Optical Fiber Waste Used as Reinforcement for Concrete with Recycled Marble Aggregate

Sustainable development in the 21st century depends on the reuse of materials and products, as well as on economic and environmental incentives for recycling. Several studies have sought to improve the quality of concrete, as well as its durability and strength, by adding fibers (metallic, polypropylene, carbon, vegetables) or by replacing the aggregate. The use of optical fiber waste in concrete is still incipient and few studies address the use of marble aggregate to be used in concrete. This research assesses the behavior of the mixture composed of: Portland cement + fine aggregate (sand from crushed marble waste) + coarse aggregate (gneiss) + optical fiber waste and water. The compressive strength, the tensile strength and the modulus of elasticity of concrete and the durability of optical fiber waste in alkaline composite were tested through microstructural evaluation. The results present an increase of about 20% in the mechanical properties and a reduction in the rigidity of the mixture, making the material more ductile. The superficial protection of the fibers made them more resistant to the alkaline attack of the cement. The knowledge acquired would allow the creation of sustainable concrete reinforced with optical fiber in a much more efficient way.


Introduction
Currently, most industrial processes are sources of waste, causing major environmental degradation and compromising sustainable development (Cox et al., 2014;Almada et al., 2020). The current challenge concerns the rational use of waste arising from that system (Aliabdo, Elmoaty & Auda, 2014;Ashish, 2018, Almada et. al., 2020. In such context, building industry, due to its large volume of resources consumed-either renewable or not-is one of the main technological sectors recommended for carrying out this absorption of solid waste (Munir, Kazmi & Wu, 2017;Vardhan et al., 2015). The technical and scientific areas have experienced a boom with regard to studies that seek to find alternatives regarding the total replacement of the natural fine aggregate in order to face the growing shortage of supply and the resulting increase in its price, as well as the reuse of other waste from different sectors (Kou & Agrela, 2011;Medina, Rojas & Frias, 2013;Arel, 2016;Preira-Mercado et al., 2016).
In such context, studies developed in the early 2000's have evidenced the reuse of crushed marble waste as a fine aggregate for the manufacture of concrete and mortar (Barbosa, Santos & Coura, 2018;Ashih, 2018). The consumption of ornamental marble stones (metamorphic rocks composed of calcite and dolomite) by the building industry raises a concern about the environmental impacts caused by the waste generated in the extraction and processing steps. It is estimated that 20−30% of the raw material is turned into waste (Buyuksagis, Uygunoglu & Tatar, 2017). The largest amount of waste was obtained in the phases of extraction and primary processing (cutting the plates) and secondary processing (cutting the final pieces for ornamentation), and such waste can be used as aggregates (Ashish, 2019).
aggregates as a substitute for fine aggregate, having been observed that there was a lower water demand in mortars containing up to 50% of those wastes. This can be explained by the marble powder thixotropy, which caused the mortar to flow with less energy when compared to the reference mixture. The increase in the incorporation content tends to increase the demand for water, reducing workability (Ashish, 2018). It was observed that the use of marble powder in the proportion of 10 to 15% in concrete increases the tensile and compressive strength by about 15 to 20%, due to the reduction of the water/cement ratio (Singh, Srivastava, & Bhunia, 2018). Moreover, lower w/c ratios promote lower porosity, which can be achieved with the use of plasticizer or superplasticizer admixtures (Sardinha, Brito & Rodrigues, 2016). This is corroborated by the observation of the microstructure, which shows that the concretes with marble waste in the mixtures were denser, with a lower formation of crystals (calcium hydroxide and ettringite), reduced ITZ thickness and lower presence of large pores (Alyamaç & Ince, 2009).
Other studies have sought to improve the quality of concrete, as well its durability and strength, by adding fibers such as metallic fibers (Yap et al., 2015a;Yap et al., 2015b), polypropylene (Lanzoni, Nobili & Tarantino, 2012), carbon (Viana et al., 2020) and vegetables (coconut, sisal, among others). The fibers contribute to a greater toughness to tensile strength and to a reduction of the porosity and of the shrinkage cracks of concrete. Glass recycling is an environmental issue due to its high volume, the excessive cost of recycling it and its insolubility in the environment (Sales et al., 2017). There are many studies on the use of glass waste, such as: in cement mortars applied as pozzolan (Chen et al., 2002) and aggregate applied in concrete (Shayan & Xu, 2006). In most research related to the addition of glass to concrete, the glass contains fine particles with a diameter of about 150 μm (different percentages of glass that increase in use as a replacement powder of cement and aggregates) (Ling, Poon & Kou, 2012). The results of the application of glass powders in different percentages (lower than 20%) indicated a slight increase in the compressive strength (Lee et al., 2011) and the tensile strength of the concrete. Research (Sales et al., 2017;Alvarenga et al., 2019) on the incorporation of glass waste as a fine aggregate in mortar or as a substitute for cement concluded that the presence of glass tends to reduce the heat conduction in these composites, improving thermal insulation properties.
The use of optical fiber waste in concrete is still incipient (Ge et al., 2015). The addition of that waste  results in a product with some qualities, specifically technical (quality proven by mechanical tests), environmental (high consumption of fiber residues from optical fiber waste) and economic (due to the possibility of reducing the size of the concrete pieces as opposed to the increase in tensile strength, consequently reducing the cost of the material). Also, from their implementation up to the end of their lifespan, optical fibers generate waste, as they are extremely thin and fragile. The handling of such fibers is delicate and, once broken, their waste cannot be fully reused for the same purpose, which promotes studies to reuse them in other industrial sectors. Besides that, the fiberglass is damaged by the degradation that can occur due to the alkalinity of Portland cement (Reis et al., 2009). Therefore, this study aimed to analyze the durability of optical fiber waste (composed of dielectric, crystalline and homogeneous materials forming concentric capillaries) in an alkaline environment. Analyses of the mechanical properties of concrete were carried out, in which the incorporation of recycled aggregates and optical fiber waste and the behavior of the microstructure of glass fibers were evaluated after 5 years of exposure to alkaline compounds in concrete.

Characterization of Materials
It was used the high initial strength Portland cement (CPV-ARI-95% clinker) (NBR 5733, 1991). Table 1 presents the general characterization of the cement. A coarse aggregate of gneiss was used, with the characteristics expressed in Table 2.  It has also been produced the artificial fine aggregate, from the marble waste crushing (WA). The chemical composition (Table 3) of the marble waste was determined by X-ray fluorescence (XRF), method XRF79C: fusion with lithium tetraborate, with 0.01% detection limit and using the CLA70C method for the determination of volumetric iron (II) -FeO (silicate) oxide. Loss on ignition was determined by calcination of the sample at 405°C and/or 1000°C, using the method PHY01E: LOI (Loss on ignition). The XRD (Table 4) was performed in a Bruker D2 Phaser with CuKa (1.54184 A) radiation; 30 kV and 10 mA; a step size of 0.018° and step time of 1s; a 2 tetha range of 8-70°. The Rietveld refinement was carried out using the software X'Pert HighScore Plus 3.0 and the Crystallography Open Database (COD 2020). The samples were passed through a #325 (45 microns) mesh sieve. Zincite (analytical grade, purity > 99%) was used as an internal standard (20 wt%, homogenized in an agate mortar for 10 minutes). The reinforcement of the concrete was made with optical fiber waste. The size of the test specimen and the percentage of fiber significantly influence the indirect tensile strength, that is, for the same test specimen dimension, increasing the percentage of fiber will increase the strength ( . To assist in the morphological analyses of the fibers, a digital microscope with up to 1600x magnification was used.

Characterization of Concrete
The amount of fiber waste in the concrete was 1.4% in relation to the volume of concrete. The ratio of the concrete mixtures was 1: 1.83: 2.83 (cement: fine aggregate: coarse aggregate), whereas the water/cement ratios were 0.50 (for slump test = 140 mm) and 0.46 (for slump test = 100 mm). The experimental program consisted of analyzing the chemical and mechanical properties, namely: Raman spectra of the materials (cement, marble waste and concrete), compressive strength, tensile strength, flexural strength and modulus of elasticity.
A detailed investigation concerning the benefits of employing crushed marble waste as a fine aggregate in

Analys
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Charac
Marble wa (Table 3 (Dhir et al., 2004;Reis et al., 2009;Fathi et al., 2017;Dehghan, Peterson, & Shvarman, 2017): i) The increase in fiber resistance is achieved by using alkali-resistant (AR) fibers, that is, fibers composed of about 16% of zirconium oxide (ZrO 2 ); however, their cost is relatively high (approximately 3 times the cost of the conventional version), thus making it economically unviable; ii) Surface protection of fibers (allowing "encapsulation" of the fiber), preventing contact with hydrated surfaces, that is, making them more resistant to the alkaline attack of cement; iii) Change of the cement matrix through the use of low alkalinity cements and/or the addition of minerals capable of reducing the pH of porous solutions, or by eliminating the formation of calcium hydroxide, or by modifying the mixture of concrete and latex (reducing the formation of Ca (OH) 2 ).

Conclusions
The recycling process of waste should follow an appropriate methodology so that the products developed may present performances compatible with the technical standards and do not cause environmental risks. With regard to waste, the object of this research, it is clear that the results are compatible with, and even higher to, the technical standards, reinforcing the great benefits. The main conclusions drawn regarding the use of marble waste as an aggregate and optical fiber waste for the production of mixed concrete was that the concrete with marble waste has good mechanical properties and the addition of optical fiber waste improved the performance of the concrete.
As an indicator of long-term sustainability, it can be used in concrete to achieve one or several of the following: replacement of non-renewable materials with recycled materials or waste; reduction in energy requirements in the manufacture of concrete; improvement in the durability of concrete in a particular environment; reduction in the cost of materials; utilization of local unconventional raw materials (for example: marble waste and/or optical fiber waste) in low-cost concrete.
It can be concluded that the inclusion of waste: i) Results in an increase of about 20% in mechanical properties. Such fact can be justified by the higher density and by the tensile effect, preventing the micro-cracks from spreading and propagating in the rupture of the studied specimen; ii) Regarding flexural strength and modulus of elasticity, these properties are extremely important in very specific areas: impact forces, support conditions, among others, being that, for the reinforced composite, the results of the addition of fibers tend to result in a reduction in the rigidity of the mixture, making the composite material more ductile; iii) Surface protection of the fibers prevented contact with hydrated surfaces, making them more resistant to the alkaline attack of the cement.
Specifically, for the building industry, the accumulation of construction and demolition waste in inappropriate or scarce areas, not to mention the growing demand for materials, is considered a current social and economic problem. The "green" materials, which have high durability, high properties, low maintenance requirements and contain a large proportion of recycled or recyclable materials, are important for the planet and this composite includes the concept of sustainability (environmental, social and economic).