The output of compact discs (CDs) manufacturing is growing annually by about 10% worldwide. Approximately 25% of them can be regarded as waste right away since they are misprints or were supplied alongside commercial material that was never intended to be used. If each CD weighs 20 g, several hundred thousand tons of garbage must be produced each year on a global scale. Globally, enormous volumes of compact discs are thrown away annually. The discs’ heavy metals, toxic colours, and non-biodegradable polycarbonate cause significant environmental issues. Compact disc recycling has just recently gained traction in a few nations because the existing method is ineffective, fraught with problems, uses hazardous chemicals, and ignores colour treatment [1,2,3,4].
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Lightweight concrete plays a significant role in construction, merited by its low density and high insulation capacity, which are particularly advantageous in structural applications for reducing the self-weights of structural members and, as a consequence, the sectional dimensions of these members [5]. Lightweight concrete is made from light coarse or fine aggregates, the lightweight aggregates (LWA) are produced from clay, shale, natural pumice, or industrial wastes [6,7,8]. Various studies in the field of construction are focusing on modern sustainable materials for replacing natural aggregate (NA) with recycling aggregate [9, 10]. Many types of waste materials have been proven to be reusable in achieving normal strength in sustainable concrete [11,12,13]. Additionally, some studies attempted to improve the mechanical and physical properties of sustainable concrete by using admixtures to reduce the water demand of the mixture, thereby reducing the effect of the water-cement ratio (W/C) [14, 15],mix design [16],use of fibres [17]; or binder replacement with a stronger binder like epoxy [18] and polyester resin, resulting in polyester concrete [19].
Polyester concrete (PC) is a composite material composed of aggregate with organic resin as a binder. PC developed from the s until the s to become widely used in the construction field, merited by its distinctive features such as high compressive strength, high flexural strength, high splitting strength, low porosity, low water absorption, high chemical resistance, and quick hardening. It has been used to produce precast elements [20,21,22]. The performance of PC strongly depends on various factors like the mix proportion, type and grading of aggregate (fine and cores) and type and percentage of filler [23]. Avci et al. [24] studied the effect of grading of aggregate on the mechanical properties of the PC mix. Varughese and Chaturvedi [25] studied the influence of adding fly ash with river sand and aggregate, reporting that the fly ash improved the mechanical properties and decreased the percentage of water absorption up to 75% by weight of fly ash. Many researchers [26] reported the advantages of using recycled aggregate (fine, coarse or both) with polyester resin, polymer and epoxy, and studied the benefits of adding recycled aggregate on the mechanical properties, physical properties and the benefits to the environment by reducing the manufacturing of cement, keeping and reducing the consumption of natural aggregate, and expending industrial and construction waste [27,28,29]. Natural aggregate (fine and coarse) can be replaced in different percentages by many types of waste such as construction, electronic, plastic, glass or sub-products such as plastic waste polyethylene terephthalate (PET), electronic plastic waste (EPW), glass, recycled concrete, crushed brick, reclaimed asphalt pavement and marble [30]. This study focuses on plastic waste, in particular EPW, which has low cost, high strength-to-weight ratio, high durability (deterioration resistance), easy workability and shaping, and low density. Plastic waste is generated worldwide, and frequently ends up in rivers, on coasts and beaches, and in landfills. Only approximately 25% of plastic waste around the world is recycled. This insufficiency in recovery and recycling and the associated contamination of land and oceans [31] necessitate more efficient recycling strategies to mitigate the environmental impact of waste plastics.
This research introduces an innovative approach by utilizing EPW, specifically discarded digital video discs (DVDs) and CDs, to develop a novel type of concrete termed “lightweight polyester concrete” (LWPC). The binding agent in this LWPC is polyester resin, while attapulgite clay, a resource found in the Tar Al-Najaf region (Injana) of Iraq, serves as the filler material. By incorporating this locally sourced clay, the study aims to evaluate its effectiveness and impact on the properties of LWPC, potentially offering a sustainable and environmentally friendly alternative in concrete production.
The recycled aggregate is electronic plastic waste crashed and sieved before use. The DVDs and CDs were prepared in stages that included cutting the DVDs and CDs with a shredder and then crushing the pieces into smaller pieces. Table 1 and Fig. 1 show the grading of the DVDs and CDs. The properties were within the requirements of ISO Zone 1 [32]. Table 2 and Fig. 2 show the structures of EPW, which is composed of a clear polycarbonate plastic substrate, a reflective metallic layer (alumina), and a clear protective coating of acrylic plastic.
Polyester Resin Typically, the binding material in concrete is cement and water. To produce the LWPC, this study used polyester resin as a binding material. This type of resin was used due to its physical and mechanical properties such as compressive and flexural strengths, availability, rapid setting, and low cost compared with other types of resins. The physical and mechanical properties (as given in the technical sheet provided by the manufacturer) are listed in Tables 3 and 4. The polyester resin is used as the binder in this study (Farapol O 115 by Farapol Company) [33] is unsaturated polyester resin composed of orthophthalic anhydride and standard glycols and has good flexibility, moderate reactivity and low viscosity. The setting time for the binder was recorded to be 14–18 min.
Accelerator The accelerator (Hardener GP50) is a colorless oily liquid called methyl ethyl ketone peroxide (MEKP). It is an organic peroxide with the formula [(CH3)·(C2H5)·C(O2H)]2O2. It is used with polyester resin to initiate a chemical reaction. The accelerator used in this study has the chemical formula C8H18O6, with a density of 1.170 g/cm3, active oxygen of approximately 8.1–9.1%, melting point below 30 °C, flash point of 75 °C, and clear appearance. The accelerator is stable at 20 °C. The dosage rate was 4.5% of the resin weight as recommended by the manufacturer.
Attapulgite is a clay mineral with fibrous silicate and acidic properties that render it useful as an adsorbent and catalyst. The Attapulgite clay was obtained from the Tar Al-Najaf region (Injana) in Iraq. The HRA was produced from raw materials that exist as rocks and, therefore, need to be ground using a crusher. The attapulgite chemical formula is Mg5Si8O20(OH)2(OH2)4.4H2O and contains aluminium silicate minerals and hydrous magnesium with water in its structure. After looking at the ideal calcination temperature and calculation time, Al Amide () [34] found that 750 ºC was the ideal temperature. The temperature increased at a pace of 4 ºC per minute, and 30 min was the ideal time for computations.
This study used three calcination temperatures, 300 °C, 600 °C and 900 °C, to compare with Attapulgite produced without burning, as shown in Fig. 3. An electric furnace was used to burn the attapulgite clay for 30 min as soaking time at the required calcination temperatures. The heating rate was 10 °C/min [34,35,36] resulting in HRA time of 30 min, 60 min and 90 min, for the temperatures 300 °C, 600 °C and 900 °C, respectively, until reaching the calcination temperatures. The chemical analysis of attapulgite before burning shows a high percentage of SiO2 (51.8%), with the other composition being CaO (7.16%), Al2O3 (8.55%), Fe2O3 (4.91%), MgO (5.58%), TiO2 (0.45%), SO3 (0.51%), Na2O (1.01%), K2O (1.8%) and losses of ignition (LOI) (18.23%). The physical properties of HRA are a specific gravity of 2.33 and a fineness of m2/kg. The chemical analysis of attapulgite after calcination at 900 °C shows an increase in SiO2 to 60.40%, with the other oxides being CaO (8.56%), Al2O3 (13.85%), Fe2O3 (6.81%), MgO (5.58%), TiO2 (0.65%), SO3 (0.45%), Na2O (1.13%), K2O (2.47%) and LOI (0.1%). This increase is due to the absence of LOI after burning, where the LOI brews of water within clays and feldspar and CO2 within calcite and dolomite [34].
Following calcination at several temperatures, the morphology and appearance of the calcined attapulgite, including its colour and particle diameter, underwent considerable changes. Attapulgite samples that are calcined between 200 and 500 °C do not sinter; nevertheless, at 600 and 700 °C, sintering partially occurs. At 800 °C, the phenomenon of fully melted sintering takes place. The uncalcined attapulgite has a grey-white color that progressively turns orange as the calcination temperature rises. Magnetite (Fe3O4) generated by high-temperature calcination may be the reason for the color shift of attapulgite [37].
In the first stage, all the molds were oiled before casting to facilitate de-molding. In the second stage the dry materials (recycled fine aggregates EPW and HRA) were mixed well then, the liquid mixture polyester resin and accelerator were added to the dry mix, and all the components were mixed for 3–4 min until a homogenous mix was obtained. The mixtures were carefully cast in molds in two layers, ensuring to completely fill the molds; some of the mixtures needed vibration to ensure that the mold is completely filled. The LWPC specimens need 2 h to be hardened and de-molded. Testing the hardened samples (cubes and prisms) was carried out to evaluate their physical properties (water absorption, density and ultrasonic pulse velocity) and mechanical properties (compressive and flexural strengths) 24 h after casting.
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Figure 6 shows the density results for hardened polyester concrete with 100% aggregate substitution with EPW (DVDs and CDs). All the density values were under kg/m3. Therefore, all the mixtures are considered lightweight concrete. Additionally, the figure shows that the M6 (no EPW) mixtures have the highest density (– kg/m3). M6 shows the change in properties in the absence of EPW, the effect of HRA calcination temperature [34], and the properties of lightweight aggregates used [43, 44]. The density decreased with the increase in the calcination temperature. The clay particles (HRA) will eventually lose all of their water when the calcination temperature rises, increasing HRA’s water requirement. More liquid will be required to saturate the calcined clay particles. According to research by AL Amide () [34], the ideal calcination temperature was 750 °C. Below that temperature, compressive strength, density, and strength activity index (SAI) dropped below that temperature. Due to the low polyester resin content and higher resin demand, M1-900 displayed the maximum porosity and water absorption, while the M1 set demonstrated the effect of the calcination temperature. By contrast, Fig. 6 shows that for the mixes M1–M4, the density increases with the increase in the polyester resin-to-EPW ratio.
Figures 7 and 8 show the porosity and absorption results for all the specimens. The highest porosity and absorption are seen for the mixes with the lowest polyester resin content (M1 set). Additionally, the results show that M2, M3, M4, M5 and M6 have water absorption percentages under 1% due to the low viscosity and low molecular weight of the polyester resin. This means that using polyester resin in the proper amount allows the production of void-free concrete [13]. The results showed the effect of polyester resin content, but not those of the HRA calcination temperature or filler content, and therefore the density test has been more informative in evaluating the aforementioned parameters.
The UPV test helps find the homogeneity of the mixture and detect voids or cracks in the concrete specimen. It is used to indirectly predict the development of compressive strength in concrete.
Figure 9 shows the test results of all mixtures. The test results show a decrease in UPV with a decrease in the polyester resin-to-EPW ratio. The lowest velocity is seen for the M1 set, while the highest is achieved in the M6 set, in which the HRA used as a filler material gives the mixtures more homogeneity and higher density with low voids [45]. Within the M6 mixtures, the M6-0 and M6-300 were 2.65 and 2.67 km/sec, respectively. The raw materials of the concrete have many characteristics that directly influence the results of UPV.
Figure 10 shows the influence of polyester resin dose on EPW (recycled aggregate) and the effect of HRA for all calcination temperatures on the dynamic elastic modulus of LWPC. The dynamic elastic modulus calculated based on the UPV results represents the stress-to-strain ratio under dynamic loading [46]. The results show that the HRA increases the dynamic elastic modulus as shown clearly for the M6 set, where the highest value was obtained for M6-300 which has the highest density and UPV.
Figure 11 shows that all the mixtures failed after reaching the maximum load. In the M6 set, the specimens undergoing the test show high strain without fracture, and elongation in the specimen edges until the failure point. The polyester resin has many advantages such as high compressive strength, high flexural strength, and high ductility. All these properties made hardened polyester more flexible and more ductile during the compressive strength test [47]. The dynamic elastic modulus (Ed) for all the mixtures gradually increased with the HRA calcination temperature, with the M2 and M4 showing the same performance due to the effect of the resin-to-EPW ratio. The shear modulus (µd) or modulus of rigidity is defined as the ratio of shear stress to shear strain and is determined by multiplying the UPV by the squared mass density [40].
Figure 12 shows the results of the shear modulus, which shows the resistance of the LWPC to shearing deformation. The results show high shearing deformation resistance; the shear modulus ranged between 4.7 and 9.8 GPa, and the results show a gradual increase in the shear modulus with the increase in HRA calcination temperature. The M6 set shows the highest shear modulus for all mixtures, and the values for the mixtures calcinated at all four temperatures ranged between 9.1 and 9.8 GPa, which shows the influence of EPW on the shear modulus of the mixtures; the EPW reduces the shear modulus specifically when the polyester resin content is not sufficient to completely coat the EPW, which weakens the bonds between the components (EPW + filler) and the binder. The optimum shear modulus is observed for the sets M2 and M4 (in the ranges 7.1–7.6 GPa and 6.7–7.3 GPa, respectively). On the other hand, the M5 set shows decreasing resistance to shearing deformation, because the HRA filler fills the pores with small particles and decreases the interfacial transition zone (ITZ) [48] and makes the mixture homogenous [49, 50] and more ductile.
Figure 11 shows the type of failure for all specimens after the compressive strength test, while Fig. 13 shows the results of compressive strength is affected by EPW, resin-to-EPW ratio, and the HRA calcination temperatures for the mixtures containing both EPW and HRA, compared with the reference mixtures (M5 set without HRA and M6 set without EPW). The results show that many mixtures are considered high-strength concrete because the compressive strength values are higher than 41 MPa according to the guidelines (ACI 363, ) [51]. The M6 set showed the highest values (74.99, 64.29, 63.58 and 51.44 MPa for M6-0, M6-300, M6-600 and M6-900, respectively), which indicates the effectiveness of HRA filler with optimum quantity of polyester resin that gives the mixture high workability during the mixing process, flowability during casting, and homogeneity. The HRA is a clay with small particles that has the ability to reduce the ITZ by filling the voids and decreasing the porosity, which strengthens the bonds between the components [48, 52, 53]. The M6 set showed a gradual decrease in compressive strength with increasing HRA calcination temperature [34, 35]. This trend is reversed for the sets containing both EPW and HRA. Three mixtures in the M5 set (M5-A40:60, M5-A50:50 and M5-A60:40) had higher compressive strength (47.7,47.1 and 44.7 MPa) compared with the other mixes, due to the effect of sufficient polyester resin content,the effect of this factor is seen for the M5-A30:70 with compressive strength of only 25 MPa. Figure 13 shows the surface texture of all mixtures, where the M1 set and M5-A30:70 (without HRA) have rough surfaces and high porosity, and show high water porosity and absorption percentage, according to Figs. 7 and 8, respectively.
The M2-900 and M3-900 mixtures have the highest compressive strength values (53.7 and 63.2 MPa, respectively). Such high-strength concrete is generally used in the erection of high-rise structures and rigid pavement construction. For example, the requirement specifies a compressive strength higher than 30.7–32 MPa for light aircraft pavement [54, 55] , 35–45 MPa for precast products like interlocking concrete block pavements (which is classified under the medium traffic category), and 45–55 MPa for the very heavy traffic category [56]. Therefore, many factors affect LWPC properties, which requires optimizing the quantity of polyester resin, EPW percentage, HRA filler, HRA calcination temperatures and the ITZ between the binder and the dry material to produce the optimum mixture.
The flexural strength was calculated for the 24 mixtures according to the ASTM C348–08 [42], and the results are shown in Fig. 14. The flexural strength test evaluates the ability of an unreinforced concrete beam or slab to withstand bending or load without failure. This property is required in pavement products, specifically because the flexural strength measures a paving material’s ability to handle heavy traffic over time and resist breaking when pressure is applied. In this test, the third point load method was used for the prism specimens 40 × 40 × 160 mm3 in dimensions as recommended by ASTM C348–08 [42]. The results show that all the mixtures are within the requirement of ASTM C348–08 and that the lowest value is 10.5 MPa while the highest is 35.3 MPa. Accordingly, the results are within the Federal Aviation Administration (FAA) recommendation for pavement design, where the flexural strength must be higher than 4.1 MPa [55] and the US Army and Air Force requirement that specifies a minimum value of 4.5 MPa [54]. Moreover, Fig. 14 shows that the flexural strength values satisfy the specifications for precast interlocking concrete blocks for paving [57], which limits the minimum flexural strength breaking load to 2 kN for 40 mm thickness and 3 kN for 60 mm for residential pathways and public pedestrian paths, and the IS guidelines [57] that specify a maximum breaking load of 7 kN for heavy duty and industrial roads. The M1 set and the M6 set showed the effect of HRA calcination temperatures, as the flexural strength decreased with increasing the calcination temperatures. On the other hand, and similarly to compressive strength, the calcination temperature does not visibly affect the flexural strength of mixtures that have a sufficient polyester resin content. The highest flexural strength values were obtained for the M2 set. This is attributed to the adequate amount of polyester resin used as a binding material, which coated the EPW and HRA and resulted in homogenous mixtures with high bending load resistibility.
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