Pyrolysed cork-geopolymer composites: A novel and sustainable EMI shielding building material

Construction and Building Materials 229 (2019) 116930

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Pyrolysed cork-geopolymer composites: A novel and sustainable EMI shielding building material Rui M. Novais a,⇑, Manfredi Saeli a, Ana P.F. Caetano a, Maria P. Seabra a, João A. Labrincha a, Kuzhichalil P. Surendran b, Robert C. Pullar a,⇑ a b

Department of Materials and Ceramic Engineering/CICECO-Aveiro Institute of Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal Materials Science and Technology Division, CSIR-NIIST, Industrial Estate, Trivandrum 695019, India

h i g h l i g h t s  Recycled cork wine stoppers were used as a highly sustainable carbon source.  An industrial waste was used as main raw material in the geopolymers production.  First ever report on cork-geopolymer composites for EMI shielding applications.  Maximum total shielding effectiveness values ranging from

13.8 to

15.9 dB.

 This highly sustainable material may ensure large-scale electromagnetic protection.

a r t i c l e

i n f o

Article history: Received 4 February 2019 Received in revised form 22 August 2019 Accepted 9 September 2019

Keywords: Inorganic polymer Construction Microwave absorption Cork Composite

a b s t r a c t In this investigation, and for the first time, pyrolysed sustainable cork was used to produce waste-based geopolymer-cork composites with enhanced electromagnetic interference (EMI) shielding properties. The influence of the pyrolysed cork amount and the geopolymer porosity on the EMI shielding ability of the composites was studied. The maximum total shielding effectiveness (SET) values achieved by these novel building materials ( 13.8 to 15.9 dB) are equal to any other reported geopolymer microwave (MW) absorbers over the X-band, despite containing much lower carbon content. In addition, our composites were produced using an industrial waste (biomass fly ash) as raw material and recycled wine stoppers as a carbon source (2.5–3.75 wt%). This strategy is different from those implemented in the only other reported MW absorbing geopolymers, which used standard commercial chemical precursors, and the added carbon component is also a non-renewable commercial product, added in much greater quantities (10 more). Therefore, our approach not only decreases the consumption of virgin raw materials (e.g. kaolin), but also enhances the global sustainability of the construction sector. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The exponential increase in the use of electronic devices observed over the past years, associated with the era of the Internet of things (IoT) [1], has raised concerns regarding the unprecedented levels of electromagnetic radiation pollution [2] which may affect human health [3,4], besides increasing the risk of electromagnetic interference (EMI) [5,6] in sensitive areas such as aerospace and the aviation industry, the military and transport. It is imperative to develop materials with the ability to shield/absorb electromagnetic waves, not only to alleviate potential human

⇑ Corresponding authors. E-mail addresses: [email protected] (R.M. Novais), [email protected] (R.C. Pullar). https://doi.org/10.1016/j.conbuildmat.2019.116930 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

exposure to such radiation, but also to ensure the protection of electronic devices. Of particular interest are the microwave (MW) frequencies, with wavelengths in the cm range (300 MHz– 30 GHz), including the X-band (8–12 GHz) used for military radar and communications, some commercial and civil wireless and satellite communications, motion sensors and speed detection devices. 5G wireless devices will also operate at frequencies above 5 GHz. Most research into microwave absorbing materials is aimed at lightweight materials for coatings and applied panels, usually based on foams or polymers containing some form of carbon [7,8], conductive metals or magnetic absorbers such as ferrites [9]. Amorphous and graphitic carbon is a well-known GHz/MW absorbing material [10] and has been used as a powder (carbon black, charcoal, graphite) [11,12], carbon fibres [13], carbon

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nanotubes (CNTs) [14,15], carbon nanorods [16] and graphene [17]. However, the development of building materials with enhanced EMI shielding properties would also be an excellent strategy to restrict the electromagnetic radiation levels inside buildings, and to ensure large-scale electromagnetic protection. Clearly, in such materials low weight is less of an issue, secondary to mechanical properties and chemical stability. Highly electrically conductive carbon fibres and filaments [18–20], carbon nanotubes (CNTs) [21,22] and graphene-coated CNTs [23] have been added for EMI shielding to electrically insulating building materials such as cement, other additives include metal fibres [24] and metalcoated carbon fibres [25]. CNTs have also been added to concrete [26,27]. Generally, a reasonable/practical microwave absorbing material should have a shielding effectiveness (SE) of at least 10 dB, while an excellent shielding material should have a SE of at least 30 dB (i.e. shielding efficiency 99.9%), and in cements containing 0.5–1.5 vol% carbon filaments SE values of 29 to 37 dB have been achieved [18,20]. The feasibility of using Portland cement composites (the cement matrix itself presents poor SE) for EMI shielding applications has been specifically considered, using pyrolysed peanut and hazelnut shells [28], carbon fibres and filaments [18,20,23], Fe3O4-carbon fibres [29], carbon nanotubes [22], nano-Fe3O4 [30] and other additives [24,31,32]. Ordinary Portland cement is a highly used binder, being a common ingredient of concrete and mortars, and as such it is one of the most widely used materials in construction. It is made mostly from limestone. However, its production is responsible for a great amount of greenhouse gases (5–7% of the total anthropogenic carbon dioxide emissions) [33], which has led to the pursuit of alternative and lower carbon footprint materials. One promising alternative is the use of geopolymers [34–37] (also known as inorganic polymers) [38–40], alkali activated aluminosilicate binders, which may have a much lower carbon footprint [41] when the raw materials and activator nature and content is carefully considered. However, the possibility of using geopolymer-carbon composites for electromagnetic radiation shielding, and in particular for microwave shielding, has been rather neglected. In fact, there is virtually no literature on this, the only example of a study on alumino-silicate geopolymers being the work of Zhang et al. [42], who added graphite to a metakaolin-based geopolymer prepared using a potassium silicate solution as alkaline activator, and evaluated the microwave absorption properties of the composites. They looked at samples between 1.25 and 4.97 mm thick, with a massive graphite content of up to 50 wt%, and over the frequency range of 2–18 GHz. A peak attenuation of 64.8 dB at 5.1 GHz was observed when adding 40 wt% of graphite with a thickness of 4.15 mm. However, in general high amounts of graphite were required to achieve reasonable SE values, with narrow effective ranges of only a few GHz, and none were very effective over the key X-band region of 8–12 GHz. Although this demonstrated the feasibility of using geopolymer composites for EMI shielding applications, the very high graphite contents used (up to 50 wt%) were dictating the microwave attenuation and the geopolymeric specimens were cured at 70 °C for 7 days. In this study, pyrolysed cork, coming from recycled wine stoppers, was added in much lower amounts (up to 3.75 wt%), to a waste-based geopolymer. The pyrolysed cork was directly added to the geopolymeric slurry without the need for complex and expensive procedures (e.g. ball-milling and ultrasonic stirring) such as those used in [42]. The cork-composites were also synthesised and cured at room temperature (20 ± 1 °C) using an industrial waste (biomass fly ash), instead of commercial metakaolin, as the main aluminosilicate source (70 vs 30 wt%), this being aligned with the circular economy concept and contributing towards the sustainment development.

Cork is a fully sustainable and 100% renewable precursor harvested without damaging the tree [43,44]. This is a crucial advantage over other carbonaceous materials [45] and, therefore, its incorporation after pyrolysis into building materials for EMI shielding applications, instead of other environmentally unsustainable precursors (e.g. carbon nanotubes, graphene, carbon fibre), is an environmentally friendlier strategy. In fact, cork pyrolysis can be considered a null carbon footprint process, since the CO2 emitted during pyrolysis will be neutralised by that absorbed by the tree to regenerate the cork bark. Moreover, in this study recycled cork wine stoppers, instead of natural/unprocessed and high quality cork (known as reproduction cork [46]), were used to produce the pyrolysed cork, which further decreases the process production cost and increases its global sustainability. Recycled cork wine stoppers may be a highly abundant and low cost precursor for EMI shielding applications. In fact, worldwide stoppers production reaches a striking 12 billion units/year, which may be recycled and reused in other applications [47]. Over 12 billion cork stoppers are produced each year. Yet, despite being recyclable, cork wine stoppers cannot be reused by the wine industry due to their lower quality and contamination after use, and the small proportion which are recycled are used (after processing) in the production of other products such as badminton and tennis rackets, and cricket balls, amongst others. In addition, recycled cork wine stoppers can be bought by the cork industry, and then after being ground and heat treated, can be used to produce cork agglomerates for insulation applications. Nevertheless, their use as a carbon source in the production of eco-friendly EMI shielding materials, as proposed here, will attribute value to this material. In addition, and despite recent efforts promoting the collection and recycling of cork wine stoppers (e.g., ‘‘Greencork” in Portugal, ‘‘ReCORK” in USA and ‘‘Cork Recycling Program” in Australia) [48], this process is still in its early days. In fact, over the past 11 years the ‘‘ReCork” program has collected and recycled roughly 105 million corks, corresponding to around 10 million/year and to 0.08% of the worldwide stoppers production per year. This shows that the majority of used wine corks is still not recycled, and therefore cannot be used by the industry. This means that most of the used corks are still lost in landfills. For that reason, innovative management strategies that might encourage the recycling of used cork stoppers are eagerly pursued. Besides the remarkable sustainability of cork and it’s very low apparent density, both crucial features in EMI shielding materials, cork also has a unique microstructure consisting of hollow polyhedral cells, with a hexagonal honeycomb shape (15–20 lm diameter) when viewed from the radial direction coming out of the tree, and a rectangular shape (45 lm length, resembling a brick wall) when viewed from the transverse side or top directions. The hexagonal honeycombs are expected to aid the internal reflection and absorption of the microwaves within the structure. Further, for very high shielding performances, honeycomb panel wafers are commonly used which are usually costly and heavy. Only in such structures can important design considerations like ventilative cooling and shielding be combined. Our attempt is to explore a similar air permeable shielding structure using naturally occurring honeycomb materials. This investigation intends to act as a proof-of-concept, demonstrating the possibility of using green, low cost, and environmental friendly cork-geopolymer composites as an innovative EMI shielding building material. The influence of the cork amount and the geopolymer porosity on the EMI shielding ability of the composites was studied. The results reported here are expected to be extended to other cork sources besides wine stoppers. One exciting possibility could be the use cork powder wastes produced in substantial amounts by the cork industry, which have low economic value and are mainly used to produce energy by their combustion [46].

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having a geometry in accordance with DIN 1164, coupled with a flat paddle.

2. Experimental conditions 2.1. Materials Recycled cork wine stoppers were used in this study. The cork samples were pyrolysed under nitrogen in a graphite furnace, following previous works by the authors [44,49]. The samples were heated at 10 °C/min up to 900 °C, kept for 30 min at this temperature and then cooled down to room temperature, all while under a flow of N2. The pyrolysed cork was then ground in a mortar and sieved below 75 mm. Biomass fly ash waste was used as the main source (70 wt%) of reactive silica and alumina, while metakaolin (ArgicalTM M1200S; Univar) was used in lower amounts (30 wt%) as an additional aluminosilicate source in order to achieve suitable SiO2/Al2O3 molar ratios. The fly ash waste, supplied by a Portuguese paper and pulp company, was used as-received, despite the coarse particle size (containing particles of a few mm) which is detrimental to the geopolymer’s compressive strength. The potential of these ashes as an aluminosilicate source for geopolymer production, has been demonstrated by the authors previously [50–52]. The direct use of the ashes in the geopolymer synthesis, without performing any expensive sieving or milling step, decreases the geopolymer production cost, and may allow an easier industrial scale-up application. The chemical activation of the aluminosilicate sources was performed using a mixture of sodium silicate (SiO2/Na2O = 3.2; Quimialmel) and 10 M sodium hydroxide (ACS reagent, 97%, Honeywell) in 3:1 wt% proportion. The SiO2/Na2O ratio of the activating solutions was 1.7. This ratio was selected considering preliminary tests in which the influence of the sodium silicate to sodium hydroxide in the geopolymers’ compressive strength was evaluated. A hydrogen peroxide solution (Alifar, 30 vol%, Laboratório Aliand) was used as a foaming agent. This foaming agent was used for the first time to induce porosity in geopolymers by Bell and Kriven [53].

2.2. Cork-geopolymer composite synthesis In this study, and following previous work by the authors [54], a reference composition without added pyrolysed cork was synthesised (see details in Table 1). Then, six other compositions were prepared containing various amounts of cork and foaming agent, to evaluate the influence of the pyrolysed cork and foaming agent amount on the EMI shielding behaviour of the composites. The cork-geopolymer composites synthesis involved two steps: in the first, the geopolymeric slurry was prepared, while in the second, the pyrolysed cork and the foaming agent (amount depending on the composition) were added to the slurry. Manufacture involved five steps: the 1st step was performed manually, while the others were carried out using an intensive mixer (KichenAidÒ),

a) hand mixing of biomass fly ash and metakaolin for 1 min inside a plastic bag to ensure a uniform blend; b) homogenisation of sodium hydroxide and silicate at 60 rpm for 5 min; c) mixture of the alkaline solution with the solid precursors at 95 rpm for 9 min; d) adding the foaming solution (amount depending on the composition, ranging from 1 to 2 wt%) and the cork to the slurry and mixing at 135 rpm for 1 min to ensure homogeneity and bubbling; e) pouring the slurry into the standard metallic moulds (160 mm  40 mm  40 mm) and sealing with a plastic film. The hardened specimens were unsealed, demoulded after 24 h, and cured for 28 days until their characterisation. The manufacturing process was all performed at ambient conditions (20 °C, 65% RH) using a simple, reproducible and low-cost technique. 2.3. Materials characterisation The microstructures of recycled cork wine stoppers, pyrolysed cork and the geopolymer composites were evaluated by scanning electron microscopy (SEM – Hitachi SU 70) equipped with energy dispersion spectroscopy (Bruker EDS). Samples were coated in gold/palladium. Optical analysis (Leica EZ4HD microscope) was also performed on the cork-geopolymer composites to study their morphology and pore size distribution. The mechanical performance of the cork-geopolymer composites was determined at 28 days of curing, according to the European Standard EN 1015-11:1999 by means of compressive strength tests. A universal testing machine (Shimadzu, AG-A), provided with a 20 kN load cell, running at a displacement rate of 0.5 mm/min, was used. Three replications were used to calculate the mean values. The apparent density was calculated by measuring the samples’ weight and volume. Three specimens were used, and the arithmetic mean value was presented. For the microwave shielding measurements in the X-band (8.2– 12.4 GHz), the studies were carried out on a vector network analyser (Agilent E5071C) using the waveguide method. Total shielding effectiveness (SET = SER + SEA) was calculated from reflection (SER) and absorption (SEA) shielding effectiveness. Solid geopolymer samples were machined into 10.1  22.9 mm rectangles, with 3, 5 and 7 mm thickness, using a Struers polishing machine (Struers Tegramin 25), in order to make the sample suitably fit into the waveguide slot coupled to the vector network analyser. SSE (specific shielding effectiveness) was calculated from density. Elemental analysis for carbon, hydrogen and nitrogen was carried out with a Tuspec Micro CHNS 630-200-200 elemental analyzer.

Table 1 Mixture composition of the cork-inorganic polymer composites. Sample ID

Matrix C2.5 C3.75 LC2.5_1 LC3.75_1 LC2.5_2 LC3.75_2

Description

Geopolymer Composite with 2.5% cork Composite with 3.75% cork Lightweight composite with Lightweight composite with Lightweight composite with Lightweight composite with

Mixture proportion (wt%)

2.5% cork and 1% H2O2 3.75% cork and 1% H2O2 2.5% cork and 2% H2O2 3.75% cork and 2% H2O2

Additives

Metakaolin

Biomass fly ash

Alkaline activator

Cork (wt%)

H2O2 (wt%)

15

35

50

– 2.5 3.75 2.5 3.75 2.5 3.75

– – – 1 1 2 2

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3. Results and discussion 3.1. Cork and pyrolysed cork characterisation Fig. 1 presents a digital photograph of the cork stopper before and after pyrolysis. The pyrolysis induced some volume changes, but the cork’s weight loss (70 wt%) is the most relevant feature promoted by the thermal treatment under N2. The SEM micrographs reveal the characteristic microstructure of cork, consisting of hexagonal cells (radial direction, 20 mm diameter), which are preserved in the pyrolysed cork. These features are in line with previous reports on the pyrolysis of cork [49,55]. Not surprisingly, the EDS maps show that carbon and oxygen are the most abundant elements – nevertheless, trace amounts of sodium, potassium and chlorine were also detected. However, considering the poor sensitivity of the EDS technique for light elements such as carbon and oxygen, these maps should only be considered as a rough estimate. After pyrolysis, the cork was sieved below 75 mm, the apparent density of this powder being 140 kg/m3. The result of the elemental analysis shows that the pyrolysed cork is composed of carbon (90.74 wt%), hydrogen (0.77 wt%) and nitrogen (1.67 wt%). The EDS map shown in Fig. 1 and the EDS spectrum (not shown here) show very similar oxygen and potassium content in comparison with those seen in the cork precursor, 24% and 0.5%, respectively. 3.2. Cork-geopolymer composites characterisation A digital photograph of the reference geopolymer (matrix) and two composites containing different amounts of pyrolysed cork powder is shown in Fig. 2. As observed, the colour of the specimens changes from light (left side specimen) to dark grey (right side specimen) due to the incorporation of pyrolysed cork. To evaluate the pyrolysed cork distribution within the matrix, EDS elemental mapping was performed on the specimens, and representative maps for dense and lightweight composites are shown in Fig. 3. As observed, carbon is homogeneously distributed in all the composites, which is beneficial for their EMI shielding ability. Silicon and aluminium, the backbone elements of the geopolymeric network, are also homogeneously distributed in the samples. Fig. 4 presents optical micrographs of the pure geopolymer and the various cork-containing composites. The micrographs of the dense composites (prepared without foaming agent) show the presence of some air voids introduced during mixing. Indeed, the incorporation of pyrolysed cork powder into the geopolymeric

Fig. 2. Digital photograph of the geopolymeric matrix and two dense corkgeopolymer composites (C2.5 and C3.75).

matrix increased the slurry viscosity, hindering the release of air bubbles after mixing. It should be highlighted that the specimens were not vibrated after mixing, in an attempt to simplify the composite synthesis. When the foaming agent was added to the composites, a substantial increase in the number and volume of pores was observed, due to the oxygen release coming from the foaming agent decomposition in the alkaline medium. Interestingly, the amount of cork powder affected the pore size distribution of the composites produced, with a coarser pore size distribution being observed for higher foaming agent content (2 wt%) in the composites containing less added cork (2.5 wt%). This was associated with the higher viscosity of the slurry observed for the composite containing 3.75 wt% cork, in comparison with its lower-cork-containing counterpart, which prevented pore growth and coalescence. Despite this, the apparent densities of both composites (coded as LC2.5_2 and LC3.75_2) were similar as demonstrated in Fig. 5. The apparent density of the cork-geopolymer composites is extremely relevant when considering their use as building material. The lowest value here reported (750 kg/m3) might be further reduced by increasing the amount of foaming agent [56,57], decreasing the molarity of the activator [58], and by decreasing the solid-to-liquid ratio of the mixture composition [59,60]. This will be considered in future work. Nevertheless, this density value is lower than that in other reports on geopolymer foams [61] and mortars [62]. Fig. 5 presents the compressive strength and the apparent density of the various composites and of the reference composition. In the dense composites, the incorporation of 2.5 wt% of pyrolysed cork decreased the compressive strength by roughly 31%, from

Fig. 1. Digital photograph, SEM micrographs and EDS elemental mapping of cork before and after pyrolysis.

R.M. Novais et al. / Construction and Building Materials 229 (2019) 116930

5

Fig. 3. EDS elemental mapping of dense (a and b) and lightweight (c) cork-geopolymer composites containing a) 2.5 wt% (C2.5) and (b and c) 3.75 wt% cork (C3.75 and LC3.75_2, respectively).

Fig. 4. Optical microscopy micrographs of the matrix and the various dense and lightweight cork-geopolymer composites.

22.2 MPa (geopolymer matrix) to 15.3 MPa. This was expected, and is attributed to the significantly lower density and strength of this additive in comparison with the geopolymeric matrix. In line with this remark, the apparent density of this composite (C2.5) was 8% lower than that of the matrix. A further increase in the cork content (to 3.75 wt%) did not significantly modify the strength or density. As expected, the lightweight composites showed much lower strength than the matrix, a fourfold (to 5 MPa) and a six fold (to 3.5 MPa) decrease being observed when adding 1 and 2 wt% foaming agent to the composites, respectively, regardless of the amount of pyrolysed cork. A similar trend was also observed for apparent density, as shown in Fig. 5. A SEM micrograph and the corresponding EDS spectrum of the fracture surface of the 3.75 wt% cork-containing composite are

Fig. 5. Compressive strength and apparent density of the matrix and the various cork-geopolymer composites (measured at the 28th day). Note: for clarity the dense composites are highlighted in light-red, while the lightweight ones are in lightgreen. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

presented in Fig. 6. The micrograph reveals an excellent interface between the pyrolysed cork and the matrix. Moreover, it also demonstrates that the pyrolysed cork preserved its microstructure despite the high alkalinity of the geopolymeric matrix, showing that the composites are durable and stable.

3.3. Microwave shielding capabilities The total shielding effectiveness (SET) of the 3 mm thick samples is shown in Fig. 7a, and the range of values of SER, SEA and SET for each 3 mm thick sample over 8.2–12.4 GHz are given in Table 2. The first thing to note is that for all samples, SEA remained more-or-less constant across the frequency range, while SER showed a steep decline with increasing frequency and in all cases converged towards similar values of around 2 dB above 12 GHz,

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Fig. 6. SEM micrograph and EDS spectrum of the fracture surface of the cork-geopolymer composite containing 3.75 wt% cork (C3.75) after the compressive strength test.

GP Matrix C2.5 LC2.5_1 LC2.5_2

b)

9

2% FA 12

3 mm thickness

8

7

6

5

10 9 8 7 6 5 4 8

10

11

9

10

12

Frequency (GHz)

7 mm thickness

LC3.75_2 7mm

LC3.75_2 5mm

12

LC3.75_2 3mm

11

LC3.75_1 5mm

10

Frequency (GHz)

LC3.75_1 7mm

9

LC3.75_1 3mm

8

LC2.5_2 5mm

6

LC2.5_2 7mm

GP 3mm

7

LC2.5_2 3mm

8

LC2.5_1 7mm

9

C3.75 5mm

10

C2.5 7mm

11

C3.75 3mm

12

C2.5 5mm

13

GP 7mm

14

17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 C2.5 3mm

15

SET (-dB)

Total Shielding Effectiveness SET (-dB)

16

12

d)

C3.75 LC3.75_1 LC3.75_2

GP 5mm

GP Matrix C2.5 LC2.5_1 LC2.5_2

c)

11

Frequency (GHz)

LC2.5_1 5mm

9

5 mm thickness

11

4 8

C3.75 LC3.75_1 LC3.75_2

C3.75 7mm

10

1% FA C3.75 LC3.75_1 LC3.75_2

LC2.5_1 3mm

GP Matrix C2.5 LC2.5_1 LC2.5_2

Total Shielding Effectiveness SET (-dB)

Total Shielding Effectiveness SET (-dB)

a)

Fig. 7. Total shielding effectiveness (SET) over the X-band of the matrix and the various cork-geopolymer composites for thicknesses of a) 3 mm, b) 5 mm and c) 7 mm. The maximum SET value of every composite sample is compared in d). FA = addition of foaming agent only to geopolymeric (GP) matrix.

whatever the sample was made from. It was this trend in decrease of SER which contributed most to the decrease in SET observed with increasing frequency in all samples. SEA was significantly higher in all cork-inorganic polymer composites compared to the matrix,

and those with more cork added (3.75 wt%) had higher SEA, and hence SET, values. This signifies that a greater quantity of pyrolysed cork within the composite increased its microwave absorbing ability within the bulk. In the filler material, multiple internal

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Table 2 Reflection (SER), absorption (SEA), total (SET) and specific (SSE) shielding effectiveness of the geopolymer (GP) matrix and cork-GP composites, all of 3 mm thickness, over the Xband (8.2–12.4 GHz). Sample ID

SER ( dB)

SEA ( dB)

SET ( dB)

Density (g cm

Matrix C2.5 C3.75 LC2.5_1 LC3.75_1 LC2.5_2 LC3.75_2

2.3–3.4 2.6–5.2 2.2–5.2 2.5–4.1 2.9–4.8 2.3–3.5 2.5–4.6

2.4–2.6 4.1–4.2 4.4–4.6 3.2–3.7 4.6–4.7 3.4–3.7 4.9–5.1

4.7–6.1 6.8–9.3 7.2–9.8 5.8–7.8 7.2–9.5 5.7–7.2 7.4–9.7

1.307 1.200 1.180 0.949 0.923 0.745 0.748

reflection of EM waves occurs at the regularly arranged carbon honeycomb microstructure which forms an electrically conducting network. Interestingly, there was little variation in reflection characteristics throughout the samples, since absorption shielding is the dominant mechanism here. Similar trends were observed in the 5 and 7 mm thick samples. The geopolymeric matrix has low attenuation values of 4.7 to 6.1 dB, which compares well to previous reports on the microwave absorption of geopolymers, which showed SET values over the X-band of around 1 to 5 dB for 1.25 to 4.97 mm thick samples [42], and similar values for alumino-phosphate geopolymers [63]. We also measured samples of the geopolymeric matrix made with just the foaming agents (FA) added, and without pyrolysed cork, as shown in Fig. 7a. They were actually slightly worse than the unadulterated matrix, probably because they contained less mass of the alumino-silicate material in the same volume, and so had even less absorbing ability. This suggests that it is the pyrolysed cork additives which have the major effect on increasing microwave absorption, as was demonstrated by the results for C2.5 and C3.75 in Fig. 7a and Table 2. Both show SET significantly larger than the matrix across the X-band, by a factor of around 50% higher for C2.5, and nearly 60% higher for C3.75. For a conductive honeycomb structure, the shielding efficiency is given by [64], SE = 27 al 20logN where l is the length of the waveguide (i.e., the length of the honeycomb cell) and a is the radius of the N apertures. Hence, the shielding efficiency increases with increasing waveguide length over its diameter. Interestingly, natural cork has a microstructure made of hollow hexagonal honeycomb cells, whose length (45 lm) is 2–3 times that of their diameter (15–20 lm). Such structures are likely to have high EMI shielding. Since pyrolysed cork contains honeycombs made of conductive carbon nanoparticles, they can generate micro-currents inside the system on exposure to the EM waves, leading to eventual damping of the incoming wave. Additionally, the porous microstructure enables EM waves to undergo multiple reflections and consequent attenuation before passing through the shield. Thus, the unique microstructure of the cork is supplementing its superior shielding behaviour. When 1 or 2 wt% foaming agent was added to C3.75, it made very little difference to the SET. However, when the foaming agent was added to C2.5, the values became significantly worse, although still superior to the matrix (Fig. 7a). This would seem to be related to the observation made above, that a coarser pore size distribution was seen with foaming agent added to the composites containing 2.5 wt% pyrolysed cork content than in those with 3.75 wt% cork. This was attributed to the higher viscosity of the precursor slurry with more cork preventing bubble/pore growth and coalescence, resulting in more, and smaller pores with the higher cork content (Fig. 4). These smaller pores appear to slightly aid absorption in the X-band, whereas the fewer, larger pores up to 1 mm in size reduce shielding effectiveness.

3

)

SEE ( dB g

1

cm3)

3.7–4.7 5.7–7.8 6.1–8.3 6.2–8.2 6.1–7.7 7.8–10.3 9.9–13.0

When the thickness of the sample was increased to 5 mm and 7 mm, the SET values of all samples increased as the greater bulk volume increased the ability of the material to absorb microwaves. As can be seen in Fig. 7b–c, the shielding response became more irregular, but this was not random noise, as the same minor peaks and troughs were seen in all samples of the same thickness, even though these were all individual measurements. Since the filler material (pyrolysed cork) is electrically conducting, they can generate micro-currents inside the system on exposure to the EM waves, leading to eventual damping of the incoming wave. Since the microstructure of the geopolymer-pyrolysed cork composite is porous, the variation of micro-current can be very rapid which was giving rise to fluctuating shielding effectiveness in Fig. 7b–c. With 5 mm samples, the geopolymer matrix still showed a general downwards trend with frequency, although the values were slightly higher (see Table 3). The addition of 2.5 and 3.75 wt% pyrolysed cork led to increases in SET of between 34 and 60% and 52–87% over the matrix, respectively, and the downwards trend with increasing frequency had actually become reversed, with a dip at around 9.5 GHZ but an overall increase with frequency. As before, LC2.5_1 and LC2.5_2 had worse SET than the C2.5 composite without foaming agents, but LC3.75_1 and LC3.75_2 were equal with, or superior-to, C3.75 (Fig. 7b). Similar, but more enhanced, trends were observed with a thickness of 7 mm (Fig. 7c), for which all samples, even the geopolymer matrix, showed an overall increase in SET with frequency. C2.5 had SET between 67 and 72% higher than the matrix, and C3.75 (with no foaming agent added) had the greatest SET of all samples, between 104 and 126% greater than the geopolymeric matrix, at this thickness. The maximum SET values for each sample are compared in Fig. 7d, and it is clear that the outstanding composites are those with 3.75 wt% pyrolysed cork and a thickness of 7 mm, with peak SET values of 14.6 to 15.9 dB. These values compare well to those of cements and concretes with similarly low amounts of carbonaceous absorber added. Cements with 0.6 wt% CNTs added (a much more costly product, financially and environmentally, than ours) had reflection losses of around 8 to 10 dB over the X-band for samples between 25 and 25 mm thick, thinner samples giving very uneven MW absorption through the frequency range [21]. Three centimetre thick concrete with 3 wt% CNTs added achieved SET of 12 dB, but only at frequencies of 2 GHz [27], and 15 wt% CNTs were required to give SET of 27 dB over the X-band in 2 mm thick Portland cement composites surpassing the value here reported for the corkgeopolymer composites, but using a four-times higher carbon amount [22]. Finite element modelling suggested that a thickness of 7 cm would be needed to achieve SET values of 20 dB in concrete with 3 wt% CNTs added [26], and even then at frequencies of only 800 MHz. Carbon fibre seems to be more effective than the use of pyrolysed cork, and 0.5 wt% carbon fibre in 4 mm thick Portland cement gave SET of 30 dB at low frequencies of 1– 2 GHz [18] and 1.5 wt% gave 40 dB at 1 GHz [20]. A high SET of

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Table 3 Minimum and maximum values of total (SET) and specific (SSE) shielding effectiveness of the GP matrix and cork-GP composites with thicknesses of 5 mm and 7 mm over the Xband (8.2–12.4 GHz) (density as in Table 2). Sample ID

5 mm thickness

Matrix C2.5 C3.75 LC2.5_1 LC3.75_1 LC2.5_2 LC3.75_2

7 mm thickness

SET ( dB)

SEE ( dB g

4.6–7.5 7.4–9.6 8.6–11.4 6.1–8.4 8.4–10-5 6.1–8.5 9.1–10.9

3.5–5.7 6.2–8.0 7.3–9.7 6.4–8.9 9.1–11.4 8.2–7.2 12.2–14.6

1

cm3)

14

C3.75 LC3.75_1 LC3.75_2

4.7–6.0 8.8–10.8 11.7–13.5 8.5–11.4 13.5–15.8 8.5–10.5 17.6–19.9

cm3)

5 mm thick 20 18

13

16

8

LC3.75_2 7mm

LC3.75_2 5mm

LC3.75_1 7mm

LC3.75_2 3mm

LC3.75_1 5mm

LC2.5_2 7mm

LC3.75_1 3mm

12

LC2.5_2 5mm

11

LC2.5_2 3mm

10

Frequency (GHz)

LC2.5_1 7mm

9

LC2.5_1 5mm

8

C3.75 7mm

3

LC2.5_1 3mm

0

4

C3.75 3mm

2

5

C3.75 5mm

4

6

C2.5 5mm

6

7

C2.5 7mm

8

10

GP 7mm

9

12

C2.5 3mm

10

14

GP 3mm

3

11

GP 5mm

12

-1

SSE (-dB g cm )

-1

3

Specific Shielding Efficiency, SSE (-dB g cm )

15

6.1–7.8 10.5–13.0 13.8–15.9 8.1–10.8 12.5–14.6 7.8–10.0 13.2–14.9

1

b)

GP Matrix C2.5 LC2.5_1 LC2.5_2

16

SEE ( dB g

list of wastes. Several efforts have been made over the past years to attempt to valorise this waste stream, including its incorporation in Portland cement [65,66], and its use as soil amendment/fertiliser [67]. Nevertheless, concerns regarding the leaching of trace elements restrict the use of fly ashes as fertilisers [67], while their alkalis and chloride content [68] is in some cases prohibitive for clinker production. Regrettably, the ashes are mostly discarded in landfills [66,67,69], with associated costs. The landfill tax strongly varies within the EU28 from 3 €/ton (in Lithuania) to more than 100 €/ton in Belgium (reference year: 2017) [70]. Landfill taxes are still expected to rise in the next few years to encourage waste reuse and recycling, but also due to the exhaustion of existing landfill space which could be particularly challenging in developing countries (e.g. India). The latter could be a huge driving force in the pursuit of novel and environmentally friendlier waste management strategies. Taking this into account, it seems a reasonable assumption to consider that the pulp and paper industry (producer of the biomass fly ash waste) would provide the waste at a lower price than that of commercial virgin raw materials (e.g. metakaolin), which would strongly reduce the geopolymers’ production costs in comparison with the production of geopolymers based on virgin raw materials [42]. Besides the expected economic advantage, our strategy would also mitigate wastes landfilling and prevent the depletion of virgin raw materials, in line with the circular economy concept and contributing towards cleaner production. In fact, the commercial price of their coal fly ash counterpart, which is a by-product not a waste, has been reported to be 4 times lower than that of metakaolin [41]. Nevertheless, the economic advantage of the proposed strategy in comparison with

26 dB was found for 5 mm thick Portland cement composites with 0.4 wt% carbon fibre, which was increased to 32 dB when the fibre was coated with graphene oxide [23], but again this was a vastly more complex and costly process than ours, and still involved Portland cement. Our inorganic polymer composites based on valorising waste materials, were produced simply and easily, and do not release CO2 formation or curing. The only other reported MW absorbing geopolymers involve materials made from standard commercial chemical precursors, not wastes, and the added carbon component is also a non-renewable, commercial product. Alumino-silicate geopolymer composites with an enormous addition of 14–50 wt% graphite powder and 5 mm thickness had SET values over the Xband similar to our samples, despite containing up to 20 times more carbon [42]. Another article by the same group, on 1–5 mm thick alumino-phosphate GP composites with added carbon felt, also required large amounts of 26–40 wt% carbon to achieve similar average SET values of over 10 dB throughout the X-band, and only narrow peaks of absorbance reached the high values greater than 20 dB that they reported [63]. Therefore, not only are our sustainable inorganic polymer-cork composites produced from wastes equal to any other reported geopolymer MW absorbers over the X-band, but they contain an order of magnitude less added carbon. In this study, the geopolymer was produced using mainly biomass fly ash (70 wt%) as precursor, while in [42] metakaolin was prepared by calcining kaolin (at 800 °C for 3 h). This is extremely relevant as it might ensure lower production costs. In fact, the biomass fly ashes are currently considered a waste (not a by-product), according to the European

a)

SET ( dB)

Fig. 8. a) Specific shielding effectiveness (SSE) over the X-band of the matrix and the cork-geopolymer composites for samples 5 mm thick. The maximum SSE value of every composite sample is compared in b).

R.M. Novais et al. / Construction and Building Materials 229 (2019) 116930

other literature studies must be supported by a cost-benefit analysis, and this will be considered in future work. Anyway, a scenario considering a negative cost for the biomass fly ashes, value depending on the countries’ landfill tax, when assessing the composites cost, cannot be ruled out, but other factors such as the transportation costs also need to be accounted for. Furthermore, the composites made with foaming agents are also lightweight materials, with densities under 1 g cm 3 (see Table 2), meaning that they have enhanced specific shielding effectiveness (SSE, in units of dB g 1 cm3). This is significant in applications where weight is an issue, especially for construction materials which are, by their nature, reasonably dense. All of the geopolymers and concrete/cement composites discussed above will have densities greater than 1 g cm 3, so their SSE values will be smaller than their SET values. As can be seen in Tables 2 and 3, this is also true for our inorganic polymer-cork composites without added foaming agent (FA), but those with foaming agents added (1 or 2 wt%) have lower densities, and enhanced SSE values, particularly for LC2.5_2 and LC3.75_2 with 2 wt% FA added. The range of SSE values for all samples are shown in Tables 2 and 3, and the plots of SSE over the X-band for the 5 mm thick samples are shown in Fig. 8a. The problems of the lower MW absorption of the lower density composites with 2.5 wt% cork, and larger pore sizes, means that the advantage of the lower density is not fully exploited for LC2.5_2, which is no better than the denser LC3.75_1 at lower frequencies, and significantly worse above 9.5 GHz. However, LC3.75_2 has an SSE much greater than all other composites, and the difference is even greater with 7 mm thick composites. The peak SSE values for all composites are shown in Fig. 8b, and it is apparent how much better the LC3.75_2 samples are. 4. Conclusions In this investigation, pyrolysed cork-geopolymer composites were synthesised and then evaluated, for the first time, as ecofriendly EMI shielding building materials. Cork is a natural and highly sustainable material, therefore its incorporation after pyrolysis into geopolymer composites envisioned for EMI shielding applications, instead of other environmentally unsustainable precursors (e.g. carbon nanotubes, graphene, carbon fibre), is an environmentally friendlier strategy. Results showed that all cork-geopolymer composites presented enhanced shielding effectiveness (SET) ability in comparison with the geopolymer matrix ( 4.7 to 6.1 dB; 3 mm thickness), while an increase in the pyrolysed cork amount increased the composites microwave absorbing ability. The maximum SET values over the Xband ( 13.8 to 15.9 dB) is similar to the only other study on alumina-silicate geopolymers, despite the much lower amount of carbon used here (3.75 instead of 40 wt%). Furthermore, the use of foaming agents effectively reduces the composites density highly enhancing their SSE (specific shielding effectiveness). The maximum SSE being 19.9 dB g 1 cm3 for the 3.75 wt% corkcontaining composite. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors would like to thank FCT project H2CORK (PTDC/ CTM-ENE/6762/2014), R.M. Novais wishes to thank FCT grant

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(Ref. CEECIND/00335/2017) and R.C. Pullar wishes to thank FCT grant IF/00681/2015, for supporting this work. This work was developed in the scope of the project CICECO - Aveiro Institute of Materials UID/CTM/50011/2019 (Compete Reference: POCI-010145-FEDER-007679), Associated Laboratory of University of Aveiro, financed by national funds through the FCT/MCTES. M. Saeli wishes to thank Portugal 2020 project PROTEUS - POCI-01-0247FEDER-017729 co-financed by national funds through the FCT/ MEC. K.P. Surendran acknowledges Indo-Portuguese bilateral project (INT/PORTUGAL/P-09/2013) for mutual visits and fruitful discussions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.116930. References [1] L. Bittencourt, R. Immich, R. Sakellariou, N. Fonseca, E. Madeira, M. Curado, L. Villas, L. da Silva, C. Lee, O. Rana, The internet of things fog and cloud continuum: integration and challenges, Internet Things 4 (2018) 134–155, https://doi.org/10.1016/j.iot.2018.09.005. [2] S. Xie, Z. Ji, Z. Shui, B. Li, J. Wang, G. Hou, J. Wang, Design and manufacture of a dual-functional exterior wall structure for 1.1–5 GHz electromagnetic radiation absorption, Compos. Struct. 201 (2018) 608–615, https://doi.org/ 10.1016/j.compstruct.2018.06.079. [3] A.K. Singh, A. Shishkin, T. Koppel, N. Gupta, A review of porous lightweight composite materials for electromagnetic interference shielding, Composites Part B 149 (2018) 188–197, https://doi.org/10.1016/ j.compositesb.2018.05.027. [4] X. Liang, W. Liu, Y. Cheng, J. Lv, S. Dai, D. Tang, B. Zhang, G. Ji, Review: recent process in the design of carbon-based nanostructures with optimized electromagnetic properties, J. Alloys Compd. 749 (2018) 887–899, https:// doi.org/10.1016/j.jallcom.2018.03.344. [5] M.-S. Cao, Y.-Z. Cai, P. He, J.-C. Shu, W.-Q. Cao, J. Yuan, 2D MXenes: electromagnetic property for microwave absorption and electromagnetic interference shielding, Chem. Eng. J. (2018), https://doi.org/10.1016/j. cej.2018.11.051. [6] C.J. von Klemperer, D. Maharaj, Composite electromagnetic interference shielding materials for aerospace applications, Compos. Struct. 91 (2009) 467–472, https://doi.org/10.1016/j.compstruct.2009.04.013. [7] F. Qin, C. Brosseau, A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles, J. Appl. Phys. 111 (2012), https://doi.org/10.1063/1.3688435. [8] R. Narasimman, S. Vijayan, K.S. Dijith, K.P. Surendran, K. Prabhakaran, Carbon composite foams with improved strength and electromagnetic absorption from sucrose and multi-walled carbon nanotube, Mater. Chem. Phys. 181 (2016) 538–548, https://doi.org/10.1016/j.matchemphys.2016.06.091. [9] R.C. Pullar, Hexagonal ferrites: a review of the synthesis, properties and applications of hexaferrite ceramics, Prog. Mater. Sci. 57 (2012) 1191–1334, https://doi.org/10.1016/j.pmatsci.2012.04.001. [10] G.-S. Wang, X.-J. Zhang, Y.-Z. Wei, S. He, L. Guo, M.-S. Cao, Polymer composites with enhanced wave absorption properties based on modified graphite and polyvinylidene fluoride, J. Mater. Chem. A. 1 (2013) 7031, https://doi.org/ 10.1039/c3ta11170a. [11] K.H. Wu, T.H. Ting, G.P. Wang, W.D. Ho, C.C. Shih, Effect of carbon black content on electrical and microwave absorbing properties of polyaniline/carbon black nanocomposites, Polym. Degrad. Stab. 93 (2008) 483–488, https://doi.org/10.1016/j.polymdegradstab.2007.11.009. [12] W. Liu, S. Tan, Z. Yang, G. Ji, Hollow graphite spheres embedded in porous amorphous carbon matrix as lightweight and low-frequency microwave absorbing material through modulating dielectric loss, Carbon N.Y. 138 (2018) 143–153, https://doi.org/10.1016/j.carbon.2018.06.009. [13] X. Liu, X. Yin, L. Kong, Q. Li, Y. Liu, W. Duan, L. Zhang, L. Cheng, Fabrication and electromagnetic interference shielding effectiveness of carbon nanotube reinforced carbon fiber/pyrolytic carbon composites, Carbon N.Y. 68 (2014) 501–510, https://doi.org/10.1016/j.carbon.2013.11.027. [14] A. Saib, L. Bednarz, R. Daussin, C. Bailly, X. Lou, J.M. Thomassin, C. Pagnoulle, C. Detrembleur, R. Jérôme, I. Huynen, Carbon nanotube composites for broadband microwave absorbing materials, IEEE Trans. Microwave Theory Tech. 54 (2006) 2745–2753, https://doi.org/10.1109/TMTT.2006.874889. [15] S.K.D.S. Pande, B.P. Singh, R.B. Mathur, T.L. Dhami, P. Saini, Improved electromagnetic interference shielding properties of MWCNT–PMMA composites using layered structures, Nanoscale Res. Lett. 4 (2009) 327–334, https://doi.org/10.1007/s11671-008-9246-x. [16] H. Yu, T. Wang, B. Wen, M. Lu, Z. Xu, C. Zhu, Y. Chen, X. Xue, C. Sun, M. Cao, Graphene/polyaniline nanorod arrays: synthesis and excellent electromagnetic absorption properties, J. Mater. Chem. 22 (2012) 21679– 21685, https://doi.org/10.1039/c2jm34273a.

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