<figure><img src="image_112.png" /><figcaption>Fig. 5. (a) DHT<sub>λ,exp</sub>, (b) DHR<sub>λ,exp</sub> and (c) DT<sub>λ,exp</sub> of polyester batting versus wavelength for different thicknesses.</figcaption></figure>

This value corresponds to the reflection coefficient of the reference material to close the hole of the sphere in the absence of sample. In case of transmission this procedure is not valid, because the ratio between the reference and sample measurements gives directly the absolute transmittance. The spectral absorptivity of the samples is then deducted and analyzed as a function of the wavelength. The conservation of energy equation (the radiative energy balance) is written as:

$$ \alpha_{\lambda,\text{exp}} + \text{DHT}_{\lambda,\text{exp}} + \text{DHR}_{\lambda,\text{exp}} = 1 \quad (43) $$

# 4. Results and discussion

## 4.1. Hemispherical spectral transmittance and reflectance

Hemispherical-directional spectral transmittance and reflectance have been measured using a Brucker FTIR spectrometer with integrating sphere, which all the measurements have been made in a vacuum vessel, at different infrared wavelengths ranging from 2 to 21 µm (i.e. from 500 to 5000 cm<sup>-1</sup>). These spectral measurements cover a large number of wavelengths in order to observe the overall evolution of the properties and to reduce the computing time. We observed that no spectral information was lost of the evolution of the spectral measurements. Thus, the similar dependence on the wavelength was noticed for each thickness of the polyester batting. Fig. 5(a) and (b) shows DHR and DHT measurements of the

<figure><img src="image_113.png" /><figcaption>Fig. 6. Radiative properties of polyester batting versus wavelength for different thicknesses: (a) the optical thickness of polyester batting, (b) identified extinction coefficient, and (c) calculated extinction coefficient.</figcaption></figure>

polyester batting for different thicknesses of 1.28 mm, 1.45 mm, 1.58 mm, 1.65 mm, 1.84 mm, 2.30 mm, 2.40 mm and 2.64 mm. The DHT (Fig. 5(a)) varied between 90% and 8% and that of DHR varied between 0% and 32%. The direct transmittance measurements can help assessment of the phase function shape [31] and allows to identify the extinction coefficient by the law Beer Lambert (Eq. (31)). The results of DT related to the thicknesses are plotted in Fig. 5(c) we noticed the presence of three peaks of transmission for thicknesses of 1.28 mm, 1.65 mm and 1.45 mm; the peaks are observed around the wavelengths of 7, 9 and 14 µm, respectively, which they are due to the spectral attenuation of the albedo and the phase function. A significant decrease of the transmission was observed for each thickness between 15 µm and 19 µm (Fig. 5(c)), which is due to the effect of diffusion. Thus, the falls of hemispherical directional reflectance is observed around the wavelength of 5.5 µm and 7.5 µm was noticed, which is corresponds to the Christiansen effect. The reflectance and transmittance are extremely low around the wavelengths 5.75, 7.5 and 13.76 µm; the polyester batting strongly absorbs the infrared radiation in these wavelengths. In the spectral band 15–20 µm, the polyester batting is purely diffusing or slightly absorbing. We concluded that the sample thickness and the fiber density are two parameters that govern the transmission and reflection.

## 4.2. Optical thickness and albedo

By using the experimental measurements of DHT and DHR obtained for all samples, an inversion method was applied to find
<figure><img src="image_114.png" /><figcaption>Fig. 7. (a) Variation of spectral extinction coefficient with thickness and (b) error (%) of spectral extinction coefficient with thickness.</figcaption></figure>

<figure><img src="image_115.png" /><figcaption>Fig. 8. Albedo of polyester batting versus wavelength for different thicknesses.</figcaption></figure>

the spectral optical thickness $\tau_{\lambda}$, extinction coefficient $\beta_{\lambda}$ and the albedo $\omega_{\lambda}$ of each material. Fig. 6(a) shows the optical thickness $\tau_{\lambda}$ of the polyester batting material for different thicknesses (1.28, 1.45, 1.65, 1.58, 2.30, 2.40, 2.64 mm). We noticed that the optical thickness depends mainly on the wavelength. In addition, we have identified the extinction coefficient experimentally and theoretically; Fig. 6(b) and (c) presents the extinction coefficient, calculated from the Beer–Lambert law, and that identified by the inverse method versus wavelength and for different thicknesses. We observed that the values of the extinction coefficient reported by the inverse method are identical to those calculated by the Beer–Lambert law. We also noticed the presence of the peaks of extinction for each thickness around wavelengths of 6, 7.5, 8.6, 10 and 13.8 µm. These peaks correspond to the absorption peaks, beyond the evolution of extinction is monotonous.

The average value of the extinction coefficient have been determined experimentally and theoretically for the wavelength of 18.75 µm. Fig. 7(a) shows the extinction coefficients, which depends on the thickness of the material. We noticed (Fig. 7(b)) that the value is very low except for thickness of 2.4 mm, which the error reached 30%; this not significant. Fig. 8 shows the relative values of the albedo for different thicknesses. The maximum albedo value is about 0.98 for the thickness of 1.58 mm, and the lowest value is of 0.34 for 2.30 mm. We noticed that the same

<figure><img src="image_116.png" /><figcaption>Fig. 9. (a) Absorptance of the polyester batting versus wavelength for different thicknesses and (b) the phase function versus wavelength for different thicknesses.</figcaption></figure>

<figure><img src="image_117.png" /><figcaption>Fig. 10. The scattering coefficient of the polyester batting versus wavelength for different thicknesses.</figcaption></figure>

<figure><img src="image_118.png" /><figcaption>Fig. 11. The absorption coefficient versus wavelength for different thicknesses.</figcaption></figure>

behavior for the different thicknesses, and the observed peaks correspond to the albedo fall, that shows the influence of extinction on the scattering around the wavelengths of 5.8, 8, 9, 10, 11.8, and 13.8 µm. Beyond this, the medium is scattering and absorbing for all thicknesses. Finally, a slight decreases in albedo for wavelength higher than 18 µm was observed for all thicknesses.

Generally, the incident radiation on the material surface may be absorbed, transmitted or reflected. The contributions of the three
<figure><img src="image_119.png" /><figcaption>Fig. 12. Thickness effect on the radiative conductivity and the phonic conductivity with a fiber diameter of 31 µm.</figcaption></figure>

phenomena at a wavelength is given by spectral coefficients absorbance $α_λ$, directional transmission $DHT_λ$ and directional reflection $DHR_λ$. The determination of the absorbance $α_{λ,exp}$ (i.e. from Eq. (43)) is important, knowing that at equilibrium temperature, the Kirchoff's law states that the directional absorbance is equal to the directional emissivity [31]:

$$α_{λ,exp} = ε_{λ} \qquad (44)$$

The spectral absorbance of each thickness of the material is plotted in Fig. 9(a). The polyester fiber are strongly absorptive for wavelengths above 3 µm, 5.5 µm, 6.5 µm, 7 µm, 9 µm, 10 µm, 11 µm, 12 µm, 13 µm, 14 µm, 16 µm and 20 µm (Fig. 9(a)). For the Henyey-Greenstein coefficient g (or the anisotropy coefficient) (Fig. 9(b)), the inverse method for different thicknesses was investigated. We noticed the presence of 6 main peaks around wavelengths of about 5.8, 7, 9.8, 10.5, 11.5 and 13.8 µm, which correspond to the albedo fall (Fig. 9(a)) or the scattering decreases; forward scattering for all thicknesses.

### 4.3. Absorption and scattering coefficients

From Eq. (25), we can extract the diffusion coefficient. Fig. 10 shows the evolution of the diffusion coefficient versus wavelength for different thicknesses. We noticed a fall behavior of the diffusion at the wavelength near 14 µm for all thicknesses. The thermal absorption coefficient may be deduced from Eq. (25); it depends on the wavelength and the medium temperature. Fig. 11 shows that the variations, of the absorption coefficient $κ_λ$ of each thickness of polyester batting material, are extremely interesting, which will appear according to the wavelength $λ$ (of about 1700 m⁻¹). Thus, we noticed the presence of absorption peaks at 5.8, 7.5, 9, 11.6, and 13.8 µm with similar behaviors.

### 4.4. Effective radiative conductivity

As mentioned above, the radiative conductivity is estimated from the Rosseland approximation (Eq. (33)) with the wavelength range $Δλ = 2–20 µm$ for the polyester batting material, then the phonic conductivity is deduced from Eq. (5). The apparent or effective conductivity is determined experimentally at room temperature using Eq. (39) taking into account the uncertainty measurement (Eq. (40)). Fig. 12 shows the radiative conductivity variation and the phonic conductivity with the material thickness. We observed that the radiative conductivity vary with the phonic conductivity, this variation can be explained by the different density of the fibers for each sample. Thus, the radiative conductivity represents around 26% of the overall thermal conductivity at 20 °C. The uncertainty on the radiative conductivity was estimated to less than 2% with the measured spectral hemispherical reflectance and transmittance.

## 5. Conclusion

The present work reports the numerical and experimental studies of physical and thermal properties of polyester batting insulator. The investigated media is non-gray, anisotropically absorbing, emitting and scattering. We introduced an inverse method with the aim of determining radiative properties of polyester batting insulation. The radiative conductivity was determined according to the Rosseland approximation. Experimental measurements in directional hemispherical transmittance and reflectance, using a Brucker FTIR Spectrometer with integrating sphere allowed us to determine the absorption, scattering, extinction and phase function coefficients of the polyester batting. The spectral properties of polyester batting insulation have been found strongly dependent on the wavelength in 2–21 µm range. Thus, we observed the radiative conductivity increases with thickness. In our future work, we will investigate the radiative properties for comparison with other materials such as glass wool already being studied by other authors as well with hemp wool.

## Acknowledgements

The authors acknowledge here the Institute for Electronics, Microelectronics and Nanotechnologies (IEMN) of the University of Lille – France, for taking microscopic pictures of polyester batting fibrous material.

## References

[1] A. Patnaik, M. Mvubu, S.r. Muniyasamy, A. Botha, R.D. Anandjiwala, Thermal and sound insulation materials from waste wool and recycled polyester fibers and their biodegradation studies, Energy Build. 92 (2015) 161–169.

[2] R. Bhattacharyya, Heat transfer model for fibrous insulations, Thermal Insulation Performance, vol. 718, American Society for Testing and Materials STP, 1980, pp. 272–286.

[3] D. Baillis, M. Raynaud, J.-F. Sacadura, Determination of spectral radiative properties of open cell foam: model validation, J. Thermophys. Heat Transfer 14 (04) (2000).

[4] D. Baillis, M. Arduini-Schuster, J.-F. Sacadura, Identification of spectral radiative properties of polyurethane foam from hemispherical and bi-directional transmittance and reflectance measurements, J. Quant. Spectrosc. Radiat. Transfer 73 (2002) 297–306.

[5] L. Hespel, S. Mainguy, J.J. Greffet, Radiative properties of scattering and absorbing dense media: theory and experimental study, J. Quant. Spectrosc. Radiat. Transfer 77 (2003) 193–210.

[6] H.C. Zhou, Y.B. Hou, D.L. Chen, C.G. Zheng, An inverse radiative transfer problem of simultaneously estimating profiles of temperature and radiative parameters from boundary intensity and temperature measurements, J. Quant. Spectrosc. Radiat. Transfer 74 (2002) 605–620.

[7] D. Baillis, J.-F. Sacadura, Thermal radiation properties of dispersed media: theoretical prediction and experimental characterization, J. Quant. Spectrosc. Radiat. Transfer 67 (2000).

[8] A. Milandri, F. Asllanaj, G. Jeandel, Determination of radiative properties of fibrous media by an inverse method comparison with the Mie theory, J. Quant. Spectrosc. Radiat. Transfer 74 (5) (2002) 637–653.

[9] J.F. Sacadura, Thermal radiative properties of complex media: theoretical prediction versus experimental identification, Heat Transfer Eng. 32 (9) (2011) 754–770.

[10] N.L. McKay, T. Timusk, B. Farnworth, Determination of optical properties of fibrous thermal insulation, J. Appl. Phys. 55 (11) (1984) 4064–4071.

[11] S. Zhao, Z. Boming, D. Shanyi, Effects of contact resistance on heat transfer behaviors of fibrous insulation, Chin. J. Aeronaut. 22 (5) (2009) 569–574.

[12] V. Nicolau, M. Raynaud, J. Sacadura, Spectral radiative properties identification of fiber insulating materials, Int. J. Heat Mass Transfer. 37 (Suppl. 1) (1994) 311–324.

[13] D. Baillis, M. Arduini-Schuster, J.F. Sacadura, Identification of spectral radiative properties of polyurethane foam from hemispherical and bi-directional transmittance and reflectance measurements, J. Quant. Spectrosc. Radiat. Transfer 73 (2002).

[14] S.-Y. Zhao, B.-M. Zhang, S.-Y. Du, An inverse analysis to determine conductive and radiative properties of a fibrous medium, J. Quant. Spectrosc. Radiat. Transfer 110 (13) (2009) 1111–1123.

[15] S. Lee, G. Cunnington, Conduction and radiation heat transfer in high-porosity fiber thermal insulation, J. Thermophys. Heat Transfer 14 (April) (2000) 121–136.

[16] S. Lee, Effect of fiber orientation on thermal radiation in fibrous media, Int. J. Heat Mass Transfer 32 (2) (1989) 311–319.
[17] A. Kaemmerlen, F. Asllanaj, H. Sallée, D. Baillis, G. Jeandel, Transient modeling of combined conduction and radiation in wood fibre insulation and comparison with experimental data, Int. J. Therm. Sci. 49 (11) (2010) 2169–2176.

[18] F. Intini, S. Kuhtz, Recycling in buildings: an LCA case study of a thermal insulation panel made of polyester fiber, recycled from post-consumer pet bottles, Int. J. Life Cycle Assess. 16 (05) (2011).

[19] M. Bomberg, S. Klarsfeld, Semi-empirical model of heat transfer in dry mineral fiber insulations, J. Build. Phys. 6 (3) (1983) 156–173.

[20] A. Karamanos, S. Hadiarakou, A. Papadopoulos, The impact of temperature and moisture on the thermal performance of stone wool, Energy Build. 40 (8) (2008) 1402–1411.

[21] V.G. Petrov-Denisov, V.S. Zholudov, V.V. Gur'ev, A method for estimating heat-shielding properties of mineral wool insulation based on silicate fibrous materials, Glass Ceram. 57 (09) (2000).

[22] R.V.M. Spinnler, E.R.F. Winter, T. Sattelmayer, Theoretical studies of high-temperature multilayer thermal insulations using radiation scaling, J. Quant. Spectrosc. Radiat. Transfer 84 (2004).

[23] K. Daryabeigi, Analysis and testing of high temperature fibrous insulation for reusable launch vehicles, in: 37th AIAA Aerospace Sciences Meeting and Exhibit, AIAA, USA, 1999, pp. 99–1044.

[24] K. Daryabeigi, G.R. Cunningham, J.R. Knutson, Combined heat transfer in high-porosity high-temperature fibrous insulation: theory and experimental validation, J. Thermophys. Heat Transfer 25 (2011).

[25] M.F. Modest, Radiative Heat Transfer, Academic Press, 2013.

[26] M. Necati Özışık, Radiative Transfer and Interactions with Conduction and Convection, Wiley-Interscience Publication, Wiley, 1973.

[27] Dingyu Xue, YangQuan Chen, Solving Applied Mathematical Problems with MATLAB, CRC Press, 2008. 446 pp.

[28] R. Siegel, Thermal Radiation Heat Transfer, fourth ed., Taylor & Francis, 2001.

[29] H. Hottel, A. Sarofim, Radiative Transfer. McGraw-Hill Series in Mechanical Engineering, McGraw-Hill, 1967.

[30] L.O. Duthoit, Application des méthodes de la thèorie des systèmes à la simulation de l'évolution des flux thermiques sur les faces d'entree et de sortie d'une paroi multicouches PhD thesis, University Lille, 1985.

[31] A. Celzard, G. Tondi, D. Lacroix, G. Jeandel, B. Monod, V. Fierro, A. Pizzi, Radiative properties of tannin-based, glasslike, carbon foams, Carbon 50 (7) (2012) 4102–4113.
Research Paper

# Characterization of the thermal properties of fibrous insulation materials made from recycled textile fibers for building applications: Theoretical and experimental analyses

<figure><img src="image_120.png" /></figure>

Amine Tilioua<sup>a,*</sup>, Laurent Libessart<sup>b</sup>, Stéphane Lassue<sup>c</sup>

## HIGHLIGHTS

* The inverse method is used to obtain the radiative properties of the studied material.

* Radiative properties strongly depend on both wavelength and material thickness.

* Phonic thermal conductivity is deduced from the effective thermal conductivity.

* Radiative thermal conductivity is of very little effect.

* Material thickness exerts an influence on radiative and phonic thermal conductivity.

## GRAPHICAL ABSTRACT

<figure><img src="image_121.png" /></figure>

<figure><img src="image_122.png" /></figure>

<figure><img src="image_123.png" /></figure>

<figure><img src="image_124.png" /></figure>

## ARTICLE INFO

### Keywords:

Insulation

Recycled textile

Radiative properties

Inverse method

Radiative conductivity

Phonic conductivity

## ABSTRACT

The present study focuses on the thermal characterization of an insulation material made from recycled textile fibers for building applications, which qualifies as a semi-transparent medium. Experimental and numerical studies have been carried out to determine the radiative flux ratio for such a recycled textile fiber-based insulation for three thicknesses (5.35 mm, 5.67 mm and 6.64 mm). The inverse method, which relies on reflection and transmission measurements using a Fourier-Transform Infrared Spectrometer coupled to an integrating sphere, has been applied along with a least squares procedure. The relevant radiative properties of recycled textile insulation material are obtained by minimizing the deviation between experimental and theoretical data. The effective thermal conductivity of the fibrous insulation is measured at room temperature by means of a fluxmeter device. The radiative thermal conductivity is estimated by implementing the Rosseland model, while phonic conductivity is derived from the effective thermal conductivity. The radiative thermal conductivity displays a very limited effect in comparison with phonic thermal conductivity; the latter varies according to thickness, which in turn is determined by fiber density and size. Phonic thermal conductivity accounts for 26% of the effective thermal conductivity and moreover constitutes a large share (74%) relative to the radiative conductivity. The maximum radiative thermal conductivity value equals 0.0102 W m⁻¹ K⁻¹ for a thickness of 5.67 mm, with this value dropping slightly to 0.010 W m⁻¹ K⁻¹ for a thickness of 6.46 mm, and to a minimum value of 0.0096 W m⁻¹ K⁻¹ at a 5.35-mm thickness.

<footnote><sup>a</sup> MMESA, Department of Physics, Faculté des Sciences et Techniques d'Errachidia, Université Moulay Ismail, BP 509 Boutalamine, Errachidia, Morocco</footnote>

<footnote><sup>b</sup> Laboratoire Génie Civil et géo-Environnement (LGCgE)-Lille Nord de France (EA 4515), IUT de Béthune, 1230 Rue de l'Université, F-62400 Béthune, France</footnote>

<footnote><sup>c</sup> Laboratoire Génie Civil et géo-Environnement (EA-4515), Faculté des Sciences Appliquées-Université d'Artois, Technoparc Futura, 62400 Béthune, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: tiliouamine@yahoo.fr, a.tilioua@fste.umi.ac.ma (A. Tilioua).</footnote>
## Nomenclature

<table><tr><td>T</td><td>temperature, K</td></tr><tr><td>T<sub>c</sub></td><td>temperature of cold plate, K</td></tr><tr><td>T<sub>h</sub></td><td>temperature of hot plate, K</td></tr><tr><td>ΔT</td><td>temperature difference, K</td></tr><tr><td>k<sub>eff</sub></td><td>effective thermal conductivity, W m<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>k<sub>c</sub></td><td>phonic thermal conductivity, W m<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>k<sub>conv</sub></td><td>convective thermal conductivity, W m<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>k<sub>f</sub></td><td>radiative thermal conductivity, W m<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>k<sub>g</sub></td><td>gas thermal conductivity, W m<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>k<sub>s</sub></td><td>solid thermal conductivity, W m<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>k<sub>F</sub></td><td>thermal conductivity due to conduction in fibers, W m<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>k<sub>G</sub></td><td>thermal conductivity due to the gas in the material, W m<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>P<sub>λ</sub></td><td>spectral phase function</td></tr><tr><td>I<sub>λ</sub></td><td>spectral radiation intensity, W m<sup>-2</sup> µm<sup>-1</sup> sr<sup>-1</sup></td></tr><tr><td>I<sub>o</sub></td><td>incident radiation intensity, W m<sup>-2</sup></td></tr><tr><td>I<sup>o</sup></td><td>blackbody radiation intensity, W m<sup>-2</sup></td></tr><tr><td>T<sub>λ</sub></td><td>spectral transmittance</td></tr><tr><td>g</td><td>acceleration due to gravity, 10 m s<sup>-2</sup></td></tr><tr><td>C<sub>p</sub></td><td>specific heat of air at constant pressure, kJ kg<sup>-1</sup> K<sup>-1</sup></td></tr><tr><td>P</td><td>permeability of the solid skeleton</td></tr><tr><td>R<sup>*</sup></td><td>modified Rayleigh number</td></tr><tr><td>T<sup>dh</sup><sub>λ,th</sub></td><td>directional hemispherical transmittance</td></tr><tr><td>R<sup>dh</sup><sub>λ,th</sub></td><td>directional hemispherical reflectance</td></tr><tr><td>T<sup>d</sup><sub>λ,exp</sub></td><td>direct transmittance</td></tr><tr><td>FTIR</td><td>Fourier Transform Infrared Spectroscopy</td></tr><tr><td>P<sub>r</sub></td><td>reflective power, %</td></tr></table>

## Greek symbols

$\beta_{\lambda}$ spectral extinction coefficient, m$^{-1}$

$\beta_{\lambda}^{*}$ weighted extinction coefficient, m$^{-1}$

$\sigma_{\lambda}$ scattering coefficient, m$^{-1}$

$\tau_{\lambda}$ optical thickness of the medium

$\omega_{\lambda}$ albedo

$\kappa_{\lambda}$ absorption coefficient, m$^{-1}$

$\alpha_{\lambda}$ spectral absorptance, %

$\lambda$ wavelength, m

$\mu$ cosine of the polar angle

$\mu_{1}$ first direction

$\mu_{\text{air}}$ dynamic viscosity of air, kg m$^{-1}$ s$^{-1}$

$\rho$ density of the medium, kg m$^{-3}$

$\rho_{\text{air}}$ masse density of air, kg m$^{-3}$

$\eta$ volumetric thermal expansion coefficient of air, K$^{-1}$

$d\Psi_{0}$ solid angle, sr

$\varepsilon_{S}, \varepsilon_{P}, \alpha$ structural parameters dependent of the porosity

$\xi_{0}$ half-angle of incident beam

$\phi_{\text{reference}}$ reference measurement

$\phi_{0}$ reflection measurement without sample

$\phi_{\text{sample}}^{T}$ transmission measurement with sample

$\phi_{\text{sample}}^{T}$ reflection measurement with sample

## Superscripts

* weighted coefficient

## Subscripts

$\lambda$ spectral

$E$ thickness, m

## Physical constants

$\sigma$ Stefan-Boltzmann Constant, $5.67040 \times 10^{-8} \text{ W m}^{-2} \text{ K}^{-4}$

$\chi_{1}$ radiation constant, $1.19 \times 10^{-16} \text{ W m}^{-2}$

$\chi_{2}$ radiation constant, $1.4388 \times 10^{-2} \text{ m K}$

# 1. Introduction

Fibrous insulation materials perform well in controlling temperature and moisture for building applications [1]. Such materials allow minimizing the energy input required to maintain a fixed temperature. A low-density fibrous material thus behaves like a semi-transparent medium that remains entirely capable of both absorbing strongly anisotropic scattering and emitting thermal radiation [2]. The semi-transparent behavior of fibrous insulation materials at room temperature is the subject of a large body of studies aimed at describing their thermal properties [3–8]. Generally speaking, heat propagation can take place via various heat transfer modes, namely: (i) conduction in the interstitial fluid, mainly consisting of motionless air, as well as in a fibrous matrix; and (ii) radiation that propagates within the pores and interstices through the dual mechanism of fiber absorption, emission and scattering. It has been demonstrated that convection is often neglected in fibrous insulation materials [9], thus leaving two main modes for the heat transfer of insulation materials, namely radiation and conduction. Thermal radiation is the predominant heat transfer mode in a semi-transparent medium [7]; it is described by radiative properties, such as the extinction coefficient, scattering coefficient, absorption coefficient and scattering phase function, all of which have been defined for each wavelength. Radiative properties can be determined either by predictive methods based on theoretical models or by identification methods based on transmission and reflection measurements. In most previous works, the thermal radiation of fibers was examined by use of a direct method based on the interaction between fiber and radiation via the resolution of Maxwell's equations with the Mie theory

[3–4,10–14].

In addition, determining the radiative properties of insulation materials with the inverse method has been reported in many recent investigations [5–6,15–19]. The inverse method is based on experimental infrared measurements of reflection and transmission in order to determine the radiative properties (i.e. albedo, optical thickness and phase function coefficients) of fibrous insulation. Several studies offering varying degrees of complexity were carried out to identify the radiative properties of fibrous insulation. Yajnik et al. [20] developed a powerful method for deriving the spectral scattering and absorption coefficients for two different medium (i.e. glass fibers and expanded polystyrene) from monochromatic directional-hemispherical reflectance and hemispherical-directional reflectance measurements. The nonlinear least squares method seeks to minimize the difference between reflectivity measurements and their theoretical values within the 2–40 µm wavelength range in an effort to identify the coefficient values. It was noted that the scattering albedo is low for glass fibers and high for expanded polystyrene foams. Baillis et al. [8] thus determined the spectral radiative properties (i.e. absorption coefficient, scattering coefficient and phase function) of open-cell polyurethane foam from parameter identification methods. Spectral transmittance and reflectance measurements are conducted in the infrared wavelength region of 2–15 µm with different identification strategies and different types of measurements (directional-hemispherical, combination of directional-directional and directional-hemispherical). The discrete ordinate method was implemented in order to solve the Radiative Transfer Equation (RTE). Similarly, the RTE is also solved by means of a direct method (matrix exponential method) with minimization being
performed by the least squares method using Matlab software. Moreover, Zhao et al. [21] proposed an inverse method that employs the discrete ordinate method to solve the RTE, as well as steady-state and transient temperature measurements to estimate the conductive and radiative properties. The Levenberg-Marquardt iteration and Gauss linearization methods have also been used for performance optimization [5,22,23]. Both are accessible to measure the effective thermal conductivity, yet the radiative thermal conductivity estimation is unable to easily evaluate the radiation contribution in the fibrous insulation material. In our previous work [23], we investigated the effect of the thickness of homogeneous polyester batting fibrous insulation material, whose radiative thermal conductivity, according to our observation, depends on many parameters such as insulation material thickness, fiber density, porosity, fiber size, distribution and arrangement.

The present work reports on the investigation of radiative heat transfer behavior of fibrous insulation materials made from inhomogeneous recycled textile waste (i.e. featuring different fiber types and sizes) used in building applications. The radiative properties of insulation materials are identified in order to determine both the radiative thermal conductivity and effective thermal conductivity. The inverse method yields the radiative properties of recycled textile fibers. Direct calculations like in Mie theory are not applicable to obtaining for inhomogeneous structures the radiative properties from a single fiber of the studied fibrous insulation material because the fibers contain different circular cylinders with complex geometry. Three recycled textile fiber thicknesses have been investigated to evaluate the average radiative thermal conductivity value of fibrous insulation materials. The

following section will describe the studied material along with the most effective experimental method for characterizing the thermal properties of a fibrous insulation material made from recycled textile fibers. Next, the various models will be presented as regards the determination of radiative properties according to the inverse method. Lastly, the results obtained for recycled textile fiber samples will be displayed. Furthermore, to the best of our knowledge, this work represents the first time that the radiative properties and both the radiative and phonic conductivity of a fibrous insulation material with an inhomogeneous structure have been investigated using the inverse method.

## 2. Description of the studied material

In France, 85% of discarded clothing end up in a landfill; hence, each French person throws out on average 12 kg of clothes, shoes and linen every year. Plenty of such clothing waste can be recycled after completing a selection according to condition and composition. Next, the waste is frayed separately depending on fiber type. The fibrous insulation material based on recycled textile serves to reduce energy consumption relative to the heating requirements of every single building in taking into consideration users' comfort and health. The studied insulation material (from “le Relais”) [24–26] is an inhomogeneous medium obtained from discarded recycled textile fibers. It is composed of 85% recycled fibers from the textiles collected (70% cotton, 15% wool and acrylic) [27] and 15% of polyester fibers stemming from post-consumer waste plastic bottles [28], thus ensuring a density of 50 kg m⁻³. The recycled textile fiber agglomerates homogeneously by entrapping the air in the fiber network, thus constituting

<figure><img src="image_125.png" /><figcaption>Fig. 1. SEM pictures of fibrous insulation based on recycled textile fibers at magnifications of 100× (a, b), 200× (c) and 500× (d), respectively.</figcaption></figure>
an insulation material. The fibrous structure of the studied material is shown in Fig. 1. From pictures of the SEM (Scanning Electron Microscope) analyses (see Fig. 1a–d), we noted that the fibrous structure was indeed inhomogeneous. The fibrous structure containing various fiber sizes is shown in the Pictures 1a), 1b), 1c) and 1d). Three samples of the recycled textile insulation material were prepared with different thicknesses (5.34 mm, 5.68 mm and 6.50 mm) in order to characterize and compare its thermal properties. A thickness determination is solely based on density, the lateral dimensions of each sample, and the weight method in using a high-precision electronic scale.

### 3. Physical analysis

The thermal performance of insulation materials can be evaluated by means of thermal conductivity $k_e$ for a steady-state and thermal diffusivity $D$ in an unsteady state [25]. For the thermal insulation of buildings, the insulation material should be chosen for its low thermal conductivity values. Low thermal conductivity values are required in order to minimize heat losses [29]. However, this physical quantity remains one of the most difficult to measure, with many factors capable of influencing the thermal conductivity of this insulation material, including temperature, density, porosity, moisture and fiber direction. The heat transfer taking place inside the fibrous insulation material is composed of: (1) conduction in the solid matrix and interstitial fluid; and (2) natural convection via movement of the interstitial fluid and radiation [12]. The effective or equivalent thermal conductivity within fibrous materials is thus the sum of three conductivities (conduction (phonic), convection, and radiation) [30–32]:

$$k_{\text{eff}} = k_r + k_{\text{conv}} + k_c \quad (1)$$

In a fibrous material, convection is present thanks to air movement in the pores between fibers. The existence of convection in a fibrous medium may be estimated by calculating the modified Rayleigh number $Ra^*$ [9,33]; if $Ra^*$ is less than 40, then the convection transfer may be neglected. The modified Rayleigh number is defined by the following formula [9,33]:

$$Ra^* = \frac{g\eta\rho_{\text{air}}^2 C_p}{\mu_{\text{air}}} \times \frac{P}{k_e} \times \Delta T \times E \quad (2)$$

where $\Delta T= 20$K is the temperature differential between two sides of the sample. We calculated the modified Rayleigh number of recycled textile insulation material; these conditions have led to a modified Rayleigh number that lies around 2.12, so convection can be neglected. The effective thermal conductivity of the fibrous material can then be given as:

$$k_{\text{eff}} = k_r + k_c \quad (3)$$

The effective thermal conductivity of fibrous materials can now be divided into various thermal conductivities due to: radiation, solid conduction, and gas conduction [34]:

$$k_{\text{eff}} = k_r + k_G + k_F \quad (4)$$

where $k_c$ is the phonic thermal conductivity due to conduction in solids (fibers), while $k_F$ is the conduction due to gas $k_G$ [35]:

$$k_c = k_F + k_G \quad (5)$$

Fiber density is responsible for 60% of the local thermal conductivity and the fiber orientation perpendicular to the heat flux, and fiber tortuosity accounts for approximately 25% of the effect of thermal conductivity [36]. Several models [1,37–39] have been developed to evaluate the thermal conductivity due to solid conduction and gas conduction; Bankvall [40] proposed a mathematical approach to correlate the effective thermal conductivity of the two-phase solid and gas:

$$k_c = \alpha [\varepsilon_p k_g + (1-\varepsilon_p)k_S] + (1-\alpha) \frac{k_s k_g}{\varepsilon_s k_s + (1-\varepsilon_s)k_g} \quad (6)$$

with the thermal conductivity $k_F$ due to conduction in fibers and contact fibers being:

$$k_F = \alpha(1-\varepsilon_p)k_S \quad (7)$$

and $k_G$ the thermal conductivity due to gas in the material being:

$$k_G = \alpha \varepsilon_p k_g + (1-\alpha) \frac{k_s k_g}{\varepsilon_s k_s + (1-\varepsilon_s)k_g} \quad (8)$$

#### 3.1. Radiative heat transfer

This section reports on the determination of the radiative properties of recycled textile materials so as to enable both estimating the radiative thermal conductivity by use of the Rosseland approximation and understanding the spectral behavior of insulating materials. As mentioned above, the radiative properties of a fibrous material can be determined in two primary ways: (i) from theoretical models describing the interaction of radiant energy with a particulate medium, based on Maxwell's equation solutions and geometric optics combined with diffraction theory (microscopic approach); and (ii) from identification methods based on minimizing the quadratic differences between the theoretical values and directional measurements of radiation transmitted, reflected or emitted from samples, combined with RTE solutions (macroscopic approach) [7] to describe the radiation inside a semi-transparent medium. The studied medium is inhomogeneous and anisotropic; it is composed of air lying randomly in planes parallel to the boundaries. Our medium is considered to be semi-transparent, and the internal medium emission has been neglected. For a plane slab of a cold semi-transparent medium and an absorbing-scattering-emitting medium, in assuming transfer in the y-direction with azimuthal symmetry, the RTE can be written as follows [6,23]:

$$\mu \frac{\partial I_{\lambda}(y, \mu)}{\partial y} = I_{\lambda}(y, \mu) + \frac{\omega_{\lambda}}{2} \int_{-1}^{1} P_{\lambda}(\mu' \to \mu) I_{\lambda}(y, \mu') d\mu' \quad (9)$$

for $0<y<E$, $\mu \in [-1,1]$, $\lambda>0$ where $\mu = \cos\theta$, with $\theta$ being the polar angle and $I_{\lambda}$ the spectral intensity of radiation.

The radiative properties of the medium are characterized by the monochromatic coefficients of absorption $\kappa_{\lambda}$, diffusion $\sigma_{\lambda}$ and phase function $P_{\lambda}$ [5,22]. However, the same boundary conditions described in [23] to solve the RTE have been investigated herein. The RTE is indeed difficult to solve since it is an integro-differential equation and involves independent variables in the most general case; also, radiation intensity may depend on the spatial coordinates, angular direction and wavelength. The discretization of RTE is distributed at both angular and spatial discretizations. The angular discretization for choosing a given number of directions is associated with a weighting factor. In our case, the angular discretization is similar to that used in [22,23] with a constant step size of 4° on 44 points between 0° and 180°. This discretization replaces the integral term by a quadrature sum performed on the luminance with respect to the directions. In order to calculate the radiative intensity variation in directions characterized by $\mu$, the same method used in [22,23] to solve RTE (Eq. (9)) has been employed. The solution to the RTE is the spectral intensity field $I_{\lambda}$. This quantity $I_{\lambda}$ makes it possible to calculate both the theoretical directional hemispherical reflectance ($R_{\lambda,th}^{\text{dh}}$) (Eq. (10)) and the directional hemispherical transmittance ($T_{\lambda,th}^{\text{dh}}$) (Eq. (11)), as well as the direct transmittance ($T_{\lambda,th}^{\text{d}}$) (Eq. (12)) of the fibrous material.

#### 3.2. Definition of directional hemispherical transmittance and reflectance

$R_{\lambda,th}^{\text{dh}}$ denotes the ratio of the radiative flux emitted by the fibrous material toward the rear hemisphere to the incident flux, which is theoretically defined as in [22,23] by:
$$R_{\lambda,\text{th}}^{\text{dh}} = -2\pi \frac{\int_{-1}^{0} I_{\lambda}(0,\mu) d\mu}{I_{0} d\Psi_{0}} \qquad (10)$$

and $T_{\lambda,\text{th}}^{\text{dh}}$ denotes the ratio of the radiative flux emitted by the fibrous material toward the front hemisphere to the incident flux; it is theoretically defined as:

$$T_{\lambda,\text{th}}^{\text{dh}} = 2\pi \frac{\int_{0}^{1} I_{\lambda}(E,\mu) d\mu}{I_{0} d\Psi_{0}} \qquad (11)$$

Moreover, the direct transmittance $T_{\lambda,\text{th}}^{\text{d}}$ is given by:

$$T_{\lambda,\text{th}}^{\text{d}} = \frac{I_{\lambda}(E,\mu_{1})}{I_{0}} \qquad (12)$$

where $I_{\lambda}(E,\mu)$ and $I_{\lambda}(0,\mu)$ are the spectral intensity being transmitted and reflected, respectively. $d\Psi_{0}$ is the solid angle containing the incidence radiation, which is given by the following formula [5]:

$$d\Psi_0 = \frac{2\pi \sum_{i} (\phi_{\text{sample}}^r / \phi_{\text{reference}}^r)_{i} \sin\mu_i \Delta\mu}{R_{\lambda,\text{exp}}^{\text{dh}}(0,\mu)} \qquad (13)$$

The solid angle value can be determined from the bidirectional and hemispherical reflectance measurements using the FTIR spectrometer. After a numerical application (Eq. (13)), it becomes possible to obtain the solid angle value $d\Psi_{0} = 4.88 \times 10^{-3} \text{sr}$, where the divergence half-angle of the incident beam $\xi_{0}$ is correlated with the incident solid angle [5,22]:

$$d\Psi_0 = 2\pi(1 - \cos \xi_0) \qquad (14)$$

We then found $\xi_{0} = 2.35^{\circ}$ for the sample illumination.

### 3.3. The radiative diffusion of energy

In an optically dense medium, radiation will only traverse a very small distance before being absorbed, because the mean free path of the photons is small compared to the dimensions of the medium. The Rosseland model (or diffusion model) is mainly defined when the highly absorbing medium is optically dense and isotropic. The radiative conductivity can be deduced from experimental measurements, in particular the mean extinction coefficient, by adopting a Rosseland model; radiative properties could be identified. Application of Beer

<figure><img src="image_126.png" /><figcaption>Fig. 2. Investigation mode, (a) procedure for measuring hemispherical transmittance $T_{\lambda,\text{exp}}^{\text{dh}}$, (b) reflectance $R_{\lambda,\text{exp}}^{\text{dh}}$ and (c) direct transmittance $T_{\lambda,\text{exp}}^{\text{d}}$.</figcaption></figure>
Lambert's Law [41] to direct transmission results yields the initial value of extinction coefficient $β_λ$ as:

$$T_{\lambda} = \exp\left(-\int_{0}^{E} \beta_{\lambda} dy\right) \qquad (15)$$

The radiative conductivity $k_r$ of the porous medium is typically given as [42,43]:

$$k_r = \frac{16\sigma T_m^3}{3\beta_R} \qquad (16)$$

where $β_R$ is the Rosseland-mean extinction coefficient (m⁻¹) over all wavelengths, as obtained from the monochromatic extinction coefficients by the following formula:

$$\frac{1}{\beta_R} = \int_{\lambda=0}^{\infty} \frac{1}{\beta_{\lambda}^*} \frac{dI_{\lambda}^{\circ}(T_m)}{dI^{\circ}(T_m)} d\lambda \approx \int_{b}^{a} \frac{1}{\beta_{\lambda}^*} \frac{dI_{\lambda}^{\circ}(T_m)}{dI^{\circ}(T_m)} d\lambda \qquad (17)$$

where $I_{\lambda}^{\circ}(T)$ is the black-body intensity at temperature T, given by Planck's Law as:

$$I_{\lambda}^{\circ}(T) = \frac{\lambda_{1}}{\lambda^{5}\left(\exp\left(\frac{\lambda_{2}}{\lambda_{T}}\right)-1\right)} \qquad (18)$$

and $β_λ^*$ is a weighted spectral extinction coefficient [44,45].

## 4. Experimental methodology and radiation parameters

In order to characterize the radiative transfer of an insulation material made from recycled textiles, the determination of the radiative properties of the medium can be investigated according to an identification method based on both experimental measurements and the inverse method. Hence, to better understand the radiative heat transfer through fibrous insulation materials made from recycled textiles, three samples with different thicknesses were characterized using an FTIR

<figure><img src="image_127.png" /><figcaption>Fig. 3. (a) Experimental directional hemispherical transmittance, (b) reflectance and (c) direct transmittance of insulation based on recycled textile versus wavelength for different thicknesses.</figcaption></figure>
<figure><img src="image_128.png" /></figure>

<figure><img src="image_129.png" /></figure>

<figure><img src="image_130.png" /></figure>

<figure><img src="image_131.png" /><figcaption>Fig. 4. Radiative properties of insulation material based on recycled textile versus wavelength for different thicknesses, (a) identified optical thickness, (b) calculated optical thickness, (c) identified extinction coefficient and (d) calculated extinction coefficient.</figcaption></figure>
spectrometer for a wavelength range of 2–20 µm. The studied physical quantities are transmittance and reflectance directional-hemispherical monochromatic, plus direct transmittance. The experimental set-up used for the measurement campaign is an FTIR spectrometer built by Bruker, i.e. the VERTEX 80 V model, coupled with an integrating sphere produced by LABSPHERE featuring a coating of the “Infragold” type, which combines a highly reflective surface with a reflective power of 0.95 (i.e. manufacturer's specifications). This sphere has an internal diameter of 15 mm along with an opening diameter of 20 mm. The radiation introduced into an integrating sphere strikes the reflective walls and reproduces multiple diffuse reflections (Fig. 2a and b). The integrating sphere collects the radiation dispersed and measures the flux by use of a detector. The FTIR spectrometer is adapted to the near and mid-infrared measurements (1.5–25 µm), thus allowing for the measurement of directional-hemispherical transmittance and reflectance as well as direct transmittance. It contains a Globar MIR source for producing a broadband beam modulated through a Michelson interferometer [46]. This modulated beam is transmitted throughout the heated cell and is finally detected by a detector. The detector used is an MCT-referenced HgCdT, which is highly sensitive to noise, in particular of a thermal origin. It is cooled by liquid nitrogen, and the maximum range wavelength lies between 0.83 and 22 µm. The detector signal is then converted through a dual-channel A/D converter and ultimately processed and analyzed by OPUS (software for Bruker's FTIR) so as to display the transmittance and reflectance over a range of wavenumbers. The spectral distribution of the source flow is derived by a Fourier transform of this signal. Results are obtained from calculation of the ratio of the signal transmitted or reflected in the presence of the sample to the basic signal characterizing the source radiation, which is measured without a sample. Measurements were performed from 5000 scans averaged to limit the measurement noise. The spectrometer allows acquiring spectral directional transmittance and reflectance at a normal incidence. The interpretation of these spectral output serves to determine a greater number of parameters, in particular the phase function. The main steps of the measurement procedure or protocol of T<sup>dh</sup><sub>λ,exp</sub> and R<sup>dh</sup><sub>λ,exp</sub>, as well as T<sup>d</sup><sub>λ,exp</sub>, are well depicted in [23]. Let's briefly recall the procedure and equipment employed. We successively placed the three samples of recycled textile material into a sample holder in order to maintain them perpendicular to the input window of the integrating sphere, where the direction of incidence is normal to the sample. On the recycled textile material samples, the spectral results of directional-hemispherical transmittance and reflectance were collected by an integrating sphere after transmission or reflection within a wavelength ranging from 2 to 20 µm. T<sup>dh</sup><sub>λ,exp</sub> and R<sup>dh</sup><sub>λ,exp</sub> were then however calculated using the following relationships [23]:

$$R_{\lambda,\text{exp}}^{\text{dh}} = p_r \frac{\phi_{\text{sample}}^r - \phi_0}{\phi_{\text{reference}}^r - \phi_0} \qquad (19)$$

$$T_{\lambda,\text{exp}}^{\text{dh}} = \frac{\phi_{\text{sample}}^{\text{t}}}{\phi_{\text{reference}}^{\text{t}}} \qquad (20)$$

Fig. 2 provides a diagram of the measurement protocols for the three physical quantities of R<sup>dh</sup><sub>λ</sub>, T<sup>dh</sup><sub>λ</sub> with an integrating sphere using FTIR, and T<sup>d</sup><sub>λ</sub>. As regards the detector, we recovered the radiation reflected or transmitted through the sample in all directions. In order to obtain an absolute reflectivity value, a reflectance value of 0.95 was introduced. The experimental data for the transmission and reflectance given by the FTIR were obtained for wavelengths of 2333. Moreover, we selected 50 values to present the total evolution of the experimental transmission and reflectance based on the spectrum.

The incident radiation can be absorbed, transmitted or reflected. The contributions of these three phenomena, according to wavelength, are given by absorptance α<sub>λ</sub>, T<sup>dh</sup><sub>λ</sub> and R<sup>dh</sup><sub>λ</sub>, where the radiative energy balance is written as:

$$\alpha_{\lambda} + R_{\lambda}^{\text{dh}} + T_{\lambda}^{\text{dh}} = 1 \qquad (21)$$

## 5. Results and discussion

Our contribution seeks to identify the various thermal properties in the studied insulation material by using a similar previous approach. The radiative properties are identified for each wavelength from the following three physical quantities: R<sup>dh</sup><sub>λ,th</sub> (Eq. (10)), T<sup>dh</sup><sub>λ,th</sub> (Eq. (11)), and T<sup>d</sup><sub>λ,th</sub> (Eq. (12)). The radiative properties of the medium have thus been characterized by: the optical thickness (calculated for y = Ein Eq. (9)), the albedo, and the phase function [22]. Moreover, the Henyey-Greenstein phase function [47] was adopted in this model for numerical inversion [22,23]. The iterative identification method of the radiative properties of semi-transparent medium minimizes the F<sub>m</sub> function, which represents the average standard difference between theoretical and experimental investigations: R<sup>dh</sup><sub>λ</sub>, T<sup>dh</sup><sub>λ</sub> and T<sup>d</sup><sub>λ</sub> (Eq. (22)). The minimization routine was performed by the “lsqcurvefit” (least squares curve fit) MATLAB function; it's been provided in the Optimization Toolbox available for solving least squares curve-fitting problems [48]. A method has thus been developed with the MATLAB software, whose minimization calculation involves the F<sub>m</sub> gradient calculated from a derivation of RTE with respect to each target parameter. The function to be minimized is then written as:

$$F_m = \left[ \frac{R_{\lambda,th}^{\text{dh}} - R_{\lambda,\text{exp}}^{\text{dh}}}{R_{\lambda,\text{exp}}^{\text{dh}}} \right]^2 + \left[ \frac{T_{\lambda,th}^{\text{dh}} - T_{\lambda,\text{exp}}^{\text{dh}}}{T_{\lambda,\text{exp}}^{\text{dh}}} \right]^2 + \left[ \frac{T_{\lambda,th}^{\text{d}} - T_{\lambda,\text{exp}}^{\text{d}}}{T_{\lambda,\text{exp}}^{\text{d}}} \right]^2 \qquad (22)$$

The function to be minimized was presented in [22], whose theoretical values of R<sup>dh</sup><sub>λ,th</sub>, T<sup>dh</sup><sub>λ,th</sub> and T<sup>d</sup><sub>λ,th</sub> were calculated by solving Eq. (9) relative to μ and λ.

The measurements of directional-hemispherical transmittance and reflectance, as well as direct transmittance, of three thicknesses of insulation made from recycled textiles are plotted in Fig. 3a-c. We have observed that spectral measurements depend on wavelengths. The three samples of recycled textile insulation material all show the same spectral behavior with respect to wavelength with varying amplitudes. In Fig. 3a, we noted that the directional-hemispherical transmittance for the three thicknesses varied between 0% and 50%, which is very weak over the expected 3.03, 3.44, 7.33, 8.65–10.18, 13.75–14.24, 14.72–15.11 and 15.57–20 µm wavelengths. In Fig. 3b however, the directional-hemispherical reflectance varies considerably with wavelength; its minima were observed at wavelengths of 2.91, 3.42, 5.76, 6.86, 7.25, 7.58, 8.82, 9.14, 9.54 and 13.82 µm. Generally speaking, researchers [49,50] attribute the reflectance minima at these wavelengths to the Christiansen effect. The minimum directional-hemispherical transmittance value is observed at around the 3, 3.5, 7–10 and 14–20 µm wavelengths. The insulation material strongly absorbs radiation over this wavelength, as opposed to the 3–3.5, 3.5–4.5, 5–5.5 and 6.25–7 µm wavelengths; the insulation material is purely scattering and less absorbing. Fig. 3c shows the direct transmittance variation of the insulation material vs. wavelength; we noted the presence of peaks around the 2, 2.21, 2.61, 3.3, 4.25, 4.54, 5, 5.3, 6.42 and 8.4 µm wavelengths, with all these peaks corresponding to attenuation of the albedo and to the phase function of the insulation material.

### 5.1. Radiative properties of insulation made from recycled textiles

By application of the inverse method described in [22,23], the radiative properties (e.g. optical thickness, albedo, phase function, absorptance) of insulation made from recycled textile have been identified. Fig. 4a-d illustrate the variation in both calculated and identified optical thickness for the three thicknesses of recycled textile insulation material. Let's note that optical thickness depends on the wavelength and curves of the three thicknesses, which feature a similar shape at different amplitudes. It was also noted that the appearance of noise in the calculated optical thickness is observed between the 8 µm and 20 µm wavelengths (Fig. 4a). The drop in optical thickness observed
around the 5, 5.5, 6.8, 8.5, 10.8 and 12 µm wavelengths thus results in a radiation decrease at these wavelengths. However, the presence of some optical thickness peaks around the 3, 3.5, 4.5, 5.5, 7.5, 9.5 and 11 µm wavelengths could be noted, as these peaks correspond to high radiation extinction by the medium [22]. Fig. 4c presents the extinction coefficient variation of all three thicknesses of the studied material vs. wavelength. The extinction coefficient is calculated by means of Beer-Lambert's Law (Eq. (15)), and the coefficient identified from the inverse method applied to the three thicknesses of the studied material has been plotted in Fig. 4d. We found that the calculated and identified extinction coefficients showed the same spectral variation vs. wavelength, with the decrease in extinction coefficient being observed around wavelengths 3.2, 4–6, 6.5, 8.5, 10.5 and 12 µm. The extinction coefficient drop at these wavelengths is due to a significant increase in transmittance, while the lowering amplitudes for these wavelengths remain distinct. It was also reported that amplitude dispersions can be observed as of a wavelength of 14 µm. These amplitude variations are due to the upper detection limit established by the MCT detector.

The spectral absorptance of the three thicknesses has been deduced from Eq. (21). Fig. 5 shows the absorptance variation in insulation made from recycled textile vs. wavelength. Different absorptance behaviors dependent on both thickness and wavelength have been observed. The insulation made from recycled textile materials is highly absorbent at wavelengths of 2.16, 2.5, 2.8, 3.4, 4.46, 5.7 and 6.97 µm. Furthermore, it is important to know that the absorptance increase is due to the effects of decreased transmittance and reflectance at these wavelengths, while the absorptance drop is due to an increase in transmittance and reflectance. In addition, absorptance varies slightly between 13.64 and 20 µm and fluctuates from 0.94% to 0.96% at these wavelengths.

An estimation of radiative properties considers the adoption of a model to calculate the phase function. In order to minimize the standard deviation between experimental and theoretical approaches, the theoretical values have been calculated via the direct resolution of RTE. The asymmetry coefficient of phase function $g_\lambda$ has been obtained by means of identification from measurements of the $R_{\lambda,exp}^{\text{dh}}$, $T_{\lambda,exp}^{\text{dh}}$ and $T_{\lambda,exp}^{\text{d}}$ of the insulation material. Measurements were carried out in the vacuum vessel of an FTIR spectrometer. From these measurements, we identified the following three parameters: optical thickness $\tau_\lambda$, albedo $\omega_\lambda$, and anisotropy coefficient $g_\lambda$ of Henyey-Greenstein's phase function. In general, the phase function may be isotropic or anisotropic. Coefficient $g_\lambda$ sets values between -1 and 1, with: $g_\lambda = -1$ meaning complete backscattering, $g_\lambda = 0$ isotropic scattering, and $g_\lambda = 1$ complete forward scattering. For $g_\lambda > 0$, forward scattering is predominant, while for $g_\lambda < 0$, backscattering prevails. The identified anisotropy coefficient $g_\lambda$ is plotted in Fig. 6 for all three insulation material thicknesses. Let's note that the anisotropy coefficient $g_\lambda$ of all three

thicknesses display values within the 0.45–0.8 interval, which means that radiation propagation through a sample is directional and moreover that forward scattering dominates.

The albedo describes the proportion of incident radiation reflected by the studied insulation material. Fig. 7 shows the albedo variation in insulation material vs. the wavelength for various thicknesses. Let's note that the three thicknesses of studied material all exhibit the same spectral behavior. The albedo decreases have been recorded around wavelengths 2.9, 7.6, 9.02, 14.1 and 17.7 µm and are due to the extinction influence on the scattering, which corresponds to the Christiansen effect. The albedo peaks are observed around wavelengths of 2.53, 5.42, 6.14, 7.94, 10.83 and 16.94 µm for three thicknesses, owing to the predominant absorption features of the studied material.

The evolution in scattering coefficient relative to wavelength of all three material thicknesses is plotted in Fig. 8; the few drops in the scattering coefficient were repeated at 2.9, 7.6, 9.02, 14.1, 15.9 and 17.7 µm wavelengths for the three thicknesses in comparison with albedo. The scattering coefficient peaks have been observed however at 2.2, 5.42, 6.5, 7.94, 10.47, 15.15 and 16.94 µm.

The absorption coefficient indicates the reciprocity of radiation penetration depth into an insulation material; specifically, it can estimate the amount of absorbed thermal radiation. Fig. 9 shows the absorption coefficient variation $\kappa_\lambda$ vs. wavelength. Note that the absorptance coefficient reaches its maximum value at 880 m⁻¹ and peaks at wavelengths of 2.9, 6.14, 7.6, 9.02, 14.1, 17.7 and 19.45 µm. This coefficient decreases were recorded around wavelengths of 2.53, 3.97, 5.42, 6.5, 8.3, 10.83 and 11.91 µm, with a smaller absorption coefficient suggesting a greater thermal radiation distance being propagated before absorption.

## 5.2. Thermal conductivities

This section investigates the radiative and effective conductivities, whereby the radiative part was first determined by the Rosseland model. The effective thermal conductivity is determined by an experimental device according to the fluxmeter method. This same device had been used by the authors [24,51–53] to characterize the thermal properties of a phase change material. The sample is placed between a hot plate $T_h$ and a cold plate $T_c$, with the heat flux created by the temperature differential being measured using a heat flux sensor on one of the sample faces. The effective thermal conductivity can thus be obtained under a high thermal gradient. A steady-state heat flux through a fibrous insulation material includes contributions from both conduction and radiation; the thermal measurement technique is detailed in [23]. The effective thermal conductivity equals 0.039 W m⁻¹ K⁻¹; this value ranges between 0.036 and 0.042 W m⁻¹ K⁻¹ for commercial recycled textile, as proposed in [25]. The radiative thermal conductivity values

<figure><img src="image_132.png" /><figcaption>Fig. 5. Absorptance of insulation based on recycled textile versus wavelength for different thicknesses.</figcaption></figure>
<figure><img src="image_133.png" /><figcaption>Fig. 6. Phase function versus wavelength for different thicknesses.</figcaption></figure>

have been obtained from a reasonably accurate Rosseland approximation (Eq. (16)) within a range of wavelengths between 2 and 20 µm for the three samples of studied material. As expected, 45% of the black-body radiation is neglected over the wavelength $λ = [2–21] µm$ range when calculating radiative conductivity. In other words, the wavelength range is recommended to estimate the radiative conductivity of fibrous insulation materials at room temperature ($λ = [2–45] µm$ [54], for which only 5% of the black-body radiation was neglected [55]). It should be recalled herein that the three different thicknesses of the studied material all have the same porosity and density. Fig. 10a presents the radiative thermal conductivity values for each thickness. The maximum value of radiative thermal conductivity equals $0.0102 W m⁻¹ K⁻¹$ for a thickness of 5.67 mm. The radiative thermal conductivity thus slightly decreases to the value $0.010 W m⁻¹ K⁻¹$ at a thickness of 6.46 mm, and to a minimum value of $0.0096 W m⁻¹ K⁻¹$ at a thickness equal to 5.35 mm. This decrease may be due to the fiber arrangements; however, it is worth mentioning that the various values obtained of radiative thermal conductivity are also due to the fiber density and different fiber sizes in the three samples.

Once again, these findings are obviously due to the fact that the highly porous fibrous material serves to increase the radiative heat transfer contribution and, as a consequence, increase the radiative conductivity in fibrous insulation. In contrast, the phonic thermal conductivity was deduced from Eq. (1) for the three samples. This conductivity is due to atomic interactions and phonon collisions in the various fibers [56]. Fig. 10b shows the radiative and phonic thermal conductivity relative to the thickness of the studied material. It is observed that this phonic thermal conductivity is predominant compared to radiative thermal conductivity. The average radiative thermal

conductivity value equals approx. $0.01 W m⁻¹ K⁻¹$; this value will be used subsequently as a reference to evaluate the impact of radiative thermal conductivity on transient heat transfer through multilayered building walls.

## 6. Conclusion

This work has presented a study on the thermal characterization of a fibrous insulation material made from recycled textile waste clothing for building applications. The studied material therefore is not homogeneous but rather composed of different fiber sizes. Moreover, the determination of radiative properties depends on the actual characteristics of the fibrous materials. In our study, the inverse method has been adopted since it allows investigating complex fibrous materials featuring various fiber sizes compared to the Mie theory. The radiative properties have been identified (albedo, optical thickness, phase function) by means of the inverse method based on experimental measurements of hemispherical reflectance and transmittance. These properties also strongly depend on wavelengths and material thicknesses. In addition, our medium is optically thick, and the Rosseland approximation serves to calculate the radiative thermal conductivity through the use of hemispherical measurements with an isotropic phase function. The radiative thermal conductivity of fibrous insulation made from recycled textile has been calculated with the Rosseland mean extinction coefficients, whereby the effective thermal conductivity has been measured by a fluxmeter device, with the phonic thermal conductivity also being deduced from the effective thermal conductivity. An indication of the effect of fibrous insulation thickness on both radiative and phonic thermal conductivities could thus be observed. As

<figure><img src="image_134.png" /><figcaption>Fig. 7. Albedo of insulation based on recycled textile versus wavelength for different thicknesses.</figcaption></figure>
<figure><img src="image_135.png" /><figcaption>Fig. 8. The scattering coefficient of insulation based on recycled textile versus wavelength for different thicknesses.</figcaption></figure>

<figure><img src="image_136.png" /><figcaption>Fig. 9. The absorption coefficient versus wavelength for different thicknesses.</figcaption></figure>

<figure><img src="image_137.png" /><figcaption>Fig. 10. (a) The radiative thermal conductivity as a function of samples thickness, (b) thickness effect on the radiative and phonic thermal conductivity.</figcaption></figure>

expected, the radiative thermal conductivity is very low compared to phonic thermal conductivity, i.e. representing 26%. However, the phonic thermal conductivity can be improved in order to raise the level of the thermal resistance of fibrous insulation materials by increasing porosity and to reduce the material fiber diameter. The radiation contribution is not negligible when radiative heat transfer is taken into account in the insulation. Future studies will investigate the effect of

thermal properties (i.e. radiative, phonic and effective thermal conductivity) of fibrous insulation materials on the thermal behavior of building envelopes.

## Acknowledgements

The authors acknowledge here Le Relais for its contribution in this
project as well as the Institute for Electronics Microelectronics and Nanotechnologies (IEMN) of the University of Lille - France, for taking microscopic pictures of fibrous insulation material based on recycled textile. Authors thank Prof. Gérard Jeandel from LEMTA, University of Lorraine for useful discussion of the present manuscript.

## References

[1] A. Karamanos, S. Hadiarakou, A. Papadopoulos, The impact of temperature and moisture on the thermal performance of stone wool, Energy Build. 40 (2008) 1402-1411.

[2] G. Wei, Y. Liu, X. Zhang, X. Du, Radiative heat transfer study on silica aerogel and its composite insulation materials, J. Non-Cryst. Solids 362 (2013) 231-236.

[3] G.H. Tang, Y. Zhao, J.F. Guo, Multi-layer graded doping in silica aerogel insulation with temperature gradient, Int. J. Heat Mass Transfer 99 (2016) 192-200.

[4] R. Arambakam, H. Vahedi Tafreshi, B. Pourdeyhimi, Dual-scale 3-D approach for modeling radiative heat transfer in fibrous insulations, Int. J. Heat Mass Transfer 64 (2013) 1109-1117.

[5] A. Milandri, F. Asllanaj, G. Jeandel, Determination of radiative properties of fibrous media by an inverse method-comparison with the Mie theory, J. Quant. Spectrosc. Radiat. Transfer 74 (2002) 637-653.

[6] S. Le Foll, A. Delmas, F. André, Identification of radiative properties for heterogeneous materials at high temperature, Int. J. Therm. Sci. 120 (2017) 314-320.

[7] D. Baillis, J.F. Sacadura, Thermal radiation properties of dispersed media: theoretical prediction and experimental characterization, J. Quant. Spectrosc. Radiat. Transfer 67 (2000) 327-363.

[8] D. Baillis, M. Arduini-Schuster, J.F. Sacadura, Identification of spectral radiative properties of polyurethane foam from hemispherical and bidirectional transmittance and reflectance measurements, J. Quant. Spectrosc. Radiat. Transfer 73 (2002) 297-306.

[9] R. Arambakam, H. Vahedi Tafreshi, B. Pourdeyhimi, Modeling performance of multi-component fibrous insulations against conductive and radiative heat transfer, Int. J. Heat Mass Transfer 71 (2014) 341-348.

[10] Y.-J. Dai, Y.-Q. Tang, W.-Z. Fang, H. Zhang, W.-Q. Tao, A theoretical model for the effective thermal conductivity of silica aerogel composites, Appl. Therm. Eng. 128 (2018) 1634-1645.

[11] J. Yang, H. Wu, M. Wang, S. He, H. Huang, Prediction and optimization of radiative thermal properties of ultrafine fibrous insulations, Appl. Therm. Eng. 104 (2016) 394-402.

[12] A. Milandri, F. Asllanaj, G. Jeandel, J.R. Roche, Heat transfer by radiation and conduction in fibrous media without axial symmetry, J. Quant. Spectrosc. Radiat. Transfer 74 (2000) 585-603.

[13] J. Yang, H. Wu, M. Wang, Y. Liang, Prediction and optimization of radiative thermal properties of nano TiO2 assembled fibrous insulations, Int. J. Heat Mass Transfer 117 (2018) 729-739.

[14] H.T. Tagne Kamdem, Radiative characteristics of high-porosity media containing randomly oriented fibers in space, J. Therm. Sci. Eng. Appl. 9 (2017) 021014.

[15] H. Liu, X. Xia, X. Xie, Q. Ai, D. Li, Experiment and identification of thermal conductivity and extinction coefficient of silica aerogel composite, Int. J. Therm. Sci. 121 (2017) 192-203.

[16] P. Udayraj, A. Talukdar, R. Das, Agalirusamy, estimation of radiative properties of thermal protective clothing, Appl. Therm. Eng. 100 (2016) 788-797.

[17] J.F. Sacadura, D. Baillis, Experimental characterization of thermal radiation properties of dispersed media, Int. J. Therm. Sci. 41 (2002) 699-707.

[18] S. Zhao, W. Zhang, X. He, J. Li, Y. Yao, X. Lin, Simultaneous reconstruction of thermal degradation properties for anisotropic scattering fibrous insulation after high temperature thermal exposures, Appl. Therm. Eng. 78 (2015) 507-515.

[19] J. Randrianalisoa, D. Baillis, Improved inverse method for radiative characteristics of closed-cell absorbing porous media, J. Thermophys. Heat Transfer 20 (2006).

[20] S. Yajnik, J.A. Roux, Determination of radiative properties of fiberglass and foam insulations, Technical Report, Mississippi University (USA), 1987.

[21] S.Y. Zhao, B.M. Zhang, S.Y. Du, An inverse analysis to determine conductive and radiative properties of a fibrous medium, J. Quant. Spectrosc. Radiat. Transfer 110 (2009) 1111-1123.

[22] A. Kaemmerlen, F. Asllanaj, H. Sallée, D. Baillis, G. Jeandel, Transient modeling of combined conduction and radiation in wood fiber insulation and comparison with experimental data, Int. J. Therm. Sci. 49 (2010) 2169-2176.

[23] A. Tilioua, L. Libessart, G. Jeandel, S. Lassue, Determination of radiative properties of polyester batting insulation material from hemispherical transmittance and reflectance measurements, Appl. Therm. Eng. 105 (2016) 594-604.

[24] C. Lacoste, R. El Hage, A. Bergeret, S. Corn, P. Lacroix, Sodium alginate adhesives as binders in wood fibers/textile waste fibers biocomposites for building insulation, Carbohydr. Polym. 184 (2018) 1-8.

[25] S. Schiavoni, F. D'Alessandro, F. Bianchi, F. Asdrubali, Insulation materials for the building sector: a review and comparative analysis, Renew. Sustain. Energy Rev. 62 (2016) 988-1011.

[26] F. Asdrubali, F. D'Alessandro, S. Schiavoni, A review of unconventional sustainable

building insulation materials, Sustain. Mater. Technol. 4 (2015) 1-17.

[27] CSTB, European Technical Approval ETA-12/0009 2011.

[28] A. Patnaik, M. Myvubu, S. Muniyasamy, A. Botha, Thermal and sound materials from waste wool and recycled polyester fibers and their biodegradation, Energy Build. 92 (2015) 161-169.

[29] W.N. Dos Santos, J.A. De Sousa Jr, R. Gregorio, Thermal conductivity behaviour of polymers around glass transition and crystalline melting temperatures, Polym. Test. 32 (2013) 987-994.

[30] J.J. Zhao, Y.Y. Duan, X.D. Wang, B.X. Wang, Radiative properties and heat transfer characteristics of fiber-loaded silica aerogel composites for thermal insulation, Int. J. Heat Mass Transfer 55 (2012) 5196-5204.

[31] J. Wang, J. Kuhn, X. Lu, Monolithic silica aerogel insulation doped with TiO₂ powder and ceramic fibers, J. Non-Cryst. Solids 186 (1995) 296-300.

[32] O.J. Lee, K.H. Lee, T.J. Yim, S.Y. Kim, K.P. Yoo, Determination of mesopore size of aerogels from thermal conductivity measurements, J. Non-Cryst. Solids 298 (2002) 287-292.

[33] J. Lux, A. Ahmadi, C. Gobbé, C. Delisée, Macroscopic thermal properties of real fibrous materials: volume averaging method and 3D image analysis, Int. J. Heat Mass Transfer 49 (2006) 1958-1973.

[34] S.A. Lurie, Y.O. Solyaev, D.V. Lizunova, L.N. Rabiniskiy, V.M. Bouznik, O. Menshykov, Influence of mean distance between fibers on the effective gas thermal conductivity in highly porous fibrous materials, Int. J. Heat Mass Transfer 109 (2017) 511-519.

[35] C.G. Bankvall, Heat Transfer in Insulation and Insulated Structure, Divison of Building Technology, Lund Institute of Technology, 1972.

[36] M. Faessel, C. Delisée, F. Bos, P. Castéra, 3D Modelling of random cellulosic fibrous networks based on X-ray tomography and image analysis, Compos. Sci. Technol. 65 (2005) 1931-1940.

[37] J.-H. Hao, Q. Chen, K. Hu, Porosity distribution optimization of insulation materials by the variational method, Int. J. Heat Mass Transfer 92 (2016) 1-7.

[38] X. Cao, J. Liu, X. Cao, Q. Li, E. Hu, F. Fan, Study of the thermal insulation properties of the glass fiber board used for interior building envelope, Energy Build. 107 (2015) 49-58.

[39] K. Daryabeigi, Heat transfer in high-temperature fibrous insulation, 8thAIAA/ASME Joint Thermophysics and Heat Transfer Conference, St. Louis, MO 24-26 June 2002.

[40] C.G. Bankvall, Heat transfer in fibrous materials, J. Test. Eval. 1 (1973) 235-243.

[41] R. Siegel, Thermal Radiation Heat Transfer, fourth ed., Taylor & Francis, 2001.

[42] C. Jang, H. Jung, J. Lee, T.H. Song, Radiative heat transfer analysis in pure scattering layers to be used in vacuum insulation panels, Appl. Energy 112 (2013) 703-709.

[43] H. Zhang, X. Wang, Y. Li, Measuring radiative properties of silica aerogel composite from FTIR transmittance test using KBr as diluents, Exp. Therm Fluid Sci. 91 (2018) 144-154.

[44] D. Baillis, M. Raynaud, J.F. Sacadura, Determination of spectral radiative properties of open cell foam: model validation, J. Thermophys. Heat Transfer 14 (2000) 137-143.

[45] A. Öchsner, G.E. Murch, M.J.S. de Lemos, Cellular and Porous Materials: Thermal Properties Simulation and Prediction, WILEY-VCH, 2008.

[46] A. Simon, G. Zachmann, Automatic beamsplitter exchange unit for a research vacuum FT-IR spectrometer, Vib. Spectrosc. 60 (2012) 98-101.

[47] M.F. Modest, Radiative heat transfer, second ed., Academic Press, San Diego, 2003.

[48] D. Xue, Y.Q. Chen, Solving Applied Mathematical Problems with MATLAB, CRC Press, 2008.

[49] J.W. Salisbury, L.S. Walter, Thermal infrared (2.5-13.5µm) spectroscopic remote sensing of igneous rock types on particulate planetary surfaces, J. Geophys. Res. 94 (1989) 9192-9202.

[50] J.W. Salisbury, Chapter 4-Mid-infrared spectroscopy: laboratory data, in: C.M. Pieters, P.A.J. Englert (Eds.), Remote Geochemical Analysis: Elemental and Mineralogical Composition, Cambridge University Press, 1993.

[51] P. Tittelein, S. Gibout, E. Franquet, K. Johannes, L. Zalewski, F. Kuznik, J.P. Dumas, S. Lassue, J.P. Bédécarrats, D. David, Simulation of the thermal and energy behaviour of a composite material containing encapsulated-PCM: Influence of the thermodynamical modelling, Appl. Energy 140 (2015) 269-274.

[52] A. Joulin, Z. Younsi, L. Zalewski, S. Lassue, D.R. Rousse, J.P. Cavrot, Experimental and numerical investigation of a phase change material: Thermal-energy storage and release, Appl. Energy 88 (2011) 2454-2462.

[53] A. Joulin, L. Zalewski, S. Lassue, H. Naji, Experimental investigation of thermal characteristics of a mortar with or without a micro-encapsulated phase change material, Appl. Therm. Eng. 66 (2014) 171-180.

[54] C. Stark, J. Fricke, Improved heat-transfer models for fibrous insulations, Int. J. Heat Mass Transfer 36 (1993) 617-625.

[55] A. Kaemmerlen, Transfert de chaleur à travers les isolants thermiques du bâtiment, Thèse de doctorat, Université Henri-Poincaré, Nancy 1 (Université de Lorraine), 2009.

[56] N. Burger, A. Laachachi, M. Ferriol, M. Lutz, V. Toniazzo, D. Ruch, Review of thermal conductivity in composites: mechanisms, parameters and theory, Prog. Polym. Sci. 61 (2016) 1-28.
# Annexe 3

Cette annexe présente les articles :

(E) Libessart L., Djelal C., De Caro P., "Influence of the type of release oil on steel formwork corrosion and facing aesthetics", *Construction and Building Materials*, 68, 391-401, 2014

(F) Libessart L., Djelal C., de Caro P., Laiymani I., "Comparative study of the tribological behaviour of emulsions and demoulding oils at the concrete/formwork interface", *Construction and Building Materials*, 239, 2020

Ces articles illustrent les apports faits à l'interface coffrage/béton sur la caractérisation physico-chimique d'agents de démoulage, sur l'esthétique des parements en béton, ou encore sur le frottement du béton. J'indique par la suite, les mots clefs importants associés aux références des articles.

Mots clefs : Application des agents de démoulage (E, F), Corrosion (E), Emulsion (F), Energie d'adhésion (E, F), Epaisseur de film (F), Esthétique des parements (E), Huile (E), Interférométrie (F), Mécanisme à l'interface (E, F), Moule (E), Rugosité (E, F), Spectrométrie d'impédance électrochimique (E), Tension de surface (E, F), Tribologie (F), Viscosité dynamique (E, F).
<figure><img src="image_138.png" /></figure>

ELSEVIER

<figure><img src="image_139.png" /></figure>

# Influence of the type of release oil on steel formwork corrosion and facing aesthetics

<figure><img src="image_140.png" /></figure>

CrossMark

Laurent Libessart<sup>a,*</sup>, Chafika Djelal<sup>b</sup>, Pascale de Caro<sup>c,d</sup>

## HIGHLIGHTS

* The observed variations in wetting power with respect to the formwork do not appear to be significant for facing quality.

* The film thickness for vegetable-based oils stands out as a critical parameter in controlling the dusting phenomenon.

* The interface structure tends to favour protecting formwork surfaces against corrosion.

* The obtained results reveal a better resistance by vegetable oil-based films.

* The synthetic ester enhances the protective power of the film, resulting from the formation of a double soap layer.

## ARTICLE INFO

**Article history:**

Received 27 January 2014

Received in revised form 16 May 2014

Accepted 29 June 2014

**Keywords:**

Release agents

Formwork

Physicochemical

Electrochemical spectroscopy of impedance

Aesthetic

Corrosion

## ABSTRACT

The quality of the concrete facings results from the implementation of release agents and the casing surface quality. It is interesting to characterize the behaviour of release agents over the lifetime of the formwork. This study is based on the many ones test physicochemical and esthetics realized on two types of surface of formwork like on three types of applications. In complement, the ageing of the casing formwork was characterized by electrochemical spectroscopy of impedance and thanks to the correlation with the aesthetic tests will allow to find from new mechanisms to the interfaces related to the studied oil's formulations.

© 2014 Elsevier Ltd. All rights reserved.

## 1. Introduction

When removing formwork, two distinct elements become clearly apparent, namely the formwork itself which consists of a smooth and stiff material, and a concrete, a water-containing plastic material. From the time of concrete casting, there is an affinity between these two materials, and their interface gives rise to certain physicochemical mechanisms that are still poorly known. Once the concrete has hardened, the two elements can be separated: Such a step is commonly known as formwork removal. In the absence of an anti-adhesive separating layer (release oil), bonding would automatically occur, making its unmolding impossible without altering the facing surface [1]. Consequently, the

aesthetics of these facings is directly correlated with the type of concrete/formwork interface.

In order to limit phenomena leading to the corrosion of metal form plates, metal formwork manufacturers (PERI, OUTINORD, SATECO, to name a few) have expressed interest in using new release oil mixes, which are capable of extending the formwork life cycle [2]. For this reason, release agents must be selected on the basis of the type of formwork and its compatibility with formwork skins. Such agents must be applied evenly across the entire formwork, on a clean surface and in thin layers of uniform thickness, before installing reinforcements [3].

It is noted that mineral oils remain very widespread despite their resulting environmental impact. Release agents are actually included in the category of lost lubricants, in particular through leaching into the water table from lubrication zones. Moreover, oil-based release agents cause a number of known nuisances for

<footnote><sup>a</sup> Laboratoire Génie Civil et géo-Environnement (LGCgE) – Lille Nord de France (EA 4515), Hautes Etudes d'Ingénieur, 13 Rue de Toul, F-59046 Lille Cedex, France</footnote>

<footnote><sup>b</sup> Laboratoire Génie Civil et géo-Environnement (LGCgE) – Lille Nord de France (EA 4515), IUT de Béthune, 1230 Rue de l'Université, F-62400 Béthune, France</footnote>

<footnote><sup>c</sup> Université de Toulouse, INP, LCA (Laboratoire de Chimie Agro-Industrielle), ENSIACET, 4 Allée Emile Monso, F-31030 Toulouse, France</footnote>

<footnote><sup>d</sup> INRA, UMR 1010 CAI, F-31030 Toulouse, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: laurent.libessart@hei.fr (L. Libessart).</footnote>
<figure><img src="image_141.png" /><figcaption>**Fig. 1.** Application pattern of test oils on the mould walls.</figcaption></figure>

users, and related professional diseases have been catalogued by the French office of public health insurance (skin irritations by cutaneous exposure and inhalation of volatile organic compounds) [4]. To overcome these disadvantages, oils with vegetable-based formulations [5] and synthetic oils, both presumed to be less harmful to humans and the environment, have been developed.

To promote use of these oils in the concrete industry, the national union trade organization, which represents producers of concrete admixtures and mortars (SYNAD), has adopted a new charter on release oils in 2010 [6] having served to revise the year 2000 classification, so as to provide users with greater clarity, while pursuing the same objective of the enhanced use of vegetable oils.

Despite the entry of these new mix designs into the market, data are still insufficient to fully understand the specificities of their respective product families in terms of performance and use conditions. Therefore, the aim of the present study is to assess the influence of release oils on the quality of facings as well as on corrosion protection during outdoor formwork storage. The methodology adopted entails selecting four oil formulations: a vegetable oil (Oil 1), a synthetic oil (Oil 2), a mineral oil (Oil 3), and a vegetable oil associated with a synthetic oil (ester) (Oil 4).

## 2. Material and methods

### 2.1. The moulds

Eight 30 × 30 × 30 cm³ moulds were produced by a formwork manufacturer. The volume of the moulds is of 27 l. The moulds were built with both new and used walls. Two moulds were required per test since the oils are to be applied on mould surfaces according to the following scheme (Fig. 1).

The distribution of the moulds used in these tests was carried out as follows:

* Two new moulds (see Fig. 2) for application by spraying with conic and flat nozzles.

* Two new moulds for application by spraying followed by scraping.

* Two used moulds (see Fig. 3) for application by spraying with conic and flat nozzles.

* Two used moulds for application by spraying followed by scraping.

<figure><img src="image_142.png" /><figcaption>**Fig. 2.** New formwork surface.</figcaption></figure>

<figure><img src="image_143.png" /><figcaption>**Fig. 3.** Used formwork surface.</figcaption></figure>

### 2.2. The concrete

To avoid the kinds of physicochemical reactions not easily interpreted between release oil ingredients and the admixture sometimes present in concrete, a conventional concrete without any admixtures was prepared for our study. Other studies [7,8] are currently underway using various self-compacting concrete designs in order to examine interactions between chemical compounds in the oils and concrete. For our study, a 10-cm slump has been obtained using the Abrams cone. The composition of the test concrete is gathered in Table 1.

This concrete was produced at a 200 l capacity mixing plant. Mixing was carried out in accordance with the Standard NF P 18-404, entitled "Concretes: Mix design, suitability and inspection tests – Casting and storage of test specimens", which specifies the following operating procedure:

<figure><img src="image_144.png" /></figure>

The roughness of a surface is measured by moving a pick-up lift following a direction parallel at average surface to analyze. Measurements are carried out by a roughometer Surtronic 3+. The device precision is ±0.1 µm.

The roughness values of mould surfaces were $Ra = 1.30 \ \mu m$ for the new surface and $1.70 \ \mu m$ for the used surface. $Ra$ is the most widely used international roughness parameter: it is the arithmetic average of profile deviations with respect to an average line.

The tests were conducted in the laboratory at room temperature (about 20 °C). The concrete was cast simultaneously in all moulds, with each mould being filled in two layers followed by vibration in 5 places for 10 s per layer.

It should be noted that our study has focused on aesthetic flaws encountered in the concrete facing after formwork removal [9]. The two modes of application described above were analysed from this perspective. In addition, colour variations, micro-bubbling, dusting on the concrete facing, dirtying, and dusting and
<table><caption>**Table 1**
Concrete composition.</caption><thead><tr><th>Components</th><th>Silex 5/15</th><th>Gravel 4/8</th><th>Sand 0/4</th><th>Cement CEM I 52,5</th><th>Fillers</th><th>Water</th><th>W/B</th></tr></thead><tbody><tr><th>Quantities (kg/m³)</th><td>761</td><td>281</td><td>826</td><td>258</td><td>86</td><td>167</td><td>0.49</td></tr></tbody></table>

<table><caption>**Table 2**
SYNAD classification system [6].</caption><thead><tr><th>Type of oil</th><th>Environment</th><th>Health</th><th>Safety – fire</th></tr></thead><tbody><tr><td>Oil 1</td><td>4/5</td><td>3/5</td><td>4/5</td></tr><tr><td>Oil 2</td><td>3/5</td><td>3/5</td><td>4/5</td></tr><tr><td>Oil 3</td><td>1/5</td><td>1/5</td><td>4/5</td></tr><tr><td>Oil 4</td><td>3/5</td><td>3/5</td><td>4/5</td></tr></tbody></table>

attachment points on the formwork surface constitute the set of parameters selected to evaluate the appearance of facings and the quality of formwork walls.

## 2.3. Release oils

It is useful to recall that the classification system adopted by the SYNAD trade organisation [6] demonstrates that vegetable oil (Oil 1) display the best characteristics in terms of protecting the environment and human health (Table 2).

Table 3 lists the characteristics of test oils.

Note that the flash point has been identified, both to assess the product's volatility and to determine the associated safety risks (release of flammable volatile organic compounds).

## 2.4. Procedure adopted for applying the oils

The procedure to apply the oils on the mould wall is particularly important because it can affect the quality of the surface's facing. It should be noted that, on project sites, it has been observed that an excessive amount of oil often leads to poor facing quality [9]. The release agent has to be applied on to clean surfaces evenly, without missing any spots while avoiding the accumulation of oil build-up. In other words, this procedure should be carried out to establish a continuous and uniform film. During prefabrication, release oil is applied using automated sprayers to provide for better control over the oil deposit, relative to both its quantity and surface area. On worksites, oil is most often sprayed onto formwork or else spread with a rubber scraper after spraying in order to remove any excess oil from the single layer. For this, it is recommended to use stainless steel nozzles to generate a fine mist [10].

For our study, the application step was carried out with a Laser Viton 7 sprayer. Two nozzles have been tested: a conic nozzle 15/10 and a flat nozzle 04/110. Scraping was performed with a rubber scraper after spraying.

## 2.5. Physicochemical characteristics of the test oils

### 2.5.1. Viscosity analysis

For this testing campaign, we implemented a cone-plane type HAAKE 401W rheometer, with a cone angle of 1° and consists of a plate and cone of radius R whose axis is perpendicular to the plane of the plate and its apex lies on the plate (Fig. 4). The device precision is ±0.2 mPa s and distances are in mm.

Note that rotational movement is transmitted to the upper part of the measurement geometry via a controlled torque motor. Viscosity values were measured at 0 °C, 20 °C and 40 °C, and results are presented in Fig. 5.

Fig. 5 shows that Oils 1 and 2 display rather similar viscosity profiles, with moderate viscosity variations vs. temperature. In contrast, the viscosity of Oil 4 increases substantially at 0 °C. This behaviour may be ascribed to the ester introduced into the formulation, since it is likely to be a saturated ester (i.e. absence of a double bond on the hydrocarbon chain). Oil 3 exhibits an intermediate behaviour, in accordance with hydrocarbon-based synthetic oils, which are characterised

<figure><img src="image_145.png" /><figcaption>**Fig. 4.** Close-up of the plane cone.</figcaption></figure>

by few intermolecular interactions. Since the vegetable oil remains quite fluid at low temperature, it probably contains a high oleic fraction (a hydrocarbon chain with 18 carbons and one unsaturation), thus yielding an attractive viscosity profile.

### 2.5.2. Evaluation of wetting power relative to the formwork

It was found that insufficient oil wettability may lead to oil flows on the formwork and thus alter the oil film. The capacity of a liquid to spread over a surface can be estimated by its wettability, which is derived by measuring the angle of contact between the liquid and its support (Fig. 6). In the case of release oil, wetting power influences the evenness of the film deposited. The affinity of this film to the support also depends however on viscosity and adhesion energy [11].

The device used for measuring the wettability is known goniometer which consists of a camera, a light source, a display screen, a plate and a motorised piston. A drop of liquid is deposited onto a sample (5 × 5 cm²) cut from a new formwork wall. The drop formed using a syringe is placed on a support driven in an upward motion. A photograph is taken at the very instant the drop makes contact with the support. Then, the angle of contact between the tangent of the drop and the support surface is automatically measured. Note that the drop must be spherical and adhere perfectly to the support.

Drop angle values are given in Table 4 at 20 °C, with a dispersion of ±0.5°.

In the case of a new plate, the synthetic oil yielded the smallest drop angle, whereas the other three oils displayed very similar angle values. Oil offers greater wetting power when acting on new surfaces than on other material compositions. In contrast, for the used plate, the mixed vegetable-synthetic oil produced the smallest angle. Let us also note that a low level of wettability can cause oil to flow on the formwork. The reduction in the contact angle could be explained by a higher roughness for the used formwork surface. It is then easier for oil to get into the grooves of wear.

## 3. Results

### 3.1. Correlation between film thickness and oil viscosity

The formwork sample is 5 × 3 cm², oil was deposited by spraying and eventually followed by spreading using a scraper. The oil

<table><caption>**Table 3**
Physicochemical characteristics of the test oils.</caption><thead><tr><th>Nature</th><th>Oil 1<br>Liquid</th><th>Oil 2<br>Liquid</th><th>Oil 3<br>Liquid</th><th>Oil 4<br>Liquid</th></tr></thead><tbody><tr><th>Composition</th><td>Vegetable oil</td><td>Synthetic oil</td><td>Mineral oil</td><td>Vegetable oil associated with a synthetic oil</td></tr><tr><th>Colour</th><td>Straw-coloured</td><td>Colourless</td><td>Umber</td><td>Colourless</td></tr><tr><th>Flash point (°C)</th><td>65</td><td>62</td><td>67</td><td>180</td></tr><tr><th>Density</th><td>0.85</td><td>0.82</td><td>0.86</td><td>0.88</td></tr></tbody></table>
<figure><img src="image_146.png" /><figcaption>Fig. 5. Variation of dynamic viscosity according to temperature.</figcaption></figure>

<figure><img src="image_147.png" /><figcaption>Fig. 6. Determination of the drop angle with respect to its spread.</figcaption></figure>

<table><caption>**Table 4**
Drop angle values.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Angle drops θ</th></tr><tr><th>New formwork surface</th><th>Used formwork surface</th></tr></thead><tbody><tr><td>Oil 1</td><td>26.5</td><td>21.4</td></tr><tr><td>Oil 2</td><td>21.6</td><td>22.8</td></tr><tr><td>Oil 3</td><td>25.1</td><td>23.3</td></tr><tr><td>Oil 4</td><td>24.6</td><td>14.5</td></tr></tbody></table>

film thickness was determined by weighing. This method was validated by implementing an analytical technique based on alpha radiation [11]. Results indicated a high level of agreement between these two measurement methods.

Knowing the oil mass density and the plate surface area, the sample thickness can simply be determined through weighing.

$$V = S \times e \leftrightarrow e = \frac{V}{S} \qquad (1)$$

with

$$V = \frac{m}{\rho} \qquad (2)$$

which yields

$$e = \frac{m}{\rho \times s} \qquad (3)$$

<table><caption>**Table 5**
Film thickness determination according to the weighing method.</caption><thead><tr><th>Type of oil</th><th>Film sprayed with a conic nozzle (μm)</th><th>Film sprayed with a flat nozzle (μm)</th><th>Film sprayed, then scraped (μm)</th></tr></thead><tbody><tr><td>Oil 1</td><td>4.75</td><td>4.47</td><td>1.02</td></tr><tr><td>Oil 2</td><td>3.62</td><td>2.93</td><td>0.77</td></tr><tr><td>Oil 3</td><td>4.22</td><td>3.64</td><td>1.01</td></tr><tr><td>Oil 4</td><td>5.45</td><td>5.45</td><td>0.64</td></tr></tbody></table>

S is the sample surface area (in m²), ρ is the oil mass density (kg/m³), V is the oil volume (in m³), m is the oil film mass (kg), and e is the film thickness (m).

Table 5 presents the results of the thickness measurement at 20 °C.

Measurement uncertainty amounted to ±0.15 µm, in assuming a weighing measurement error of 5%. After spraying, we obtained film thickness values lying between 3 and 5.5 µm. The conic nozzle tended to produce a thicker film than the flat nozzle. Scraped films yielded thicknesses of between 0.6 and 1 µm. The film with Oil 2 exhibited the least thickness, while Oils 1 and 4 (containing vegetable esters) gave rise to the thickest applications.

## 3.2. Assessment of the aesthetics of rough facings

### 3.2.1. Facing grading system

It should be noted that the grading criteria adopted to evaluate facing quality are based on French Standard P18-503 [12] and listed in Table 6.

### 3.2.2. Facing appearance

The procedure below explains the sequence between casting, demoulding and the characterization of the surface.

<figure><img src="image_148.png" /></figure>

The samples were stored in a laboratory at constant temperature and constant moisture.

The concrete facing is dried two hours after the demoulding. The traces of moisture related to the demoulding have then disappeared.

Table 7 reports the scores obtained when assessing facing appearance vs. type of mould and application system. The dispersion is 0.2 for Tables 7–14.

Examining this table, it appears that, on average, it is the mixed synthetic vegetable oil that provides the best results, followed by vegetable oil.

It is found that the mineral and synthetic oils reveal significant colour variations on the facing surfaces although they always seem to be less influenced by the type of delivery system used.
<table><caption>**Table 6**
Grading criteria.</caption><thead><tr><th>Micro-bubbling, dusting, dirtying, attachment point</th><th>Degree of importance</th><th>Facing aspect</th></tr></thead><tbody><tr><td>None</td><td>1</td><td>Excellent</td></tr><tr><td>Very slight</td><td>2</td><td>Very good</td></tr><tr><td>Slight</td><td>3</td><td>Good</td></tr><tr><td>Moderate</td><td>4</td><td>Average</td></tr><tr><td>Important</td><td>5</td><td>Bad</td></tr><tr><td>Very important</td><td>6</td><td>Insignificant</td></tr></tbody></table>

<table><caption>**Table 7**
Influence of test parameters on facing appearance.</caption><thead><tr><th rowspan="2"></th><th colspan="3">New mould</th><th colspan="3">Used mould</th></tr><tr><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th></tr></thead><tbody><tr><td colspan="7">Four-test averages</td></tr><tr><td>Oil 1</td><td>3.3</td><td></td><td>3</td><td>4.3</td><td>3.8</td><td>4.8</td></tr><tr><td>Oil 4.7</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Oil 6</td><td>2</td><td>5</td><td>4.3</td><td>4.8</td><td>5.5</td><td>5.5</td></tr><tr><td>Oil 6</td><td>3</td><td>4.8</td><td>4</td><td>4.8</td><td>5.3</td><td>5.3</td></tr><tr><td>Oil 6</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Oil 4</td><td>4</td><td>2.3</td><td>2</td><td>3.3</td><td>2.8</td><td>4.3</td></tr></tbody></table>

The photographs exhibited in Figs. 7 and 8 show the appearance of siding resulting from the application of oil using a conical nozzle.

Moreover, the observation of surfaces on facings produced by applying oil with a flat nozzle indicates a lower quality, and this perception remains valid throughout the testing campaign.

### 3.2.3. Bubbling

Table 8 compares the results derived on bubbling vs. mould type and application system.

For both mould types, bubbling is more pronounced with the flat nozzle. We note that spraying through a conic nozzle produces the same bubbling values as spraying combined with scraping. The oil mixture causes a small amount of bubbles, followed by the vegetable oil and synthetic oil in this order. Mineral oil is the worst performer. It should be noted that adding a synthetic ester to Oil 4 serves to reduce bubbling.

### 3.2.4. Dusting

The dusting is observed over the facing or the formwork by the appearance of a film of cement dust. Table 9 shows the results obtained for dusting on the facing with regard to the type of mould and application system.

<figure><img src="image_149.png" /><figcaption>**Fig. 7.** Facing obtained with vegetable oil.</figcaption></figure>

<figure><img src="image_150.png" /><figcaption>**Fig. 8.** Facing obtained with mineral oil.</figcaption></figure>

Relatively speaking, less dust formed on the facing removed from the used mould than from the new mould. Note also that the spray pattern associated with scraping application reduces dust accumulation. For the mineral and synthetic oils, no dust formed. Consequently, the presence of vegetable matter in Oils 1 and 4 enhances the tendency to form dust.

## 3.3. Study of mould surface following formwork removal

### 3.3.1. Dusting of the mould

Table 10 shows the results recorded for mould dust formation according to both modes of application.

When drawing comparisons with Table 9, let us point out that more dust forms on the walls of used moulds (Figs. 9 and 10). Dust formation is more apparent with the flat nozzle. As before, the use of mineral and synthetic oils led to a smaller deposit on the mould surface.

### 3.3.2. Dirtying

The dirtying corresponds to dried cement milt.

Table 11 displays the dirtying scores broken down by mould type and application system.

On average, used moulds appear to be cleaner. The application on a mould using a flat nozzle associated with a scraping causes fouling of the mould wall, while the application of a conical nozzle gives better results, although traces oil remain persistent. Vegetable oil produced less dirtying, followed by the mixed oil type. The presence of a synthetic agent in the mixed oil composition improves results when excluding the unfavourable case of scraping. In contrast, mineral and synthetic oils tend to heavily dirtying of the mould.

### 3.3.3. Attachment points

Attachment points are very localized zones of fixing of concrete on the surface of the formwork.

Table 12 reports the scores obtained on attachment points relative to mould type and application system.

Attachment points appear to be less prevalent on the used mould. The attachment phenomenon is attenuated when spraying with a conic nozzle. The Vegetable oil generates fewest attachment points, with mixed oil responsible for the second fewest. Note that surface defects are twice as high when the other two oils are used.

### 3.3.4. Overall assessment

Tables 13 and 14 present the overall assessment derived on the basis of the 6 above criteria for the four oil formulations on both types of surfaces under study. For table column entries on a specific application, the score corresponds to the aesthetic impacts on concrete facings and the mould. The last column indicates, for
<table><caption>**Table 8**
Influence of test parameters on bubbling.</caption><thead><tr><th rowspan="2"></th><th colspan="3">New mould</th><th colspan="3">Used mould</th></tr><tr><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th></tr></thead><tbody><tr><th colspan="7">Four-test averages</th></tr><tr><th>Oil 1</th><td>4.3</td><td>3.3</td><td>5.3</td><td>3.3</td><td>3.5</td><td>5.3</td></tr><tr><th>Oil 2</th><td>3.5</td><td>3.5</td><td>5</td><td>4.3</td><td>3.3</td><td>4.3</td></tr><tr><th>Oil 3</th><td>3.8</td><td>3.8</td><td>4.8</td><td>4</td><td>4.3</td><td>5.3</td></tr><tr><th>Oil 4</th><td>2.8</td><td>2.3</td><td>3</td><td>2.3</td><td>2.5</td><td>3.7</td></tr></tbody></table>

<table><caption>**Table 9**
Influence of test parameters on facing dusting.</caption><thead><tr><th rowspan="2"></th><th colspan="3">New mould</th><th colspan="3">Used mould</th></tr><tr><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th></tr></thead><tbody><tr><th colspan="7">Four-test averages</th></tr><tr><th>Oil 1</th><td>2</td><td>1.3</td><td>2</td><td>1.5</td><td>1.5</td><td>1.7</td></tr><tr><th>Oil 2</th><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><th>Oil 3</th><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><th>Oil 4</th><td>2</td><td>1.8</td><td>2.5</td><td>2.8</td><td>1.8</td><td>2.3</td></tr></tbody></table>

<table><caption>**Table 10**
Influence of parameters on mould dust formation.</caption><thead><tr><th rowspan="2"></th><th colspan="3">New mould</th><th colspan="3">Used mould</th></tr><tr><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th></tr></thead><tbody><tr><th colspan="7">Four-test averages</th></tr><tr><th>Oil 1</th><td>3.8</td><td>3.8</td><td>5</td><td>5.3</td><td>4</td><td>4.7</td></tr><tr><th>Oil 2</th><td>1.8</td><td>2</td><td>3.3</td><td>2.8</td><td>2.8</td><td>3.7</td></tr><tr><th>Oil 3</th><td>2</td><td>2</td><td>3.8</td><td>2.5</td><td>3</td><td>3.7</td></tr><tr><th>Oil 4</th><td>5</td><td>5.3</td><td>6</td><td>5.5</td><td>4.8</td><td>5.3</td></tr></tbody></table>

<figure><img src="image_151.png" /><figcaption>**Fig. 9.** Formwork obtained with vegetable oil.</figcaption></figure>

<figure><img src="image_152.png" /><figcaption>**Fig. 10.** Formwork obtained with mineral oil.</figcaption></figure>

each oil tested, a composite score from the average of the six evaluations criteria described in detail above.

It can be concluded that Oil 4 offers the highest scores for each criterion evaluated, in significantly outperforming the three other types of oils tested.

For formwork removal with a used wall, we observe that Oils 1 and 4 tie for the best score; hence, introducing a used mould does not lead to a serious deterioration in the facing, while a conic nozzle remains the preferable mode of application.

The mixed oil definitely yields the best overall performance in terms of concrete facing and formwork quality. On the whole, oils containing a vegetable base are more highly rated, except for the dust formation criterion on the facing and mould, yet even this one-time lower score does not generate any fouling phenomena. In this case, applying the oil by scraping serves to minimize film thickness and thereby limits dust formation.

### 3.4. Mould surface corrosion study

Release oils were applied on plates (dimensions: $1 \times 0.5 \text{ m}^2$) by conic spraying, followed on some specimens by spreading using a scraper. The plates were subsequently stored outdoors (Fig. 11).

A rating system was established in order to score the level of corrosion expansion [13] on the new formwork plates (Table 15).

The steel plates were exposed to climatic conditions at the beginning of their outdoor storage period, which lasted for one month starting on March 27th 2006. The amount of precipitation falling on the city of Lille during the months of March and April 2006 is shown in Fig. 12.

The following observations could be recorded (Table 16).

Surface observations reveal that the oils containing a vegetable base better protect the metal surface from corrosion. The synthetic
<table><caption>**Table 11**
Influence of parameters on mould fouling.</caption><thead><tr><th rowspan="2"></th><th colspan="3">New mould</th><th colspan="3">Used mould</th></tr><tr><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th></tr></thead><tbody><tr><th colspan="7">Four-test averages</th></tr><tr><td>Oil 1</td><td>2.8</td><td>5.3</td><td>3</td><td>2.5</td><td>3.8</td><td>3</td></tr><tr><td>Oil 2</td><td>4</td><td>4.8</td><td>4.3</td><td>2.3</td><td>5.3</td><td>5</td></tr><tr><td>Oil 3</td><td>4.5</td><td>5.8</td><td>4.8</td><td>3.5</td><td>5.8</td><td>6</td></tr><tr><td>Oil 4</td><td>2.3</td><td>4</td><td>1.8</td><td>1.5</td><td>3.8</td><td>2.7</td></tr></tbody></table>

<table><caption>**Table 12**
Influence of parameters on attachment points.</caption><thead><tr><th rowspan="2"></th><th colspan="3">New mould</th><th colspan="3">Used mould</th></tr><tr><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th></tr></thead><tbody><tr><th colspan="7">Four-test averages</th></tr><tr><td>Oil 1</td><td>2</td><td>1.5</td><td>1.5</td><td>1</td><td>1.3</td><td>2</td></tr><tr><td>Oil 2</td><td>1.5</td><td>2.5</td><td>3.8</td><td>1.3</td><td>1.8</td><td>3</td></tr><tr><td>Oil 3</td><td>2.8</td><td>2.5</td><td>3.5</td><td>1.8</td><td>2</td><td>3.7</td></tr><tr><td>Oil 4</td><td>1.5</td><td>1.8</td><td>1</td><td>1</td><td>1</td><td>1.3</td></tr></tbody></table>

<table><caption>**Table 13**
Average concrete facing quality score/new formwork surface.</caption><thead><tr><th></th><th>Concrete facing</th><th>Mould facing</th><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th><th>Total note</th></tr></thead><tbody><tr><td>Oil 1</td><td>3.2</td><td>3.2</td><td>3.0</td><td>3.0</td><td>3.5</td><td>3.2</td></tr><tr><td>Oil 2</td><td>3.2</td><td>3.1</td><td>2.8</td><td>3.0</td><td>3.7</td><td>3.2</td></tr><tr><td>Oil 3</td><td>3.2</td><td>3.4</td><td>2.9</td><td>3.2</td><td>3.8</td><td>3.3</td></tr><tr><td>Oil 4</td><td>2.4</td><td>3.2</td><td>2.6</td><td>2.8</td><td>2.9</td><td>2.8</td></tr></tbody></table>

<table><caption>**Table 14**
Average concrete facing quality score/used formwork surface.</caption><thead><tr><th></th><th>Concrete facing</th><th>Mould facing</th><th>Conic nozzle</th><th>Spraying and scraping</th><th>Flat nozzle</th><th>Total note</th></tr></thead><tbody><tr><td>Oil 1</td><td>3.3</td><td>3.1</td><td>2.9</td><td>3.1</td><td>3.6</td><td>3.2</td></tr><tr><td>Oil 2</td><td>3.5</td><td>3.1</td><td>2.8</td><td>3.3</td><td>3.8</td><td>3.3</td></tr><tr><td>Oil 3</td><td>3.7</td><td>3.5</td><td>3.0</td><td>3.5</td><td>4.3</td><td>3.6</td></tr><tr><td>Oil 4</td><td>2.9</td><td>3.0</td><td>2.6</td><td>3.0</td><td>3.2</td><td>3.0</td></tr></tbody></table>

oil has also provided satisfactory results. Moreover, it is preferable to spray oil without scraping it in order to improve protection against corrosion. Mineral oil displays poor anti-corrosion properties. These observations will be cross-referenced with results from spectroscopic measurements of electrochemical impedance, as well as with findings from a study on interface mixes (see Figs. 13–15).

# 4. Discussion

## 4.1. An evaluation of film protection properties using electrochemical impedance spectroscopy (EIS)

EIS measurements can be conducted not only at the corrosion potential, but also within the anodic or cathodic domain.

The selected steel grade was E24. The 10 × 10 cm² plates were supplied by the same formwork manufacturer. The steel was polished to remove calamine. Electrochemical measurements were performed using a conventional three-electrode set-up [11]. The electrolyte was composed of a 0.5 M solution of KOH + 0.1 M of NaOH. This solution featured a pH of 13.2, which replicated the pH of a concrete pore solution.

The working electrode was formed by the steel plate receiving the oil, to be applied with a rubber scraper so as to obtain a film with an approx. 1-µm thickness.

<figure><img src="image_153.png" /><figcaption>**Fig. 11.** Formwork surface storage zone.</figcaption></figure>

Once the oil was placed, the plate was fastened onto a Plexiglas cylindrical tube (S = 240 cm²) for holding the electrolyte. The counter electrode was a large-sized platinum plate. Lastly, a saturated calomel electrode has served as the reference electrode (Fig. 16).

A measurement with bare steel yielded the reference curve. Each measurement was performed on a new steel plate and each oil formulation was tested twice to ensure measurement reproducibility. For purposes of comparison, measurements conducted with two of the test oils, a mineral oil (M1) and a vegetable oil (V1) with
<table><caption>**Table 15**
Average concrete facing quality score.</caption><thead><tr><th>Corrosion</th><th>Magnitude of the studied phenomenon</th></tr></thead><tbody><tr><td>None</td><td>1</td></tr><tr><td>Very slight</td><td>2</td></tr><tr><td>Slight</td><td>3</td></tr><tr><td>Moderate</td><td>4</td></tr><tr><td>Significant</td><td>5</td></tr><tr><td>Very significant</td><td>6</td></tr></tbody></table>

known impedances [11] were inserted into the graph. The dispersion is ±500 (Ω cm²).

Given the layout of these diagrams, the equivalent electrical circuit shown in Fig. 18 was introduced to obtain the characteristic parameters of the metal interface. The slope of the curve log Z = f(log f) is slightly less than 1, which justifies use of a constant phase element (CPE) instead of a capacitance in the equivalent electrical diagram.

The CPE impedance is given by:

<figure><img src="image_154.png" /><figcaption>**Fig. 12.** Precipitation (mm) during March and April 2006 [14].</figcaption></figure>

<table><caption>**Table 16**
Average concrete facing quality score.</caption><thead><tr><th rowspan="3"></th><th colspan="12">Exposure time</th></tr><tr><th colspan="2">Day + 5</th><th colspan="2">Day + 10</th><th colspan="2">Day + 15</th><th colspan="2">Day + 20</th><th colspan="2">Day + 25</th><th colspan="2">Day + 30</th></tr><tr><th>Spray</th><th>Spray + scrap</th><th>Spray</th><th>Spray + scrap</th><th>Spray</th><th>Spray + scrap</th><th>Spray</th><th>Spray + scrap</th><th>Spray</th><th>Spray + scrap</th><th>Spray</th><th>Spray + scrap</th></tr></thead><tbody><tr><td>Oil 1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>2</td><td>1</td><td>2</td><td>1</td><td>2</td><td>1</td><td>2</td></tr><tr><td>Oil 2</td><td>1</td><td>1</td><td>1</td><td>1</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>3</td><td>2</td><td>3</td></tr><tr><td>Oil 3</td><td>2</td><td>1</td><td>3</td><td>2</td><td>4</td><td>4</td><td>5</td><td>5</td><td>6</td><td>6</td><td>6</td><td>6</td></tr><tr><td>Oil 4</td><td>1</td><td>1</td><td>1</td><td>2</td><td>1</td><td>2</td><td>1</td><td>2</td><td>1</td><td>3</td><td>1</td><td>3</td></tr></tbody></table>

Micrographs recorded on March 31st 2006

<figure><img src="image_155.png" /><figcaption>**Fig. 13.** Micrographs both before and after storage of sprayed oil 3.</figcaption></figure>

Micrographs recorded on April 18th 2006

<figure><img src="image_156.png" /></figure>

<figure><img src="image_157.png" /><figcaption>**Fig. 14.** Micrographs both before and after storage of sprayed oil 2.</figcaption></figure>
<figure><img src="image_158.png" /><figcaption>Fig. 15. Micrographs both before and after storage of sprayed oil 4.</figcaption></figure>

$$Z_{CPE} = \frac{1}{Q (j\omega)^{\alpha}} \qquad (4)$$

where Q is expressed in Ω⁻¹ m⁻² s⁻¹.

The film capacitance can then be calculated by applying the following equation:

$$C = Q(Re^{-1} + R^{-1})^{(\alpha-1)/\alpha} \qquad (5)$$

C being expressed in Farads.

It is worth noting that parameters of this physical model have been adjusted. The values of parameters R (±500 Ω cm²) and C (±15 μF cm⁻²) are gathered in Table 17.

Fig. 17 shows similarities in curve trends between the formulations for the two vegetable oils (V1 and Oil 1), as well as for the two mineral oils (M1 and Oil 3). Let us note that the vegetable-based formulations produce higher resistance values than mineral formulations. The synthetic formulation lies at an intermediate value. The following ranking is thus obtained in increasing order of impedance value:

Oil 3 < Oil 2 < Oil 1 < Oil4

Conversely, capacitance values are lower for the vegetable formulations; such observations reflect the fact that vegetable-based oils demonstrate a protective power greater than what can be ascribed to the chemical characteristic of these oils (triglycerides). The presence of an ester in the mixed formulation (Oil 4) seems to enhance anti-corrosion properties. Finally, vegetable based formulations contribute to limit the corrosion of the formwork, which is likely to degrade the performances of demoulding. The lifetime of the formwork is thus longer and preserves the quality of the concrete

<figure><img src="image_159.png" /><figcaption>Fig. 16. Diagram of the experimental set-up.</figcaption></figure>

<figure><img src="image_160.png" /><figcaption>Fig. 17. EIS diagram obtained at the corrosion potential after 1 h of immersion in the electrolyte.</figcaption></figure>

<figure><img src="image_161.png" /><figcaption>Fig. 18. Example of a choice of simple circuit related to the Nyquist diagram [15].</figcaption></figure>

facing. It is preferable to protect the formwork surface with a vegetable oil during the storage.

## 4.2. Study of interface mixes

In this part, the behaviour of the oil formulations placed in contact with a pore solution was studied. Pore solution was prepared from water and cement. A mix combining 100 g of cement with 500 g of water is left stirring for 3 min and then filtered to derive
<table><caption>**Table 17**
Resistance and capacitance values for each oil formulation.</caption><thead><tr><th></th><th>R (Ω cm²)</th><th>C (μF cm⁻²)</th></tr></thead><tbody><tr><td>Polished steel</td><td>29,300</td><td>443</td></tr><tr><td>M1</td><td>48,700</td><td>362</td></tr><tr><td>V1</td><td>134,000</td><td>269</td></tr><tr><td>Oil 1</td><td>111,600</td><td>244</td></tr><tr><td>Oil 2</td><td>82,800</td><td>324</td></tr><tr><td>Oil 3</td><td>45,700</td><td>317</td></tr><tr><td>Oil 4</td><td>135,000</td><td>287</td></tr></tbody></table>

<table><caption>**Fig. 19.** Schematic configuration of the medium observed for Oils 2 and 3.</caption><thead><tr><th>Pore solution</th></tr></thead><tbody><tr><td>Oil</td></tr></tbody></table>

<table><caption>**Fig. 20.** Schematic configuration of the medium observed for Oil 1.</caption><thead><tr><th>Pore solution</th></tr></thead><tbody><tr><td>Soap</td></tr><tr><td>Oil</td></tr></tbody></table>

<table><caption>**Fig. 21.** Schematic configuration of the medium observed for Oil 4.</caption><thead><tr><th>Pore solution</th></tr></thead><tbody><tr><td>Soap 2</td></tr><tr><td>Soap 1</td></tr><tr><td>Oil</td></tr></tbody></table>

<figure><img src="image_162.png" /><figcaption>**Fig. 22.** Diagram depicting the sliding of concrete on the oil film.</figcaption></figure>

a basic aqueous phase loaded with calcium ions. 50 ml of pore solution were added to 10 ml of oil. The mix was then stirred for 15 s and left to rest for one hour.

Following this rest phase, interfacial media prepared are described as follows:

* Oil 1: Formation of 3 phases (oil/soap/water) with little soap.

* Oil 2: Formation of 2 phases (oil/water).

* Oil 3: Formation of 2 phases (oil/water).

* Oil 4: Formation of 4 phases (oil/soap 1/soap 2/water).

In fact, the basic pH pore solution is likely to initiate a chemical process at the interface.

The different layers obtained in the model media are schematically represented in Figs. 19–21.

<figure><img src="image_163.png" /><figcaption>**Fig. 23.** Organization of the soap/oil interface.</figcaption></figure>

We observe that mineral-based formulations generates a small quantity of soap, as only acidifiers present in the composition as additives, are able to convert into soaps.

We check that lubricating properties are not governed by the soap film, but by the hydrophobicity and the thickness of oil film (Fig. 22).

For vegetable based oils such as Oils 1 and 4, carboxylates salt (soap) is mainly issued from the ester film. Due to their amphiphilic nature, oil and soap are chemisorbed at the interface, thus forming a double layer. This molecular organization confers specific lubricating properties [11,16].

Moreover, Oil 4 displays two soap layers; one is generated from the vegetable oil, and the other derived from a synthetic ester of the formulation. This double soap layer is probably stabilized by oleate calcium (salt of oleic acid used as an acidifier).

This interfacial structure may be at the origin of the performances observed about concrete protection and facing aesthetics (Fig. 23).

## 5. Conclusion

In this paper, the influence of release oils on the quality of facings as well as on corrosion protection during outdoor formwork storage has been assessed. This multi-criteria study was intended to correlate the choice of a release oil and its application conditions with both formwork removal performance and facing quality. Based on our conducted experiments, the main finding has emerged.

Viscosity measurements between 0 °C and 40 °C revealed that oil viscosity profiles diverge substantially below 20 °C, which can complicate spraying for the most viscous oils. We verified that film thickness was dependent on the viscosity of the oil being sprayed: the more fluid the oil, the thicker the film. Consequently, thickness varies with temperature.

The observed variations in wetting power with respect to the formwork do not appear to be significant for facing quality, at least under the standardised test conditions. Introduction of a used mould does not adversely affect facing aesthetics, though it does cause dust to form on the mould. Scraping the film may offer an attractive solution, in particular for vegetable-based oils, even if the scraping operation tends to foul the mould further.

According to the various test results, it seems that the vegetable oil and mixed oil produce the best aesthetic outcome (in terms of bubbling, colour and appearance), when accompanied by a conic nozzle application. These good findings may be explained by the concrete/formwork interface construction.

The reconstitution of pore mixes has enabled differentiating the way in which oils act. The media obtained have exposed the formation of a soap layer in the case of vegetable-based oil. Moreover, the interface resulting from the mixed oil exhibits a unique configuration due to the formation of two types of soap. This profile appears to promote the desired properties, with the exception of dust formation on the facings and moulds.

The film thickness for vegetable-based oils stands out as a critical parameter in controlling the dusting phenomenon. Depositing
a thin vegetable-based oil film also provides a way to reduce the amounts implemented in order to meet a reasonable cost target per square metre of formwork oil that remains roughly the same as conventional oil.

Furthermore, the interface structure tends to favour protecting formwork surfaces against corrosion, with these properties being evaluated by means of electrochemical impedance spectrometry measurements in the presence of pore mixes.

The obtained results reveal a better resistance with vegetable oil-based films (i.e. Oils 1 and 4). This protective power is enhanced in the presence of the synthetic ester (Oil 4), which shows the efficiency of the double soap layer for concrete hydrophobation. By improving formwork wall protection against inclement weather, it is indeed possible to extend formwork life cycle. It is clear that the performances of formulations associating a vegetable ester with a synthetic ester suggest their application when guaranteeing both the technical performance and eco-compatibility sought when introducing such products.

## References

[1] Gawrilow I. Palm oil usage in lubrication. In: 3rd Global oils and fats business forum. Interfacing with the global oils and fats business. USA, October 8; 2003.

[2] Raynaud C. Coffrage métallique; développement d'une stratégie éco-durable. France BTP, juin; 2010.

[3] OUTINORD. Parements de qualité des parois en béton. Collection technique syndicat français de l'échafaudage, du coffrage et de l'étairement, avril; 2005.

[4] Aide-mémoire juridique TJ19. Les maladies professionnelles – Tableau n°36. INRS; 2012.

[5] Projet SUMOVERA. Application of vegetable-oil based concrete mould release agents. State of the art document; 1999.

[6] Syndicat national des adjuvants pour bétons et mortiers (SYNAD). Classification Synad des agents de démoulage; 2010.

[7] Bouharoun S, Vanhove Y, Djelal C, De Caro P, Dubois I. Interactions between superplasticizer and release agents at the concrete/formwork interface. Mater Sci Appl 2012;3:384–9.

[8] Feys D, Verhoeven R, De Schutter G. Influence of demoulding oil on the rheological properties of fresh self-compacting concrete and its consequences during casting. Concr Plant Int 2009;48–57.

[9] Lemaire G, Escadeillas G, Ringot E. Evaluating concrete surfaces using an image analysis process. Constr Build Mater 2005;19:604–11.

[10] Baty G, Reynolds R. Release agents – how they work. Concr Int 1997;52–4.

[11] Djelal C, De Caro P, Libessart L, Dubois I, Pebere N. Comprehension of demoulding mechanisms at the formwork.oil/concrete interface. Mater Struct 2008;41:571–81.

[12] AFNOR P18-503. Surface et parements de béton – Eléments d'identification, nov.; 1989.

[13] Ochoa de Echevarrieta N. Propriété inhibitrices d'un mélange d'amines grasses et de sels d'acide phosphonocarboxyliques vis-à-vis de la corrosion d'un acier de carbone-influence des conditions hydrodynamiques sur le mécanisme d'inhibition. Thèse de doctorat de l'Université Paul Sabatier de Toulouse; 2004.

[14] http://www.meteociel.fr/climatologie.

[15] Keddam M. Principes et application des techniques électrochimiques en corrosion – CNRS; 1991. p. 1–14.

[16] Bouharoun S, De Caro P, Dubois I, Djelal C, Vanhove Y. Effect of a superplasticizer on the properties of the concrete/oil/formwork interface. Constr Build Mater 2013;47:1137–44.
<figure><img src="image_164.png" /></figure>

ELSEVIER

<figure><img src="image_165.png" /></figure>

# Comparative study of the tribological behaviour of emulsions and demoulding oils at the concrete/formwork interface

<figure><img src="image_166.png" /></figure>

Laurent Libessart<sup>a,*</sup>, Chafika Djelal<sup>a</sup>, Pascale de Caro<sup>b</sup>, Issam Laiymani<sup>a</sup>

## HIGHLIGHTS

* Adhesion energies are higher for films generated from emulsions.

* Films from emulsions have specificities in term of thickness and uniformity.

* The vegetable-based films generate a reduction of friction on concrete.

* Tribological tests confirms a Wenzel state for emulsions on the formwork.

## ARTICLE INFO

**Article history:**

Received 31 July 2019
Received in revised form 4 December 2019
Accepted 6 December 2019

**Keywords:**

Tribometer
Friction
Emulsion
Adhesion energy
Interface
Demoulding oil

## ABSTRACT

To ensure a performing demoulding, a release agent must resist the friction of concrete on the formwork. Emulsions are interesting alternatives that leads to a reduced consumption of oil on construction sites. The behaviours of two commercial emulsions are compared with two conventional demoulding oils. The adhesion energy calculated were favourable to the emulsions. The water evaporation kinetics leads to the measurements of film thickness. Tests with the tribometer have shown that the films produced from the emulsions generate a friction reduction of 30% and 40% at the concrete/formwork interface. Such performance can be explained by Wenzel's state of wettability, which facilitates the film fit into the formwork roughness.

© 2019 Elsevier Ltd. All rights reserved.

## 1. Introduction

In the building industry, concrete formwork removal has always been a complex part of construction operations. Current regulations limit the use of formwork release oils to two hours per day and per user, which represents severe constraints for the on-site organisation. Users attempt to make this step easier by implementing new techniques, namely: the polarisation [1], the creation of self-lubricating surface coatings [2], or even the application of a controlled permeable formwork liner at the interface [3]. The adhesion of concrete on the formwork remains an open subject. Permeable coatings still need to develop their re-use to become competitive. The mechanical characteristics of waterproof polymer coatings limit their use. The use of release agents remains the most used process despite its repeatability and costs, as evidenced by

the number of published works [4]. Due to environmental and health impacts, the market for vegetable-based products has significantly grown. In order to classify these products, the SYNAD (National Union of Admixtures for Concrete and Mortar) built an abacus [5] based on the product composition and the use-related criteria (safety, health, environment). While the compositions of vegetable-based formulations contribute to an effective concrete setting in the formwork, defects (variation in hues, micro-bubbling) can nonetheless appear in the event of an overconcentration [6,7]. Emulsions contain about 70% of water, which makes it possible to apply a reduced amount of oil on the formwork surface. Previous works [4,16] have already studied the configuration of films composed of mineral and vegetable oils at the surface of a metal formwork, in connection with their mode of action. In contrast, the behaviour of emulsions has not received much attention in the civil engineering field [8]. Therefore, it seems interesting to investigate the characteristics of the film after the water evaporation. To identify the specificities of the resulting film, comparisons are done between four commercial demoulding agents: two

<footnote><sup>a</sup> Univ. Artois, EA 4515, Laboratoire de Génie Civil et géo-Environnement (LGCgE), F-62400 Béthune, France</footnote>

<footnote><sup>b</sup> Univ. Toulouse, Laboratoire de Chimie Agro-industrielle, LCA, INRA, F-31030 Toulouse, France</footnote>

<footnote>* Corresponding author.
E-mail address: laurent.libessart@univ-artois.fr (L. Libessart).</footnote>
emulsions and two conventional oils. The resistance of the film to the moving concrete was determined by a plane/plane tribometer that represents concrete casting conditions in a formwork.

## 2. Emulsions characterization

An emulsion is the dispersion of a liquid in small droplets into another immiscible liquid [9]. Thus, for an oil-in-water emulsion, the oil is the dispersed phase in the water, the continuous phase. The oil dispersion requires a mechanical action (homogeniser) in order to form minuscule droplets [10]. The addition of an emulsifier that lowers the interfacial tension between the two phases, improves the physical stability of the mixture by forming micelle [8] (Fig. 1). Creaming or sedimentation refers to a process leading to a phase separation. Emulsions are widely used in the cosmetics, food processing, paint and construction industries.

Table 1 lists the characteristics of the four demoulding oils supplied by a manufacturer. An invert mineral emulsion (IME) and direct vegetable emulsion (DVE) are compared to a vegetable oil-based product (V) and a mineral oil-based product (M).

The dynamic viscosity is determined using a HAAKE VT550 single-speed viscometer, fitted with a 1° (PK5) cone. A constant volume of oil (v = 0.6 cm³) is deposited on the plate, and the cone is lowered toward the plate. As expected, the direct emulsion has a lower viscosity than for invert emulsion, due to a lower content in oil.

### 2.1. Surface state of formwork

Metal formwork is extensively present in the building trades; it serves to streamline concrete setting and enhance worksite productivity. The potential for formwork reuse is substantial for these structural elements repeated throughout a project, i.e. vertical shear walls, floors and columns. A new steel formwork was chosen, and its surface roughness was measured. Samples were cut from the formwork surface to perform microscopy tests and to measure film thicknesses. The parameters (Fig. 2) chosen to evaluate roughness are as follows [11]:

Ra: arithmetic mean of profile deviations relative to the average line.

Rp: average depth over an evaluation length.

Rt: peak height at the maximum profile valley.

Fig. 3 displays a 3D view of the roughness profile of the formwork sample using a white-light interferometry microscope (WLIM) VEECO NT3300 over a 4 × 4 mm² area [2]. The surface

<figure><img src="image_167.png" /><figcaption>Fig. 1. Diagram of the direct emulsion O/W and invert W/O.</figcaption></figure>

was sampled at 2052 × 2052 points with a 1.9 µm step scale along X- and Y-directions. For this sample, an average roughness value Ra of 1.34 µm, a maximum Rt profile deviation of 18.31 µm, a Rp value of 3.28 µm, and a value dispersion on the image of ±0.02. The profile values between 8 and −10 µm and the low deviation, indicated that the formwork surface has a low roughness, uniform on the sample.

Further measurements have been made on a roughness meter [11] on ten measuring points to obtain a more precise representation of the formwork roughness. The average values are given in Table 2. The dispersion value is ±0.02 µm.

Roughness values obtained by the two methods are in agreement with those of a new formwork surface used on site. [6].

### 2.2. Interfacial properties

The assessment of release agent performances implies the characterization of the interface in contact with the support. The affinity of oil with the support is a significant property since the oil film acts as a protective coating for the formwork and plays the role of a lubricant during the concrete casting step [6].

As a matter of fact, the liquid/vapor surface tension of a liquid is correlated with the value of the contact angle formed by a drop on a support [12]. Contact angles are a readily observed physical phenomenon related to the more fundamental concepts of energy and surface tension. This insight helps determine the properties of wettability and adhesion of a film, which in turn makes it possible to predict the properties of a coating or detect traces of surface pollutants. As part of a theoretical approach, adhesion energy values are calculated based on contact angle data. It may be considered that the liquid/solid adhesion energy value is representative of the adhesive strength between the liquid and the support.

The liquid/solid adhesion energy value also represents the adhesive force between a liquid and a support. Adhesion energies are calculated according to Zisman's Equation [13]:

$$ \omega_{LS} = \gamma_{LV} \times (1 + \cos\theta) \quad (1) $$

The liquid/vapor surface (γ$_{LV}$) tension was measured using Wilhelmy's plate method at 20 °C. The device employed is a tension meter manufactured by the CAD firm and fitted with a platinum blade. The surface tension is indicated by the meniscus formed along the plate boundary. The material contact angle (θ) was then determined by a PGX type of mobile goniometer. This device measures the contact angle at the interface between a liquid drop and the substrate surface where the drop has been deposited [14].

Table 3 summarises adhesion energy values at 20 °C. We can observe that emulsions have higher adhesion energies than oils, which is a necessary condition to reduce frictions. Higher adhesion energy corresponds to higher surface tension and lower drop angle.

### 2.3. Formation of the oil film

Demoulding oils were applied in a regular manner at a distance of 15 cm from the surface of a formwork sample (disk with a diameter of 5 cm) using an Ecospray sprayer fitted with a conical nozzle of 1 mm diameter (Fig. 4).

A cloud of white droplets fell on the surface. Then, a latency period occurs during the evaporation of water. The time elapsed has been estimated by the supplier at 15–20 min. During this latency period, the emulsion film formation was in progress and was analysed by means of optical microscopy (Figs. 5 and 6). As the water gradually evaporated, the emulsified medium became structured. On the emulsion micrographs, the white colour denotes the emulsified phase, which tends to disappear in favour of the oily phase. The observations conducted under optical microscopy of a single drop showed that, for the invert emulsion, water disappeared more
<table><caption>**Table 1**
Properties of release agents.</caption><thead><tr><th></th><th>DVE</th><th>IME</th><th>V</th><th>M</th></tr></thead><tbody><tr><td>Type</td><td>Direct emulsion</td><td>Invert emulsion</td><td>Oil</td><td>Oil</td></tr><tr><td>Composition</td><td>Vegetable esters</td><td>Paraffinic wax</td><td>Vegetable esters</td><td>Paraffinic hydrocarbons</td></tr><tr><td>Oily phase</td><td>Vegetable</td><td>Mineral</td><td>Vegetable</td><td>Mineral</td></tr><tr><td>Content of water (%)</td><td>80</td><td>70</td><td>0</td><td>0</td></tr><tr><td>Flash point °C</td><td>>100</td><td>96</td><td>>100</td><td>>100</td></tr><tr><td>Viscosity (mPa.s)</td><td>38.5 ± 1.5</td><td>60.6 ± 3.4</td><td>66.6 ± 2.8</td><td>24.5 ± 1.7</td></tr><tr><td>Density</td><td>&lt;1</td><td>&lt;1</td><td>0.93</td><td>0.86</td></tr></tbody></table>

<figure><img src="image_168.png" /><figcaption>**Fig. 2.** Scheme of the roughness profile (height vs. length).</figcaption></figure>

<figure><img src="image_169.png" /><figcaption>**Fig. 3.** 3D image of the new formwork roughness profile.</figcaption></figure>

<table><caption>**Table 2**
Roughness parameters of the formwork sample.</caption><thead><tr><th>Test</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th><th>9</th><th>10</th><th>Average</th></tr></thead><tbody><tr><td>Ra (µm)</td><td>1.32</td><td>1.23</td><td>1.35</td><td>1.13</td><td>1.41</td><td>1.18</td><td>1.18</td><td>1.38</td><td>1.26</td><td>1.15</td><td>1.26</td></tr><tr><td>Rt (µm)</td><td>13.53</td><td>13.23</td><td>15.54</td><td>12.53</td><td>15.67</td><td>13.87</td><td>13.65</td><td>18.56</td><td>16.98</td><td>13.23</td><td>14.68</td></tr><tr><td>Rp (µm)</td><td>2.69</td><td>3.17</td><td>3.76</td><td>3.08</td><td>4.05</td><td>3.45</td><td>2.98</td><td>3.65</td><td>4.12</td><td>3.07</td><td>3.40</td></tr></tbody></table>
<table><caption>**Table 3**
Interfacial properties of release agents at 20 °C.</caption><thead><tr><th></th><th>Surface tension L/V (mN/m)</th><th>Contact angle (°)</th><th>Adhesion energy L/S (mN/m)</th></tr></thead><tbody><tr><td>DVE</td><td>40.0</td><td>33.1</td><td>73.50</td></tr><tr><td>IME</td><td>37.1</td><td>22.5</td><td>71.35</td></tr><tr><td>V</td><td>33.4</td><td>32.8</td><td>61.5</td></tr><tr><td>M</td><td>30.6</td><td>18.5</td><td>59.6</td></tr></tbody></table>

<figure><img src="image_170.png" /><figcaption>**Fig. 4.** Surface state after spraying on the formwork.</figcaption></figure>

quickly compared to direct emulsion water (white colour no longer visible). Similarly, the reverse emulsion drop diameter increased rapidly (+20% after 5 min), which indicates that the film began to form at the surface.

Micrographs were taken 20 min after spraying of DVE emulsion and vegetable oil (V) on two formwork samples. After the oil droplets has merged, DVE emulsion has generated a film with a uniform distribution over the formwork surface (Fig. 6). We can observe that the vegetable oil film has a lesser coating rate. In fact, previous works have shown that it was necessary to scrape the vegetable oil film to improve its homogeneity [15].

During the latency period, a monitoring of the emulsion deposited onto the new formwork samples was carried out

The thickness of film (e) depends on the mass (m) of product on the plate, its density (ρ) and the plate surface (S). Weighing was performed with a precision scale of 10⁻⁴ g [6].

$$V = S \times e \Leftrightarrow e = \frac{V}{S}; \quad V = \frac{m}{\rho}; \quad \text{then} \quad e = \frac{m}{\rho \times S} \qquad (2)$$

with S (m³), ρ (kg/m³), m (kg), and e the thickness (m).

Previous works have shown that values obtained by the weighing method were very close to measurements carried out by PIXE

<figure><img src="image_171.png" /><figcaption>**Fig. 5.** Evolution of a drop of (a) DVE and (b) IME emulsions during the latency period.</figcaption></figure>

<figure><img src="image_172.png" /><figcaption>**Fig. 6.** Film micrographs after spraying (a) the V oil and (b) the DVE emulsion.</figcaption></figure>
[16]. It was found that the film thicknesses for conventional oils are typically between 3 and 4 µm for a mineral oil and between 2 and 3 µm for a vegetable oil [4].

A monitoring of film thickness variation over time was carried out. (Fig. 7). The reproducibility of measurement is based on four tests.

One test was conducted by spraying 0.0168 g of DVE and 0.0101 g of IME on two formwork samples. The lower viscosity of DVE led to deposit a slightly higher mass of DVE.

The final thickness of the oil film after complete water evaporation was determined by weighing. The aqueous phase is completely evaporated when the film mass remained constant, which corresponds to the final oil film thickness. Seven minutes were required for the film formation with the invert emulsion and 16 min with the direct emulsion, which is in agreement with their water content (table 1). Thus, the direct emulsion lost 79% of its initial mass, whereas the decrease of mass was close to 67% for

on the type of oil. The performances of a mineral film are driven by its thickness. A vegetable-based film contains fatty acids contributing to an interfacial organization in layers.

### 3.1. The concrete

The concrete used is a traditional mix with a water-to-binder (W/B) ratio of 0.57 and a gravel-to-sand (G/S) ratio equal to 1.27. The workability measured with the Abrams cone, is between 12 and 14 cm, which corresponds to the S3 consistency class (according to Standard NF EN 206-1). The relevant mix design is given in Table 4.

The concrete production requires a well-defined mixing protocol. Standard NF P 18-404, entitled "Concretes - Design, suitability and inspection testing - Specimen production and conservation", was implemented. The mixing procedure was performed with dry materials according to this protocol:

<figure><img src="image_173.png" /></figure>

the invert emulsion. The direct emulsion film reached an average thickness of 1.9 ± 0.2 µm, while the invert emulsion film was 1.7 ± 0.2 µm thick (averaged over 4 tests). The difference between the two initial masses was 39.9%, but the final film thicknesses were very close, because of the higher evaporation rate of the direct emulsion. For the mineral emulsion, the reduction in thickness was 47% compared to the mineral oil, whereas the reduction was 14% for vegetable emulsion compared to the conventional vegetable oil. Indeed, the active fractions in VDE is lower compared to that of IME. Finally, in both cases, emulsions generated thin oil films with high adhesion energies, especially for the mineral oil.

Tests were carried out at a temperature of 20 °C.

### 3.2. The plane/plane tribometer

A plane/plane tribometer was developed by our team and was patented under the number PCT/EP 2017068017. The principle behind this test set-up (Fig. 8) is as follows [4]: an interchangeable plate slides between two fresh concrete samples, with the movement being carried out using a motor coupled to an auger. The plate speed is controlled by a programmable controller. The samples are pressurised by means of jacks, with the pressure lying between 50 kPa and 500 kPa. A force sensor connected to the plate measures the tangential stresses acting on the plate due to the concrete.

#### 3.2.1. Experimental conditions

For the experimental campaign, the test parameters replicated worksite conditions in order to approximate the stresses being exerted on the oil film by the concrete. The casting speed and pressures at the formwork base constitute the key parameters of the study. Tests were conducted on a plate cut-out from a formwork wall with roughness values of Ra = 1.34 µm. The emulsions were applied by spraying in full compliance with the supplier recommendations.

## 3. The friction of the concrete on the formwork

An experimental campaign has been conducted in order to assess the resistance of these films to concrete friction using a plane/plane tribometer. Previous studies [4,15] have shown that this level of resistance is connected to the aesthetic quality of concrete facing and depends on the mechanisms at the interface and

<figure><img src="image_174.png" /><figcaption>Fig. 7. Water evaporation kinetics for the deposited emulsions.</figcaption></figure>

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

Concrete formulation.</caption><thead><tr><td>Concrete</td><td>(kg/m³)</td></tr></thead><tbody><tr><td>Cement CEM I 52,5 CP2</td><td>265</td></tr><tr><td>Fillers</td><td>88</td></tr><tr><td>Sand 0/4</td><td>792</td></tr><tr><td>Gravel 4/8</td><td>271</td></tr><tr><td>Gravel 6/20</td><td>734</td></tr><tr><td>Water</td><td>201</td></tr></tbody></table>
<figure><img src="image_175.png" /><figcaption>Fig. 8. Concrete tribometer principle [4].</figcaption></figure>

### 3.2.2. Casting speed

The plate displacement speed corresponds to the casting speed of a concrete shell extending over a certain height. For our testing campaign, a speed of 3 m/h (0.84 mm/s) was used, which corresponds to the speed most commonly practiced on worksites [18].

### 3.2.3. Contact pressure

The contact pressure of the selected jack corresponds to the concrete pressures being exerted at the formwork base for a given height. The vertical formwork elements are subjected, as a result of the fresh concrete, to a thrust perpendicular to the shuttering surface. The factors capable of influencing this pressure exerted on the formwork consist of: specific gravity of the concrete, workability measured using Abrams' cone, height of fresh concrete, minimum formwork dimension, and filling speed.

Considering fresh concrete as a fluid, the distribution of the pressure is triangular. The hydrostatic thrust (P) is given in Pascal by the following formula:

$$P = \gamma \times h \qquad (3)$$

with γ the concrete density estimated at 25 kN/m³ and h the height of the formwork (m)

Shell heights ranging from 2 to 6 m have been evaluated; they correspond to pressures in the range of 50 to 150 kPa.

## 4. Analysis of the formwork/concrete interface

Friction stresses vs. contact pressure for the fresh concrete and for formwork release agents are shown in Fig. 9. A Coulomb law is applied in this pressure range:

$$\tau_f = \mu \times P \qquad (4)$$

with τf the tangential stress (Pa) and P the contact pressure (Pa).

The stress variations vs. contact pressure indicate an average decrease in friction stresses of 30% for IME and up to 40% for VDE, compared to the interface without any demoulding agent. The oil films from emulsions thus contribute to a significant reduction in concrete friction against the formwork, even despite thin films (between 1.5 and 2 µm). We can note that the vegetable-based films (oil or emulsion) prove to be more efficient in contact with concrete, compared to the both mineral films.

A previous study of the oil/concrete interface has described two behaviours: a lubricating effect mainly due to a physical action for mineral oils and a chemical action of the vegetable-based formulations [15]. Thanks to triglycerides, fatty acids and other esters as ingredients in a vegetable formulation, carboxylates (soap) are formed in contact with concrete (basic medium), which can explain the lower frictions observed.

## 5. Discussion

This study describes the film formation generated from an emulsion and its main characteristics. Before spraying, protected from the air, the direct emulsion (DVE) is stable (Fig. 10), and the oily micelles are dispersed in the emulsion (Fig. 10a). After spraying, a set of droplets forms on the formwork surface; water is still present in a large quantity, and the micelles are still mobile (Fig. 10b). As the water gradually evaporates, the film becomes reconfigured and the micelles are directed towards the formwork surface. Due to their chemical adsorption, the film can adhere to the surface (Fig. 10c). In conclusion, the polarity of the metal surface makes it possible to correctly direct the micelles (Fig. 10d); meanwhile, the droplets combine to form a homogeneous oil film

<figure><img src="image_176.png" /><figcaption>Fig. 9. Dynamic friction constraints according to the pressure (v = 0.84 mm/s).</figcaption></figure>
<figure><img src="image_177.png" /><figcaption>**Fig. 10.** Evolution diagram of the direct emulsion over time.</figcaption></figure>

<figure><img src="image_178.png" /><figcaption>**Fig. 11.** Evolution diagram of the invert emulsion over time.</figcaption></figure>

on the surface (Fig. 10e) [17]. In the case of the invert emulsion (IME) (Fig. 11), the aqueous phase is composed of smaller-sized and more numerous water micelles compared to the direct emulsion (Fig. 11a). Over time, the water micelles occupy less volume (Fig. 11b, c). This process forces the oily phase to move towards

the formwork (Fig. 11d). Ultimately, the water contained in the micelles evaporates and the film becomes homogeneous (Fig. 11e).

To understand the phenomena at the emulsion/formwork interface, it is necessary to focus on the roughness of the plate. In fact, a sprayed oil tends to remain at the surface of the formwork asperities [11]. In contrast, the micelles (whether direct or invert) in emulsion have a size (0.02 and 0.2 µm) facilitating their insertion into the plate surface roughness (Fig. 2). In this case, the Wenzel wettability state [19] has been reached, as opposed to the conventional oils, which are in a Cassie state [20] (Fig. 12).

The measured surface tensions are greater than the conventional oils, this validates the Wenzel state for emulsions. According to Wenzel state, the film obtained after evaporation of the aqueous phase is totally inserted into the roughness of the plate, thus generating a homogeneous film, covering the whole surface. According to the work of Bormashenko [21,22], Fig. 13 illustrates the Wenzel state for an emulsion. The film of IME is anchored more quickly in the formwork asperities in fact, the micelles of water evaporate

<figure><img src="image_179.png" /><figcaption>**Fig. 12.** Schematic representation of the Cassie state for conventional oil on a new formwork [11].</figcaption></figure>

<figure><img src="image_180.png" /><figcaption>**Fig. 13.** Schematic evolution of (a) DVE and (b) IME emulsion films towards Wenzel state [21].</figcaption></figure>
faster. In both cases, despite a lower weight, the consequence of
this configuration is the formation of an oily film, more resistant
to the contact with the concrete.

## 6. Conclusion

Emulsions exhibit a set of specific physicochemical properties. Their spraying on a formwork surface is followed by a water evaporation phase, along with the oily phase organization, leading to the formation of a rather thin oil film. The water evaporation profile indicated that the latency period may vary from 6 to 15 min, which is in agreement with the time period mentioned by the manufacturers. The film micrographs (20 min after spraying the emulsion) showed that the film formed as the oil droplets merge displayed a homogeneous distribution of oil on the formwork. Furthermore, the emulsions demonstrated a tribological behaviour similar to that of the conventional oils, i.e. resulting in a significant stress reduction. The formation of the lipophilic film as the aqueous phase evaporated is most likely related to micelle migration into the formwork roughness. This process helps improve the lubricating effect between concrete and formwork, as the oil film is reconfigured by interacting with the formwork roughness and do not need to be scrapped. The use of emulsions as a formwork release agent represents an efficient alternative to combine technical performance with a lower content of active ingredients. Vegetable formulations remain more effective compared to mineral formulations. They should continue to develop on worksites over the near term. Other tests could bring further data about the appearance of concrete facings demoulded with emulsions, or by involving other techniques (CPF film, wood formwork, ...).

### CRediT authorship contribution statement

**L. Libessart**: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. **C. Djalel**: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Validation, Visualization, Writing - review & editing. **P. de Caro**: Formal analysis, Methodology, Writing - review & editing. **I. Laiymani**: Data curation, Investigation, Resources.

### Declaration of Competing Interest

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

## References

[1] N. Goudjil, C. Djalel, Y. Vanhove, H. Kada, N. Heloun, Impact of temperature on the demoulding of concrete elements with a polarization process, Constr. Build. Mater. 54 (2014) 402-412.

[2] N. Spitz, N. Coniglio, M. El Mansori, A. Montagne, S. Mezghani, On functional signatures of bare and coated formwork skin surfaces, Constr. Build. Mater. 189 (2018) 560-567.

[3] S. Kothandaraman, S. Kandasamy, The effect of controlled permeable formwork (CPF) liner on the surface quality of concretes, Cem. Concr. Comp. 76 (2017) 48-56.

[4] C. Djalel, P. de Caro, L. Libessart, I. Dubois, Comprehension of demoulding mechanisms at the formwork/oil/concrete interface, Mater. Struct. 41 (2008) 571-581.

[5] Syndicat national des adjuvants pour bétons et mortiers (SYNAD) (2014) Classification Synad des agents de démoulage.

[6] L. Libessart, C. Djalel, P. de Caro, Influence of the type of release oil on steel formwork corrosion and facing aesthetics, Constr. Build. Mater. 68 (2014) 391-401.

[7] Liu, Yang, Xie, Factors influencing bugholes on concrete surface analyzed by image processing technology, Constr. Build. Mater. 153 (2017) 897-907.

[8] F.M. Leon-Martinez, E.F. Abad-Zarate, L. Lagunez-Rivera, P.F.de J. Cano-Barrita, Laboratory and field performance of biodegradable release agents for hydraulic concrete, Mater. Struct. 49 (2016) 2731-2748.

[9] F.Y. Ushikubo, R.L. Cunha, Stability mechanisms of liquid water-in-oil emulsions, Food Hydrol. 34 (2014) 145-153.

[10] M. Fingas, B. Fieldhouse, Studies of the formation process of water-in-oil emulsions, Mar. Pollut. Bull. 47 (2013) 369-396.

[11] L. Libessart, P. de Caro, C. Djalel, I. Dubois, Correlation between adhesion energy of release agents on the formwork and demoulding performances, Constr. Build. Mater. 76 (2015) 130-139.

[12] J. Chappuis, J.-M. Georges, Contribution to study of wetting - analysis of a measuring method, J. Chim. Phys. Chim. Biol. 71 (1974) 567-575.

[13] W.A. Zisman, Relation of the equilibrium contact angle to liquid and solid constitution. Advances in chemistry, Am. Chem. Soc. (1964) 1-51.

[14] L. Libessart, C. Djalel, P. de Caro, Y. Vanhove. Influence de l'énergie d'adhésion des huiles de démoulage sur le frottement du béton. 26ème Journées Internationales Francophones de Tribologie, Mulhouse, France, Presse des Mines, (2014), pp189-197.

[15] P. de Caro, C. Djalel, L. Libessart, I. Dubois, Influence of the nature of the demoulding agent on the properties of the formwork-concrete interface, Mag. Concr. Res. 59 (2007) 141-149.

[16] C. Djalel, Y. Vanhove, D. Chambellan, P. Brisset, Influence of the thickness of demoulding oils on the aesthetic quality of facings, Mater. Struct. 43 (2010) 687-698.

[17] G. Gutiérrez, J.M. Benito, J. Coca, Pazos, Evaporation of aqueous dispersed systems and concentrated emulsions formulated with non-ionic surfactants, Int. J. Heat Mass Trans. 69 (2014) 117-128.

[18] N.J. Gardener, Pressure of concrete on formwork, ACI J. (1985) 744-753.

[19] A. Lafuma, D. Quéré, Superhydrophobic states, Nat. Mater. 2 (2003) 457-460.

[20] J. Lee, S.-H. Hwang, S.-S. Yoon, D.-Y. Khang, Evaporation characteristics of water droplets in Cassie, Wenzel, and mixed states on super hydrophobic pillared Si surface, Colloids Surf. A: Phys. Eng. Asp. 562 (2019) 304-309.

[21] E. Bormashenko, Progress in understanding wetting transitions on rough surfaces, Adv. Colloids Interface Sci. 222 (2015) 92-105.

[22] E. Bormashenko, Apparent contact angles for reactive wetting of smooth, rough, and heterogeneous surfaces calculated from the variational principles, J. Colloids Interface Sci. 537 (2019) 597-603.
# Annexe 4

Cette annexe présente les articles :

(G) Tchamo Leussa, C.C., Libessart, L., Djelal, C. et al., "Pozzolanic activity of kaolins containing aluminum hydroxide", *Scientific Reports* Vol. 10, https://doi.org/10.1038/s41598-020-70146-3, 2020

(H) Page J., Libessart L., Djelal C., Gonon M., Laiymani I., "Feasibility of Using Biomass Fly Ash in Cementitious Materials", *1st International Conference on Smart Materials for Sustainable Construction—SMASCO 2019*, Luleå, Sweden, 10–12 December 2019

Ces articles illustrent les apports faits sur la valorisation des éco-matériaux et leur intégration dans les matrice cimentaires.

Mots clefs : Activité pouzzolanique (G), Argile (G), Biomasse (H), Cendres volantes (H), Compression (G, H), Demande en eau (H), DRX (G, H), Hydroxyde d'aluminium (H), MEB (G, H), Mortiers (G), Pâte de ciment (H).
<figure><img src="image_181.png" /></figure>

OPEN

# Pozzolanic activity of kaolins containing aluminum hydroxide

Claudia Charlotte Tchamo Leussa<fnref n="1" />,², Laurent Libessart<fnref n="1" />,³, Chafika Djelal<fnref n="1" />, Chantale Njiomou Djangang<fnref n="1" />,²,✉ & Antoine Elimbi²

The addition of 10 wt% aluminum hydroxide to two crude kaolinitic clays, a commercial and a natural freshly mined one, has enhanced their pozzolanic activity, more substantially in the natural sample containing gibbsite. The obtained blends were used as replacement of 20 wt% of Portland cement in the formulations of pastes and mortars which exhibited significant decrease of setting time and increase of compressive strength from early age to 28 days. Also, SEM/EDX analyses showed very heterogeneous structures with hydrated phases identified from XRD. Specific interpretation of the role played by aluminum hydroxide revealed its aptitude to promote the formation of metastable hydrated phases (CAH₁₀/C₂AH₈) at early age, which temporally inhibited the hydration of cement. This progressive transformation led to the formation of more stable hydrated phases such as C-A-S-H which favored the increase of mechanical strength of the specimens. The sequence of transformation reactions is fully obtained with limited aluminum hydroxide content in clays. Either added as synthetic or naturally occurring in clays, aluminum hydroxide has close role in the strengthening process of cement. Hence, kaolinitic clays that naturally contain gibbsite are suggested as suitable supplementary cementitious material for partial replacement of cement.

Home ownership is fundamental to man's sense of identity and his commitment to civil society. The use of natural resources to produce low cost construction materials is a cost-effective and sustainable solution to the acute shortage of adequate shelter especially in developing countries where lack of safe, decent and affordable housing is among the greatest impediments to self-fulfillment. In addition to address housing problems, the exploitation of raw minerals is fundamental for socio-economic development. In Cameroon, many studies have identified exploitable kaolinitic clay deposits that are however scarcely used by cement industries<fnref n="1" />,².

In general, the pozzolanic activity of raw clay materials is limited due to some peculiarities such as particles size, Si/Al molar ratio, lime/pozzolana ratio, water to binder ratio, specific surface area or curing conditions. From chemical and mineral points of view, natural clays contain essentially silica and alumina bounded together by strong chemical bonds that do not easily intervene in reactions such as hydration sequences of the cement phases (C₃S, C₂S, C₄AF and C₃A) responsible for mechanical strength³,⁴. Heat treatment is among the alternative methods used to efficiently activate clays to a structure where constitutive phases (silica and alumina) can be easily involved in reactions both at fresh and hardened states of mortars and concretes. From previous studies, the range of 500 – 800 °C constitutes the optimum temperatures that transform kaolinitic clay and aluminum hydroxide into amorphous reactive phases⁵⁻¹³. Continuous concern to reduce energy consumption in cement industry led to numerous investigations among which are the effect of mechanical activation and the role of some non-clayed phases in raw clays such as gibbsite in the improvement of pozzolanic activity at ambient or lower calcination temperatures⁴,⁸,¹⁴. On this line, the presence of gibbsite in raw kaolinitic clay can lead to high pozzolanic activity and to simultaneous formation of both C-S-H (Calcium Silicate Hydrate) and C-A-S-H (Calcium Aluminum Silicate Hydrate) phases, which promote good mechanical strength. Additionally, the satisfactory pozzolanic activity of amorphous aluminum was reported¹⁵. From these references, the improvement of the pozzolanicity using aluminum hydroxide to optimize kaolinitic clay properties known already as potential supplementary cementitious materials is feasible. Furthermore, since explanation on the correlation between pozzolanic activity and mechanical strength through these mineral resources is not clarified enough in literature, and/or the use of clays as supplementary cementitious material requires prior activation, all further investigations are beneficial to respond to the societal need of reducing CO₂ emissions⁴,¹⁶. Accordingly, the present study addresses the use of kaolinitic clays in cement production in the context of saving natural resources and reducing CO₂ emissions.

<footnote n="1">¹Laboratoire de Génie Civil Et géo-Environnement (LGCgE), Université D'Artois ULR 4515, F-62400 Béthune, France. ²Laboratory of Applied Inorganic Chemistry, University of Yaoundé I, POB 812, Yaoundé, Cameroon. ✉email: djangangc@yahoo.fr</footnote>
<figure><img src="image_182.png" /><figcaption>**Figure 1.** X-Ray Diffraction patterns of starting materials K, Kc and AH₃; K: Kaolinite; A: Anatase; Q: Quartz; G: Gibbsite; and M: Muscovite.</figcaption></figure>

It assesses the effect of amorphous aluminum hydroxide mixed with kaolinitic clays on the strength of pastes/mortars. For this purpose, amorphous aluminum hydroxide was mixed at 10 wt% to two kaolinitic clay materials in order to obtain blends that were used to replace 20 wt% of Portland cement. The pozzolanic activity of the starting kaolinitic clay powders along with their aforementioned mixtures was investigated using the Chapelle test, X-ray Diffraction and Scanning Electron Microscopy to evaluate the formation of hydrated phases. In addition, setting time of pastes was measured as well as compressive strength and strength activity index of mortars.

## Methods

**Materials sampling and characterisation.** Aluminum hydroxide (AH₃), from Sigma Aldrich (France), was used in addition to two crude kaolinitic clays, one from CERADEL (France) denoted as K and used as received while the other which was quarried in the South Region of Cameroon was wet-sieved <20 µm and the residue was dried at 105 °C until constant mass to get powder named as Kc. The specific surface area of K, Kc and AH₃ was measured using the BET method. Chemical composition of AH₃ was provided by its supplier, Sigma Aldrich, while those of K and Kc were determined by Inductively Coupled Plasma Optical Emission Spectroscopy using a Perkin Elmer Spectrometer (Optima 7,000 DV ICP-OES). Structural phases were determined by X-Ray Diffraction using a Siemens D 5,000 diffractometer featuring a Bragg-Brentano assembly with a back graphite monochromator with cobalt Kα 1 radiation (~ 1.78 Å). From the diffractograms obtained (Fig. 1), the crystallinity of kaolinite was determined by calculating the Hinckley index which is the ratio of the sum of the heights of the reflexions (110) and (111) measured from the inter-peaks background and the height of the (110) peak measured from the overall background (Fig. 2)⁸. In addition, thermogravimetric analyses (TG) were performed on the two clays using a NETZSCH STA 409 instrument under 75 L min⁻¹ air flux with temperature rise of 5 °C min⁻¹ (Fig. 3). The semi-quantitative phase composition of the clays was obtained by combining XRD, TG and chemical data¹⁴. The Portland cement (CEM I 52.5 N CP2) made of 97 wt% of clinker and 3 wt% of gypsum was used, its chemical and mineralogical compositions provided are given in Table 1. Sand from Seine River (France) was also used¹⁷ and its physical properties (Table 1) were calculated from its granulometric analysis done by dry sieving, using a set of sieves. Those are coefficient of uniformity (Cu) = D₆₀/D₁₀, coefficient of curvature (Cc) = D₃₀²/D₁₀ × D₆₀ (where D₆₀, D₃₀ and D₁₀ are particle diameters at 60%, 30% and 10% of passing respectively)²¹ as well as the fineness modulus and sand equivalent which were respectively determined according to ASTM C33 (1999) and ASTM D2419-95 Standards (1998).

Preliminary tests with addition of 10 and 20 wt% of amorphous aluminium hydroxide (AH₃) in the clays (K and Kc) demonstrated the decrease of mechanical strength for the case of 20 wt%. Thus, 10 wt% of AH₃ was added to K and Kc to obtain powders labelled as K₁₀ and Kc₁₀ respectively. The pozzolanic activity of K, Kc, K₁₀ and Kc₁₀ was assessed by the modified Chapelle test according to NF P 18-513 Standard¹⁸ which is based on
<figure><img src="image_183.png" /><figcaption>Figure 2. Determination of Hinckley index of (a) K, (b) Kc.</figcaption></figure>

<figure><img src="image_184.png" /><figcaption>Figure 3. TG curves of K and Kc.</figcaption></figure>

the definition of pozzolanic material as the one capable of binding to portlandite (Ca(OH)₂) in aqueous solution. For this text, 2 g of CaO were mixed with 1 g of each material (K, Kc, K₁₀ and Kc₁₀) respectively in 250 mL of distilled CO₂-free water to form solutions of hydrated lime or portlandite. Each mixture was then stirred at 90 ± 5 °C under reflux for 16 h in a 500 mL balloon flask and then allowed to cool to room temperature before filtrating. From the filtrate, the remaining unreacted portlandite was extracted with saccharose and titrated by a 0.1 N hydrochloric acid solution according to equation (Eq. 1)¹⁸,¹⁹, each value considered being the average of three test results made under the same conditions. On the other hand, the precipitated products of reactions with K, Kc, K₁₀ and Kc₁₀, denoted as K', Kc', K₁₀' and Kc₁₀', respectively, were analyzed by XRD to identify constitutive phases. A blank experiment was carried out similarly without the materials under study for comparison¹⁸.

$$CaO (\text{lime})_{\text{aqueous}} + 2HCl_{\text{aqueous}} \rightarrow CaCl_{\text{aqueous}} + H_2O \quad (1)$$

**Pastes and mortars formulations.** Considering the previous studies which indicated 20 wt% as the optimum value of cement substitution by metakaolin⁷,²⁰, pastes and mortars were designed using cement in which 20 wt% was replaced by K₁₀ and Kc₁₀ (Table 2). For each paste, the water to binder (W/B) mass ratio was 0.39, the value corresponding to the normal consistency of 0.6 ± 0.1 cm given on the Vicat apparatus and used as required by the EN F 196-3 Standard along with that of the control paste (Pc) (with only cement as binder) was 0.25. These pastes were characterized by the measurement of the setting time using the Vicat apparatus according to EN F 196-3 Standard, both at the normal orientation and at the reverse frame; the measurement was repeated three times to have average of results as retained value. Mortars were obtained by adding 1,350 g of sand to each paste in mass ratio of binder to sand (B/S) of 1/3 as indicated by the EN F 196-1 Standard. The good workability, that is the ease and homogeneity with which the fresh blend of starting materials can be mixed, placed, consolidated and finished according to the EN F 196-1 Standard was obtained after many trials for the determination of the value of W/B mass ratio and 0.55, was finally retained for its satisfactory fulfilment. Then, fresh mortars were shaped into parallelepiped test specimens of 4 × 4 × 16 cm³ in a metallic mould then vibrated for 15 s on
<table><caption>**Table 1.** Chemical, physical and semi-quantitative mineralogical characteristics of starting materials.</caption><thead><tr><th>Oxides (% wt)</th><th>Portland cement CEM I 52.5 N</th><th>Commercial Kaolinite (K)</th><th>Cameroonian Kaolinite (Kc)</th><th>Amorphous aluminum Hydroxide (AH<sub>3</sub>)</th><th>Sand</th></tr></thead><tbody><tr><td colspan="6"><strong>Chemical composition</strong></td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>3.3</td><td>37.1</td><td>40.1</td><td>98.2</td><td></td></tr><tr><td>SiO<sub>2</sub></td><td>20.1</td><td>59.0</td><td>38.0</td><td>-</td><td></td></tr><tr><td>K<sub>2</sub>O</td><td>3.0</td><td>2.5</td><td>0.2</td><td>-</td><td></td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>5.2</td><td>1.5</td><td>0.7</td><td>-</td><td></td></tr><tr><td>CaO</td><td>64.1</td><td>-</td><td>0.1</td><td>-</td><td></td></tr><tr><td>TiO<sub>2</sub></td><td>-</td><td>-</td><td>1.1</td><td>-</td><td></td></tr><tr><td>Na<sub>2</sub>O</td><td>0.3</td><td>-</td><td>0.0</td><td>1.8</td><td></td></tr><tr><td>MgO</td><td>0.8</td><td>-</td><td>0.1</td><td>-</td><td></td></tr><tr><td>SO<sub>3</sub></td><td>3.0</td><td>-</td><td>0.0</td><td>-</td><td></td></tr><tr><td>PF</td><td>1.8</td><td>13.5</td><td>18.8</td><td>-</td><td></td></tr><tr><td colspan="6"><strong>Semi-quantitative mineralogical characteristics</strong></td></tr><tr><td>Kaolinite</td><td>-</td><td>67.0</td><td>75.3</td><td>-</td><td></td></tr><tr><td>Gibbsite</td><td>-</td><td>-</td><td>15.9</td><td>-</td><td></td></tr><tr><td>Muscovite</td><td>-</td><td>21.3</td><td>-</td><td>-</td><td></td></tr><tr><td>C<sub>3</sub>A</td><td>8.6</td><td>-</td><td>-</td><td>-</td><td></td></tr><tr><td>C<sub>3</sub>S</td><td>61.0</td><td>-</td><td>-</td><td>-</td><td></td></tr><tr><td>C<sub>4</sub>AF</td><td>11.1</td><td>-</td><td>-</td><td>-</td><td></td></tr><tr><td colspan="6"><strong>Physical characteristics</strong></td></tr><tr><td>Specific surface (m<sup>2</sup>/g)</td><td>-</td><td>13.5</td><td>17.0</td><td>51.0</td><td>-</td></tr><tr><td>Density (g/mL)</td><td>3.1</td><td>2.5</td><td>2.4</td><td>2.4</td><td>2.6</td></tr><tr><td>Fineness modulus (FM)</td><td></td><td></td><td></td><td></td><td>2.2</td></tr><tr><td>Coefficient of uniformity (Cu)</td><td></td><td></td><td></td><td></td><td>1.9</td></tr><tr><td>Coefficient of curvature (Cc)</td><td></td><td></td><td></td><td></td><td>1.0</td></tr><tr><td>Sand equivalent %</td><td></td><td></td><td></td><td></td><td>97.2</td></tr><tr><td>Class index</td><td></td><td></td><td></td><td></td><td>0/4</td></tr></tbody></table>

<table><caption>**Table 2.** Pastes and mortars formulations.</caption><thead><tr><th rowspan="2">Formulations</th><th rowspan="2">Cement (%)</th><th rowspan="2">Binder/sand (B/S)</th><th colspan="2">Clay mixtures (%)</th><th rowspan="2">Water/binder (W/B)</th></tr><tr><th>K<sub>10</sub></th><th>Kc<sub>10</sub></th></tr></thead><tbody><tr><td colspan="6"><strong>Pastes</strong></td></tr><tr><td>PC</td><td>100</td><td>-</td><td>-</td><td>-</td><td>0.25</td></tr><tr><td>PK<sub>10</sub></td><td>80</td><td>-</td><td>20</td><td>-</td><td>0.39</td></tr><tr><td>PKc<sub>10</sub></td><td>80</td><td>-</td><td>-</td><td>20</td><td>0.39</td></tr><tr><td colspan="6"><strong>Mortars</strong></td></tr><tr><td>Mc</td><td>100</td><td>1/3</td><td>-</td><td>-</td><td>0.55</td></tr><tr><td>MK<sub>10</sub></td><td>80</td><td>1/3</td><td>20</td><td>-</td><td>0.55</td></tr><tr><td>MKc<sub>10</sub></td><td>80</td><td>1/3</td><td>-</td><td>20</td><td>0.55</td></tr></tbody></table>

an electrical vibrating table (M & O. type 202, N° 106) in order to remove entrapped air bubbles before storing under a plastic film in a wardrobe at 100% humidity at 20 °C until 3, 7 or 28 days respectively. Control mortar (Mc) was also designed with only cement as binder for comparison. Compressive strength was measured on test specimens using an electrohydraulic press, type ENERPAC 3R according to NF EN 1,015–11 Standard, the result presented was the average of six tests considered at a particular age. The strength activity index, Ic was calculated according to Eq. (2),

$$I_c = \frac{R_c}{R_{c(control)}} \times 100 \qquad (2)$$

where Rc is the compressive strength of the mortars, and Rc(control) the compressive strength of the control mortar respectively.
X-Ray Diffraction (XRD) was performed on powders of mortar specimens aged 28 days while their broken pieces were etched in resin, polished and coated with the gold film before being analyzed by Scanning Electron Microscopy (SEM/EDX) with JSM 5,900 LV mode apparatus.

## Results and discussion

**Characteristics of starting materials.** The chemical and the physical properties of starting materials are given in Table 1 whereas their structural phases are presented in Figs. 1 and 3 respectively. The sums of mass percentage (% Al₂O₃ + % SiO₂ + % Fe₂O₃) are 97.6% (K) and 78.8% (Kc), both are greater than 70%, proving that the two clays fit one of the main three conditions for pozzolanic materials along with the strength activity index and the amount of amorphous phase²² according to the Standard ASTM C618. Specific surface area (SSA) is 51.0 m²/g for AH₃, 17.0 m²/g for Kc and 13.5 m²/g for K, which indicated that AH₃ is the most reactive material, followed by Kc and then K. This behavior is normal for AH₃ since it is in the amorphous state and has easy solubility enabling the mobility of ionic species¹⁸. XRD patterns confirmed the amorphous state of AH₃ displaying a large amorphous halo between 2 theta = 20 and 30°¹⁸,²³. Comparatively, XRD patterns of the two clays showed characteristic peaks of crystalline phases, which are mainly kaolinite and quartz, but also muscovite in K and gibbsite in Kc. The 4 characteristic peaks of kaolinite in K between 2 theta = 20° and 30°, are well shaped proving its good crystallinity whereas in Kc, a small amorphous halo is observed characterising its poor crystallinity⁸,²⁰. Accordingly, the Hinckley index (HI) obtained is 0.8 for K and 0.2 for Kc which confirmed the more crystallinity of kaolinite in K than that in Kc. The crystallinity of the kaolinite in Kc may has been affected by the presence of associated gibbsite which has a peculiarity to decrease the crystallinity of kaolinite according to some authors⁸,²⁴ who described a competitive phenomenon during the geological formation of the two minerals.

Furthermore, the occurrence of gibbsite in Kc is associated with a high amount of Al₂O₃ (40.10 wt%). These results are confirmed by TG analyses (Fig. 3), which exhibit the mass loss of 5.5% due to the decomposition of gibbsite around 300–350 °C in Kc and the characteristic mass loss connected to kaolinite between 400 and 700 °C which is 11.5% for K and 10.5% for Kc. From the combination of mass loss values for diverse decompositions and chemical compositions, the semi quantitative mineralogical composition of the two clays was obtained (Table 1).

**Effect of amorphous aluminum hydroxide on the pozzolanic activity of clays.** The chemical reaction between portlandite (Ca(OH)₂) and aluminosilicates in aqueous solution is called pozzolanic reaction. It is always slow and leads to the formation of calcium-silicate-hydrates (C-S-H) and calcium-aluminates/silicates-hydrates (C-A-H/C-A-S-H) which are among the various compounds that possess cementing properties (Eq. 3)²⁵⁻²⁷.

$$ \mathrm{CH} (\mathrm{portlandite}) + \mathrm{AS} (\mathrm{aluminosilicate}) + \mathrm{H}_{2}\mathrm{O} (\mathrm{water}) \rightarrow \mathrm{C-S-H} + \mathrm{C-A-H} / \mathrm{C-A-S-H} \quad (3) $$

The dashes indicate that no particular composition is implied²⁷. C-S-H is commonly known as amorphous or poorly crystalline calcium silicate hydrate while calcium aluminates/silicates hydrates are usually of high crystallinity. In this generic reaction, the “aluminosilicate”, is acting as pozzolan and can correspond to kaolinite (2SiO₂·Al₂O₃·2 H₂O), alumina (Al₂O₃), silica (SiO₂), according to the chemical composition of the starting materials (Table 1), which are kaolinitic clays (K and Kc) and aluminum hydroxide (AH₃). The possible end-products (Eqs. 3–4) formed with lime can hence be calcium silica hydrates on its amorphous state or on its crystalline phase (tobermorites). The formation of tobermorites is favored by SiO₂ and heat treatment; they are the main strength-contributing components of the pozzolanic behavior, responsible of both strength and dimensional stability.

$$ \mathrm{Ca(OH)}_2 (\mathrm{portlandite}) + 2\mathrm{SiO}_2 \cdot \mathrm{Al}_2\mathrm{O}_3 \cdot 2\mathrm{H}_2\mathrm{O} (\mathrm{kaolinite}) + \mathrm{H}_2\mathrm{O} (\mathrm{water}) \rightarrow \mathrm{CaO} - \mathrm{SiO}_2 - \mathrm{H}_2\mathrm{O} (\mathrm{C-S-H}) / \mathrm{CaO} - \mathrm{Al}_2\mathrm{O}_3 - \mathrm{SiO}_2 - \mathrm{H}_2\mathrm{O} (\mathrm{C-A-S-H}) \quad (4) $$

$$ \mathrm{Ca(OH)}_2 (\mathrm{portlandite}) + \mathrm{SiO}_2 (\mathrm{silica}) + \mathrm{H}_2\mathrm{O} (\mathrm{water}) \rightarrow \mathrm{Ca} - \mathrm{SiO}_2 - \mathrm{H}_2\mathrm{O} (\mathrm{C-S-H}) \quad (5) $$

The low amount of portlandite that remained after the reaction indicates the high pozzolanic activity of the studied sample. This is in agreement with Fig. 4 which presented the increase of pozzolanic activity of the two clays (K and Kc) with addition of 10 wt% of AH₃ (from 535.0 to 724.7 mg of Ca(OH)₂/g in K and from 590.5 to 1,121.0 mg of Ca(OH)₂/g in Kc respectively), making the two clays fulfilling pozzolanic materials requirement according to NF P 18–513 Standard. As regard the improvement of pozzolanic activity of the two clays by addition of the same amount of 10 wt% of AH₃, the increase is almost double in Kc, reaching 1,121.0 mg of Ca(OH)₂/g, which is close to that of some Highly Reactive Metakaolins (1,500 mg of Ca(OH)₂/g)¹⁸. Almost the same propensity is observed on the strength activity index of the clays alone and of those of mixtures of clays with aluminum hydroxide (Fig. 5). It can be seen that the strength activity index of the commercial clay K is lesser than that required for pozzolanic material, whereas that of Kc, which naturally contains gibbsite, is above the limit line. Finally, the addition of synthetic aluminum hydroxide (AH₃) in the two clays led K into the sound zone (Fig. 4) and its activity index is even a bit greater than that of Kc according to ASTM C618 Standard¹⁶,²². This suggests the existence of a highest amount where aluminum hydroxide positively impacts the pozzolanic activity. Also, the strength activity index is a measurement of pozzolanic activity of materials aged 28 days, i.e. it measures the change of physical and mechanical properties in relation to the pozzolanic reaction. The decrease of the main peak intensity of gibbsite on XRD patterns of Chapelle test products (Fig. 6), which was evaluated at 90.8% from Kc to Kc' and 96.5% from Kc to Kc'₁', confirms that the initial gibbsite in Kc has also reacted with portlandite. This may be due to its possible low crystallinity as natural phase associated to kaolinite along with the amorphous aluminum hydroxide, which acted as an accelerator. Additionally, the Hinckley index (HI) of kaolinite in K' evaluated at 1.6, seemed to have same value as in Kc'₁', whereas HI increased from 0.6 in Kc' to 1.9
<figure><img src="image_185.png" /><figcaption>**Figure 4.** Pozzolanic activity by Chapelle test of clays (K and Kc) and of mixtures clays—10 wt% amorphous aluminum hydroxide (K₁₀ and Kc₁₀).</figcaption></figure>

<figure><img src="image_186.png" /><figcaption>**Figure 5.** Strength activity index of mortars.</figcaption></figure>

in Kc₁₀' indicating that the addition of AH₃ has no effect on the amount of amorphous phase in K. Conversely in Kc, it contributed considerably to consume the amorphous phase which led to the increase of the crystallinity of the kaolinite in Kc.

**Setting time of pastes.** Figure 7 indicates the initial and final setting times of pastes which are respectively 150 and 210 min for control paste (PC), 40 and 80 min for PK₁₀ and then 16 and 26 min in PKc₁₀. It can be observed a rapid setting of the pastes with the addition of 10 wt% of amorphous aluminum hydroxide compared to the control paste. Once more, the role of amorphous aluminum hydroxide is highlighted as a material that generally affects the setting time²⁸. It can also be stated that the amount of acting oxide (alumina) is a factor to be considered in the setting process. In fact, the amount of this oxide is greater in Kc due the presence of gibbsite and the effect is evident when considering the difference between setting time of PK₁₀ and that of PKc₁₀. This result could be connected to the value of the specific surface area (SSA) which is 51.0 m²/g for AH₃, 17.0 m²/g for Kc and 13.5 m²/g for K, hence the higher the value of specific surface area, the lower the setting time. From the low values obtained for setting time, it is obvious that PKc₁₀ and PK₁₀ are suitable for manufacturing masonry blocks via extrusion or vibro-compaction process²⁹,³⁰.

**Mechanical strength of mortars.** The variation of compressive strength with the age of the designed mortars (Mc, MK₁₀ and MKc₁₀) is shown in Fig. 8. The control mortar (Mc) specimens displayed highest values at all ages: 33.2 MPa (3 days), 36.3 MPa (7 days) and 41.0 MPa (28 days). Those of MK₁₀ were 17.9 MPa (3 days), 22.4 MPa (7 days) and finally 37.1 MPa (28 days) whereas those of MKc₁₀ were 23.9 MPa (3 days), 27.5 MPa (7 days) and finally 36 MPa (28 days). It can be observed that the compressive strength development was initially
<figure><img src="image_187.png" /><figcaption>Figure 6. XRD patterns of materials before and after the Chapelle test: (a) CaO, K, K' and K$_{10}$'; (b) CaO, Kc, Kc' and Kc$_{10}$' for which K: Kaolinite; Q: Quartz; C: calcite; M: Muscovite, G: Gibbsite and P: Portlandite.</figcaption></figure>

<figure><img src="image_188.png" /><figcaption>Figure 7. Setting time of different pastes, I: Initial setting time and F: Final setting time.</figcaption></figure>

low at early ages and then significantly increased with time. It is well known that the hardening of the cement paste refers to the presence of hydrated phases. In the case of rapid setting time as observed in PK$_{10}$ and PKc$_{10}$ (Fig. 7), in addition to the formation of ettringite, the system undergoes instable and disorder structures of hydrated aluminosilicates (Eqs. 6–9) which is characterized by low mechanical strength as obtained for specimens aged 3 and 7 days$^{15,20,31,32}$.

$$2\text{Al(OH)}_3 \text{ (aluminum hydroxide)} + 3\text{Ca(OH)}_2 \text{ (portlandite)} + 3(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}) \text{ (Gypsum)} \\ + 20\text{H}_2\text{O (water)} \rightarrow 3\text{CaO} \cdot \text{Al}_2\text{O}_3 \cdot 3\text{CaSO}_4 \cdot 32\text{H}_2\text{O} \cdot \text{(ettringite)} \quad (6)$$

$$(\text{CaO}, \text{Al}_2\text{O}_3) + \text{H}_2\text{O} \rightarrow \text{CAH}_{10} + \text{C}_2\text{AH}_8 \text{ (Cement hydrates)} \quad (7)$$

$$\text{CAH}_{10} + \text{Alumina gel} \rightarrow \text{C}_3\text{AH}_6 + \text{AH}_3 \quad (8)$$
<figure><img src="image_189.png" /><figcaption>Figure 8. Compressive strength of mortars as function of age.</figcaption></figure>

$$C_2AH_8 + Alumina gel \rightarrow C_3AH_6 + AH_3 \quad (9)$$

Furthermore, as the hydration proceeds, the initially formed metastable hydrated phases ($CAH_{10}$ and $C_2AH_8$) are transformed into more stable ones such as $C_3AH_6$ with water (Eqs. 6–9). Thus, rapid setting time and formation of metastable phases due to the presence of amorphous aluminum hydroxide limited the cement grains from interacting with the surrounding water molecules. The impending access of water molecules to the surface of cement grains remained until the exhaustion of aluminum compound. This allowed the normal transformation mechanism of cement to more stable binder phases, which controlled the final mechanical strength. The aluminum compound also favoured the hardening, which attains a steady state at a limited amount$^{15,20,32}$. Accordingly, in the formulation $MK_{10}$, it can be suggested that the added aluminum hydroxide had, however, been involved in the process, therefore the contribution of gibbsite must also be considered. In accordance with this observation, the low values of compressive strength obtained at 3 and 7 days respectively in $MK_{10}$ and $MK_{c10}$ are contiguous to the presence of temporal phases as mentioned above and their presence can only be a source of insubstantial toughness or supplementary internal stress which favours low mechanical strength. Conversely, for specimens aged 28 days, the prevalence of more stable phases is effective and this leads to the higher values of mechanical strength. If the amount of amorphous aluminum hydroxide is a restrictive factor to the increase of mechanical strength, this is different with the pozzolanic activity which increases with the amount of aluminum compounds. Costa and Massazza$^{33}$ showed that the pozzolanic reaction with lime depends on the specific surface area of the material at early ages. At later age, it depends more on the content of reactive compounds.

**Structural and microstructural characteristics of mortars.** XRD patterns of all mortars, $MK_{10}$ and $MK_{c10}$, (Fig. 9) show the presence of ettringite ($C_6AS_3H_{32}$) as the main hydrated cement phase, and residual portlandite ($Ca(OH)_2$) and quartz ($SiO_2$) from sand. Considering that ettringite, a beneficial component of the cement system with an important role in the control of setting, is often formed preferably in the presence of alumina instead of its pair tobermorite whose formation is favored in presence of silica and high temperature process, the obtained result is therefore in accordance with the effective impact of amorphous aluminum hydroxide ($AH_3$) in the mixture$^{25,26,34}$. On the other hand, differences related to the intensity of peaks (Fig. 9) showed that their height is more or less high in accordance with the relative prevalence of respective concerned phases. The halo, which is observed at 10–20° (2θ), refers to the occurrence of amorphous phases$^{35,36}$. As for the crystalline phases, slight dissimilarities can be seen with regard to the height of halo which is comparatively the same
<figure><img src="image_190.png" /><figcaption>**Figure 9.** XRD patterns of MK10 and MKc10: CH (Portlandite), E (Ettringite), and Q (Quartz).</figcaption></figure>

in MK₁₀ and MKc₁₀. In fact, the height of the halo is proportional to the amount of amorphous phases present in the system³⁷. These amorphous phases are shown on SEM images (Fig. 10) and are specified by EDX curve from matrix zones as C-A-S-H and C-S-H³⁸,³⁹. It can then be concluded that the addition of 10 wt% of AH₃ increased the formation of hydrated phases. All the SEM images of the mortars displayed a clear heterogeneous character. The background that was taken as matrix zone is constituted of the above mentioned amorphous phases. Within this matrix are erratically distributed large grains of sand and fine grains of crystalline hydrated phases which are identified on XRD patterns (Fig. 9). The overall microstructure highlights a compact tridimensional network where the matrix formed by amorphous phase gel is acting as a binder of aggregate grains along with other crystalline phases. In the network of the structure, the role of amorphous phases is mainly to control the strengthening of the mortars and their predominance is advantageous for mechanical strength of the mortars²³. Accordingly, a slight relationship, even if not accurate at all, can be established between the obtained compressive strength of mortars and the height of amorphous halo (Figs. 8 and 9). Accordingly, the compressive strength of specimens aged 28 days is higher in MK₁₀ (37.1 MPa) than in MKc₁₀ (36.0 MPa).

## Conclusion

This paper addresses the use of clays in cement production in a context of saving natural resources and reducing CO₂ emissions. The study investigated the use of commercial amorphous aluminum hydroxide in addition to raw kaolinite and the use of such mixtures as supplementary cementitious materials for partial replacement of cement in the formulation of cement mortars. The following conclusions were retained:

1. The incorporation of amorphous aluminum hydroxide favored the enhancement of pozzolanic activity of kaolinitic clays. The enhancement was greater when the clay originally contained gibbsite;

2. In comparison to the control paste made up of cement only, the presence of amorphous aluminum hydroxide in clays promoted significant decrease of initial and final setting times of cement pastes;

3. Amorphous aluminum hydroxide stimulated the formation of metastable hydrated phases by inhibiting temporally the hydration reactions of cement at the early age and with time, gradual transformation process led to the formation of more stable hydrated phases;

4. Raw kaolinitic clays containing aluminum hydroxide are promising substitute materials when blended with Portland cement. The sequence of transformation reactions with time is fully obtained with limited amor-
<figure><img src="image_191.png" /><figcaption>**Figure 10.** SEM images of polished cement mortar fracture of MK10 (a) and MKc10 (b) with EDX curve from the matrix zones.</figcaption></figure>

phous aluminum hydroxide content in clays. Even though, compressive strength is slightly reduced however the new cement meets the standard requirements for the construction sector.

5. The quick setting induced by amorphous aluminum hydroxide is an advantage for the elaboration of bricks or cement mortars using extrusion or vibro-compaction.

## Data availability

The datasets analysed during the current study are available from the corresponding author on reasonable request.

Received: 13 December 2019; Accepted: 24 July 2020

Published online: 06 August 2020

## References

1. Tchamba, A. B. et al. Use of bauxite from Cameroon for solid state sintering and characterization of calcium dialuminates (CaO₂·Al₂O₃) refractory cements. *Ceram. Int.* **40**, 1961–1970 (2014).

2. Njoya, D., Njoya, A., Ngueteu, K. A., Tchuindjang, Y. & Nkoumbou, C. Caractérisation chimique et minéralogique de quelques indices de bauxite de Foumban (Ouest-Cameroun). *Int. J. Biol. Chem. Sci.* **11**, 444–452 (2017).

3. Gonçalves, J. P., Tavares, L. M., Toledo, F. & Fairbaorm, E. M. R. Performance evaluation of cement mortars modified with metakaolin or ground brick. *Constr. Build. Mater.* **23**(5), 1971–1979 (2009).

4. Yao, G. et al. Effect of mechanical activation on the pozzolanic activity of muscovite. *Clays Clay Miner.* **67**(3), 209–216 (2019).

5. Sabir, B., Wild, B. S. & Bai, J. Metakaolin and calcined clay as pozzolan for concrete: a review. *Cement Concr. Compos.* **23**, 441–454 (2001).

6. Elimbi, A., Tchakoute, H. K. & Njopwouo, D. Effects of calcination temperature of kaolinite clays on the properties of geopolymer cements. *Constr. Build. Mater.* **25**, 2805–2812 (2011).

7. Rashad, A. M. Metakaolin: fresh properties and optimum content for mechanical strength in a comprehensive overview. *Rev. Adv. Mater. Sci.* **40**, 15–44 (2015).

8. Djangang, N. C. et al. Sustainable binder from high amounts of gibbsite associated with kaolinitic clay. *Annales de Chimie Science des Matériaux* **39**(1–2), 75–91 (2015).

9. Tironi, A., Cravero, F. C., Scian, A. N. & Irassara, E. F. Pozzolanic activity of calcined halloysite-rich kaolinitic clays. *Appl. Clay Sci.* **147**, 11–18 (2017).

10. Scrivener, K., Martirena, F., Bishnoi, S. & Maity, S. Calcined clay limestone cements (LC3). *Cem. Concr. Res.* **114**, 49–56 (2017).
11. Dhandapani, Y., Sakthivel, T., Santhanam, M., Gettu, R. & Pillai, R. G. Mechanical properties and durability performance of concretes with Limestone Calcined Clay Cement (LC3). *Cem. Concr. Res.* **107**, 136–151 (2018).

12. Douamba, W., Gildas, C., Bello, A., Lawane, L. & Messan, A. Effect of Metakaolin as partial substitution to Portland cement on the mechanical and durability properties of high performance concrete. *Am. J. Civ. Eng. Archit.* **6**, 253–259 (2018).

13. Djangang, N. C. et al. Reaction sintering and microstructural evolution in metakaolin-metastable alumina composites. *J. Therm. Anal. Calorim.* **117**(3), 1035–1045 (2014).

14. Fabbri, B., Gualtieri, S. & Leonardi, C. Modification induced by the thermal treatment of kaolin and determination of reactivity of métakaolin. *Appl. Clay Sci.* **73**, 2–10 (2013).

15. Brykov, A. S., Vasilév, A. S. & Mokeev, M. V. Hydration of Portland cement in the presence of high activity aluminum hydroxides. *Russ. J. Appl. Chem.* **85**, 1793–1799 (2012).

16. Yuanyuan, L. et al. Assessment of pozzolanic activity of calcined coal-series kaolin. *Appl. Clay Sci.* **143**, 159–167 (2017).

17. Laneyrie, C. et al. Influence of recycled coarse aggregates on normal and high performance concrete subjected to elevated temperatures. *Constr. Build. Mater.* **111**, 368–378 (2016).

18. Ferraz, E. et al. Pozzolanic activity of metakaolins by the French standard of the modified Chapelle test: a direct methodology. *Acta Geodyn. Geomater.* **12**, 289–298 (2015).

19. Ntimugura, F., Sore, S. O., Bello, L. & Messan, A. The influence of Metakaolin from Saaba (Burkina Faso) over physico-mechanical and durability properties of mortars. *J. Civ. Eng.* **7**, 389–408 (2017).

20. Xu, Q. & Stark, J. The chemical action of Al(OH)₃ "Accelerators in the Early Hydration of Portland Cements". *Zement KalkGips* **61**, 82–92 (2008).

21. Cassagnabère, F., Diederich, P., Mouret, M., Escadeillas, G. & Lachemi, M. Impact of metakaolin characteristics on the rheological properties of mortar in the fresh state. *Cement Concr. Compos.* **37**, 95–107 (2013).

22. Benkaddour, M., Kazi, F. A. & Abdelaziz, S. Durabilité des mortiers à base de pouzzolane naturelle et de pouzzolane artificielle. *Revue Nature et Technologie* **01**, 66–73 (2009).

23. Wang, K. S., Lin, K. L. & Tzeng, B. Y. Latent hydraulic reactivity of blended cement incorporating slag made from municipal solid waste incinerator fly ash. *J Air Waste Manag. Assoc.* **53**, 1340–1346 (2003).

24. Criado, M., Aperador, W. & Sobrados, I. Microstructural and mechanical properties of alkali Activated Colombian raw materials. *Materials* **9**, 158 (2016).

25. Boffoue, M. O. et al. Influence de la teneur en ciment sur les propriétés thermomécaniques des blocs d'argiles comprimée et stabilisée. *Afrique Sci.* **11**(2), 35–43 (2015).

26. Müller, J. C., Pozzolanic activity of natural clay minerals with respect to environmental geotechnics. Doctorat thesis: Swiss federal institute of technology Zurich (2005). https://doi.org/10.3929/ethz-a-005142316

27. Houston, J. R., Maxwell, R. S. & Carroll, S. A. Transformation of meta-stable calcium silicate hydrates to tobermorite: reaction kinetics and molecular structure from XRD and NMR spectroscopy. *Geochem. Trans.* **10**(1), 1–14 (2009).

28. Komnitas, K. & Zaharaki, D. Geopolymerisation: a review and prospects for the minerals industry. *Miner. Eng.* **20**, 1261–1277 (2007).

29. Khelifi, H. Matériaux argileux stabilisés au ciment et renforcés de fibres végétales: formulation pour extrusion. Thèse de doctorat, Université de Bretagne Sud. (2012).

30. Ndong, E. J. G. Développement de matériaux cimentaires à base de sous-produits bois. Thèse de doctorat : Génie Civil, Université d'Artois (2015).

31. Dupain, R., Lanchon, R., Saint-Arraman, J. C. Granulats, Sols, Ciments, Bétons. © Casteilla (2000).

32. Brykov, A. & Anna, A. Efficacy of aluminum hydroxides as inhibitors of alkali-silica reactions. *Mater. Sci. Appl.* **4**, 1–6 (2013).

33. Costa, U. & Massazza, F. Influence of the thermal treatment on the reactivity of some natural pozzolanas with lime. *Il Cemento* **3**, 105–122 (1977).

34. Cassagnabère, F., Mouret, M. & Escadeillas, G. Early hydration of clinker-slag-metakaolin combination in steam curing conditions, relation with mechanical properties. *Cem. Concr. Res.* **39**, 1164–1173 (2009).

35. Pardal, X., Pochard, I. & Nonat, A. Experimental study of Si–Al substitution in calcium-silicate-hydrate (C–S–H) prepared under equilibrium conditions. *Cem. Concr. Res.* **39**, 637–643 (2009).

36. García-Giménez, R., Villa, R., Martínez-Ramirez, S., Fernández-Carrasco, L. & Frías, M. Influence of ZnO on the activation of kaolinite-based coal waste: pozzolanic activity and mineralogy in the pozzolan/lime system. *Appl. Clay Sci.* **156**, 202–212 (2018).

37. He, C., Osbzck, B. & Makovick, E. Pozzolanic reactions of six principal clay minerals: activation. Reactivity assessments and technological effects. *Cem. Concr. Res.* **25**(8), 1691–1702 (1995).

38. Puertas, F. & Torres-Carrasco, M. Use of glass waste as an activator in the preparation of alkali-activated slag. Mechanical strength and paste characterization. *Cem. Concr. Res.* **57**, 95–104 (2014).

39. Antoni, M., Rossen, J., Martirena, F. & Scrivener, K. Cement substitution by a combination of metakaolin and limestone. *Cem. Concr. Res.* **42**, 1579–1589 (2012).

## Acknowledgments

The authors thank the Erasmus Mundus program for its financial participation towards completion of this research work, within the scope of the Dream-ACP project. The authors also wish to thank Pr Gonon Maurice, Univ. Mons, Belgium, for his expert consultation and laboratory resources.

## Author contributions

T.L.C.C.: manipulation as part of Ph.D. research work, results exploitation and article elaboration. L.L.: Follow-up of manipulations and results exploitation. D.C.: Follow-up of manipulations and results exploitation. D.N.C.: results exploitation and article elaboration. EA: Follow-up of results exploitation and article elaboration.

## Competing interests

The authors declare no competing interests.

## Additional information

Correspondence and requests for materials should be addressed to C.N.D.

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