<figure><img src="image_408.png" /><figcaption>**FIGURE 14** | Experimental and numerical results of the temperature at the top layer.</figcaption></figure>

## CONCLUSION

The OOA or VBO manufacturing of high performance thermoplastic composite laminate was investigated by *in-situ* monitoring method. Two kinds of experimental set-up were developed, viz. the oven heating method where more uniform heating could be obtained and the heating plate method coupled with the *in-situ* monitoring system. For this second set-up, carbon/PEKK prepreg laminates were fabricated by the one-side heating mold while the laminate thickness and the temperature differential along the thickness direction were monitored during the consolidation cycle. This method was very effective to produce laminates which were unaltered by the *in-situ* monitoring (no thermocouples placed inside the laminates) and to analyze the interlaminar consolidation phenomena during the manufacturing process.

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## DATA AVAILABILITY STATEMENT

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

## AUTHOR CONTRIBUTIONS

FS, CS, and PB contributed to the design of experimental works and realized them. All the authors contributed to the development of the model, the interpretation of the results, the statistical analysis, and contributed to conception of the study.

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**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Saffar, Sonnenfeld, Beauchêne and Park. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Smart Materials

<figure><img src="image_409.png" /><figcaption>Caroline Duc</figcaption></figure>

Dr. Caroline Duc received her M.Eng. in Material Sciences from Institut National de Science Appliquée, Lyon (France) and her Ph.D. degree in Micro/Nanosystems and Sensors from Lille University, France, in 2017. In 2019, she joined the Department of Energy and Environment of the Institut Mines Telecom Lille Douai as an Assistant Professor. Her research interests focus on the behavior of nanocomposites based on conductive polymers and their uses in biomedical and environmental applications.
<figure><img src="image_410.png" /></figure>

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# Hydrogen Sulfide Detection by Sensors Based on Conductive Polymers: A Review

Caroline Duc*, Mohamed-Lamine Boukhenane, Jean-Luc Wojkiewicz and Nathalie Redon

IMT Lille Douai, Institut Mines-Télécom, University of Lille, Energy and Environment Centre, Lille, France

Coming from natural and anthropogenic sources, hydrogen sulfide gas ($H_2S$) is a smelly hazardous substance at the sub-ppm level, which can lead to poisoning deaths at higher concentrations. Researchers have been working for decades to design sensors with sufficient/good/robust metrological properties and good stability in order to monitor and control in real time the risk associated with this gas. Among the devices proposed, chemiresistive sensors based on conductive polymer appear as a good alternative to the most common solutions such as electrochemical and optical sensors. They present various advantages in terms of design (easy fabrication, easy tuning of physical and chemical properties, low cost, etc.) and performances (good sensitivity, good reproducibility, room temperature operation, short response time, etc.). In this review, we summarize the progresses made on conductive polymer sensors dedicated to $H_2S$ detection, including the performance of the different materials and sensing mechanisms. Finally, we identify the limitations of these sensors and highlight the most promising approaches to enable the use of these technologies in real-world applications.

**Keywords:** hybrid polymer, conductive polymer, chemiresistive sensor, hydrogen sulfide detection, $H_2S$ sensors, gas sensing

## OPEN ACCESS

**Edited by:**

Emilia Morallon,
University of Alicante, Spain

**Reviewed by:**

Peter Kruse,
McMaster University, Canada
Haibao Lu,
Harbin Institute of Technology, China

**\*Correspondence:**

Caroline Duc
caroline.duc@imt-lille-douai.fr

**Specialty section:**

This article was submitted to
Smart Materials,
a section of the journal
Frontiers in Materials

**Received:** 27 April 2020

**Accepted:** 12 June 2020

**Published:** 30 September 2020

**Citation:**

Duc C, Boukhenane M-L, Wojkiewicz J-L and Redon N (2020) *Hydrogen Sulfide Detection by Sensors Based on Conductive Polymers: A Review*. *Front. Mater.* 7:215. doi: 10.3389/fmats.2020.00215

## INTRODUCTION

Corrosive, colorless, water-soluble, and flammable in ambient conditions, hydrogen sulfide ($H_2S$) is a smelly hazardous substance at the sub-ppm level, which can lead to poisoning deaths at higher concentrations (Malone Rubright et al., 2017). The exposure toward this gas is mainly linked to geothermal activity (e.g., crude petroleum, natural gas, hot spring) and organic decomposition from sewers, wastewater treatment plants, landfills, Sargasse seaweeds, etc. It is also produced by the industry as a by-product of paper manufacturing and tanneries or synthetized intentionally for its use as an agricultural disinfectant and an intermediate in sulfuric acid production [(National Research Council (US) Committee on Acute Exposure Guideline Levels, 2010)]. According to the Occupational Safety and Health Administration and the Bureau of Labor Statistics, $H_2S$ is one of the most dangerous gases in the workplace (Malone Rubright et al., 2017; Occupational Safety Health Administration, 2020). International public safety organizations have established several exposure limits from 1 ppb to 100 ppm in order to (i) limit the olfactory disturbance from the natural and industrial production of $H_2S$ and to (ii) protect the workers from acute and chronic exposures (Malone Rubright et al., 2017). Humans can smell $H_2S$ at low concentrations (0.5 to 300 ppb) (Beauchamp et al., 1984), and the gas can produce severe damages such as possible nausea (~2 ppm), loss of smell (~20 ppm), severe lung/nose/throat irritation (~100 ppm), and death (>250 ppm) (Habeeb et al., 2018). Moreover, $H_2S$ is present in various and complex
environments, in which the humidity as well as the concentration in co-pollutants (e.g., SO₂, CO in geothermal activity) are continuously changing (Malone Rubright et al., 2017). Therefore, monitoring equipment working in different ranges of concentration and environmental conditions is needed.

Analytical methods such as gas chromatography and/or spectrometry are commonly used to quantify H₂S concentration in air. Possessing high sensitivity toward low concentration, they present many drawbacks such as complex sampling and analyzing processes, high cost, and low temporal resolution or a posteriori measurements (Tranchida et al., 2016; Zoccali et al., 2019). Odor monitoring and safety control demand cost-effective, user-friendly, and continuous real-time measurements. As standardized methods are not fully fulfilling those requirements, many researches have been done to develop fast, portable, and low-cost devices (Llobet et al., 2017). Among the devices developed, chemiresistive, electrochemical, and optical sensors are the most popular in H₂S sensing (Pandey et al., 2012; Serban et al., 2014). Even if few sensors have been commercialized, their sensitivity and stability need to be optimized.

Among the H₂S sensors in vogue, the chemiresistive sensor is the simplest transducing system (Wong et al., 2020). Translating chemical information in an electrical signal, this device is based on a two-point contact measurement of the resistance of a sensitive material. Metal oxides (MOx) appear as a model sensitive material in gas sensing (Korotcenkov and Cho, 2017; Joshi et al., 2018) and more specifically in H₂S sensing (Guo et al., 2015; Mirzaei et al., 2018) given their high sensitivity, fast response, and easy integration in compact electronic devices. Nevertheless, MOx-based sensors present a limited selectivity and a high dependence on relative humidity and generally require a high operating temperature (>100°C).

An alternative to MOx is the use of conductive polymer (CP) as a sensitizing surface. Showing electronic properties of metals and semiconductors while retaining mechanical properties and processing advantages of polymers, CPs have appeared as promising materials in the field of gas sensor in early 1980s (Nylabder et al., 1983). Since then, many works have been done to increase their metrological performances (Bai and Shi, 2007; Fratoddi et al., 2015; Park et al., 2017). Indeed, CPs present the advantages of being easily synthesized and modified through chemical or electrochemical processes, being easily included in low-cost and miniaturized devices, and being gas sensitive at room temperature, but their selectivity toward H₂S and their long-term stability need to be enhanced.

The aim of this paper is to give an overview of the roles played by CPs in chemiresistive sensors dedicated to H₂S. After a presentation of the mechanism of detection and sensing performances of pure CPs, the discussion focuses on their hybridization with inorganic fillers. Indeed, these materials trying to combine the advantages of CPs and inorganic nano-objects have awakened extensive interest in the last decade (Hangarter et al., 2013; Kaushik et al., 2015; Park et al., 2017; Iqbal and Ahmad, 2018). Mechanisms and performances are addressed for each type of inorganic fillers (i.e., noble metal, MOx, metal chloride, and carbon-based nanostructures). Finally, a cross-comparison of the sensing properties and the potential

for future application of the different sensitive surfaces is done in order to highlight the promising approaches and the investigation needed for achieving those technologies employed in the field. The discussion is based on comparative tables (Supplementary Material) ordered by class of material (e.g., pure CPs, noble metal composites, etc.), where the sensing performance of the materials (i.e., limit of detection and response time) and properties allowing their access to real-field application (i.e., selectivity, stability, reversibility, operating temperature, behavior under humidity changes, etc.) are presented.

## SENSORS BASED ON INTRINSIC CONDUCTIVE POLYMERS

Intrinsic CPs are π-conjugated systems presenting a multilevel structure at the scale of the monomers, the chains, and the bulk of the material. This chemical organization makes their properties highly tunable at different scales from combination of different monomers and chain conformation modification by additive to formation of various morphologies at nanoscale/microscale (Skotheim and Reynolds, 2007).

The π-conjugation along the polymer chain allows the formation of delocalized electrons at the origin of the electric conduction of CPs. Actually, electrons in double bonds are free to move along the carbon skeleton given the overlap of the p<sub>z</sub> orbitals (Dongmin Kang and Jeffrey Snyder, 2017). Due to the presence of imperfections [e.g., localization of charges induced by Peierls distortion (Anderson and Roth, 1994), short conjugation length, weak degree of crystallinity (Le et al., 2017)], this phenomenon provides low conductivity to CPs in neutral state. Dopants are often used to remove or add electrons from the chain to increase conductivity and form p-type and n-type doped CPs, respectively. In p-type doping, the electrons move from the polymer to the dopant (oxidation process), creating holes along the polymer chain. Conversely, the electron density increases in the n-type doped polymer by the movement of the electron from the dopant to the polymer (reduction process) (Le et al., 2017). Thereby, CPs display conductivity which can be modulated by redox reaction or protonation from near-insulator to metallic conductor (Bai and Shi, 2007).

As a nucleophilic (i.e., reducing) gas, H₂S is expected to decrease the conductivity of p-type CPs and conversely increase the conductivity of n-type CPs (Wong et al., 2020). However, this classical behavior is observed only in few studies concerning polyaniline (PANI) deposited by thermal evaporation (Agbor et al., 1995), PANI doped in maleic acid with dodecyl hydrogen sulfate studied under nitrogen (Palaniappan and Saravanan, 2010), polycarbazole (Joshi et al., 2014), and polyazomethines studied in air (Patil et al., 2019). Those p-type materials show effectively a decrease in conductivity under H₂S (Figure 1A). On the contrary, the other sensors based on pristine CPs display an increasing conductivity in the presence of H₂S. In fact, in the presence of water at room temperature, H₂S tends to dissociate into H⁺ and HS⁻ (Agbor et al., 1995). H⁺ protonates the nitrogen atoms from PANI (Agbor et al., 1995; Dong et al., 2016), polypyrrole (PPy) (Kriván et al., 2000; Garg et al., 2015), or poly(N-propylaniline) (PNPA) (Chabukswar et al., 2013) and
<figure><img src="image_411.png" /><figcaption>**FIGURE 1** | Mechanisms of H₂S detection by CPs. (A) The reducing gas H₂S induces a reduction of the major charge carrier (holes, +) resulting in an increase in CP resistance (R₀ < R). (B) In the presence of water, H₂S releases protons, which increase the charge carrier in the polymer and thus decrease the resistivity (R₀ > R).</figcaption></figure>

leads to an increase in conductivity toward H₂S (Figure 1B). The reducing effect of H₂S and the dissociation of the gas followed by the protonation of CPs are the two mechanisms at the origin of the resistance modulation of the H₂S sensors based on CPs exclusively.

In chemiresistive sensors, the presence of gaseous analyte induces changes in electronic conductivity of a sensitive surface. Therefore, the sensing performance depends on (i) the interaction between the gas and the sensitive material and on (ii) the charge transport (i.e., capacity to transmit the changes to the electrodes) (Park et al., 2017). Each property of the material affecting one of these two parameters will alter the sensing performance of the device.

Given the mechanism of action of H₂S, the interactions of gas/polymer are mainly driven by the number of active sites (i.e., redox groups or atoms able to receive proton) capable of interacting with the gas. Beside the molecular structure of the polymer chain, which delimits the total number of active sites, the morphology controls the accessibility to the sites. This parameter can influence the sensitivity as well as the response time of the sensor. Indeed, an open structure (e.g., a porous material) facilitates the adsorption/desorption of the gas in the polymer. For example, the porous agglomerate of nanoparticles (50–100 nm) of PNPA insures a high surface-to-volume ratio leading to the best response from pure-CP sensor toward H₂S, with a modulation of –25% of the resistance at 500 ppb and a response time lower than 60 s at 100 ppm (Chabukswar et al., 2013).

Then, the charge transport in CPs is achieved by two mechanisms, the intra-chain transport (i.e., along the chain) and the inter-chain transport, which are both influenced by many factors. The intra-chain transport is fast and localized along the conjugated backbone. It depends mainly on the charge carrier and the conjugation length (Kivelson and Heeger, 1988; Phillips and Wu, 1991). The inter-chain is based on hopping or tunneling of the charge carrier and depends mainly on the conformation of the chain and their organization, which can influence the distance between chains and the degree of crystallinity (Hangarter et al.,

2013). Therefore, the disorder of the structure of the polymer can affect its sensing performance.

In the case of H₂S detection by CPs, two parameters have shown a large impact on the performance of the sensor: the dopant and the process. The dopant is known to affect the charge transport and the morphology (Krukiewicz and Katunin, 2016). Thus, the size and nature of the dopant can influence the sensing performance of the CPs and their stability. For example, PANI doped with hydrochloride acid (HCl) has a response five times higher than the same material doped with sulfosalicylic acid (SSA) tested toward 100 ppm of H₂S (Dong et al., 2016). SSA anions being bigger, they lead to larger chain spacing. The inter-chain transport is less effective, resulting in a lower activity. Moreover, PANI–HCl is less stable over 50 days. Small anions (as HCl) are known to be less stable (Li and Wan, 1999). They are more likely to migrate and volatilize from the CPs, leading to a drift of the resistance with time. Moreover, functional groups, such as the presence of both polar and non-polar groups on SSA, can increase the decomposition temperature (Dong et al., 2016). Those two parameters influence directly the stability of CPs.

Besides influencing the morphology of the sensitive surface, the process of fabrication can affect even the mechanism of variation of the conductivity of CPs. As illustration, PANI in emeraldine base form deposited on Au-IDE by spin-coating (1 µm thick) or thermal evaporation (210 nm) shows, respectively, an reversible increase or an irreversible decrease in the conductivity in the presence of H₂S under dry nitrogen (Agbor et al., 1995). The increase in the conductivity from the spin-coated film is due to the protonation of PANI when H₂S dissociates into H⁺ and HS⁻ in contact with water from the film, whereas in the case of thermal evaporation, the film does not contain water and H₂S acts as a reducing gas decreasing the conductivity of the film.

CPs have shown their ability to detect H₂S from 0.5 to 1,000 ppm. They present the advantage of having a good sensitivity at room temperature and of being highly tunable chemically and physically, giving rise to many optimization approaches. However, as reported in **Supplementary Table 1**, intrinsic CPs dedicated to H₂S sensing are often instable and present a low selectivity. Each reducing or oxidizing gas can affect differently their resistance depending on their mutual affinities. For example, PPy nanowires present an interesting 53% resistance decrease in contact with 200 ppm H₂S but they also suffer a larger increase in resistance (143%) in the presence of ammonia (Garg et al., 2015). In order to overcome those main drawbacks, many works dedicated to H₂S sensing by CPs are a focus on the hybridization of those materials with inorganic fillers.

# HYBRIDS CONDUCTIVE POLYMER SENSORS: ORGANIC-INORGANIC COMPOSITES

## Noble Metal Composites

Noble metals are often used in MOx sensors as sensitizing agent (Ramgir et al., 2013). Indeed, they provide additional absorption sites, promoting a chemical reaction occurring on
the surface (chemisorption, redox). This strategy has been applied to CPs hybridized by surface functionalization or bulk inclusion of noble metal nanoparticles (NPs) or clusters in order to increase the sensitivity and/or selectivity toward H₂S gas. PANI, PPy, polycarbazole, and polyimide have been used as a matrix when Ag and Au have been used as metallic additives (Supplementary Table 2).

The mechanisms at the origin of the sensing performance of polymer/metal hybrids are not perfectly understood. As shown in **Figure 2**, the metallic particles can have two effects on the response of CPs toward H₂S. Firstly, they present high affinity with analytes carrying sulfur atoms, which facilitates the interaction gas/material (Joshi et al., 2014). Indeed, by reacting with the gas, the metallic items release protons, which dope the CP and induce an increase in conductivity. For example, in the PANI-AuNP system, the gas reacts with gold to form AuS and H⁺, leading to an enhancement in the doping level of PANI (Shirsat et al., 2009; Liu et al., 2012). Secondly, the presence of metallic particles in p-type CPs induces the formation of nano-Schottky junctions. At the interface between the two components of the material, the difference in work function between the metal and the organic semiconductor prevents the transfer of electron, resulting in an excess of negative charge at the surface of the particles and the formation of a depletion region (Joshi et al., 2014; Kaushik et al., 2015). Changes in the width of the depletion region modulate the conductivity of the sensing material by changing the resistance but also the capacitance of the material (Joshi et al., 2014; Mekki et al., 2014). The properties of the Schottky junctions are highly sensitive to adsorption phenomena, and their presence amplifies the response toward gas adsorbed, resulting in enhanced sensitivity.

Given the sensing mechanism, the efficiency of the metallic fillers is highly dependent on their nature, geometry, and distribution. The role of sensitizing agents endorsed by metallic nanoparticles has been demonstrated on PANI nanowires decorated by Au-NPs (Shirsat et al., 2009). Electropolymerization followed by electrofunctionalization of the material leads to the formation of polymeric fibers with a diameter of 250–320 nm and well-dispersed NPs (70–120 nm). This system shows an extraordinarily high response at sub-ppb level (−20% at 0.1 ppb) when the unfunctionalized nanofibers have a limit of detection four orders of magnitude higher (>50 ppm). The high performance of the system is mainly attributed to its morphology (i.e., nanofibers decorated by nanoparticles) and the good dispersion of the metallic particles.

Moreover, the addition of metal can enhance the selectivity toward H₂S. Compared to the pristine polymer, electropolymerized polycarbazole covered by a 1-nm layer of gold shows a response toward 40 ppm H₂S doubled when its response toward 40 ppm NH₃ is divided by a factor of 7.5 (Joshi et al., 2014). The slower response and recovery of the functionalized surface endorse the authors' assumption, i.e., this specific behavior is due to the enhanced interaction between H₂S and the material.

Briefly, metal/CP composites have shown their ability to detect H₂S from 0.1 ppb to 800 ppm. PANI nanofibers decorated with gold particles provide the lowest detection limit

from the literature dedicated to organic chemiresistive sensors for H₂S detection (Shirsat et al., 2009). The origin of this good performance can be attributed to three factors: (i) the morphology insures the accessibility to a high number of active sites, (ii) the presence of gold leads to the release of protons by reacting with H₂S, and (iii) the formation of Schottky junction in the bulk of material leads to the amplification of the response toward H₂S. In order to optimize the sensitive materials, the exact role of the metallic items in hybridized CPs needs to be clarified and, most importantly, the effects of environmental parameters such as humidity need to be considered. Indeed, those parameters have never been investigated on the hybridized metal/CPs when they are crucial for the end-use of gas sensors.

## Metal Oxide and Sulfide Composites

Metal oxides (MOx) are the most commonly employed material in chemiresistive gas sensors in the academic and industrial field (Korotcenkov and Cho, 2017; Llobet et al., 2017; Joshi et al., 2018). The hybridization of such material with CPs often aims to overcome some of their drawbacks, e.g., their low selectivity and high operating temperature, and to optimize their sensing performances (Hangarter et al., 2013). For H₂S detection, mechanical mixing and *in situ* polymerization have been used to prepare MOx/CPs hybrids based on PPy, PANI, or polythiophene and WO₃, SnO₂, or the metal sulfide CdS (Supplementary Table 3).

The presence of H₂S can induce a decrease in the resistance of both CPs and n-type semiconductors. This fact tends to support the assumption that the conjoint effect of gas on the different moieties of the material is at the origin of the increased sensitivity

<figure><img src="image_412.png" /><figcaption>**FIGURE 2** | Mechanisms of H₂S detection by CPs hybridized with metallic nanoparticles (Au-NPs). R<sub>CP</sub> and R<sub>Schott</sub> represent the resistance contribution of the polymer and depletion layer (in pale yellow) at the interface with the metal (Schottky junction), respectively. Those resistances tend to decrease in contact with H₂S gas.</figcaption></figure>
<figure><img src="image_413.png" /><figcaption>FIGURE 3 | Mechanisms of H₂S detection by CPs hybridized with metal oxide (MOx). R<sub>CP</sub>, R<sub>MOx</sub>, and R<sub>p-n</sub> represent the resistance contribution of the polymer, the MOx, and the depletion layer at the interface between the two materials (p-n junction), respectively. The depletion layer is represented by lighter colors. The size of the resistance represents their tendency to decrease in contact with H₂S.</figcaption></figure>

with metal salts (MCl₂) in solution to form a metal sulfide (MS) and a strong acid (HCl) (Skoog and West, 1963), as illustrated in the following Equation:

$$H_2S + MCl_2 \rightarrow MS + 2 HCl \quad (3)$$

n air, metal and chloride ions are strongly bound, making the reaction with H₂S unfavorable. The association with PANI by coordination of metal ions facilitates the reaction with gas. Therefore, the presence of the two components is necessary for the detection of H₂S gas. In contact with metal salts lightly coordinated to nitrogen atoms from the polymer, H₂S releases a strong acid, which dopes PANI resulting in a drastic increase in polymer conductivity (Virji et al., 2005).

Given the mechanisms of action, the hybrids based on metal chloride present two specificities. Showing a variation of resistance of a few orders of magnitude at 10 ppm, they have the highest response from literature but their responses are quasi-systematically irreversible. For example, screen-printed PANI-CuCl₂ from an aqueous solution shows a resistance divided by 10⁵ toward 15 ppm H₂S at room temperature but the signal presents no recovery during the ventilation of the sample even after 20 h (Sarfraz et al., 2013). The enhanced response is attributed to the doping effect from HCl released, associated with a favorable morphology, facilitating the diffusion of the analyte in the bulk of the material. Unlike the doping of PANI, the reaction of metal chloride with H₂S is irreversible, resulting in the formation of stable copper sulfides (CuS and/or Cu₂S), which are good conductors at room temperature (Silvester et al., 1991).

The unique hybrid material based on metal chloride showing recovery are surface-printed layer-by-layer alternating PANI NPs and CuCl₂ deposited on silver electrodes (Crowley et al., 2010). The reversibility of the signal depends on the nature of the electrode. Indeed, the signal is irreversible on carbon electrodes and completely reversible on silver for several exposures at a high concentration (100 ppm). The presence of silver postpones the end of the stock of reactants able to form strong acid. Silver chlorides (AgCl) can be formed in the presence of copper chlorides (CuCl₂), leading to the formation of copper (Cu) and/or copper(I) chloride (CuCl). Under exposition to H₂S, both

chlorides and metal can react, resulting in the release of proton, electron, and HCl according to the following equations:

$$H_2S + 2Cu(s) \rightarrow Cu_2S + H_2S + H^+ + e^- \quad (4)$$

$$H_2S + 2AgCl \rightarrow Ag_2S + 2HCl \quad (5)$$

The release of electrons is assumed to be at the origin of the reversibility of the signal. However, due to the finite reservoir of free copper, the sensor response results to be irreversible over time.

Shortly, CPs hybridized with metal chloride have shown their ability to detect H₂S from 2.5 to 15 ppm. The use of metal chloride insures a high response toward H₂S. However, due to the irreversible reaction at the origin of the enhanced response, those hybrid materials can be used only for short-time applications.

## Graphitic Composites (CNT/Graphene)

Due to their good transducing properties, efficient adsorption of gas molecules, and high aspect ratio (Joshi et al., 2018), carbon nanotubes (CNT) and graphene are the most explored additives for the hybridization of CPs in gas sensing (Hangarter et al., 2013; Varghese et al., 2015). However, only few carbon-based composites are dedicated to H₂S sensing (Supplementary Table 5). Graphitic hybrid CPs are exclusively composed of PANI decorated by single-wall nanotubes (SWNT), graphene oxide (GO), or reduced graphene oxide (rGO).

In the literature dedicated to H₂S, carbon fillers have never been employed as sensitizing agent. Firstly, they are used to enhance analyte and/or charge transport. For example, in PSS-doped PANI hybridized with GO, H₂S is not detected by pristine graphene sheets when the response of the composites is ~20% toward 1 ppm of gas (Cho et al., 2014). Moreover, the conductivity is changing from 11.2 to 168.4 S/cm with the addition of 30 wt% of graphene.

Secondly, carbon fillers can be used as an amplifier in a ternary system based on the p-n junction by adding new p-n interfaces and thus increasing the effect of the depletion region on the signal. In the SnO₂/RGO/PANI nanocomposite, the heterojunction is the main contributor to sensing enhancement (Zhang et al., 2019). The band gaps of SnO₂, RGO, and PANI are 3.5, 4.39, and 4.49 eV, respectively, resulting in the formation of
<figure><img src="image_414.png" /></figure>

**FIGURE 4** | (A) Relative responses of different materials at the lowest H₂S concentration tested. The relative response is defined as (R<sub>g</sub>-R<sub>0</sub>)/100/R<sub>0</sub>, where R<sub>g</sub> and R<sub>0</sub> correspond to the resistance of the sensor in H₂S and in the reference medium (air or nitrogen). It has been directly extracted or calculated from the papers presented in **Supplementary Tables 1–4**. (B) Response times of different materials directly extracted from the literature. Metal, MOx, MCI, and C refer to hybrid composites based on conductive polymers (CPs) mixed with noble metal particles and metal-oxide, metal-chloride, and carbon-based items, respectively.

a p-n junction between the two p-type materials (i.e., RGO and PANI) and the metal oxide (SnO<sub>2</sub>).

Finally, carbon-based hybrids have shown their ability to detect H₂S from 50 ppb to 30 000 ppm. However, the graphitic composites often result in low selectivity toward H₂S with a higher response toward NH₃, as shown for PANI hybridized with SWNT (Lim et al., 2010) or GO (Gaikwad et al., 2017).

## SENSING PERFORMANCES

The first characteristics of sensing performance are the amplitude and rate of the response. **Figure 4** summarizes the relative response of the materials discussed previously as a function of the lowest concentration tested and the response time at different concentrations (the values and references are detailed in **Supplementary Tables 1–5**).

Considering the relative response (**Figure 4A**), the hybrids filled with metal chloride present an extraordinarily high amplitude of response about several orders of magnitude. This performance can be explained by the sensing mechanism, resulting in the release of a strong acid in the presence of H₂S jointly to the increase in filler conductivity. Tested between 2.5 and 15 ppm, those sensors show the highest relative response, but the contact with the target gas induces irreversible changes in the material giving rise to devices with a short lifetime.

In contrast, odor disturbance is felt below 1.5 ppm (Malone Rubright et al., 2017), and the guideline value likely to protect the public from H₂S odor nuisance established by the World Health Organization is 7 µg/m³ (i.e., 4 ppb for an averaging time of 30 min) (World Health Organization, 2000); only few studies characterize the sensors at the sub-ppm level. The lack of characterization at lower concentrations is not specific to CP-based sensors. Indeed, a similar trend is observed on MOx sensors (Mirzaei et al., 2018). According to **Figure 4A**,

PANI-nanofibers decorated with gold nanoparticles are the lonely CP-based material characterized below 50 ppb (Shirsat et al., 2009). Presenting a good response even at a lower concentration (–20% at 0.1 ppb), this nanostructured hybrid material highlights the high potential of CP-based sensors. Indeed, it largely outperforms chemiresistive sensors based on metal oxide nanostructures, whose best combination (i.e., highest response at the lowest concentration) is –33% at 10 ppb for CuO nanowires (Mirzaei et al., 2018).

Given the necessity of real-time measurement, the rapidity of the sensor is also an important parameter to consider. This parameter is often characterized by the response time and less systematically by the recovery time, which are the times to reach 90% of the resistance change upon injection of the target gas and reference gas (air or nitrogen), respectively. As shown in **Figure 4B**, the response time of pure CPs comprised the range 100–300 s. In general, the addition of MOx allows reducing the response time, to values lower than 120 s. The lowest response time (2.5 s at 5 ppm) is observed on nanostructured WO₃ functionalized with polythiophene. Despite the increase in the response time with the concentration of H₂S, it is still lower than 35 s even at 200 ppm (Bai et al., 2014). In addition, this material shows a fast recovery (<15 s). The hierarchical structure of the material can be one reason for this high rapidity. Here, the hybridization of CPs and the fabrication of an open hierarchical structure appear as functional approaches to enhancing the velocity of CP-based devices.

Beyond the sensitivity and velocity, selectivity needs to be considered in order to evaluate the performance of a device. Ammonia is the interfering species most frequently investigated on CP-based chemiresistive sensors dedicated to H₂S sensing (**Supplementary Tables 1–5**). Commonly inducing an opposite change of resistance than H₂S, ammonia can induce high response, which can disturb the ability of the sensor to quantify the target gas as seen on carbon-based hybrids and some of
pure CPs. The hybridization with metal, MOx, or metal chloride seems to enhance the selectivity toward H₂S. However, the exact benefit of the fillers on the selectivity is difficult to prove given the fact that the interfering studies are most of the time only conducted on the best composite and not on its different components. Further investigations are necessary to clarify the origin of increased selectivity in order to rationalize the research on sensitive material.

Then, the stability of the material under different environmental conditions is crucial to turning those CP-based sensors effective on the field. This pre-requisite encompasses two main capacities of the devices: (i) the preservation of its behavior under different humidities and temperatures and (ii) the conservation of its performance over time. Those two parameters undergo a lack of consideration in the literature. Indeed, <25% of the studies address the impact of humidity or time. When the increase in humidity can induce an increase or a decrease in response depending on the material, the mechanisms at the origin of this changing behavior are rarely explicated. Moreover, only short-term aging is considered. CP-based sensors seem stable for a few weeks. For example, WO₃ nanoparticles decorated by PPy maintain 83% of its response toward 1 ppm of H₂S after 54 days (Su and Peng, 2014), which corresponds to the longest aging test.

## CONCLUSION AND PERSPECTIVES

Finally, working at room temperature, easily tuned by changing monomers or dopants, easily processed, and integrated into low-cost and miniaturized devices, CPs present a large potential in H₂S sensing applications. The common approach to enhancing their sensing performance while retaining the advantages from their polymeric nature is to hybridize them with inorganic fillers (e.g., noble metal particles, metal oxide or chloride items, and carbon-based nano-objects). The addition of those fillers allows decreasing the limit of detection and the response time, leading to composites competing with MOx sensors. However, more investigation is needed to be able to establish a clear link between

the material properties (chemical nature and morphology) and its sensing performance. Moreover, none of the sensors developed have been employed in realistic situations. Indeed, the impact of interfering species (other reducing/oxidizing gas), environmental conditions (humidity and temperature), and time should be examined more systematically. While field measurement can be complicated to implement, experiments with continuous changes in H₂S concentration, using gas mixture as reference and controlled humidity/temperature, can easily be done in laboratory in order to validate the applicative potential of the materials under investigation.

## AUTHOR CONTRIBUTIONS

CD made the illustrations and the main part of the bibliographic, analytic, and writing work. M-LB participated in the bibliographic research. J-LW and NR participated in the reflection and correction. All authors contributed to the article and approved the submitted version.

## FUNDING

The study has been funded by Institut Mines-Télécom Lille Douai (IMT Lille Douai).

## ACKNOWLEDGMENTS

The authors gratefully acknowledge Alexis Vlandas (CNRS, IEMN) and Sabine Crunaire (IMT Lille Douai) for their corrections and fruitful discussions and Patricia Krawczak (IMT Lille Douai) for the opportunity to participate in the article collection Women in Science: Materials.

## SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fms.2020.00215/full#supplementary-material

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**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Duc, Boukhenane, Wojkiewicz and Redon. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Structural Materials

<figure><img src="image_415.png" /><figcaption>Maria Chiara Bignozzi</figcaption></figure>

Prof. Maria Chiara Bignozzi (MCB) is an associate professor in Materials Science and Technology at the School of Engineering and Architecture, Alma Mater Studiorum University of Bologna, Italy, and director of Centro Ceramico (www.centroceramico.it). She is also involved in European and international standards, being a Convener of WG1 “Test methods” for ISO TC 189 and CEN TC 67 “Ceramic tiles”. Her research activity is mainly concerned with building materials (ceramic tiles, geopolymers/alkali-activated materials, concrete, cement, etc.) and their physical-mechanical performances, durability, and sustainability features.

<figure><img src="image_416.png" /><figcaption>Chiara Magrini</figcaption></figure>

Chiara Magrini is a Ph.D. candidate in the Department of Civil, Chemical, Environmental, and Materials Engineering (DICAM), University of Bologna, Italy, under the supervision of Professor Alessandra Bonoli. Her research focuses on waste prevention and management, both at the European and regional levels (Emilia-Romagna region, Italy). She has worked on projects co-funded by the European Institute of Innovation and Technology-EIT, Climate-KIC.

<figure><img src="image_417.png" /><figcaption>Alessandra Bonoli</figcaption></figure>
Prof. Alessandra Bonoli is an associate professor at the University of Bologna. Her main research topics are in Environmental Engineering with particular regard to treatment, recovery, and recycling of raw materials and solid waste; the life cycle assessment and circular economy; and the dissemination of urban green technologies and unconventional building materials for climate change resilience in urban areas. She founded and currently coordinates the research group of Transition Engineering whose activities are aimed at building a sustainable future, from an environmental, social, and economical point of view, through conservation and valorization of natural resources, raw materials, water, and energy. She is a delegate of UNIBO at the UN Sustainable Development Solutions Network (UN, New York) and UE Commission Operational Groups of the European Innovation Partnership on Raw Materials (EIP Raw Materials).

<figure><img src="image_418.png" /><figcaption>Severine A. E. Boyer</figcaption></figure>

Dr. Séverine A.E. Boyer is a CNRS Research Officer at MINES-Paris/PSL – CEMEF-CNRS 7635 (Sophia Antipolis, France), and she wrote her Ph.D. Dissertation in 2003 with IFPEN. She conducts fundamental and finalized studies on the mechanisms of the poly-morphogenesis/deformation of structural synthetic and natural organic systems to develop or recycle functional and original materials and predict their use. She has developed chains of original experimental and numerical models connecting the multi-scale structure from a chemical-molecular size to a macroscopic size in complex and extreme environments (species diffusion, thermodynamic, or not equilibrium). Her international recognitions include the following: Thesis - ‘William F. Giauque Student Award, CALCON 2003 (USA)’; Young Researcher - “ICTAC Young Scientific Award, ICTAC 2012 (Japan)” (first time awarded to a French researcher); ‘First Place Best Poster, PPS 2019 (South Africa)’; and awards given to her collaborative colleagues.

<figure><img src="image_419.png" /><figcaption>Vilma Ducman</figcaption></figure>

Dr. Vilma Ducman, head of the Laboratory for Cements, Mortars, and Ceramics at ZAG, has been involved in the research of ceramic building products and the development of recycled (lightweight) aggregate and composites. In the last 10 years, she has also been actively involved in the research and development of alkali-activated materials. She has led or been involved in many projects (both national and international) on recycling and the development of new building materials or products. She has published over 50 articles in scientific journals and over 100 in professional magazines and conference proceedings, and she has also acquired four SI patents on recycling.
<figure><img src="image_420.png" /><figcaption>Stefania Manzi</figcaption></figure>

Prof. Stefania Manzi is an Associate Professor in Materials Science and Technology at the School of Engineering and Architecture, University of Bologna, Italy. She has a Master's degree cum laude in Building Engineering and a Ph.D. in Materials Engineering. Her research activity mainly concerns materials for civil and building engineering, architectural restoration, and the most innovative production and setting processes. She has taught various courses in the field of Materials Science and Technology to students of Building Engineering, Building Engineering/Architecture, Civil Engineering, Industrial Design, Chemical and Process Engineering, and Environmental Engineering at the University of Bologna.
<figure><img src="image_421.png" /></figure>

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# Gender Balance in Construction Material Research: The Analysis of Alkali-Activated Materials by a Bibliometric Study Using Scopus Database

Giulia Masi, Stefania Manzi and Maria Chiara Bignozzi*

Department of Civil, Chemical, Environmental and Materials Engineering, University of Bologna, Bologna, Italy

Research in alkali-activated materials (AAMs) is an innovative and dynamic material science topic. This is due to the relevant performances in terms of physico-mechanical properties comparable to traditional construction materials (e.g., ordinary Portland cement). The sustainability of AAMs is often highlighted since they can be developed by using many natural or industrial by-products-based precursors. This leads to challenges with the optimization of AAM production due to their different performances, availability, and costs. However, they are flexible and locally adapted materials. The research interest in AAMs has rapidly increased in the early 2000s. In recent years, about 1,000 international articles have been published each year. This study aims at assessing the gender balance of this specific research topic to investigate the relevance of women's participation. Author lists of the published articles were analyzed throughout the Scopus database, applying "alkali-activated material" and "geopolymers" as keywords in the titles, abstracts, and keywords search. The last 10 years (2009–2019) were taken into account. The gender of the most prolific authors was analyzed, and a focus of European authors on this topic was considered, as more than 25% of the research in AAMs have been carried out in Europe. The analysis of 5,900 publications in the last decade shows that there is currently a shift toward men in the gender balance in this specific research, and female authors only covered 22% of the most productive authors worldwide. Considering European authors, a more equal gender distribution is reached, when the first author is considered, with a concentration of women in the range of 36–56%, recorded over 1,396 articles.

**Keywords:** geopolymers, gender balance, bibliometry, alkali activation, Scopus database, alkali activated materials

**OPEN ACCESS**

**Edited by:**
Claire Emily White,
Princeton University, United States

**Reviewed by:**
Ailar Hajimohammadi,
University of New South Wales,
Australia
Francisca Puertas Maroto,
Eduardo Torroja Institute for
Construction Sciences (IETcc-CSIC),
Spain

*Correspondence:
Maria Chiara Bignozzi
maria.bignozzi@unibo.it

**Specialty section:**
This article was submitted to *Structural Materials*, a section of the journal *Frontiers in Materials*.

**Received:** 23 June 2020
**Accepted:** 26 August 2020
**Published:** 17 September 2020

**Citation:**
Masi G, Manzi S and Bignozzi MC (2020) Gender Balance in Construction Material Research: The Analysis of Alkali-Activated Materials by a Bibliometric Study Using Scopus Database. *Front. Mater.* 7:572514
doi: 10.3389/fmats.2020.572514

## INTRODUCTION

### Literature Review on Alkali-Activated Materials and Geopolymers

Nowadays, all around the world concrete is the main widely employed construction material. Moreover, its consumption is constantly increasing, due to its rising demand in developing countries. On the other hand, the production of cement has a huge impact on the environment. Worldwide, it has been estimated that the cement sector is responsible for nearly 8% of all carbon dioxide (CO₂) emissions caused by anthropic actions. Indeed, it is well known that the manufacturing of 1 ton of Portland cement implies the emission of almost the same amount of CO₂ in atmosphere, although
during its entire life cycle, carbonation of cement allows for CO₂ sequestration (for a cumulative amount of 4.5 GtC measured between 1930 and 2013) and determines an adjustment of CO₂ emissions (43% calculated considering the same period) related to cement production (Xi et al., 2016). Moreover, taking into account the period of time from 1990 to 2050, a rise in cement manufacturing of nearly 250% is expected (Imbabi et al., 2012; Scrivener et al., 2018), even if economic crisis due to external factors (e.g., COVID emergency) may locally modify this scenario.

The first use of the term “alkali activation” dates back to 1908, but only in the 1930s–1950s Purdon carried out some work on this technology in Belgium (Palomo et al., 2014; Provis, 2014). The real development of alkali activation was due to an extensive research program by Glukhovsky starting from the 1950s (Provis, 2014). The term “geopolymer” was introduced in the 1970s by Davidovits, indicating the alkali activation of metakaolinite, limestone, and dolomite (Palomo et al., 2014).

Especially in the last two decades, alkali-activated materials (AAMs), including geopolymers, have quickly caught people's interest, in order to reduce CO₂ emissions due to cement and ceramic material productions (Provis, 2014; Pacheco-Torgal et al., 2017; Sandanayake et al., 2018; Asim et al., 2019; Rakhimova and Rakhimov, 2019b; Shi et al., 2019; Singh and Middendorf, 2020). Alkali activation is the general expression used for the reaction of a precursor (i.e., a solid aluminosilicate) in an alkaline environment sourced by alkali activators to obtain a hardened binder mainly based on hydrous alkali aluminosilicate. This explanation can even include low amount of traditional cement together with elevated combinations of blast furnace slag or pozzolans. However, when cement is the principal origin of alkalinity, these kinds of binders are not considered in the alkali activation nomenclature (Bignozzi and Bonduà, 2011). Moreover, the term geopolymer is largely used, sometimes also in a quite indiscriminate way, to account for low calcium AAMs. Finally, hybrid binders containing both cement and a source of alkalis together with aluminosilicate elements have also been developed. Nevertheless, in general, all these systems may be considered as part of the wider field of alkali activation (Provis and Bernal, 2014; Provis et al., 2015; Provis, 2018; Provis and van Deventer, 2019; Shi et al., 2019).

AAMs can be synthesized by two main routes: i) a one-part mix (dry precursors and activators activated by water) and ii) a two-part mix (dry powder combined with activating solutions) procedure. Alkali activation by the two-part mix approach has been for sure the most investigated and used in the first applications of alkali activation-based products introduced in the market. The two-part mixture is used in the precast production, where packaging and transporting of chemical substances, together with curing conditions, can be managed at a close range (Provis, 2018; Singh and Middendorf, 2020). On the other hand, studies are being carried out to improve the one-part methodology and to develop products suitable for the market, so this methodology would be widely employed henceforth (Luukkonen et al., 2018).

As solid precursors for the production of AAMs, typical aluminosilicate supplementary cementitious precursors can be

used (Rakhimova and Rakhimov, 2019a), including blast furnace slag, coal fly ash (Zhuang et al., 2016), calcined clays, and natural pozzolans (Robayo-Salazar and Mejía de Gutiérrez, 2018) that have shown high-quality outcomes. Blast furnace slags used in alkali activation need to be granulated and ground. Additions exhibiting pozzolanic behavior, generally utilized in cement blends only to a lower extent, can be suitable for alkali activation as well. Various types of slags (cooled until a reactive condition and finely ground), sludges coming from industrial processing, ashes of different nature and origin (Hertel and Pontikes, 2020), etc., can also be employed in alkali activation, ensuring their safe use in terms of leachability of potentially toxic elements (Provis, 2018; Singh and Middendorf, 2020). As alkali activators, M₂O₃·rSiO₂ and MOH (with M = Na or K) are the most utilized chemicals (Silva et al., 2017; Provis, 2018; Rakhimova and Rakhimov, 2019a).

Inter-grinding or co-calcination of different aluminosilicate powder precursors and solid activators have been demonstrated to be effective in the synthesis of one-part AAMs, thus promoting their development (Luukkonen et al., 2018; Provis, 2018).

The great variability of precursors and activators available worldwide does not allow one to design one or more model mixes that are always applicable; thus, studying fundamental rules to properly set up *ad hoc* mix designs together with a precise quality control and characterization of the starting materials is a challenge that needs to be addressed. The control of setting time, the choice of suitable admixtures, and the curing conditions are some of the features under investigation. For example, when a low quantity of calcium was present in the fly ash used as precursor, alkali activation at room temperature obtaining outstanding mechanical properties was demonstrated (Bignozzi et al., 2014). At the same time, also clay-based binders can harden quickly at room temperature; however, the amount of water added, as function of particle size, can strongly influence the setting time and workability. Generally speaking, sealed curing is highly indicated for AAMs.

A large number of AAMs and geopolymers have been studied so far, from the point of view of chemical, microstructural, physical, mechanical, and durability performances of precursors and hardened products (Bignozzi et al., 2013; Masi et al., 2014; Singh et al., 2015; Ma et al., 2018; Zhang et al., 2018; Hassan et al., 2019; Li et al., 2019; Tang et al., 2019; Wu et al., 2019; Saccani et al., 2020; Mugahed Amran et al., 2020); however, it is still difficult to think that alkali-activated binders will soon be ready for complete substitution of cement in all the applications where it is normally involved. AAMs are surely attractive for their potentially high degree of sustainability, especially when they are obtained with raw materials and/or waste locally available, without requiring thermal and/or specific separation treatments (Pacheco-Torgal et al., 2017; Rakhimova and Rakhimov, 2019b; Shi et al., 2019; Singh and Middendorf, 2020).

The field of application of AAMs is broad (Wu et al., 2019), and is still increasing, with exploitations in renders, grouts, mortars, lightweight and foamed concrete (Carabba et al., 2019a), precast concrete elements, plain and reinforced concrete (Kamseu et al., 2014; Monticelli et al., 2016a;
Monticelli et al., 2016b; Mo et al., 2016; Carabba et al., 2017b; Tittarelli et al., 2018; Hassan et al., 2019), pipes, or matrix for nuclear or toxic waste immobilization (Rasaki et al., 2019; Singh and Middendorf, 2020). Moreover, features such as high temperatures and chemical attack resistance make AAMs particularly attractive for specific applications such as refractory blocks, passive fire protection systems, and sewer pipes (Pacheco-Torgal et al., 2012; Masi et al., 2015; Carabba et al., 2017a; Carabba et al., 2019b; Rashidian-Dezfouli and Rangaraju, 2017; Zhang et al., 2017; Ameri et al., 2019; Carabba et al., 2019b; Lahoti et al., 2019; Wang et al., 2020).

## Gender Equality in Material Science

Gender equality represents a major issue in all science, technology, engineering, and mathematics (STEM) fields. Traditionally, women are underrepresented in these sectors. Among the others, women underrepresentation in material science and engineering is an historical problem (The University Materials Council, 2008; Larivière et al., 2013). In addition, the trend related to the presence of women decreases especially at the highest level of academic careers or when high responsibility positions are considered within scientific decision-making bodies (European Commission, 2008; European Commission, 2010).

Considering all the fields of life, in the last 5 years in Europe, some improvements have been achieved (European Commission, 2015): women's employment reached 64% (the highest rate ever recorded) and an improvement in the presence of women was recorded in decision-making positions in many spheres of life (+9% of women as board members in the biggest listed companies). These results are related to some of the policies sponsored by the European Commission in the first years of the 2000s. In particular, the Commission's 2010-2015 strategies for bridging the gender gap consisted in the following principles: equal economic independence for women and men; work of equal value shall be equally paid; decision-making, dignity, integrity, and ending gender-based violence are equally assessed; and last, gender equality is promoted beyond European Union (EU) (European Commission, 2015). It was demonstrated that major improvements were achieved in those countries where introduced legislative measures were properly addressed (European Commission, 2015). In addition, in the early 2000s, a group of women scientists was involved in a European Technology Assessment Network group to explore and make recommendations on the issue of women in science in Europe. The European Technology Assessment Network group proposed three models to promote gender equality, which consisted of equal treatment between men and women, positive action, and mainstreaming (Rees, 2001). In particular, they identified gender mainstreaming as the most positive action for a long-term strategy, in order to contribute to bridging the gender balance. This was because equal treatment refers to individuals' rights with the aim of equality, and a positive action can partly improve the gender gap. On the contrary, gender mainstreaming can recast the systems and structures themselves that currently advantage men. In addition, several recommendations, mainly focused on the science, engineering, and medicine sectors, have been

proposed to promote gender equity, in terms of direct economic support, strategies related to psychology and culture, and major collaborative and international motivations (Smith et al., 2015).

However, gender inequalities are still present in education and in occupations, especially in STEM and related careers in the EU and worldwide (Shen, 2013; European commission, 2015; Kang et al., 2019; Stoet and Geary, 2018). Many factors influence these differences. First, gender inequalities in education still persist, in terms of study subject preferences, performance, and participation (Kang et al., 2019; Stoet and Geary, 2018). At the age of 11 or 12 years, students begin their career aspirations which will be developed on the basis of their studies during the secondary school. In particular, the perception of STEM careers by females is generally stereotyped, as it is perceived with less time for oneself and as an obstacle for achieving the traditional life goals (family, children, etc.). For these reasons, higher involvement in scientific subjects in schools could help bridge the gender gap in the selection of future careers (Kang et al., 2019). In addition, women are more likely to have a higher education (even, if according to the United States National Science Foundation, grants for women are about half of the grants for men during their doctorates in the United States), but female scientists still face major challenges [e.g., in United States, women represent only 21 and 5% of full science and full engineering professors, respectively (Shen, 2013); in Italy, female full professors in the scientific sector of “materials science and technology” are only 26% (https://cercauniversita.cineca.it/)]. One of the most persistent issues is that a large part of qualified female researchers quit in the early stages of their science careers. This is mainly linked to several aspects, such as family values (women in postdoc positions drop out of research careers when they become parents or plan to have children, up to twice as often as men with families), life issues and discriminations, and pay problems (in the EU public sector, women in science earned 25-40% less than male scientists in 2006) (Shen, 2013).

One of the tools to investigate gender distribution in any field of science and engineering is the study of journal authorship, journal editorial boards, and the extent of collaborations, due to the fact that generally journal publications are the main outcomes indicating scientific and technological productivity, as well as the main channel for communicating new knowledge (Larivière et al., 2013; Mauleón et al., 2013; West et al., 2013). A cross-disciplinary bibliometric study on the relationship between gender and research outputs by Larivière et al. highlighted that men dominated in scientific production in nearly every country worldwide and women were underrepresented, especially when first authorship was considered (Larivière et al., 2013). Moreover, another research analyzed the gender distribution in paper authorship, as well as the compositions of editorial boards of Spanish journals in the years between 1998 and 2009 (Mauleón et al., 2013). They found that female presence was lower that of men, but the gender gap was narrowing in the last period of the analysis, indicating a higher participation and recognition of women. Finally, a bibliometric study based in the United States using JSTOR corpus highlighted an increase of women
in the authorships (27% in 1990–2012 vs. 15% in 1665–1989) and in obtaining higher education degrees (64% of PhD recipients are women in 2004), especially in the first years of the 2000s (West et al., 2013). However, women were still underrepresented in tenure track and research university faculty positions (only 39% in full-time positions). Finally, West et al. demonstrated that whereas, historically, women were underrepresented in the first author positions in journal-article author lists, the situation was more equal in terms of first author, while a gender gap remained when last author positions and single-author articles were considered (West et al., 2013).

## Aim of the Study

As previously mentioned, research in AAMs is nowadays a proficient and very active research sector of material science, which is a very broad research field. In addition, this specific topic has recently seen a quick increase in terms of scientific interest and number of published articles and involves many aspects, applications, and research backgrounds (chemistry, material and civil engineering, etc.). AAMs exhibited strong development as a possible replacement of ordinary Portland cement in the construction sector, due to the sustainability features of the production of these materials (Provis, 2018). Traditionally, construction and construction material production are a male-dominated sector. A study by Paul et al. (2020) on gender balance in the European cement industry showed that female authors gain some achievement in terms of presence in scientific publications. Especially it was found that women are very active and involved in the development of strategies to increase sustainability features in cement production. However, women are still underrepresented in this specific sector, and only 33% of contributions are from female authors. Last, due to the relatively recent sensitivity to the gender balance issue and to the increasing involvement of women in research, a quick increase in the selection of a topic such as AAMs in the last two decades may represent a very good opportunity for monitoring the growing presence of female researchers in material science, specifically in the building sector. For all these reasons, the analysis of gender distribution based on a bibliometric study was performed using the Scopus database, including indexed articles on peer-reviewed journals, conference proceedings, and books. This shall be a representative scenario describing gender balance in one specific field of material science. Beyond numerical and geographical distribution of the research outputs, analysis of the gender of the most prolific authors (first 100) in the last decade, between 2009 and 2019, was reported. Last, the European scenario was also discussed in detail, analyzing gender equity in terms of first and last positioned authors in the author lists. This choice was because usually the most important contributors to articles are found in the first and last positions (with one usually overlapping with the name of the corresponding author), especially when some scientific areas (including material science) are considered (Kosmulski, 2012; Simoes and Crespo, 2020). Accordingly, in this study, the corresponding author role was excluded in the analysis as it often coincides with the first author. In addition, the first author is generally the most involved researcher in a specific study, i.e.,

performing laboratory testing (if any) and writing the main body of the paper, while the last author is usually the one who conceptualizes the research study and/or the director of the research laboratory. Moreover, even if different author orders can be also applied, for example, the alphabetic order when all authors provide the same contribution to the study, this represents more the exception rather than the rule in article publishing. Finally, the impact of the research based in Europe was considered through analysis of the citations of the research outputs, as well as the correlation with gender distribution.

## MATERIALS AND METHODS

The bibliometric study was carried out using the Scopus database (https://www.scopus.com). This database indexes content from 24,600 titles and 5,000 publishers which are selected by an independent review board. Scopus generates precise citation results and detailed researcher profiles. Moreover, the selected database takes into consideration only indexed research products; thus, non-peer-reviewed papers published in conference proceedings and articles published in non-indexed journals are not considered in this study.

The keywords used for the bibliometric research were “alkali activated materials” or “geopolymers.” In order to refine the results and be sure that all the documents were related to the specific topic, a first refinement of the results was carried out selecting only English language articles, reviews, and conference papers in order to analyze international research studies. A second refinement was based on selecting further keywords with the aim of a more precise delimitation of the topic. In particular, the following keywords were considered: “inorganic polymer,” “geopolymer(s),” “alkali activated,” “geopolymer concrete,” “geopolymer composites,” “geopolymer binders,” “alkali-activated binder,” “alkali-activated cement,” “geopolymerization,” “alkali-activated slag,” “geopolymer mortars,” “alkaline activation,” “geopolymer materials,” “geopolymer matrix,” “geopolymer cement,” and “alkali-activated fly ash.” These refinements allowed us to bring down the number of documents to be analyzed from more than 8,000 to 5,900 documents.

The analysis was carried out as follows: i) the trend of the published articles and the geographical distribution of the authors were processed by the “Analyze search results” tool available in Scopus. ii) The analysis of gender balance was carried out for the first 100 most prolific authors for each year. In order to correctly define the gender of the authors, each author profile was checked using the Scopus database and/or ResearchGate web page (https://www.researchgate.net/) and/or Google Scholar database (https://scholar.google.com/), in which author profile pictures are also usually reported, thus making gender recognition easier. When no information about the author profile (e.g., first name and picture) was available, the specific author was categorized as “undefined.” iii) Only for European authors, gender analysis of the first and last authors was carried out considering each author profile. As previously reported, first and last positioned authors usually play the most important roles in manuscript writing and in research development. Moreover, a
correlation with the number of citations was also considered to
analyze the impact of the publications related to gender
distribution. Number of citations for each article was recorded
on May 6th, 2020, on the Scopus database web page.

## RESULTS AND DISCUSSION

The following section reports the results of the bibliometric study on gender balance in the field of AAMs, including geopolymers. First, numerical and geographical distribution of published articles is reported. Then, gender analysis of the most prolific authors worldwide is discussed. Last, analysis of the gender balance of European authors is shown, considering first and last authors in the author list, as well as the correlation between author gender and number of citations.

## Numerical and Geographical Distribution of Published Articles

The first publication on geopolymers dates back to 1979 by Davidovits. However, only in the first years of the 2000s, this specific research topic started attracting research interests, as depicted in **Figure 1**, probably due to an increasing attention to environmental pollution problems and the consequent need to reduce CO₂ emission in building material production. Especially in the last decade, there has been an exceptional increase in research interest about AAMs (including geopolymers), as demonstrated by the increasing linear trend of published articles between 2009 and 2019 (inlay of **Figure 1**).

Specifically, in 2019 almost 1,000 articles were published in scientific peer-reviewed journals and international conference proceedings, highlighting that research on this topic is still strongly active all over the world. Due to this trend in the last 10 years, further analyses about geographical distribution and gender balance were carried out considering only the period between 2009 and 2019.

Geographical distribution of the articles on AAMs published between 2009 and 2019 is shown in **Figure 2**. Most of the articles were published by Chinese (13%), Australian (10%), and Indian (8%) researchers. In detail, about half of the publications on AAMs were based in Asia (47%), mainly in China, India, Malaysia, Thailand, and Indonesia. In addition, 26% of the publications were based in Europe, most of them in Italy, United Kingdom, France, the Czech Republic, and Spain, while 12% were publications from America and Oceania. Finally, only 3% of the publications were from African researchers, mainly based in Egypt, Cameroon, Tunisia, Nigeria, and South Africa.

## Gender Balance of Most Prolific Authors

A record of about 5,900 publications was analyzed to investigate the gender balance of the most prolific authors with a focus on each year in the last decade (2009–2019). The analysis took into account 595 authors, and among them, only 144 authors were females (24%), against 434 male authors (73%) (for 17 authors (about 3%), the gender was not identified). The distribution of author gender of the most productive researchers for each year is reported in **Figure 3**. No specific trend was observed. The gender

<figure><img src="image_422.png" /><figcaption>**FIGURE 1** | Distribution of the published works on the topic of alkali-activated materials (including geopolymers). In the inlay, distribution of publications in the last 10 years is reported (2009–2019).</figcaption></figure>
<figure><img src="image_423.png" /><figcaption>**FIGURE 2** | Geographical distribution of the published works (2009–2019) on the specific topic of alkali-activated materials and geopolymers: representation of country-based and continent-based distribution.</figcaption></figure>

<figure><img src="image_424.png" /><figcaption>**FIGURE 3** | Gender distribution of the first 100 most prolific authors publishing on alkali-activated materials in the years 2009–2019.</figcaption></figure>
distribution is similar to the one determined for the 5,900 publications, showing a female author rate ranging between 19% in 2013 and 30% in 2014. Surprisingly, in the last 5 years, an increase in women as most prolific authors was not observed, thus indicating that women have less opportunity or interest in publishing the results of their research or, alternatively, the number of women involved in building material science is still rather limited. Moreover, the fact that no specific trend of gender distribution was observed can be ascribed to the limited range of time considered in this study, due to the recent development of the research topic taken into account. Even if this specific topic has been mainly prolific in the last decade, the analyzed situation exhibited only one generation of researchers. Thus, this kind of analysis should be repeated for the next decades in order to monitor the gender distribution, and hopefully a significant increase in female authors among the most prolific researchers could be recorded. This result would be significant to reach a more equal gender distribution in this specific field of material science.

## The Case Study of Publications by European Authors

In order to deeply examine the gender distribution of authors involved in AAM research, a case study of publications with at least one European author was considered. In this specific case, a total record of 1,386 publications in the period 2009–2019 was analyzed: **Table 1** reports the type and distribution of the considered publications. In particular, most of the outputs are articles published in international peer-reviewed journals (74%). The remaining part is split as follows: 23% publications in international conference proceedings and only 3% of the publications are reviews.

Generally speaking, the number of all the publication types increased from 2009 to 2019, except for conference proceedings where a more alternate trend is detected. This peculiar behavior can be ascribed to the fact that international conferences are rarely scheduled every year, with biennial frequency being the most popular solution for many important congresses in the material sector (e.g., International Conferences on Modern Materials and Technologies, conferences organized by the European Ceramic Society and the American Ceramic Society, American Concrete Institute conventions, and International Conference on Advanced Ceramics and Composites). Moreover, at the moment, AAMs (including geopolymers) are usually included in one-day specific sessions inside conferences covering broader topics (such as ceramics, cement, and concrete)

and specific congresses on these are only a few (e.g., Engineering Conference International on AAMs, the Geopolymer Camp organized by the Geopolymer Institute, and International Conference on Non-Traditional Cement and Concrete at BRNO in Czech Republic).

It is further highlighted that open access (OA) publications represent 18% of publications by European authors, with an increasing trend in the last decade. In particular, from 2013, the number of OA publications continuously increased up to 26% in 2018 and 2019. OA publications provide free online access of scientific information, thus following European recommendations. Nowadays, in fact it is recognized that more accessible research results have the positive impact to produce better and more efficient science. Especially the EU Commission policies have encouraged and supported OA publications at the international level as demonstrated in the framework of its Research and Innovation Programmes, such as Horizon 2020 (European Commission, 2017).

Finally, only 1.4% of the total publication by European authors are by single authors, highlighting the interdisciplinary and collaboration research aspects characteristic of the material science sector.

Regarding the gender distribution of the European authors in 2009–2019, analysis of the record was carried out considering both first and last author positions in the author lists. These are key roles in research, as previously explained, usually representative of the active and responsible figure and the manager of the specific research, respectively. For this reason, generally speaking, it is possible that most of the first authors are researchers in an early-stage career, while expert researchers and professors are usually located in the last author position.

**Figure 4** reports the gender distribution regarding the European authorships of the first (Figure 4A) and last authors (Figure 4B) in articles published in 2009–2019. Unlike that reported for most productive authors publishing on AAMs worldwide (Figure 3), the case of European authors shows a higher participation of women in publications. In particular, for first authors in the author lists, a concentration of women in the range 36–56% was recorded (Figure 4A). Even if no specific trend was exhibited in the last decade, these results are encouraging as they are almost double of those observed worldwide and can be considered an interesting result of the policies of EU only recently strongly addressed to promote gender balance. As previously mentioned, first authors generally correspond to researchers in an early-stage career; thus, the actual and future European policies for enhancing excellence in research would help and encourage

<table><caption>TABLE 1 | Type of publications based in Europe on alkali-activated materials (including geopolymers) in the last decade (2009–2019).</caption><thead><tr><th></th><th>2009</th><th>2010</th><th>2011</th><th>2012</th><th>2013</th><th>2014</th><th>2015</th><th>2016</th><th>2017</th><th>2018</th><th>2019</th><th>Total</th></tr></thead><tbody><tr><td>Total publications</td><td>30</td><td>31</td><td>48</td><td>51</td><td>110</td><td>111</td><td>128</td><td>165</td><td>205</td><td>273</td><td>234</td><td>1,386</td></tr><tr><td>Journal articles</td><td>17</td><td>20</td><td>30</td><td>39</td><td>86</td><td>74</td><td>104</td><td>123</td><td>148</td><td>193</td><td>194</td><td>1,028</td></tr><tr><td>Conference proceedings</td><td>13</td><td>11</td><td>14</td><td>7</td><td>24</td><td>31</td><td>21</td><td>38</td><td>50</td><td>72</td><td>34</td><td>315</td></tr><tr><td>Review</td><td>—</td><td>—</td><td>4</td><td>5</td><td>—</td><td>6</td><td>3</td><td>4</td><td>7</td><td>8</td><td>6</td><td>43</td></tr><tr><td>Open access</td><td>1</td><td>—</td><td>—</td><td>—</td><td>11</td><td>12</td><td>17</td><td>38</td><td>41</td><td>70</td><td>62</td><td>252</td></tr><tr><td>Single author</td><td>—</td><td>—</td><td>2</td><td>2</td><td>3</td><td>2</td><td>3</td><td>—</td><td>6</td><td>5</td><td>6</td><td>19</td></tr></tbody></table>
<figure><img src="image_425.png" /></figure>

<figure><img src="image_426.png" /><figcaption>**FIGURE 4** | Gender distribution of publications with European authors in the last decade (2009–2019): (A) first author and (B) last author in the publication author lists.</figcaption></figure>

the most valued researchers to continue in their careers. On the other hand, analysis of the gender distribution of the last authors (Figure 4B) shows a lower concentration of women in the last author position than in the first author one, thus confirming that women in high positions (full professor, manager of laboratory, etc.) are still a minority. This situation is, however, better than that reported for most prolific authors worldwide, being the range between 20 and 38% (Figure 3). The trends in Figures 4A, B (showing near equity in the first author position and the low

amount of women in the last author position) agree with those reported in a previous study developed in the United States regarding several research fields (West et al., 2013).

Moreover, an analysis of the citations of publications about AAMs in which the first or last European authors were represented by women was carried out to highlight the impact of the publications. Results are reported in Table 2 considering the last decade (2009–2019). Generally speaking, higher percentages of women are found for number of citations lower

<table><caption>**TABLE 2** | Correlation between female authors and the number of publication citations (data refer to papers published in 2009–2019).</caption><thead><tr><th rowspan="3">Year</th><th rowspan="3">Author list position</th><th colspan="8">Number of citations</th></tr><tr><th colspan="2">&gt;100</th><th colspan="2">50-100</th><th colspan="2">10-50</th><th colspan="2">&lt;10</th><th colspan="2">None</th></tr><tr><th>Publications</th><th>Women (%)</th><th>Publications</th><th>Women (%)</th><th>Publications</th><th>Women (%)</th><th>Publications</th><th>Women (%)</th><th>Publications</th><th>Women (%)</th></tr></thead><tbody><tr><td rowspan="2">2009</td><td>First author</td><td>4</td><td>25.0</td><td>5</td><td>60.0</td><td>10</td><td>50.0</td><td>8</td><td>87.5</td><td>3</td><td>0.0</td></tr><tr><td>Last author</td><td></td><td>25.0</td><td></td><td>20.0</td><td></td><td>30.0</td><td></td><td>12.5</td><td></td><td>0.0</td></tr><tr><td rowspan="2">2010</td><td>First author</td><td>5</td><td>20.0</td><td>6</td><td>66.7</td><td>7</td><td>28.6</td><td>11</td><td>54.5</td><td>2</td><td>50.0</td></tr><tr><td>Last author</td><td></td><td>40.0</td><td></td><td>16.7</td><td></td><td>28.6</td><td></td><td>45.5</td><td></td><td>50.0</td></tr><tr><td rowspan="2">2011</td><td>First author</td><td>8</td><td>50.0</td><td>7</td><td>71.4</td><td>17</td><td>58.8</td><td>12</td><td>58.3</td><td>4</td><td>25.0</td></tr><tr><td>Last author</td><td></td><td>0.0</td><td></td><td>42.9</td><td></td><td>23.5</td><td></td><td>25.0</td><td></td><td>50.0</td></tr><tr><td rowspan="2">2012</td><td>First author</td><td>2</td><td>0.0</td><td>10</td><td>20.0</td><td>24</td><td>41.7</td><td>14</td><td>57.1</td><td>1</td><td>0.0</td></tr><tr><td>Last author</td><td></td><td>0.0</td><td></td><td>10.0</td><td></td><td>37.5</td><td></td><td>57.1</td><td></td><td>100.0</td></tr><tr><td rowspan="2">2013</td><td>First author</td><td>4</td><td>100.0</td><td>23</td><td>39.1</td><td>56</td><td>30.4</td><td>23</td><td>30.4</td><td>7</td><td>42.9</td></tr><tr><td>Last author</td><td></td><td>0.0</td><td></td><td>21.7</td><td></td><td>39.3</td><td></td><td>13.0</td><td></td><td>28.6</td></tr><tr><td rowspan="2">2014</td><td>First author</td><td>9</td><td>44.4</td><td>8</td><td>37.5</td><td>48</td><td>47.9</td><td>40</td><td>32.5</td><td>7</td><td>57.1</td></tr><tr><td>Last author</td><td></td><td>22.2</td><td></td><td>50</td><td></td><td>27.1</td><td></td><td>27.5</td><td></td><td>28.6</td></tr><tr><td rowspan="2">2015</td><td>First author</td><td>3</td><td>0.0</td><td>15</td><td>46.7</td><td>69</td><td>53.6</td><td>36</td><td>58.3</td><td>5</td><td>60</td></tr><tr><td>Last author</td><td></td><td>33.3</td><td></td><td>33.3</td><td></td><td>37.7</td><td></td><td>44.4</td><td></td><td>40</td></tr><tr><td rowspan="2">2016</td><td>First author</td><td>—</td><td>—</td><td>15</td><td>26.7</td><td>86</td><td>37.2</td><td>51</td><td>43.1</td><td>13</td><td>53.8</td></tr><tr><td>Last author</td><td></td><td>—</td><td></td><td>33.3</td><td></td><td>33.7</td><td></td><td>29.4</td><td></td><td>38.5</td></tr><tr><td rowspan="2">2017</td><td>First author</td><td>—</td><td>—</td><td>2</td><td>50.0</td><td>109</td><td>31.2</td><td>50</td><td>44.0</td><td>13</td><td>53.8</td></tr><tr><td>Last author</td><td></td><td>—</td><td></td><td>50.0</td><td></td><td>29.4</td><td></td><td>30.0</td><td></td><td>38.5</td></tr><tr><td rowspan="2">2018</td><td>First author</td><td>1</td><td>0.0</td><td>1</td><td>0.0</td><td>91</td><td>38.5</td><td>134</td><td>36.6</td><td>36</td><td>41.7</td></tr><tr><td>Last author</td><td></td><td>0.0</td><td></td><td>0.0</td><td></td><td>34.1</td><td></td><td>33.6</td><td></td><td>44.4</td></tr><tr><td rowspan="2">2019</td><td>First author</td><td>—</td><td>—</td><td>1</td><td>0.0</td><td>18</td><td>27.8</td><td>138</td><td>41.3</td><td>76</td><td>48.7</td></tr><tr><td>Last author</td><td></td><td>—</td><td></td><td>0.0</td><td></td><td>27.8</td><td></td><td>31.2</td><td></td><td>31.6</td></tr></tbody></table>
than 100 and female author in the first position. However, in the years 2011, 2013, and 2014, noteworthy percentages of European women as first authors are found in publications with more than 100 citations. Less relevant is the percentage related to women in the last author position with a high number of citations, even if some exceptions are observed (e.g., 2017: 50% of European female last authors with number of citations in the range of 50–100; 2015: 33.3% of European female last authors with number of citations >100). These results agree with previous results indicating that researchers in the early-stage careers, represented by first authors, are more equally gender distributed also in highly cited publications.

## CONCLUSIONS

The following conclusions can be drawn from this study:

* AAMs, including geopolymers, is a new material science field in which the research is prolific and innovative. Especially in the last decade, a continuous growth in publications in international peer-reviewed journals and in international conference proceedings was detected;

* publications on AAMs are mainly based in Asia (about 50%), even if Europe (about 26%) is largely involved in this specific topic;

* the analysis of gender distribution of the most productive authors worldwide showed that only 22% of the total authors are women;

* considering European authors, a more equal gender distribution is reached when the first author is considered. This trend is also highlighted when citations

are related to the gender: a significant concentration of women, especially in the first author positions, with citations >50, was found.

The analysis of the gender distribution reported in this study takes into consideration only indexed research products (articles published in peer-reviewed journals and conference proceedings) reported in the Scopus database. Moreover, it is representative of just one relative recent field of material science, so further gender-based bibliometric researches should be carried out to have a broader scenario for the material science sector. Results from this specific study can be used as the basis for future studies to bridge the gender gap and may contribute to enhancing the sensitivity toward gender issues. Finally, a continuous increase in the participation of female researchers is constantly registered in national and international conferences on AAMs and more in general in the construction material sector, thus showing an encouraging signal for narrowing the still existing gender gap.

## DATA AVAILABILITY STATEMENT

All datasets presented in this study are included in the article.

## AUTHOR CONTRIBUTIONS

MB conceptualized the topic of the study. GM elaborated the statistical data. GM, SM, and MB drafted the manuscript. All authors contributed to the article and approved the submitted version.

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**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Masi, Manzi and Bignozzi. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
<figure><img src="image_427.png" /></figure>

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# A Case Study of Industrial Symbiosis to Reduce GHG Emissions: Performance Analysis and LCA of Asphalt Concretes Made With RAP Aggregates and Steel Slags

Alessandra Bonoli*, Anna Degli Esposti and Chiara Magrini*

Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), University of Bologna, Bologna, Italy

The concept of sustainability in the road construction sector is a complex issue because of the various steps that contribute to the production and release of greenhouse gas (GHG) emissions. Addressing this issue, the European Commission has put various policy initiatives in place to encourage the construction industry to adopt circular economy (CE) and industrial symbiosis (IS) principles e.g., the use of recycled materials. Cooperativa Trasporti Imola (CTI), a company located in the Emilia-Romagna region (Italy), has been chosen for the current case study to examine practices, management, and the industrial symbiosis network among various companies in the road construction and rehabilitation sector. In this regard, the use of steel slags, obtained by an electric arc furnace (EAF), and reclaimed asphalt pavement (RAP), obtained by the deconstruction and milling of old asphalt pavement have been investigated. Two mixtures of recycled hot Mix Asphalt (HMA) i) were prepared incorporating different recycled material percentages for the wearing and binder course, respectively, ii) were characterized in terms of size distribution, strength modulus and volumetric properties, iii) and finally were compared to the performances of two mixtures entirely designed by virgin materials for the wearing and binder course, respectively. Therefore, the Life Cycle Assessment (LCA) tool was chosen to evaluate the environmental impacts that affect the designed road life cycle. The results show that recycling RAP and EAF slags in a CTI batch plant provides benefits by reducing the consumption of virgin bitumen and aggregates and by reducing CO₂eq emissions. Finally, practical implications on the use of recycled materials in new asphalt mixtures from a life cycle and industrial symbiosis perspective are provided.

**Keywords:** life cycle assessment, circular economy, industrial symbiosis, road construction, electric arc furnace steel slags, reclaimed asphalt pavement, recycled aggregates, standard characterization

## OPEN ACCESS

**Edited by:**
Emilia Morallon,
University of Alicante, Spain

**Reviewed by:**
Luigi Coppola,
University of Bergamo, Italy
Marco Pasetto,
University of Padua, Italy

**\*Correspondence:**
Alessandra Bonoli
alessandra.bonoli@unibo.it
Chiara Magrini
chiara.magrini7@unibo.it

**Specialty section:**
This article was submitted to *Structural Materials*, a section of the journal *Frontiers in Materials*.

**Received:** 09 July 2020
**Accepted:** 09 November 2020
**Published:** 10 December 2020

**Citation:**
Bonoli A, Degli Esposti A and Magrini C (2020) A Case Study of Industrial Symbiosis to Reduce GHG Emissions: Performance Analysis and LCA of Asphalt Concretes Made With RAP Aggregates and Steel Slags. *Front. Mater.* 7:572955
doi: 10.3389/fmats.2020.572955

## INTRODUCTION

Greenhouse gas (GHG) emissions in infrastructure projects are a key indicator when sustainability is being assessed (Gasparatos et al., 2008; Fernández-Sánchez and Rodríguez-López, 2010). The road sector, due to its characteristics (high energy consumption; use of resources, raw material, and surface; generation of high volumes of waste; quantity of linked transports and long service life) is
one of the main sectors that contributes to global warming (GW) (Cass and Mukherjee, 2011). Significant GHG emissions result from many stages of the road life cycle. Santero and Horvath (2009) stated that GHG emissions could range from negligibly small values to 60,000 tons of equivalent carbon dioxide ($CO_{2eq}$) per lane-kilometer over a service life of 50 years. Similarly, the emission factor per meter per year associated with the construction of road infrastructure has been estimated at 14.7 kg of $CO_{2eq}$ (Hill et al., 2012). In this sense, regarding road construction and maintenance, several steps contribute to the production and release of GHG emissions, i.e., site clearing, preparation of the sub-grade, production of construction materials (i.e., granular sub-base, base course, surfacing), site delivery, construction works, ongoing supervision, and maintenance activities. As European road infrastructure includes a growing network, with 4.8 million km at the end of 2013 (European Union Road Federation, 2017) and 5.5 million km at the end of 2016 (European Union Road Federation, 2019), this sector has broad margins for environmental improvement (Santero et al., 2011a; Santero et al., 2011b; JRC, 2016). For these reasons, this study is of particular interest.

Road construction is one of three main drivers of resource use in the European Union (Steger and Bleischwitz, 2011). Road construction not only requires large quantities of materials, but also their maintenance is highly material intensive. Construction works and regular maintenance of roads require materials that are produced through highly carbon-intensive and energy-demanding processes (Santos et al., 2015; Jiang and Wu, 2019). Previous studies have suggested that the life-cycle of GHG emissions associated with building roads can account for 10–20% of the emissions associated with the lifetime usage of the road by vehicles (Chester and Horvath, 2009; Hanson and Noland, 2015; Noland and Hanson, 2015). Previous research has also shown that the bulk of the emissions related to road construction and maintenance activities is often associated with the upstream emissions embodied in the materials used (Hanson and Noland, 2015; Huang et al., 2015; Noland and Hanson, 2015). Hence, the choice of materials impacts local pollution and environmental degradation. These materials primarily include asphalt, concrete, and steel (Hanson and Noland, 2015). Therefore, also considering the increase of landfilling restrictions on CDW, the use of alternative materials such as industrial by-products has gained greater significance and attention from academia and industrial sectors (Jamshidi et al., 2017).

The European Commission (EC) has put various policy initiatives in place to encourage the construction industry toward circular economy (CE) principles. The overall idea is to reconsider the whole life cycle of resources, to make the European Union (EU) a “circular economy” based on recycling, and the use of waste as a resource (EC, 2011).

The use of alternative materials for the construction and rehabilitation of roads would therefore be a strategy to be boosted, establishing regional industrial symbiosis (IS) agreements which can support companies to gain

competitiveness and reduce the environmental impact associated to their day to day business activities (Martin-Portugues Montoliu et al., 2019). For that reason, the EC has recently launched an industry-led IS reporting and certification system (EC, 2020). In this sense, symbiotic activities can be applied at different levels. According to Roberts (Roberts, 2004), they can involve a single firm or organization (micro level); companies co-located in the same area (meso level); and finally the entire regional or national production system (macro level). The greatest benefits are achieved at the meso level, where the clustering of complementary companies provides a complexity of functions (Roberts, 2004; Taddeo, 2016).

## Background of Hot Mix Asphalt, Recycling of Reclaimed Asphalt Pavement and Electric Arc Furnace Steel Slags

Roads are built in layers and three main types of road pavements can be identified: flexible, semi-rigid, and rigid pavements. In Europe, the main pavement type is flexible (asphalt) (Sherwood, 2001; Garbarino et al., 2016). As shown in **Figure 1**, the main road layers for flexible pavement are:

* surface, binder, and base courses, which consist of bituminous mixtures;

* road base and sub-base courses, which consist of cement bound or unbound aggregates.

Asphalt mixtures are typically composed of approximately 95% of mineral aggregates mixed with about 5% paving bitumen, with bitumen functioning as the glue that binds the mineral aggregates in a cohesive mix (EAPA, 2011). In general, three types of asphalt mixtures can be used: hot mix asphalt (HMA), warm mix asphalt (WMA), and cold mix asphalt (CMA).

Some aggregates can, usefully, be created by recycling processes.

In this study, the use of steel slags obtained by an electric arc furnace (EAF), and reclaimed asphalt pavement (RAP) obtained by the deconstruction and milling of old asphalt pavement have been investigated. The HMA technology was used in the production process.

While recycling HMA results in a reusable mixture of aggregates and aged asphalt binders known as RAP (Al-Qadi et al., 2007; Noferini et al., 2018), recycling steel slags produces artificial aggregates, containing 90% iron oxide and smaller quantities of other oxides (calcium, magnesium, silicon, etc.), derived from additives used in steel production. Moreover, on the basis of production technology, steel slags can be classified as basic oxygen furnace (BOF), electric arc furnace (EAF), and ladle refining (LF) slags (Meng and Liu, 2000; Gu et al., 2018).

As is widely known, steel slags, produced during the separation of molten steel from impurities in a steel-making furnace, are one of the most common industrial wastes and they can be used for several applications. Thanks to their high hardness and cementing properties, they are commonly used in the road sector (Rashad, 2019). In steel plants, high-grade steel
<figure><img src="image_428.png" /><figcaption>**FIGURE 1** | Flexible pavement layer system (Garbarino et al., 2016). The sizes of each course represented in the figure do not necessarily correspond to the actual ones.</figcaption></figure>

is melted, obtaining a sinking heavy liquid metal fraction, which agglomerates into a metal phase, separating metals from some light fractions such as chromium by reducing agents and liberating chromium from its compounds. Slag composition can be optimized in relation with application requirements and then it can be cast. In some processes (i.e., RecArc project, http://www.recarc.bam.de) the high-grade steel slags can be converted into a chromium-rich metal phase, which can be used as a raw material in high-grade steel production.

As far as the recycling of RAP and of EAF steel slags are concerned, several studies have shown that the use of these materials is common in pavement construction because of their technical performances and economic value. Miliutenko et al. (2013) have shown that HMA mixes with RAP content have the same technical characteristics (stiffness, fatigue, and deformation resistance) as virgin ones. Thanks to the incorporation of RAP in new asphalt mixtures, the need of neat bitumen is reduced, making RAP recycling economically attractive (Noferini et al., 2018; Pantini et al., 2018).

Similarly, other studies have demonstrated that steel slags with proper pre-processing and sufficient in-place quality control procedures can perform credibly well as asphalt aggregates (Del Fabbro et al., 2001; Ahmedzade and Sengoz, 2009; Gu et al., 2018). In particular, EAF slags have been frequently used as pavement aggregates due to their excellent mechanical properties, which make them suitable for asphalt layers with any kind of traffic load (Santos et al., 2015).

Moreover, Pasetto and Baldo (2017) studied the stiffness and the fatigue performance of five different base-binder bituminous mixtures, made with RAP and EAF steel slag, up to 70% by weight of the aggregate. They reported that the resulting mixes with RAP and EAF slag were characterized by improved stiffness and fatigue performance compared to the control asphalt concrete, made exclusively with natural aggregate.

<figure><img src="image_429.png" /><figcaption>**FIGURE 2** | LCA phases (EC et al., 2010).</figcaption></figure>

## The Life Cycle Assessment and the Circularity Approach

The Life Cycle Assessment (LCA) is a considerable method to evaluate the environmental impacts of a system, a product, or a process. All the inputs (such as energy and resources) are identified, with the aim of quantifying the relevant emissions, the consumed resources, and the related environmental impacts. Considering a product, the impacts do not only arise during the manufacturing stage, but along its entire life cycle, including the extraction and transportation of raw materials, use and maintenance, possible reuse, and end of life. Therefore, the approach encompasses the whole life cycle of a product, “from cradle to grave,” as the first definition stated (SETAC, 1993): from “cradle,” where raw materials are extracted, put into production, and used, to “grave,” i.e., waste disposal, with the aim to provide a comprehensive picture of the environmental impacts of the system.
According to a circularity perspective, a new philosophy, referred to as “from cradle to cradle”, is taking hold: at their end of life, materials are not considered as waste to be discarded, but as secondary raw material, thanks to an appropriate recycling process. In this way, a cradle-to-cradle closed loop is outlined.

According to the ISO14040 standard, the four steps to perform a Life Cycle Assessment (LCA) are: the definition of the goal and scope of the analysis, the inventory analysis, the impact assessment, and finally, the interpretation of the results (Figure 2).

## Goal and Scope Definition

The context of the study and its purpose are set. The goal of the LCA states the intended application and the reasons for carrying out the study, the intended audience, and whether the results are to be used for internal purpose or for disclosure to the public. The scope includes the following items: functional unit, system boundary, allocation procedure, data requirements, impact assessment method, assumption, and data quality. In particular, the functional unit, that defines the quantification of the identified function of the product, has the primary purpose to provide a reference to which the inputs and outputs are related, ensuring the comparability of the LCA results. The system boundary defines the unit processes to be included in the system. Criterions for the choice of the system boundaries are physical (description of the productive cycle), geographical (reference area), and temporal (reference period).

## Inventory Analysis or Life Cycle Inventory

It lists all the inputs (materials and energy) and outputs (products, co-products, and emissions) to be used to compare standards and processes. Inventory analysis involves data collection and calculation procedures, aiming at quantifying the relevant inputs and outputs of a product system. The life cycle inventory uses both primary and specific as well as literature and secondary data from international databases.

## Impact Assessment

The life cycle impact assessment (LCIA) includes the following mandatory elements: the selection of impact categories and characterization models; the assignment of LCI results to the selected impact categories (classification); and the calculation of category indicator results (characterization).

## Interpretation

Finally, the life cycle interpretation aims at the identification of substantial issues, based on the results of the previous steps. The evaluation includes considerations about the completeness and the consistency of the study, conclusions, limitations, and recommendations.

In Figure 2, the two-way arrows highlight the iterative approach of an LCA. During the analysis, the results and the assumptions in subsequent stages might lead to the revision of what has been done in previous stages, in a process of continuous

improvement. Information which was not available during the compilation of the previous phases can be added afterward.

## The Life Cycle Assessment in the Road Construction Sector

Due to the high amount of GHG emissions generated during road construction, rehabilitation, and operation, the evaluation and reduction of the environmental impact related to the road sector have become an international challenge (Espinoza et al., 2019). In this sense, a systematic approach has emerged to assess the environmental impact of pavements. LCA is considered a relevant methodology to evaluate the environmental impacts that affect the road life-cycle (Espinoza et al., 2019) and the International Organization for Standardization (ISO) has established the principles, requirements, and guidelines to regulate the LCA analysis (ISO 14040, ISO 14044, ISO 14020, ISO 14024, and ISO 14025). Moreover, the Joint Research Centre (JRC) in 2016 proposed an LCA as an assessment methodology of road environmental performance, with reference to ISO 14067 or equivalents and ISO 14040/14044. Finally, according to Espinoza et al., (2019), the use of an LCA for evaluating the environmental performance of the construction of road projects allows construction companies to obtain information that can be used to predict the performance of their projects and to evaluate compliance with environmental requirements. Similarly, it allows the selection of optimal materials and construction processes, reducing the GHG emissions and permitting a more sustainable approach.

Therefore, LCA analysis performed by Espinoza et al., (2019) highlighted that HMA production generates the greatest environmental impact, considering the extraction of raw material and the construction of the HMA layers. Previous research has shown that HMA emits up to 18-22 kgCO₂/t (Agentschap Wegen en Verkeer, 2012; JRC, 2016) and a recent LCA literature review for roads, carried out by JRC (2016), shows that the second largest source of environmental impact after the use phase is the production of construction materials. In addition, in low traffic roads, this can in fact be the most significant source of environmental impact (JRC, 2016). Moreover, the durability of road materials is a key factor that will influence the requirement for maintenance. The impacts of maintenance activities themselves are dominated by impacts from material production and transportation. Consequently, special attention to HMA production and construction materials is required in order to minimize GHG emissions. For these reasons, several studies have pointed out the environmental benefits of using recycled materials, such as RAP and EAF steel slags. Hasan et al. (2020) argued that RAP obtained after milling and screening existing asphalt pavements is a viable alternative to mitigate the high GHG burdens of bitumen and aggregates (Praticò et al., 2015; Guo et al., 2018) and transport agencies (AASHTO, 2012; Hasan et al., 2020). In particular, the use of steel slags in asphalt mixtures saves natural resources, by reducing the consumption of natural and non-renewable aggregates and the quantity of slag deposited on landfill sites (Ferreira et al., 2016) and the reduction of the landfill space requirements associated with the need to landfill industrial
wastes and by-products (Carpenter et al., 2007; Huang et al., 2009; Miliutenko, et al., 2013, Mladenovic et al., 2015). The EAF is a less energy intensive process where electricity is used to melt steel into the end product. This could be a promising alternative that may have close to zero CO₂ emissions, theoretically (Ferreira et al., 2016; Morfeldt et al., 2015). Finally, Giani et al. (2015) explored the replacement of virgin asphalt by 10% RAP in a HMA surface course and by 20% RAP in a HMA binder course of a 1 km asphalt pavement section in Italy. They found that the HMA RAP alternative exhibited 688 tons of CO₂eq (6.8%) GHG emissions reductions, considering that the environmental burdens of asphalt significantly depend on the bitumen content (Häkkinen and Mäkelä, 1996).

## Description of Context: Recycling Asphalt Pavement in “Cooperativa Trasporti Imola Scrl” Company

The “Cooperativa trasporti Imola Scrl” (CTI) company has four plants for the production of asphalt mixes, three batch plants and one drum plant. In both the typologies, the mineral aggregates are dried and heated in a rotating drum. Nowadays, the predominant plant type in the U.S. and New Zealand is the drum-mix plant, while batch plants prevail in Europe, South Africa, and Australia (EAPA, 2018). While in batch plants aggregates are stored in hot bins to mix them with bitumen in discrete batches, in drum plants the mixing of aggregates with bitumen takes place in the same drum. After those processes, the mixtures are stored in silos or loaded into trucks for delivery. Afterwards, aggregates, temporarily stored in a silo, are transported by mechanical shovels, and loaded on hoppers for pre-dosage. The CTI plants have seven hoppers, five for aggregates and two for milled materials, with the possibility of introducing RAP. In the drum plant, the drum acts both as a dryer and a mixer, whereas in the batch plant the mixing of aggregates with bitumen takes place in different machines. The bitumen, heated to 130–150°C by an oil-fired oleothermal boiler, is kept at a constant temperature in the storage tanks. Considering hot in plant recycling, while the most conventional drum plants can accommodate up to 50% RAP, and the percentage of reusable RAP in batch plants ranges from 10 to 30% (Kandhal and Mallick, 1997; Noferini et al., 2018), nowadays multiple readily available for production technologies can accommodate up to 100% of recycled hot mix asphalt (Zaumanis et al., 2014; Noferini et al., 2018). The CTI batch plants might accommodate up to 45–50% RAP, while the percentage of reusable RAP in drum plants is approximately 50%. Hence, nowadays there is no technical limit on RAP content in new asphalt mixtures, as long as an adequate performance is achieved. However, it is a common practice to set a maximum value, to guarantee the durability of asphalt mixes in the long term (JRC, 2016), due to the possible compromising effect of the aged bitumen in RAP on the final mix. Moreover, the defined optimum content of RAP in asphalt mixtures varies widely from country to country, from 7 to 50% (up to 66%) by mass (Kalman et al., 2013; Garbarino et al., 2016). On average, western European countries have 40% RAP

content in HMA and WMA mixtures, while Eastern European countries have 6% (BIOIS and EC, 2011; Blankendaal et al., 2014; Garbarino et al., 2016).

Moreover, according to the European Commission, steel slags can be used in road construction, meeting the requirements of European and national legislations and standards, although a specific recycling target is not set (JRC, 2016).

In Italy, the steel slags resulting from steelmaking are considered by-products, whereas RAP, as a result of the milling operations of existing road pavements at their end of life stage, is not considered to be waste, as long it is re-used within the domain of the asphalt sector (Italian M.D. 69/2018). Due to the fact that in Italy the use of steel slags and RAP for road construction is allowed, in this study, the content of RAP and EAF slags in the mixtures was designed to allow for the production of the mixtures in the CTI batch plant and to achieve acceptable values of physical and mechanical properties, in compliance with national legislations and technical standards.

## Objectives and Research Approach

This study aims at testing the use of EAF steel slags and RAP in two mixtures, for wearing and binder courses, respectively. The physical and mechanical properties and the environmental performances have been evaluated. The objectives of the research study are summarized below:

* define a standard characterization of mixtures in order to evaluate the physical and mechanical performances related to the use of virgin and recycled materials;

* assess the environmental impacts associated with the mixtures and model a best-case scenario for the CTI batch plant with the maximum percentages of steel slags and RAP;

* identify practical implications of the use of recycled materials in new asphalt mixtures, from a life cycle and industrial symbiosis perspective.

The research study is divided into two phases: in the first phase, the effects of recycled materials on asphalt mixture proprieties are investigated. Two specific types of asphalt mixtures are produced with different compositions:

(1) 35% RAP and 16% steel slags for the wearing course, by weight;

(2) 40% RAP and 15% steel slags for the binder course, by weight.

Asphalt materials are characterized in terms of size distribution, strength modulus (indirect tensile strength), and volumetric properties (air voids content).

The second phase aims at evaluating environmental impacts by applying LCA methodology to the geographical context of the CTI company. The novelty of this study is the integration of the technical analysis of material characterization, assessed by laboratory experiments, with the analysis of the environmental impacts.
<table><caption>**TABLE 1** | Composition of asphalt mixtures, percentages of aggregates by weight.</caption><thead><tr><th>Material</th><th>Fraction (mm)</th><th>MixW0 (control)</th><th>MixW1<br>(35% RAP, 15%<br>EAF steel<br>slags)</th><th>MixB0 (control)</th><th>MixB1<br>(40% RAP, 16% EAF steel slags)</th></tr></thead><tbody><tr><td>Gravel</td><td>14/20</td><td>—</td><td>—</td><td>15</td><td>6</td></tr><tr><td>Gravel</td><td>10/16</td><td>—</td><td>—</td><td>20</td><td>—</td></tr><tr><td>Gravel</td><td>8/12</td><td>—</td><td>—</td><td>10</td><td>—</td></tr><tr><td>Gravel</td><td>4/8</td><td>19</td><td>10</td><td>7</td><td>—</td></tr><tr><td>Gravel</td><td>3/6</td><td>30</td><td>9</td><td>12</td><td>10</td></tr><tr><td>Sand</td><td>0/4</td><td>45</td><td>30</td><td>32</td><td>25</td></tr><tr><td>Filler</td><td>—</td><td>6</td><td>1</td><td>4</td><td>3</td></tr><tr><td>RAP</td><td>0/8</td><td>—</td><td>35</td><td>—</td><td>—</td></tr><tr><td>RAP</td><td>8/12</td><td>—</td><td>—</td><td>—</td><td>40</td></tr><tr><td>EAF slag</td><td>4/8</td><td>—</td><td>15</td><td>—</td><td>16</td></tr></tbody></table>

<figure><img src="image_430.png" /><figcaption>**FIGURE 3** | MixW1 gradation and limits.</figcaption></figure>

# MATERIALS AND METHOD

## Performance Analysis of Asphalt Mixtures

Four mixtures were analyzed:

* A control mixture for the wearing course (MixW0)

* An experimental mixture for the wearing course (MixW1)

* A control mixture for the binder course (MixB0)

* An experimental mixture for the binder course (MixB1)

A description of the four mixtures can be found in **Table 1**. The design of the aggregate distribution was based on gradation limits specified in the UNI 13108 Italian technical specification for bituminous layers, as shown in **Figures 3, 4**, with cumulative percentage passing on the y axis and logarithmic sieve size on the x axis. On the graphs, the sieve size scale (x axis) is logarithmic.

The experimental program can be divided into three different phases. In order to evaluate the physical and mechanical performances of the designed mixtures, MixW1 and MixB1 were characterized in terms of particle size distribution (1), volumetric properties (2), and strength modulus (3) according to the standard UNI EN 933-1 (2012), UNI EN 12697-12 (2018), UNI EN 12697-23 (2003), and UNI EN 12697-26 (2012). Asphalt mixtures were manufactured in a laboratory with design neat bitumen content of 6% for the wearing course and 5% for the binder course (these percentages include aged bitumen contained in RAP, 3.85% for the wearing course and 3.30% for the binder course, respectively). A neat binder was incorporated into the mixes, taking into account the presence of the aged binder in the RAP fractions. At the same time, the inclusion of recycled materials in the mixtures requires the addition of rejuvenating agents (ACF) to improve the adhesion properties, thermal susceptibility, viscosity, and workability of the mixes. The ACF
<figure><img src="image_431.png" /><figcaption>FIGURE 4 | MixB1 gradation and limits.</figcaption></figure>

is incorporated in the commercial bitumen. Both aggregates and bitumen can directly replace their equivalent virgin products in the new mixtures (MixW1 and MixB1) at a ratio of 1:1. Aggregates were heated at 160°C. The physical and mechanical characterizations were then carried out. Asphalt mixes were tested for particle size distribution (EN 933 – 1), air void content (EN 12697-8), and indirect tensile strength (EN 12697 – 23).

### Determination of Particle Size Distribution (EN 933 – 1)

The sieve analysis was carried out in a laboratory to define the particle size distribution of MixW1 and MixB1. According to the EN 933-1 standard, a representative weighed sample for each mixture was separated on sieves of different sizes (Series 2). To find the percentage of the aggregate passing through each sieve, Eq. 1 was used:

$$ \% \text{ retained} = \frac{W_{\text{Sieve}}}{W_{\text{Total}}} \times 100\% \quad (1) $$

where: $W_{\text{Sieve}}$ is the mass of the aggregate in the sieve; $W_{\text{Total}}$ is the total mass of the aggregate.

In order to find the cumulative percentage of the aggregate retained in each sieve, Eq. 2 was used. The total amount of the aggregate retained in each sieve and the amount in the previous sieves were added up. Then, the cumulative percentage passing of the aggregate was found by subtracting the percentage retained from 100%.

$$ \% \text{ cumulative passing} = 100\% - \% \text{ cumulative retained} \quad (2) $$

The % cumulative retained ($P_i$) used was calculated using Eq. 3:

$$ P_{c1,i} \cdot \alpha_{C1} + P_{c2,i} \cdot \alpha_{C2} + \dots + P_{cj,i} \cdot \alpha_{Cj} + \dots + P_{cm,j} \cdot \alpha_{Cm,j} = P_i \quad (3) $$

where: $P_{c1,i}$ is the passing at sieve j; $\alpha_{C1}$ is the percentage by weight of the total of the sieve j.

To solve Eq. 3, Eq. 4 was provided:

$$ \sum_{j=1}^{m} \alpha_{Cj} = \alpha_{C1} + \alpha_{C2} + \dots + \alpha_{Cj} + \dots + \alpha_{Cm} = 1 \quad (4) $$

### Determination of Air Voids Content (EN 12697-8)

Once the mix design for MixW1 and MixB1 was defined, the following step in the research program considered their physical analysis. The compactability and workability properties of the HMAs were evaluated against gyratory compactor samples (EN 12697-31). For both mixtures, three specimens per MixW1 were compacted up to 180 times more than the gyratory compactor, and three specimens per MixB1 were compacted up to 210 times more than the gyratory compactor. The air voids content (v) of each specimen was evaluated according to the EN 12697-8 standard.

### Determination of Indirect Tensile Strength (EN 12697-23)

Finally, for each mixture, according to the EN 12697-23 standard, the indirect tensile strength (ITS) was performed at 25°C.

### Life Cycle Assessment Study

The present study has assessed the impacts arising from the hot-mix batch plant by applying an LCA methodology to the geographical context of the CTI plant, located in the Emilia-Romagna region. As previously described, an LCA study consists of four stages: 1) goal and scope definition, 2) inventory analysis, 3) impact assessment, and 4) results and interpretation.

### Goal and Scope Definition

Quantitative and comparative life cycle assessment results on road construction materials are essential first steps toward making informed decisions and toward more sustainable practices in road construction (Chowdhury et al., 2010). The present LCA study aims at evaluating the potential environmental impacts related to asphalt mixtures: 1) MixW0 compared to MixW1 and 2) MixB0 compared to MixB1. The final aim is to
<figure><img src="image_432.png" /><figcaption>**FIGURE 5** | Diagram flow of recycling reclaimed asphalt pavement (RAP) and electric arc furnace (EAF) in the CTI batch plant.</figcaption></figure>

provide recommendations to the CTI for the improvement of technologies and regulations, based on environmental considerations. The functional unit (FU) of LCA is 1 km of secondary suburban road (with a width of 10.5 m, and a thickness of 4 cm for the wearing course, and 6 cm for the binder course). The system boundary includes all the treatment processes, starting from virgin material mining, and secondary and virgin materials entering the CTI batch plant (diesel, electricity), until when they leave the plant as an (solid, liquid, or gaseous) emission or as a new material. The final disposal or recycling processes are out of the boundary. Hence, this LCA is a cradle-to-gate analysis. As depicted in **Figure 5**, the system and processes involved in the present study are:

- The raw material transportation from the mining site/quarry to the CTI plant;

- The RAP transportation from road worksites to the CTI plant;

- The RAP pre-processing, which includes crushing and screening;

- The avoided production and transportation of natural aggregates (replaced by recycled aggregates), including extraction, processing, and transportation to the CTI batch plant;

- The avoided production and transportation of virgin bitumen (replaced by recycled bitumen).

The geographical scope is local. The study focuses on the conditions and CTI technologies used in 2018. The potential environmental impacts were evaluated using the software SimaPro®. This analytical tool works in accordance with the ISO 14040 standard (ISO, 2006a). The impact assessment baseline, performed by the Institute of Environmental Sciences of the Leiden University (CML) in version 3.05, was selected as a method for the environmental impact assessment, using the LCI “Ecoinvent 3.5” and “Europe & Denmark input output” databases. The following impact categories were evaluated: abiotic depletion, acidification, eutrophication, global warming potential, ozone layer depletion, and photochemical oxidation.

## Inventory Analysis

Data regarding the core processes, i.e., transportation, hot recycling, and energy consumption, are primary data. For analyzing the CTI HMA batch plant, data were collected directly from the CTI company. Data related to other foreground processes, i.e., bitumen production, extraction of natural mineral resources, and pre-processes of waste asphalt, were instead taken from the LCA software SimaPro databases (Ecoinvent and Europe & Denmark databases). Therefore, the avoided impacts, due to the avoided consumption of natural virgin aggregates because of the EAF steel slags and RAP addition into hot mixes, are modelled using secondary data on quarry activities in Europe.

Inventory data about the transportation of the raw materials, asphalt waste, and bitumen are modelled using the primary data on CTI transports, as shown in **Table 2**. **Table 3** shows the inventory data on energy consumption in the CTI batch plant.

## Impact Assessment

In the LCIA, the CML impact assessment baseline calculation method was adopted. The consumption of materials and energy as well as the emissions to air, water, and soil were gathered according to the effects they can have on the environment. According to ISO 14044 (2006b), the LCIA consists of classifications into impact categories, normalization, and the weighting of impacts. In this standard, a distinction between mandatory elements (classification and characterization) and optional elements (normalization, grouping, ranking, and weighting) was pointed out. In the current LCA study, classification and characterization were performed to assess the environmental impacts of MixW1 compared to MixW0 and of MixB1 compared to MixB0. No optional elements were evaluated.

Therefore, this methodology aims to assess the environmental impacts of the processes identified in the inventory analysis. Hence, all substances were measured and assigned to an impact category. The results are represented by single midpoints.
<table><caption>**TABLE 2** | Inventory data about the transportation of the asphalt waste, the by-products, and the primary materials to the plant.</caption><thead><tr><th>Material</th><th>Transport distance (km)</th><th>Description</th><th>Lorry type</th><th>Source</th></tr></thead><tbody><tr><td>EAF slags</td><td>150</td><td>Road distance between company - CTI batch plant</td><td>32 metric tons, EURO 6</td><td>Ecoinvent 3.5</td></tr><tr><td>Asphalt waste</td><td>40</td><td>Road distance between RAP site - CTI batch plant</td><td>32 metric tons, EURO 6</td><td>Ecoinvent 3.5</td></tr><tr><td>Natural aggregates</td><td>190</td><td>Road distances between quarry site - CTI batch plant</td><td>32 metric tons, EURO 6</td><td>Ecoinvent 3.5</td></tr><tr><td>Virgin bitumen</td><td>230</td><td>Road distances between bitumen plants - CTI batch plant</td><td>28 metric tons, EURO 6</td><td>Ecoinvent 3.5</td></tr></tbody></table>

<table><caption>**TABLE 3** | Inventory data about the energy consumption in the CTI batch plant.</caption><thead><tr><th>Processes</th><th>Energy type</th><th>Energy consumption/ton (kWh/ton)</th><th>Methane (m³)</th><th>Source</th></tr></thead><tbody><tr><td>Line 0</td><td>Electricity</td><td>6131</td><td>8.5</td><td>Ecoinvent 3.5</td></tr><tr><td>Line 1</td><td>Electricity</td><td>273</td><td>8.5</td><td>Ecoinvent 3.5</td></tr></tbody></table>

<table><caption>**TABLE 4** | Environmental impacts related to MixW1 and MixB1.</caption><thead><tr><th rowspan="2">Impact categories</th><th rowspan="2">Unit</th><th colspan="2">Total</th></tr><tr><th>MixW1</th><th>MixB1</th></tr></thead><tbody><tr><td>Global warming potential</td><td>kg CO<sub>2</sub> eq.</td><td>4.60E + 04</td><td>5.80E + 04</td></tr><tr><td>Human toxicity</td><td>kg 1.4 - DB eq.</td><td>1.82E + 04</td><td>2.62E + 04</td></tr><tr><td>Acidification</td><td>kg. SO<sub>2</sub> eq.</td><td>3.09E + 02</td><td>3.35E + 02</td></tr><tr><td>Eutrophication</td><td>kg PO<sub>4</sub> eq.</td><td>8.31E + 01</td><td>1.09E + 02</td></tr><tr><td>Ecotoxicity</td><td>kg 1.4 - DB eq.</td><td>1.53E + 03</td><td>1.55E + 03</td></tr><tr><td>Photochemical oxidation</td><td>kg C<sub>2</sub>H<sub>4</sub></td><td>1.53E + 01</td><td>1.72E + 01</td></tr><tr><td>Ozone layer depletion</td><td>kg CFC - 11 eq.</td><td>3.18E - 02</td><td>3.39E - 02</td></tr><tr><td>Abiotic depletion</td><td>kg Sb eq.</td><td>1.45E - 01</td><td>1.49E - 01</td></tr><tr><td>Abiotic depletion fossil fuels</td><td>MJ</td><td>2.52E + 06</td><td>2.73E + 06</td></tr></tbody></table>

<table><caption>**TABLE 5** | Mechanical and volumetric properties of MixW1 and MixW0.</caption><thead><tr><th rowspan="2">Specimen</th><th rowspan="2">Avg. ITS (MPa)</th><th colspan="3">Avg. void (%)</th></tr><tr><th>10</th><th>120</th><th>210 (v)</th></tr></thead><tbody><tr><td>MixW1</td><td>2.68</td><td>11.2</td><td>2.7</td><td>1.8</td></tr><tr><td>MixW0</td><td>1.19</td><td>13.2</td><td>4.0</td><td>2.5</td></tr></tbody></table>

<table><caption>**TABLE 6** | Mechanical and volumetric properties of MixB1 and MixB0.</caption><thead><tr><th rowspan="2">Specimen</th><th rowspan="2">ITS (MPa)</th><th colspan="3">Void (%)</th></tr><tr><th>10</th><th>100</th><th>180</th></tr></thead><tbody><tr><td>MixB1</td><td>1.88</td><td>9.6</td><td>4.6</td><td>1.6</td></tr><tr><td>MixB0</td><td>1.35</td><td>13.6</td><td>4.9</td><td>2.9</td></tr></tbody></table>

# RESULTS

## Standard Characterization Test Results: Performances of the MixW1 and MixB1 Mixtures

In order to evaluate the physical and mechanical performances of the mixtures incorporating different recycled aggregate percentages for the wearing and binder courses, MixW1 and MixB1 were characterized in terms of air void content (v), indirect tensile strength (ITS), indirect tensile stiffness modulus (ITSM), and indirect tensile strength ratio (ITSR).

The determination of the air void content of MixW1 and MixB1 can be found in the **Supplementary Material** as well as the results of the determination of ITS, ITSR, and ITSM of MixW1 and of MixB1, respectively. According to the UNI EN 12 697 - 12 standard, the ITSR value represents the ratio of the indirect tensile strength of wet (water conditioned) specimens to that of dry specimens expressed as percentages, calculated by using the following equation (Eq. 5):

$$ITSR = 100 \cdot \frac{ITS_w}{ITS_d} \qquad (5)$$

Where: $ITS_w$ is the indirect tensile strength of wet (water conditioned) specimens; $ITS_d$ is the indirect tensile strength of dry specimens.

## Life Cycle Assessment Results: Performances of the MixW1 and MixB1 Mixtures

The LCA was chosen to evaluate the environmental impacts that affect the designed road life cycle (production and treatment processes and transportation of the involved materials). The overall environmental impacts related to the production of asphalt mixtures MixW1 and MixB1 in the CTI batch plant are shown in Table 4. The analysis was supported by the LCA in compliance with the ISO 14040 standard and the ISO 14044 standard.

# DISCUSSION OF STANDARD CHARACTERIZATION OF MIXTURES AND LIFE CYCLE ASSESSMENT

To discuss the results of the standard characterization of the designed mixtures, a comparison of the performances of the designed mixtures and control mixtures was first performed.

Tables 5, 6 show the results for the four mixtures, in terms of average indirect tensile strength (ITS) and average air void percentages.
<figure><img src="image_433.png" /><figcaption>FIGURE 6 | Contribution analysis related to recycling of RAP and EAF slags in CTI batch plant for wearing course (above) and for binder course (below).</figcaption></figure>

The mechanical analysis was supported by the ITS test in
compliance with the EN 12697-23 standard. For each mixture,
three samples were prepared with a gyratory compactor (180 and
210 times) and then conditioned at 25°C for 4 h before testing.
According to the scientific literature, an ITS test is generally used to
assess the level of tenacity of the aggregate-filler-bitumen bond
(Sangiorgi et al., 2019) and the ITS value strongly depends on the
medium-high amount of aggregates, bitumen, and recycled materials.
From the analysis of data, there was a difference in terms of indirect
tensile resistance between the two experimental mixtures (MixW1
and MixB1) compared to the control ones (MixW0 and MixB0). It
could be argued that the results indicate a hardening of the composite
blend caused by the presence of the aged bitumen. Therefore, the two
mixtures show different air void contents.

To note, the average value of ITS recorded for both
experimental mixtures was considerably higher than the limit
suggested by the Italian technical specifications (ANAS, 2019),
which ranges between 0.72 and 1.60 MPa per wearing course, and
between 0.72 and 1.40 MPa per binder course.

Similarly, if the Italian technical specification is taken into
account, the air void content (v) was lower than the suggested
one, which ranges between 3 and 8%.

The standard characterization of the mixtures evaluated the
hardening effect of the old bitumen on the content blend.
According to Noferini et al. (2018), the hardening effect
becomes relevant when the RAP binder content is above 20%
by weight of the mixture. In particular, the new bituminous
mixtures per binder and wearing course (MixB1 and MixW1)
<table><caption>**TABLE 7** | Environmental impacts related to MixW0 and MixW1, MixB0 and MixB1.</caption><thead><tr><th rowspan="2">Impact categories</th><th rowspan="2">Unit</th><th colspan="4">Total</th></tr><tr><th>MixW0</th><th>MixW1</th><th>MixB0</th><th>MixB1</th></tr></thead><tbody><tr><td>Global warming potential</td><td>kg CO<sub>2</sub> eq.</td><td>7.51E + 04</td><td>4.60E + 04</td><td>9.37E + 04</td><td>5.80E + 04</td></tr><tr><td>Human toxicity</td><td>kg 1.4 - DB eq.</td><td>2.86E + 04</td><td>1.82E + 04</td><td>4.02E + 04</td><td>2.62E + 04</td></tr><tr><td>Acidification</td><td>kg. SO<sub>2</sub> eq.</td><td>5.30E + 02</td><td>3.09E + 02</td><td>6.36E + 02</td><td>3.35E + 02</td></tr><tr><td>Eutrophication</td><td>kg PO<sub>4</sub> eq.</td><td>1.30E + 02</td><td>8.31E + 01</td><td>1.73E + 02</td><td>1.09E + 02</td></tr><tr><td>Ecotoxicity</td><td>kg 1.4 - DB eq.</td><td>2.30E + 03</td><td>1.53E + 03</td><td>2.33E + 03</td><td>1.55E + 03</td></tr><tr><td>Photochemical oxidation</td><td>kg C<sub>2</sub>H<sub>4</sub></td><td>2.61E + 01</td><td>1.53E + 01</td><td>3.25E + 01</td><td>1.72E + 01</td></tr><tr><td>Ozone layer depletion</td><td>kg CFC - 11 eq.</td><td>4.74E - 02</td><td>3.18E - 02</td><td>5.97E - 02</td><td>3.39E - 02</td></tr><tr><td>Abiotic depletion</td><td>kg Sb eq.</td><td>2.44E - 01</td><td>1.45E - 01</td><td>2.23E - 01</td><td>1.49E - 01</td></tr><tr><td>Abiotic depletion fossil fuels</td><td>MJ</td><td>3.74E + 06</td><td>2.52E + 06</td><td>4.73E + 06</td><td>2.73E + 06</td></tr></tbody></table>

<figure><img src="image_434.png" /></figure>

were more rigid than the traditional ones (MixB0 and MixW0), but they were not excessively thickened because the final void value was less than 2%. Hence, the final void content of the two new mixtures was optimal.

Moreover, as the stiffness and the fatigue performance were not tested, further research might investigate these aspects.

Secondly, a comparison of LCA results between the two mixtures for the wearing course (MixW0 and MixW1) and the two mixtures for the binder course (MixB0 and MixB1) was performed. Hence, **Figure 6** and **Table 7** show the results of the contribution analysis related to the recycling of RAP and EAF slags in the CTI batch plant. Interestingly, significant differences were found between MixW0 and MixW1, and between MixB0 and MixB1. LCA allows the authors to evaluate the environmental benefits related to the use of recycled aggregates. Due to the reduction of emissions and natural resources used, MixW1 and MixB1 provide environmental benefits in all impact categories (abiotic depletion, abiotic depletion fossil fuels, global warming potential, ozone layer depletion, human toxicity, terrestrial ecotoxicity, photochemical oxidation, acidification, and eutrophication). The avoided impacts associated with the use of recycled material and with the reduction in the consumption of bitumen and aggregates overcome the impacts related to the waste transportation and the pre-treatment processes, resulting in

a total reduction in environmental impact. Hence, it can be argued that recycling RAP and EAF steel slags in the CTI batch plant provides environmental benefits, besides reducing the consumption of virgin bitumen and natural aggregates.

These results are supported by the ones obtained in a preliminary and more general study, previously undertaken (Degli Esposti et al., 2020), showing that the experimental mixtures have fewer environmental impacts than the control ones. According to the LCA results, a reduction in all impact categories occurred, and mainly in human toxicity (–30.5%) and eutrophication (–24%), related to the intensive energy consumption and the utilization of non-renewable sources, during both the extraction and transportation phases. In particular, a robust reduction in CO<sub>2eq</sub> emissions was demonstrated by the better performance of the category global warming potential (–21%), as it was estimated at 46 tons of CO<sub>2eq</sub> for the experimental mixtures (MixW1 and MixB1) and at 58 tons of CO<sub>2eq</sub> for the control mixtures (Mix W0 and MixB0). To summarize, the use of RAP and EAF steel slags in 1 km of suburban road allows the CTI to reduce the content of virgin bitumen by weight of the total mixture by 2.15% (by total weight) for the wearing course and by 1.70% (by total weight) for the binder course. Moreover, the use of recycled materials in 1 km of suburban road allows the company to save 438.0 tons of natural aggregates and 18.2 tons of virgin bitumen for the
wearing course, 826.2 tons of natural aggregates and 22.1 tons of virgin bitumen for the binder course, as shown in **Figure 7**.

## CONCLUSION

The inert nature of RAP, and the excellent mechanical properties of the EAF slags make them two potentially useful materials in a wide variety of applications, including re-use or recycling in new asphalt pavements. This case study demonstrates the high potential for recycling RAP and EAF steel slags in the road construction sector, as a secondary raw material and a by-product, respectively.

As a result of testing the use of EAF steel slags and RAP in new bituminous mixtures, the physical and mechanical properties as well as the environmental performances of the two mixtures have been evaluated for wearing and binder courses, respectively. In order to maximize the environmental sustainability of the road pavement, the use of RAP and EAF steel slags can be recommended.

Moreover, the authors believe that LCA results and indicators are appropriate tools to compare and communicate the environmental performances of different asphalt mixtures in road construction.

By reducing the global environmental impact and recycling by-products, the CTI and the co-located companies are a real case study of industrial symbiosis at the meso-level.

The authors believe that the development of industrial symbiosis projects provides the opportunity to promote waste reduction, reuse, and recycling, while reducing the environmental impacts, as well as increasing companies' competitiveness, in particular in countries like Italy, where there are already several large industrial clusters. Moreover, information sharing among stakeholders would facilitate the development of industrial symbiosis networks.

Future research efforts could focus on investigating other recycled materials, for the same applications as virgin ones,

with the purpose of reaching the same quality level and performances. In this issue, no economic evaluation was carried out. As a future research direction, the economic sustainability will be evaluated.

## DATA AVAILABILITY STATEMENT

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

## AUTHOR CONTRIBUTIONS

AB and AE designed the study. AE collected information and materials from the company. AB and AE conceived and planned the experiments for the characterization of asphalt mixtures. AE carried out the experiment. All the authors contributed to the interpretation of the results. AE and CM designed the LCA model and analysed the data. All the authors contributed to the interpretation of the LCA results. AB took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis and manuscript.

## ACKNOWLEDGMENTS

The authors gratefully acknowledge contributions from the CTI company (Imola, Italy).

## SUPPLEMENTARY MATERIAL

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2020.572955/full#supplementary-material.

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**Conflict of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Copyright © 2020 Bonoli, Degli Esposti and Magrini. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
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# 3D-Extrusion Manufacturing of a Kaolinite Dough Taken in Its Pristine State

Séverine A. E. Boyer<fnref n="1" />*, Lucie Jandet<fnref n="2" /> and Alain Burr<fnref n="1" />*

Ceramic is among the complicated materials to use in the design of fine objects. Complex shapes without any major defect are not easy to produce. In most of the cases, the production of ceramic parts is the results of three steps. Firstly, the “sculpture” of the raw piece by adding raw materials to lead to the final object. Secondly, the “drying” and finally the “high temperature oven-dry” of the dried raw object to transform the granular dough into a nice consistent compact material. Exploiting the special characteristics of ceramic is not only a thing of the past. Nowadays new possibilities, i.e., shapes and styles, can be offered in the use of ceramics, and especially where it concerns the application of the Additive Manufacturing (AM) concept. The combination of Computer Aided Design (CAD) to AM opens a completely new means of finding novel ways of processing final objects. By choosing to use kaolin clay without any chemical additions (or improvers) as “a model material,” the ability to produce controlled structures with freedom in design by additive deposition modeling is exposed. Discussions relate to the concomitant control of the process parameters, the kaolin hydration and the complexity of printed structures. The optimization of process parameters (nozzle speed, layer thickness, wall thickness) were defined with the calibration of the material flow. Both windows adjusting water content in dough (%wt) and imposing pressure in the tank of the 3D printer have been defined accordingly. The role of layer impression support was also found to be important. This study credits to use the state-of-the art technique (3D printing) to explore sustainable manufacturing of potteries.

**Keywords:** ceramics, 3D dough printing, metastable state, kaolin hydration, artistic pottery

## INTRODUCTION

The three-dimensional (3D) printing is transforming the manufacturing technology space. It brings original concepts and promotes the emergence of native materials (and taken in their pristine state), such as biomaterials, within new possibilities of CAD engineering (Guvendiren et al., 2016; Bourell et al., 2017; Bose et al., 2018; Gutierrez-Heredia et al., 2018; Ngo et al., 2018; Harris et al., 2019; Jovic et al., 2019; Wasti and Adhikari, 2020). As example, Bose et al., in 2018 classified selected Additive Manufacturing (AM) (binder jetting, direct energy deposition techniques, material extrusion and jetting, powder bed fusion and vat polymerization) and presented some combination of AM and biomaterials (ceramics, metallic biomaterials, polymers) to finally focus on applications of AM in biomaterials and biomedical devices (metallic implants to improve osteointegration, hard tissue engineering, soft tissue engineering). Gutierrez-Heredia et al., in 2018 presented a method for

**OPEN ACCESS**

**Edited by:**

Patricia Krawczak,
IMT Lille Douai, France

**Reviewed by:**

David Olubiyi Obada,
Ahmadu Bello University, Nigeria
Mohd Mustafa Al Bakri Abdullah,
Universiti Malaysia Perlis, Malaysia

**\*Correspondence:**

Séverine A. E. Boyer
severine.boyer@mines-paristech.fr
Alain Burr
alain.burr@mines-paristech.fr

**Specialty section:**

This article was submitted to
Structural Materials,
a section of the journal
Frontiers in Materials

**Received:** 13 July 2020

**Accepted:** 10 February 2021

**Published:** 21 April 2021

**Citation:**

Boyer SAE, Jandet L and Burr A (2021)
3D-Extrusion Manufacturing of a
Kaolinite Dough Taken in Its
Pristine State.
Front. Mater. 8:582885
doi: 10.3389/fmats.2021.582885

<footnote n="1">¹MINES Paris, CEMEF-Centre de Mise en Forme des Matériaux, PSL Research University, Sophia Antipolis Cedex, France,</footnote>

<footnote n="2">²MINES Paris Engineering School, PSL Research University, Paris Cedex, France</footnote>