Test Beds

In Table 3, the results of gammadensimeter testing are reported for the four test beds. During the setup of the materials, the objective was to achieve a near identical compactness for the four test beds.

For the 0/20 amphibolite aggregate (NA B), the setup is in accordance with the sought characteristics. The water content

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** (a) Evolution of the bearing index CBR of RA, NA A, and NA B after compaction at $w_{OPN}$ for endogenous conditions; and (b) evolution of the bearing index CBR of RA, NA A, and NA B after compaction at $w_{OPN}$ after immersion under water.</figcaption></figure>
<table><caption>**Table 3.** Water content and specific weight of the studied materials after test bed compaction</caption><thead><tr><th>Test<br>bed N°</th><th>Material</th><th>ω<sub>n</sub><br>(%)</th><th>ρ<sub>d</sub><br>(kN/m<sup>3</sup>)</th><th>ρ<sub>s</sub><br>(kN/m<sup>3</sup>)</th><th>Compactness<br>(%)</th></tr></thead><tbody><tr><td>1</td><td>RA</td><td>6.5</td><td>19.0</td><td>24.1</td><td>79</td></tr><tr><td>2</td><td>RA-CS</td><td>5.8</td><td>18.7</td><td>23.4</td><td>80</td></tr><tr><td>3</td><td>NA A</td><td>9.4</td><td>17.7</td><td>26.5</td><td>67</td></tr><tr><td>4</td><td>NA B</td><td>5.2</td><td>24.0</td><td>28.6</td><td>84</td></tr></tbody></table>

and the specific density are identical to those of the Proctor optimum. The compactness is greater than 80%.

However, for the 0/31.5 limestone aggregate (NA A), even if the water content is compliant, the specific density is much lower (17.7 kN/m³ instead 20.6 kN/m³) than the Proctor optimum. The compactness is therefore much lower than the previous one (67% instead of 79%). Therefore, unlike the NA B test bed, the NA A test bed can be considered as a poor quality reference.

For RA, as well as for that with a dosed addition of ceramics sand (RA-CS), the results of the setup are satisfactory, because in both cases the compactness achieved is close to 80%. Even though RA-CS is not very sensitive to water during compaction, these results could have been optimized with a better control of the water content, because in both cases the natural water content was lower than that obtained at the Proctor optimum.

## Evolution of the Mechanical Performance of Test Beds

Before setting up the aggregates, 12 plate tests were completed on the upper part of the earthwork. On average, an EV2 modulus of 57 MPa was obtained on the whole surface.

Fig. 6 shows the results of the plate tests carried out on the four test beds as a function of time at $t_0$, $t_0 + 3$ months, and $t_0 + 8$ months.

As expected, on the day of compaction ($t_0$), the results are rather homogeneous. The RA-CS presents the lowest EV2 modulus. Surprisingly, the highest EV2 modulus was found for the NA A, although its compactness was the lowest at $t_0$ (Table 3).

Three months after compaction, the EV2 modulus of RA, RA-CS, and NA B have significantly increased, about 2.5 times higher, and three times higher for the recycled aggregates test beds. Conversely, the NA A seems to be more sensitive to weather

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Evolution over time of the EV2 modulus for the studied aggregates over the eight months following compaction ($t_0$).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Evolution over time of the needle penetration ratio (R) determined with setting tests for F RA and F RA-CS slurries.</figcaption></figure>

conditions (the stiffness test at three months was completed in the winter).

Eight months after compaction, the four EV2 modulus have further increased, even for NA A. However, this increase is in no way as high as that of the RA, whose modulus exceeds 500 MPa (and even 1,500 MPa for RA-CS). Note that beyond a certain bearing capacity (about 500 MPa), the determination of the EV2 modulus is not easy, because the settlement measurement becomes inaccurate. For this reason, the ordinate axis was capped at 500 MPa in Fig. 6. In any case, the overall evolution of the EV2 modulus observed in situ for the RA confirms its potential reactivity.

The increase in bearing capacity of the RA tests beds is in line with literature results, e.g., Arm (2001), and the previous results obtained in laboratory with the CBR tests. These significant mechanical evolutions are all probably due to a chemical activity of the fines fraction in RA and CS.

## Reactivity of the Recycled Aggregates

Fig. 7 shows the results of the setting tests obtained for two slurries of F RA and F RA-CS. As defined previously, R is the ratio between the needle penetration and the total height of the material.

First of all, we note that R evolves as a function of time for the two slurries studied. This means that a solidification of the slurry happens over this period. It is a good indication of the potential reactivity of the fine particles, because the tests were carried out under endogenous conditions, i.e., without water exchange with the ambient environment. In the short term, the setting kinetics is fairly similar for both slurries, although for F RA-CS, the setting effect is more pronounced. However, in the longer term, the behavior of F RA-CS differs greatly from F RA, because the setting of this material continues beyond 21 days. These results therefore underline the influence of ceramic particles on the overall reactivity of RA, and show a clear reaction with the basic solution of the material.

Nevertheless, these results must be qualified. The R ratio only reaches a maximum of 0.25, which is very far from a significant hydraulic setting value. This is also the conclusion that can be drawn from the microcalorimetric tests. Fig. 8 presents the heat flux, expressed in mW/g of dry 0/100 µm RA, recorded over the first four days of testing. This test was carried out on a slurry of F RA alone to highlight the potential hydraulic reactivity of residual anhydrous cement contained in the cementitious fine particles of RA.

We observe an initial release of heat during the early hours that can be attributed to the thermal stabilization of the sample.
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Heat flux determined during the first four days of isothermal microcalorimetry testing carried out on slurries of fines from recycled aggregates (F RA) and portland cement (OPC). Two replicates were tested for F RA.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Evolution over time of the pH of lime solutions containing ceramic sand (CS) and sand from recycled aggregates (RAs).</figcaption></figure>

This heat release can also come from the dissolving of certain mineral phases, as commonly observed in the case of portland cement. Then, the heat flux increases progressively to reach a maximum after 60 h. This heat peak resembles that observed during the hydration of cement. However, the heat flux released by the fines differs from that of cement by much slower kinetics (because it needs three to five times more time to reach the maximum flux). If this flux is really the consequence of the hydration of cement, we can suppose that the grains of cement that are still anhydrous are barely accessible (because they are partly covered by already formed hydrates, or else by calcium carbonates resulting from the atmospheric carbonation of the fines).

Furthermore, the amplitude of the heat flux is much lower in the case of the fines. In total, at the end of the test, the heat released does not exceed 5J for 1 g of fines. By way of comparison, the complete hydration of 1 g of portland cement releases up to 500. This provides us an order of magnitude of 1% for the anhydrous cement content in the studied fines. This low anhydrous cement content (or content accessible for hydration) suggests that the hydraulicity of the fines is limited.

To support this conclusion, two tests were carried out to assess the potential pozzolanic reactivity of ceramic particles contained in, or added to RA. Fig. 9 presents a first result obtained for sands (CS and sand from RA) placed in a lime solution. Over 20 days, a significant decrease in pH is observed for both sands. This pH evolution cannot result from an acidification induced by the CO₂ present in atmospheric air, because the experiments used air tight reactors (only opened for pH measurements). It can therefore be a consequence of the consumption of calcium hydroxide by the elements present in CS or RA in solution.

Under a pH of 12.4, the portlandite [Ca(OH)₂] is unstable. It dissolves and can intervene in pozzolanic reactions for the formation of new hydrates. According to this hypothesis, the silica, present in large quantities in ceramics, might react with portlandite and lead to the formation of hydrated calcium silicates (CSH), leading to a decrease in pH. The decrease in pH can be attributed to the precipitation with cations. We also note in Fig. 9 a decrease in the pH of the solution containing the RA sand. This highlights a certain pozzolanicity in the recycled aggregate itself.

The same protocol was used with a determination of the Ca²⁺ concentration in the solution at different timepoints. The tests were carried out on fines in solution with Ca(OH)₂. Fig. 10 reports the evolution over time of the consumption of the Ca²⁺ ions by the

<figure><img src="image_12.png" /></figure>

$$fixed Ca^{2+} = 100 \frac{n_0 - n(t)}{n_0}$$

Where: n₀ is the initial Ca²⁺ content in the solution and n(t) the content determined at time t.

<caption>**Fig. 10.** Evolution over time of the consumption of Ca²⁺ ions by fine elements (under 100 µm) of RA, RA-CS, and CS in a lime solution.</caption>
samples. The relative concentration of fixed Ca²⁺ is calculated with the equation given in Fig. 10.

The consumption of Ca²⁺ ions present in the lime solution seems to confirm the pozzolanic character of the studied fines, especially the fines in CS (F CS). This remains a hypothesis, as other elements may also consume the Ca²⁺ ions. In particular, RA and CS contain a certain amount of sulfate (Vegas et al. 2011). The SO₄²⁻ ions can react with the calcium to form anhydrite (CaSO₄), gypsum (CaSO₄, 2H₂O) or even more complex forms with the anhydrous particles of the cement, such as ettringite. However, the soluble sulfate contents of the study materials assessed according to European standard NF EN 1744 1 are low. For instance, as shown in section 4.2, the content is equal to 0.17% in the case of CS. In a 100 g of solution, the amount of sulfate brought by 10 g of CS is therefore equal to 1.77 × 10⁻⁴ mol. Because the lime con centration of the solution is 17.68 × 10⁻³ mol × L⁻¹, the amount of Ca²⁺ ions is 1.77 × 10⁻³ mol. Finally, the ratio between the Ca²⁺ amount and the sulfate amount is approximately 10. This rough calculation shows that sulfates cannot account for the total con sumption of Ca²⁺ observed in the case of CS. Of course, miner alogy investigations on before and after contact with the lime solution would be necessary to determine the nature of the products formed by the consumption of the Ca²⁺ ions.

## Conclusion

The use of recycled aggregates instead of natural aggregates in road applications can have a significant environmental impact. In re gions where natural resources are scarce, the reuse of recycled ag gregates would reduce the need to source natural aggregates from more remote regions, thus cutting CO₂ emissions linked with its transportation. Apart from these well known benefits, this study further shows that recycled aggregates can be a high technical value added material compared to the natural aggregates. The re sults of the present study can be summarized as follows:

* At the macroscopic scale, the mechanical properties of the test beds containing recycled aggregates increased much more over the months than those of natural aggregates and exceed 500 MPa in both of case (recycled aggregates and recycled ag gregates with ceramic sand). The results of CBR testing in lab oratory confirmed these tendencies.

* The setting tests were carried out on slurries of fines obtained by sieving of the recycled aggregates. The results show a slow but significant solidification over time that is faster in the case of a high ceramic particles content.

* The heat released during microcalorimetric tests on slurries of recycled aggregates fines was approximately equivalent to that of a 1% anhydrous cement content. It can be concluded that the residual hydraulicity of recycled aggregates fines is very limited and cannot explain the growth of the mechanical performance over time.

* The pozzolanic reactivity was investigated using lime solutions containing fines from recycled aggregates and ceramic sand. The monitoring of the Ca²⁺ ions concentration show that these ions are consumed by fines. The consumption is much higher in the case of the ceramic fines. These results highlight that the latter are pozzolanic material.

From a practical point of view, ceramic sand obtained by crush ing terracotta tiles could prove a valuable material that could be mixed with recycled aggregates in order to increase their mechani cal properties, and consequently their added value. Moreover, the design of pavements and road embankments could be revised to take into account the reactivity of recycled aggregates at long term.

## Data Availability Statement

Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request. In fact, this research was within the framework of the RECYMENT project and the report is available in French.

## Acknowledgments

This research was carried out as a part of the RECYMENT pro ject (contract number: 13 06 C0085), cofinanced by LaSIE and ADEME (French agency for the ecological transition). The authors would like to acknowledge Mr. Laurent CHATEAU (ADEME), Mr. Valery FERBER (Charier Company), Ms. Mariane AUDO (post doc at the LaSIE), Mr. Flavien GABILLY and Mr. Mathieu MOREAU (trainees).

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