Effective mechanical reinforcement of inorganic polymers using glass fibre waste
Graphical abstract
Introduction
Recent data suggests that global CO2 emissions have slightly decreased (∼0.1%) in 2015 to around 36.2 billion tons, which is in line with the slowing trend in annual emissions growth over the past three years (Oliver et al., 2016). Nevertheless, to significantly mitigate anthropogenic climate change, further decrease is mandatory. In this context, a reduction of the massive emissions arising from Portland cement production, accounting for around 5–7% of the total CO2 anthropogenic emissions (Chen et al., 2010), could contribute to this. Indeed, cement production reached a distressing value of around 4200 million tons in 2016 (U.S. Geological Survey, 2017), generating roughly 3570 million tons of CO2 (considering that 1 ton of Portland cement generates 0.85 ton of CO2 (Ke et al., 2015)). Therefore, the development of new building materials with a reduced carbon footprint is desirable. Inorganic polymers (also known as geopolymers) emerge as a promising and environmentally friendlier alternative to Portland cement.
Inorganic polymers are synthesised by mixing aluminosilicate source materials (e.g metakaolin) with alkaline (or acidic) activators. Geopolymerization occurs through complex chemical reactions involving dissolution, transportation and polymerisation (Duxson et al., 2007, Komnitsas and Zaharaki, 2007), which are affected by the nature and concentration of the activator, aluminosilicate precursor nature, curing temperature, and mixture composition. Several waste streams have been used as aluminosilicate sources (e.g. fly ash, waste glass, red mud) (Ascensão et al., 2017, Novais et al., 2016b, Novais et al., 2017) and as alkaline activators (Peys et al., 2016), which further reduces the geopolymer's environmental impact. Besides their lower carbon footprint, geopolymers show other advantages over Portland cement, such as higher resistance towards freezing/thawing cycles and acids. Despite these interesting properties, geopolymers are brittle under applied force. To overcome this limitation, various fibres have been studied as reinforcement agents, namely steel (Al-Majidi et al., 2017), glass (Al-Majidi et al., 2017, Alomayri, 2017), cotton (Alomayri et al., 2014), wool (Alzeer and Mackenzie, 2012), poly (vinyl alcohol) (Al-Majidi et al., 2017, Nematollahi et al., 2017), polyethylene (Nematollahi et al., 2017) and flax (Assaedi et al., 2015) fibres.
Despite the increasing interest in geopolymers, the number of investigations regarding reinforced geopolymers is scarce, in comparison with common fibre reinforced cement concrete. Alzeer and Mackenzie, 2012reported a 40% improvement in flexural strength for wool-reinforced geopolymer in comparison with the unreinforced matrix, while Assaedi et al. (2015) observed an impressive 369% enhancement in compressive strength when using a flax fabric (corresponding to 4.1 wt.% flax fibres) reinforced fly ash geopolymer. These studies demonstrate that fibre reinforced geopolymers exhibit enhanced mechanical properties which may extend the geopolymers' application range.
The use of glass fibre in geopolymers is less common, despite its obvious suitability (e.g. high tensile strength and specific modulus) (Yan et al., 2016). Moreover, the existing literature is unclear: some studies report a drop in mechanical strength (Nematollahi et al., 2014), others minor changes (Al-Majidi et al., 2017) or significant improvement (Natali et al., 2011) when using glass fibre. Nematollahi et al. (2014) observed a slight increase in compressive strength using 0.5 and 1.25 vol.% fibres, while the compositions containing 0.75 and 1.00 vol.%, showed the opposite behaviour. Al-Majidi et al. (2017) stated that the addition of 1 vol.% glass fibre to a geopolymer matrix did not significantly alter the compressive strength, while Natali et al. (2011) reported 30% increase in flexural strength when using 1 wt.%. The reason for these results was not sufficiently addressed by the authors, and for that reason further studies considering the use of glass fibres are required to provide a deeper knowledge on the influence of glass fibres (content and length) on geopolymer properties.
In this work, glass fibre-reinforced geopolymers were produced using glass fibre waste (GFW) coming from wind turbine blade production. The influence of the fibre content and length on the geopolymers' microstructure, apparent density, compressive and tensile strength was evaluated. To the best of our knowledge, this is the first investigation where GFW was used as a reinforcement agent in a geopolymer matrix. Metakaolin (MK) was selected as the aluminosilicate source, since it is widely used as a geopolymer precursor, and for that reason can be considered as a model system. This investigation aims to provide a deeper knowledge of the influence of glass fibre (content and length) on geopolymer properties, thus reducing the existing knowledge gap.
The GFW used in this investigation is currently disposed of in landfills, which is unsustainable in the circular economy concept. Therefore, its use as a reinforcement agent in geopolymers production, as proposed here, may reduce/prevent the contribution to landfill of this waste.
Section snippets
Materials
Geopolymers were prepared using a single geopolymer composition (MK-0.0%), consisting of 38.18 wt.% MK, 30.30 wt.% sodium silicate, 19.40 wt.% NaOH and 12.12% water, and with varying proportions (0–3 wt.%) of added GFW as a reinforcement agent. MK was purchased under the name of Argical™ M1200S from Univar®, while GFW was supplied by a Portuguese company “Riablades S.A.”, located in Aveiro (Portugal), and comes from wastes generated during wind turbine blade production. The GFW, received as
Glass fibre waste (GFW) characterisation
The chemical composition of the GFW, presented in Table 2 (XRF), shows that the most abundant oxides are SiO2, CaO and Al2O3, followed by MgO and K2O.
Fig. 2 presents a typical SEM micrograph and the EDS spectrum of the GFW. The micrograph shows that the fibres' diameter is roughly 18 μm, while the elements detected by EDS are in agreement with the chemical composition determined by XRF.
XRD characterisation
The XRD patterns of the unreinforced and GFW-reinforced geopolymers are presented in Fig. 3. For comparison,
Conclusions
In this investigation, the reinforcing potential of glass fibre waste in inorganic polymers production was evaluated for the first time. The influence of glass fibre (content and length) on the inorganic polymers' microstructure, apparent density, compressive and tensile strength was considered. Results show an impressive increase both in compressive (up to ∼162%) and tensile strength (up to ∼77%) for the GFW-reinforced geopolymer in comparison with the unreinforced matrix, demonstrating the
Acknowledgements
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. R.C. Pullar thanks the FCT for funding under grant IF/00681/2015.
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