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Recycling carbon and glass fiber polymer matrix composite waste into cementitious materials
Resources, Conservation & Recycling 155 (2020) 104659
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Full length article
Recycling carbon and glass fiber polymer matrix composite waste into cementitious materials
T
Edward Clarka, Monika Bleszynskia, Frank Valdezb, Maciej Kumosaa,* a b
Industry/University Cooperative Research Center for Novel High Voltage/Temperature Materials and Structures, University of Denver, United States United States Bureau of Reclamation, Lakewood, CO, United States
ARTICLE INFO
ABSTRACT
Keywords: Fiber polymer matrix composite waste Recycling Portland cement Salt aging Molecular dynamics
Recycling options for fiber polymer matrix composites (FPMCs) are limited since they typically cannot be re-used or re-processed. One source of FPMCs is the utility industry, which is increasingly using hybrid carbon and glass FPMC core high voltage (HV) conductors for energy transmission. As there are currently no recycling methods for these composite cores, we investigated if hybrid carbon and glass FPMC waste could be recycled as an admixture for cementitious materials in order to improve their properties for seawater applications. We assessed the compression strength of ordinary Portland cement (OPC) with 6% wt of various FPMC admixtures before and after accelerated salt-water aging. The experimental part of this research was strongly supported by molecular dynamics simulations (MD) to examine the effect of FPMC admixtures on moisture diffusion in OPC. It was established that recycled finely ground FPMC admixtures can provide a benefit to cementitious materials by decreasing void content and slowing the diffusivity of corrosive compounds such as salt water.
1. Introduction The use of fiber polymer matrix composite (FPMC) materials is steadily increasing across the globe, with large amounts FPMC waste being generated every year. Within the next two decades the projected demand for FPMC materials is expected to grow between 10–15 (USD Billion) for new and evolving industrial applications, including aircraft, automobiles, consumer goods, advanced medical devices and others (Oliveux et al., 2015; Bhadra et al., 2017; Witik et al., 2013; PachecoTorgal et al., 2017; Correia et al., 2011). As a result, advances in research and development of novel composite materials has led to the production of many different types of FPMCs, specifically those containing glass and carbon fibers (Bhadra et al., 2017; Pacheco-Torgal et al., 2017; Correia et al., 2011; Asokan et al., 2010). Unfortunately, the continuous generation of composite preproduction waste and postwaste (end-of-service-life) has resulted in enormous amounts of waste material being sent to landfills for disposal (Oliveux et al., 2015; Bhadra et al., 2017). The mass worldwide production of polymeric composite materials including thermoset plastics, thermoplastics, adhesives, resins, and specialized coatings climbed to over 320 million metric tons in 2015, with continuous growth of ∼10% per year (Oliveux et al., 2015; Bhadra et al., 2017). Thus, the large demand for FPMC parts, components, and materials has resulted in large quantities of waste materials, many of which are non-decomposable (Oliveux et al., 2015; Bhadra ⁎
et al., 2017; Witik et al., 2013). One source of FPMC waste comes from utilities and power generation and distribution companies, which are experiencing increasing demand (Asokan et al., 2010). Some of these utilities use Polymer Core Composite Conductors (PCCC), such as, for example, the hybrid carbon and glass fiber Aluminum Conductor Composite Core (ACCC) for their overhead high voltage (HV) transmission lines (Fig. 1a). These conductors have superior electrical energy transmission properties and are gradually replacing traditional steel core conductors (Asokan et al., 2009; Mastali et al., 2016a; Abdollahnejad et al., 2017; Mastali et al., 2018, 2016b; Mastali et al., 2017; Burks et al., 2009). In certain situations, however, if not properly utilized, PCCCs can be damaged either during installation or in service by excessive bending, impact, aging at extreme temperatures, corrosion, excessive Aeolian vibrations, galloping, excessive snow and ice loading, etc. (Anon, 2019; Mastali et al., 2016a; Abdollahnejad et al., 2017; Mastali et al., 2018, 2016b; Mastali et al., 2017; Burks et al., 2009). If prematurely damaged, PCCCs will have to be replaced before their expected life creating a large amount of composite waste. While the aluminum helical strands in these conductors can easily be recycled, the recycling of the PCCC cores remains a major challenge (Fig. 1b). The separation of glass and carbon fibers from their thermoset resin in the PCCC cores is not straightforward. Therefore, large amounts of preproduction composite material waste, damaged cores, and end-of-service-life material could be
Corresponding author. E-mail address: [email protected] (M. Kumosa).
https://doi.org/10.1016/j.resconrec.2019.104659 Received 10 October 2019; Received in revised form 18 December 2019; Accepted 20 December 2019 0921-3449/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. a, b. Standard and ACCC conductors (left), and ACCC conductor core rods (right).
Fig. 2. Estimated mass ton of ACCC material as a function of length (km).
generated by the industry in the future. This is especially important, as the United States currently has over 250,000 km of HV transmission lines (Asokan et al., 2010). Thus, when the HV PCCCs reach end-ofservice-life, a large amount of waste could be produced (Fig. 2). Reclaimed fiber materials (rFM) from products such as HV PCCC cores are estimated to produce over 8 million kg of carbon and glass fiber material, and these amounts are expected to increase every year (Oliveux et al., 2015; Bhadra et al., 2017; Witik et al., 2013; Pacheco-Torgal et al., 2017; Correia et al., 2011). Currently, recycling hybrid fiber composites is difficult because the current methods used to separate the fibers and resins are specific to only single fiber types. Most carbon and glass composites are not readily recycled because of the harsh methods required to extract the fibers from the resins, which can severely degrade the fibers’ mechanical properties. While the most common waste management solution is disposal into landfills, there are various methods to recycle FPMC waste which have been successful, including either physical or chemical recycling methods (Witik et al., 2013). Some studies have reported that carbon and glass fibers separately added to cementitious concrete structures as admixtures can increase the strength and durability of these versatile materials (Burks et al., 2010, 2011; Middleton et al., 2013; Håkansson et al., 2015; Middleton et al., 2015; Waters et al., 2017; Saburow et al., 2017; Shao et al., 2000; Wang et al., 2008; Escudie et al., 2006; Chuang et al., 2017; Chen et al., 2006). FPMC fibers can also be used in cementitious materials as concrete fillers and binders. In addition to increasing durability and strength, there is also a need for better salt resistant cement-based materials for offshore applications, such as support columns for bridges, wind turbines, and coastal barriers. Many of these structures are comprised of high-strength reinforced metal alloys, rock, and concrete materials to deflect ocean wave impacts (Burks et al., 2010, 2011;
Middleton et al., 2013; Håkansson et al., 2015; Middleton et al., 2015; Waters et al., 2017; Saburow et al., 2017; Shao et al., 2000; Wang et al., 2008; Escudie et al., 2006; Chuang et al., 2017; Chen et al., 2006). The material composition of the concrete structures is a mixture of ordinary Portland cement (OPC) with sand and stone aggregate. OPC is the primary binding paste of aggregated concrete structures that are heavily used in marine and shore environments because of their versatility, high strengths, and chemical stability under extreme conditions (Yang, 2017; Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995; Gjørv et al., 1979). Unfortunately, although cement based materials are often durable and strong, they are not impenetrable, and in marine environments they can suffer from aggressive exposure to ocean seawater mechanical impact and the corrosive effects of saltwater (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995). Corrosion and deterioration of many concrete structures can be attributed to construction quality and design parameters. Large standard concrete structures can have open pores and voids, ranging from 11 to 18% by volume (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995). When these cement structures are continuously exposed to saltwater environments, corrosion, frictional wearing, and aging will occur. In some cases, within several years or less in service, signs of extreme deterioration ranging from surface pitting, large cracks, spalling, and structural compromises will appear (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995). Although the degree and modes of damage will vary in concrete structures, their failures can be catastrophic and very costly. While never before investigated, it is possible that the addition of FPMC waste from PCCC cores could improve the properties of cementitious materials in saltwater environments. The goal of this study 2
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Fig. 3. a–f. ACCC cut rods (a), and crushed ACCC chips (b), mixture of virgin carbon and glass fiber bundles (1:1 wt %) (c), and ground PFP (d) with magnified closeups in e) and f).
was therefore to assess the potential for reclaimed recycled conductor core hybrid FPMC materials to be used in cementitious materials for coastal offshore applications. We therefore tested OPC with and without various FPMC admixtures before and after accelerated saltwater aging to determine the effects of the admixtures on the compressive strength and aging resistance of the cement.
HV polymer core composite material used in this study was manufactured by CTC Global, and is comprised of a unidirectional hybrid carbon fiber (T700) wrapped with a surrounding glass fiber (ECR glass) shell, coated with an epoxy based thermoset resin (Fig. 1b). A total of five different waste material admixtures were considered in this study: conductor core crushed rod chips, conductor core particle FPMC powder, virgin carbon fiber bundles, virgin glass fiber bundles, and virgin carbon/glass fiber bundle mixture. The virgin carbon (Toray 700) and glass fibers (E-glass) were purchased from Epoxy World and the conductor admixtures were manufactured from conductor core rods, which had not been used in service. The fine particle fiber-powder (PFP) was collected during the cutting of the conductor core rods using a wet saw before their subsequent crushing. Fig. 3a–d shows the different fiber and FPMC admixtures used in this study. The magnified views in Fig. 3e–f show small particles in the PFP admixture. Table 1 shows the various admixtures and their specifications.
2. Experimental materials and methods Improving the performance of cementitious materials is often done by additives known as admixtures, which can alter the performance of the mix for a specific application. In this study, the cement was used as the matrix, while different conductor core FPMC materials were used as admixtures. 2.1. Fibers (admixture) Admixtures were produced from conductor core rods and virgin carbon and glass fibers with varying shapes and sizes (Fig. 3a–d). The 3
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Chopped: stranded glass fiber Chopped: stranded glass and carbon fiber (1:1 ratio)
Chopped: bundled stranded carbon fiber
Table 2 Chemical composition of typical OPC (wt%) Quickrete Portland cement, type I/ II [From manufacture].
Powder: glass, carbon, resin Neat: Ordinary Portland cement FPMC type
CaO
SiO2
Fe2O3
Al2O3
Trace Amounts
66
21
3
6
< 4%
Table 3 Imitated seawater composition. Sodium Chloride
NaCl
24.53 g/L
Magnesium Chloride Sodium Sulfate Calcium Chloride Potassium Chloride Sodium Bicarbonate Potassium Bromide Boric Acid Strontium Chloride Sodium Fluoride Water
MgCl2 Na2SO4 CaCl2 KCL NaHCO3 KBr H3BO3 SrCl2 NaF H2O
5.20 g/L 4.09 g/L 1.16 g/L 0.695 g/L 0.201 g/L 0.101 g/L 0.027 g/L 0.0025 g/L 0.003 g/L 988.968 g/L Total: 1025 g/L
2.2. Cement (matrix) OPC is a finely ground powder material that is mixed with water before setting. Sand and cement are mixed to create mortar, while mixing aggregates and sand with cement results in concrete. Mixtures are specified to meet required characteristics and cement binder is typically not used alone. For example, concrete often includes 60–80% of course or fine aggregate admixtures such as sand and gravel (Aïtcin, 1998; Dvorkin et al., 2006). Curing, or hardening, of OPC comes from heat-hydration, which is a chemical reaction of the cement compounds, and is commonly conducted in several stages (Aïtcin, 1998; Dvorkin et al., 2006; Becker et al., 2019). The raw chemical compounds of cement are calcium oxide, silica, alumina, along with trace amounts of iron oxide and other common calcium derivatives of limestone, clays, and shale. Calcium oxides consist of largely tri-calcium and di-calcium silicate clinkers from gypsum and calcium oxide derivatives with trace amounts of aluminum and ferrous oxides (Aïtcin, 1998; Dvorkin et al., 2006). The compounds and ratios for OPC are listed in Table 2. After approximately 2–3 days, the hydration (curing) process is complete; however the cement will continue to cure as long as water and non-hydrated compounds are present. In some cases, the curing process can take many years before the cement reaches maximum compression strength, but it is common to test strengths after 7–56 days of curing (Aïtcin, 1998; Becker et al., 2019). Water is an important component in cement manufacturing. Too much water will reduce strength, and too little can reduce workability thus creating incomplete formation within a mold.
Crushed: HV conductor core
Virgin glass fiber Particle fiber powder Ordinary Portland cement Admixture
Table 1 FPMC admixtures.
Chips
Virgin glass/carbon fiber
Virgin carbon fiber
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2.3. Sample preparation The various FPMC admixtures were added to the cement at 6.0 wt %. The cement and admixtures were thoroughly dry mixed with a handscoop and rotary mixing paddle for homogeneity and consistency before adding water. The water ratio was kept constant at 33.33% of the combined total weight of all constituents, as cement paste mixture workability decreases with the addition of the FPMC admixtures. The cement paste without (neat) and with admixtures was poured into 50 mm3 molds and allowed to pre-cure for 24 h according to the ASTM C109/C109 M standard (Norma, 1999). Afterwards, the cubes were submerged in clean tap water for 56-days at ∼23 °C before compression tests or saltwater aging experiments were performed. A total of 36 OPC specimens were created. 18 (3 of each type of admixture) were tested 4
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Fig. 4. Compression strengths of virgin OPC with and without FPMC admixtures.
MPa). This may be due to glass fibers providing hydration and pozzolanic reactive properties, which consequently correlate to beneficial performance in cementitious materials (Gjørv et al., 1979; Norma, 1999; Chung et al., 2002; Ogi et al., 2005). The compression strength of the virgin carbon/glass fiber admixture was an average of the individual carbon fiber/cement and glass fiber/cement composites. Without further analysis is not clear to why the strength of OPC with particle fiber material was higher than for the cement with the other FPMC admixtures. However, the relative size effect, voids/air gaps and dispersion within the cement matrix likely affect the mechanical strengths as larger FPMC materials have much lower homogeneity compared to smaller particles (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995; Gjørv et al., 1979; Aïtcin, 1998). Thus, the smaller the admixture particle size, the more evenly it will distribute throughout the cement paste matrix. This may account for the lower average compression strengths seen in the OPC samples with chips, virgin glass/carbon fibers and virgin carbon fiber admixtures. Optical microscope images of the sample surfaces after the compression tests are shown in Fig. 5a–f. The fracture surfaces of the neat cement compared to the cement with admixtures are visibly different. Crack initiation and final rupture of cement is dependent on the existing voids and micro-gaps within the solid matrix (Aïtcin, 1998; Dvorkin et al., 2006; Becker et al., 2019; Norma, 1999; Chung et al., 2002; Ogi et al., 2005). However, in Fig. 5 e, the existence of larger gaps and voids within the cement matrix is limited in the admixture with PFP, which may contribute to the retained compressive strengths. There is therefore a high probability of even particle fiber material dispersion throughout the cement, which can mitigate strength loss through increasing homogenous mixing as shown in Fig. 5b. ACCC chips and virgin carbon fiber admixtures performed much worse than neat OPC and cement with PFP admixture, and this is supported visually by Fig. 5c, d, where the virgin carbon fibers and polymer resin bound chips acted as crack initiators. Additionally, the presence of fiber clumping was seen in the virgin carbon fiber admixture specimens, which contributed to the weak mechanical bonding between the cement and admixture, and consequently it performed the poorest of all the specimens tested. Rockwell hardness results for the samples from all six admixtures are shown in Fig. 6, with each data point consisting of 9 test indentations. The neat OPC sample had a hardness of 84.1, and all admixture samples except for the ACCC chips exhibited an increase in hardness. The addition of PFP admixtures increased the hardness the most to 93.5, an increase of over 11% over the neat cement, while the chips
for compression strengths and hardness as virgin samples. The remaining 18 (3 of each type) were exposed to salt-water aging and subsequently tested for compression as aged samples. 2.4. Saltwater aging Salt-aging was performed to evaluate the resistance of the various cement mixtures to accelerated sulfate chemical attack caused by seawater and marine/coastal environments (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016). After curing for 56 days in clean tap water at 23 °C, the manufactured cement specimens were placed in a 3% simulated seawater solution at 80 °C for 30 days. The seawater solution contained 24.53 g/L of sodium chloride, as well as other compounds commonly found in ocean water, as detailed in Table 3. 2.5. Compression and hardness testing Virgin and aged OPC samples with and without FPMC and fiber admixtures were subjected to compression tests using an Instron hydraulic testing system until failure. The compression tests were conducted according to the specifications listed in the ASTM Standard C109/C109 M (Norma, 1999; Chung et al., 2002; Ogi et al., 2005). After the testing, optical microscope images of the broken sample surfaces were taken. The virgin cement samples with the admixtures were also tested for their hardness using a Rockwell hardness tester with a half inch diameter carbide ball (HR15Y). The crosshead speed was 5.08 cm/ min. 3. Experimental results and discussion 3.1. Virgin test results The compressive strengths of OPC with various admixtures are shown in Fig. 4. All cement/admixture composites exhibited lower strengths than the neat Portland cement, which had an average strength of 74 MPa. A much lower average strength can be observed for the carbon fiber/OPC combination compared to the glass fiber/cement and carbon/glass/cement combinations. The strength of the PFP/cement composite exhibited the highest compression strength values of all the admixture samples. The ACCC chips and virgin carbon fiber cement mixtures performed the worst. The virgin glass fiber/cement samples had an average compression strength of 53 MPa, while strength of the virgin carbon fiber/cement composite was significantly lower (33 5
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Fig. 5. a–f. Optical microscopy images of 50X neat (a), 40X PFP (b), 10X chips (c), 10X virgin carbon/glass fiber (d), 10X virgin glass fiber (e), 10X virgin carbon fiber (f). Embedded carbon fiber bundles, glass fiber bundles, and composite core chips are denoted by red, yellow, and blue arrows, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
decreased hardness to 83.3, a decrease of 0.95%. Scatter was smallest for the particle fiber powder sample, but greatest for the chips.
compression strengths after aging. The recycled composite core chips and virgin glass/carbon fiber admixture samples also performed poorly compared to OPC with PFP. Virgin glass/carbon fiber and chips lost ∼12% and ∼24% of their compression strengths after aging, respectively. The glass/carbon/cement composite again performed near the average of the glass/cement and carbon/cement samples. The reduction in strength is possibly due to the internal ingression of salt species such as sodium chloride, which can damage the cementitious matrix (Aïtcin, 1998; Dvorkin et al., 2006). The addition of PFP admixture, however,
3.2. Salt-aged test results The compression strength results from the samples after salt aging are shown in Fig. 7. While neat cement lost approximately 30% compression strength after aging, the cement with the PFP admixture lost only ∼6% compression strength after aging (Fig. 7), leading to similar 6
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Fig. 6. Rockwell HR15Y hardness values for OPC mixtures.
Fig. 7. Compression strengths of OPC/admixture composites after saltwater aging.
may reduce or delay these deleterious effects. This is further investigated in Section 4 by using molecular dynamics simulations.
with the FPMC. 4.1. Methods
3.3. SEM image analysis
The MD models were generated in Materials Studio using built-in library structures to generate nanoscale OPC and PFP admixture particles, and results were analyzed using previously utilized methods (Bleszynski et al., 2016). Because the experimental PFP admixture contains small fragments of glass and carbon fibers and epoxy, nanoscale cylinders of glass and carbon fibers were created using Materials Studio Nanostructure Module using pre-constructed carbon and glass compounds. For simplicity the epoxy was simulated in the models as bisphenol-A (BPA) molecules, a major constituent of epoxy resins. Two nanoscale OPC models were created: standard density OPC (neat) and OPC with the PFP admixture. As previously shown in Fig. 3e, f, the PFP admixture consisted of microscopic glass, carbon, and epoxy particulates. The OPC was created using nanoscale particles of calcium oxide, silicon dioxide, ferric oxide, and aluminum oxide. The individual nanoparticles and components included in the MD models are shown in Figs. 9a–e and 10 a–c. These particles were placed in a 78 Å x 78 Å x 78 Å lattice, or box, using Amorphous Cell in a wt% equal to the typical
Following compression testing, SEM image analysis was conducted on the remaining PFP virgin and saltwater aged sample fragments (Fig. 8a–c). SEM analysis shows clear fracturing of the carbon fiber and micro-fracturing of the OPC in the virgin sample (Fig. 8a). By contrast, the saltwater aged sample (Fig. 8b, c) shows protruding carbon fibers with no significant micro-fracturing in either the fibers or the OPC. 4. Molecular dynamics simulations and discussion To better understand and assess how cement with and without FPMC and fiber admixtures would withstand ocean environments, a series of Molecular Dynamics (MD) diffusion models were created. The diffusion of water through nanoscale OPC was investigated to assess how rapidly water, or seawater, would diffuse using a Mean Squared Displacement (MSD) analysis. This was done in order to quantify the interaction between cement molecules and the compounds associated 7
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composition of OPC, Table 1. Forcite dynamics at NVT (a constant number of molecules, volume, and temperature) and Mean Squared Displacement analysis was then conducted to assess the diffusion of water molecules through the different cement mixtures. The two models, neat OPC and OPC with particle fiber powder are shown in Fig. 11. The overall MD methodology and build process is shown in Fig. 12. 4.2. Molecular dynamics diffusion analysis The diffusion analysis was conducted using Forcite analysis to generate the mean squared displacement (MSD) data according to the formula below, where N is the number of diffusive atoms in the model and D is the diffusion coefficient (Bleszynski et al., 2016; Kalinichev et al., 2007).
D=
1 d lim 6Na t dt
Na
[ri (t )
ri (0)]2
i=1
Mean squared displacement is a technique used in studying the dynamics of molecule collisions and observing the trajectory of any given molecule as it randomly wanders erratically through an available volume. Although there is no directed motion, a molecule will not remain indefinitely close to its initial position and will wander depending on factors such as particle or atom charge, material density, and temperature. This mechanism is relevant to transport processes in materials, most notably in diffusion (Bleszynski et al., 2016). 4.3. Diffusion results Fig. 13 illustrates the diffusion of water through OPC with and without PFP admixture. The MD simulations indicate that the MSD of water through neat OPC is several times higher than in OPC with the PFP admixture. The dynamic diffusion model can be used to explain the higher degradation of the compression strength of the neat cement samples in the salt-aging experiments, as water will carry salt ions through the cement network. The lower rate of water diffusion observed in the PFP/ cement model could be beneficial for cement used in offshore environments, as seawater will diffuse through the material at a decreased rate. Neat cement, which has large pores and voids, exhibits faster water diffusion. The addition of PFP admixtures to OPC decreased porosity, which reduced the rate and transport of salt ions. This could decrease the chances of chloride and sulfide attacks within the cement structure (Kalinichev et al., 2007; Chen et al., 2008). Magnesium sulfate reacts with the constituent calcium oxides in OPC forming calcium sulfate and magnesium hydroxide precipitates, which causes deleterious leaching effects and ultimately failure of the overall structure (Chen et al., 2008). The addition of PFP admixture therefore promotes durability by delaying the time of aging by chloride or sulfide attacks, giving the material a longer lifespan in seawater. 5. Concluding remarks Considering the various FPMC admixture/cement combinations considered in this study, the 6 % PFP/composite was most advantageous in terms of retaining compression strength after exposure to accelerated salt-water aging. By contrast all other admixtures types and neat OPC lost significant compression strength after aging. The addition of PFP to cement also significantly improved hardness by over 11%, compared to neat OPC. This is beneficial for applications where increased durability and wear resistance is critical. The retained properties of the OPC / PFP admixture after aging in ocean salt-water supports the notion of recycling FPMC waste into cementitious materials for off shore and other corrosive environments. As long as the recycled FPMC waste is finely ground, the new OPC
Fig. 8. a–c. SEM images of PFP admixture samples showing protruding carbon fibers; (a) virgin at 10 μm, (b) saltwater aged at 10 μm, and (c) saltwater aged at 50 μm.
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Fig. 9. a–e. Molecular Dynamics nanoparticles used in the OPC diffusion simulation from top left to bottom right: aluminum oxide (a), ferric oxide (b), water (c), silicon dioxide (d), calcium oxide (e).
Fig. 10. a–c. Primary nanoparticles used in Molecular Dynamics to represent the FPMC admixture material from left to right: bisphenol-A (epoxy) (a), carbon fiber (b), and glass fiber (c).
Fig. 11. a, b. Molecular dynamics of OPC without (a) and with (b) nanoscale particle fiber powder admixture elements.
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preliminary data analysis Maciej Kumosa – aquired funding, graduate and postdoctoral advising and supervision, experimental and numerical data analysis, major help with the rewriting and editing the final draft, help with revising the reviewed manuscript and some help with responding to reviewers 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. Acknowledgments This work was funded by the National Science Foundation I/UCRC Center for Novel High Voltage Materials and Structures under #IIP 1362135 and by the NSF Grant Opportunities for Academic Liaison with Industry program under #CMMI-123252. The authors would also like to thank Dr. Jacob John, NNF at the University of Nebraska for his assistance with this research, and Dr. Joseph Hoffman for providing
Fig. 12. Build process and methodology for MD diffusion analysis of water through nanoscale OPC with and without PFP admixture.
Fig. 13. MSD diffusion of water through neat OPC vs. OPC with PFP.
editing support
composite material could have favorable properties for use in marine or coastal environments due to its lower diffusion rate of water, as shown in the molecular dynamics simulations. Therefore, it has been determined in this study that hybrid carbon/ glass polymer ACCC waste, which has either reached its end-of-service life or can no longer be used for its original purpose, could be successfully recycled as an admixture for OPC materials, thereby improving their properties. If properly processed, recycling FPMC waste into cementitious materials could be a valuable and environmentally responsible alternative to disposal in landfills.
References Oliveux, G., et al., 2015. Current status of recycling of fibre reinforced polymers: review of technologies, reuse and resulting properties. Prog. Mater. Sci. 72, 61–99. Bhadra, J., et al., 2017. Recycling of polymer-polymer composites. In Micro and Nano Fibrillar Composites (MFCs and NFCs) From Polymer Blends. pp. 263–277. Witik, R., et al., 2013. Carbon fibre reinforced composite waste: an environmental assessment of recycling, energy recovery and landfilling. Compos. Part A Appl. Sci. Manuf. 49, 89–99. Pacheco-Torgal, F. (Ed.), 2017. Eco-Efficient Repair and Rehabilitation of Concrete Infrastructures. Woodhead Publishing. Correia, J.R., et al., 2011. Recycling of FRP composites: reusing fine GFRP waste in concrete mixtures. J. Clean. Prod. 19.15, 1745–1753. Asokan, P., et al., 2010. Improvement of the mechanical properties of glass fibre reinforced plastic waste powder filled concrete. Constr. Build. Mater. 24.4, 448–460. https://www.epa.gov/greenpower/us-electricity-grid-markets, accessed September 2019. Asokan, P., et al., 2009. Assessing the recycling potential of glass fibre reinforced plastic waste in concrete and cement composites. J. Clean. Prod. 17.9, 821–829. Mastali, M., et al., 2016a. The impact resistance and mechanical properties of reinforced self-compacting concrete with recycled glass fibre reinforced polymers. J. Clean. Prod. 124, 312–324. Abdollahnejad, Z., et al., 2017. Comparative study on the effects of recycled glass–Fiber
Credit author statement Edward Clark – performing recycling experiments, experimental data analysis, help with MD simulations, preparation of original manuscript Monika Bleszynski – MD simulations, experimental and numerical data analysis, rewriting the first draft, major assistance with correcting the manuscript and responding to reviewers Frank Valdez – significant assistance with recycling experiments, 10
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E. Clark, et al. on drying shrinkage rate and mechanical properties of the self-compacting mortar and fly ash–Slag geopolymer mortar. J. Mater. Civ. Eng. 29.8, 04017076. Mastali, M., et al., 2018. Effect of different lengths and dosages of recycled glass fibres on the fresh and hardened properties of SCC. Mag. Concr. Res. 70.22, 1175–1188. Mastali, M., et al., 2016b. The impact resistance and mechanical properties of self-compacting concrete reinforced with recycled CFRP pieces. Compos. Part B Eng. 92, 360–376. Mastali, M., et al., 2017. The impact resistance and mechanical properties of the reinforced self-compacting concrete incorporating recycled CFRP fiber with different lengths and dosages. Compos. Part B Eng. 112, 74–92. Burks, B., et al., 2009. Hybrid composite rods subjected to excessive bending loads. Compos. Sci. Technol. 69.15-16, 2625–2632. Burks, B., et al., 2010. Failure prediction analysis of an ACCC conductor subjected to thermal and mechanical stresses. Ieee Trans. Dielectr. Electr. Insul. 17.2, 588–596. Burks, B., et al., 2011. Characterization of the fatigue properties of a hybrid composite utilized in high voltage electric transmission. Compos. Part A Appl. Sci. Manuf. 42.9, 1138–1147. Middleton, J., et al., 2013. The effect of ozone and high temperature on polymer degradation in polymer core composite conductors. Polym. Degrad. Stab. 98.11, 2282–2290. Håkansson, E., et al., 2015. Galvanic corrosion of high temperature low sag aluminum conductor composite core and conventional aluminum conductor steel reinforced overhead high voltage conductors. Ieee Trans. Reliab. 64.3, 928–934. Middleton, J., et al., 2015. Aging of a polymer core composite conductor: mechanical properties and residual stresses. Compos. Part A Appl. Sci. Manuf. 69, 159–167. Waters, D., et al., 2017. Low velocity impact to transmission line conductors. Int. J. Impact Engineering, Vol. 106, 64–72. Saburow, O., et al., 2017. A direct process to reuse dry fiber production waste for recycled carbon fiber bulk molding compounds. Procedia Cirp 66, 265–270. Shao, Y., et al., 2000. Studies on concrete containing ground waste glass. Cem. Concr. Res. 30.1, 91–100. Wang, C., et al., 2008. Effect of carbon fiber dispersion on the mechanical properties of carbon fiber-reinforced cement-based composites. Mater. Sci. Eng. A 487.1-2, 52–57. Escudie, R., et al., 2006. Effect of particle shape on liquid-fluidized beds of binary (and ternary) solids mixtures: segregation vs. Mixing. Chem. Eng. Sci. 61.5, 1528–1539. Chuang, W., et al., 2017. Dispersion of carbon fibers and conductivity of carbon fiberreinforced cement-based composites. Ceram. Int. 43.17, 15122–15132.
Chen, C., et al., 2006. Waste E-glass particles used in cementitious mixtures. Cem. Concr. Res. 36.3, 449–456. Yang, X., 2017. Study on slamming pressure calculation formula of plunging breaking wave on sloping sea dike. Int. J. Nav. Archit. Ocean. Eng. 9.4, 439–445. Costa, A., et al., 2002. Case studies of concrete deterioration in a marine environment in Portugal. Cem. Concr. Compos. 24.1, 169–179. Yamini, O.A., et al., 2019. Experimental investigation of using geo-textile filter layer in articulated concrete block mattress revetment on coastal embankment. J. Ocean. Eng. Mar. Energy 1–15. Luznik, L., et al., 2013. The effect of surface waves on the performance characteristics of a model tidal turbine. Renew. Energy 58, 108–114. Narayan, S., et al., 2016. The effectiveness, costs and coastal protection benefits of natural and nature-based defences. PLoS One 11.5, e0154735. Halamickova, P., et al., 1995. Permeability and chloride ion diffusion in Portland cement mortars: relationship to sand content and critical pore diameter. Cem. Concr. Res. 25.4, 790–802. Gjørv, O.E., et al., 1979. Diffusion of chloride ions from seawater into concrete. Cem. Concr. Res. 9.2, 229–238. Aïtcin, P.C., 1998. High Performance Concrete. CRC press. Dvorkin, L., et al., 2006. Basics of Concrete Science. St-Petersburg: Stroy-Beton. Becker, J., et al., 2019. Concrete pumping effects on air-entrained voids in concrete mixtures No. SPTC 14 1-38-F. Norma, A.S.T.M., 1999. C109/C109M, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50-mm] Cube Specimens), West Conshohocken, Pa. EE. UU: American Society for Testing and Materials. Chung, H.S., et al., 2002. Strength and ductility of laterally confined concrete columns. Can. J. Civ. Eng. 29.6, 820–830. Ogi, K., et al., 2005. Strength in concrete reinforced with recycled CFRP pieces. Compos. Part A Appl. Sci. Manuf. 36.7, 893–902. Bleszynski, M., et al., 2016. Silicone rubber RTV-1 aging in the presence of aqueous salt. Ieee Trans. Dielectr. Electr. Insul. 23.5, 2822–2829. Kalinichev, A.G., et al., 2007. Molecular dynamics modeling of the structure, dynamics and energetics of mineral–water interfaces: application to cement materials. Cem. Concr. Res. 37 (3), 337–347. Chen, J.K., et al., 2008. Damage evolution in cement mortar due to erosion of sulphate. Corros. Sci. 50.9, 2478–2483.
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Resources, Conservation & Recycling journal homepage: www.elsevier.com/locate/resconrec
Full length article
Recycling carbon and glass fiber polymer matrix composite waste into cementitious materials
T
Edward Clarka, Monika Bleszynskia, Frank Valdezb, Maciej Kumosaa,* a b
Industry/University Cooperative Research Center for Novel High Voltage/Temperature Materials and Structures, University of Denver, United States United States Bureau of Reclamation, Lakewood, CO, United States
ARTICLE INFO
ABSTRACT
Keywords: Fiber polymer matrix composite waste Recycling Portland cement Salt aging Molecular dynamics
Recycling options for fiber polymer matrix composites (FPMCs) are limited since they typically cannot be re-used or re-processed. One source of FPMCs is the utility industry, which is increasingly using hybrid carbon and glass FPMC core high voltage (HV) conductors for energy transmission. As there are currently no recycling methods for these composite cores, we investigated if hybrid carbon and glass FPMC waste could be recycled as an admixture for cementitious materials in order to improve their properties for seawater applications. We assessed the compression strength of ordinary Portland cement (OPC) with 6% wt of various FPMC admixtures before and after accelerated salt-water aging. The experimental part of this research was strongly supported by molecular dynamics simulations (MD) to examine the effect of FPMC admixtures on moisture diffusion in OPC. It was established that recycled finely ground FPMC admixtures can provide a benefit to cementitious materials by decreasing void content and slowing the diffusivity of corrosive compounds such as salt water.
1. Introduction The use of fiber polymer matrix composite (FPMC) materials is steadily increasing across the globe, with large amounts FPMC waste being generated every year. Within the next two decades the projected demand for FPMC materials is expected to grow between 10–15 (USD Billion) for new and evolving industrial applications, including aircraft, automobiles, consumer goods, advanced medical devices and others (Oliveux et al., 2015; Bhadra et al., 2017; Witik et al., 2013; PachecoTorgal et al., 2017; Correia et al., 2011). As a result, advances in research and development of novel composite materials has led to the production of many different types of FPMCs, specifically those containing glass and carbon fibers (Bhadra et al., 2017; Pacheco-Torgal et al., 2017; Correia et al., 2011; Asokan et al., 2010). Unfortunately, the continuous generation of composite preproduction waste and postwaste (end-of-service-life) has resulted in enormous amounts of waste material being sent to landfills for disposal (Oliveux et al., 2015; Bhadra et al., 2017). The mass worldwide production of polymeric composite materials including thermoset plastics, thermoplastics, adhesives, resins, and specialized coatings climbed to over 320 million metric tons in 2015, with continuous growth of ∼10% per year (Oliveux et al., 2015; Bhadra et al., 2017). Thus, the large demand for FPMC parts, components, and materials has resulted in large quantities of waste materials, many of which are non-decomposable (Oliveux et al., 2015; Bhadra ⁎
et al., 2017; Witik et al., 2013). One source of FPMC waste comes from utilities and power generation and distribution companies, which are experiencing increasing demand (Asokan et al., 2010). Some of these utilities use Polymer Core Composite Conductors (PCCC), such as, for example, the hybrid carbon and glass fiber Aluminum Conductor Composite Core (ACCC) for their overhead high voltage (HV) transmission lines (Fig. 1a). These conductors have superior electrical energy transmission properties and are gradually replacing traditional steel core conductors (Asokan et al., 2009; Mastali et al., 2016a; Abdollahnejad et al., 2017; Mastali et al., 2018, 2016b; Mastali et al., 2017; Burks et al., 2009). In certain situations, however, if not properly utilized, PCCCs can be damaged either during installation or in service by excessive bending, impact, aging at extreme temperatures, corrosion, excessive Aeolian vibrations, galloping, excessive snow and ice loading, etc. (Anon, 2019; Mastali et al., 2016a; Abdollahnejad et al., 2017; Mastali et al., 2018, 2016b; Mastali et al., 2017; Burks et al., 2009). If prematurely damaged, PCCCs will have to be replaced before their expected life creating a large amount of composite waste. While the aluminum helical strands in these conductors can easily be recycled, the recycling of the PCCC cores remains a major challenge (Fig. 1b). The separation of glass and carbon fibers from their thermoset resin in the PCCC cores is not straightforward. Therefore, large amounts of preproduction composite material waste, damaged cores, and end-of-service-life material could be
Corresponding author. E-mail address: [email protected] (M. Kumosa).
https://doi.org/10.1016/j.resconrec.2019.104659 Received 10 October 2019; Received in revised form 18 December 2019; Accepted 20 December 2019 0921-3449/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. a, b. Standard and ACCC conductors (left), and ACCC conductor core rods (right).
Fig. 2. Estimated mass ton of ACCC material as a function of length (km).
generated by the industry in the future. This is especially important, as the United States currently has over 250,000 km of HV transmission lines (Asokan et al., 2010). Thus, when the HV PCCCs reach end-ofservice-life, a large amount of waste could be produced (Fig. 2). Reclaimed fiber materials (rFM) from products such as HV PCCC cores are estimated to produce over 8 million kg of carbon and glass fiber material, and these amounts are expected to increase every year (Oliveux et al., 2015; Bhadra et al., 2017; Witik et al., 2013; Pacheco-Torgal et al., 2017; Correia et al., 2011). Currently, recycling hybrid fiber composites is difficult because the current methods used to separate the fibers and resins are specific to only single fiber types. Most carbon and glass composites are not readily recycled because of the harsh methods required to extract the fibers from the resins, which can severely degrade the fibers’ mechanical properties. While the most common waste management solution is disposal into landfills, there are various methods to recycle FPMC waste which have been successful, including either physical or chemical recycling methods (Witik et al., 2013). Some studies have reported that carbon and glass fibers separately added to cementitious concrete structures as admixtures can increase the strength and durability of these versatile materials (Burks et al., 2010, 2011; Middleton et al., 2013; Håkansson et al., 2015; Middleton et al., 2015; Waters et al., 2017; Saburow et al., 2017; Shao et al., 2000; Wang et al., 2008; Escudie et al., 2006; Chuang et al., 2017; Chen et al., 2006). FPMC fibers can also be used in cementitious materials as concrete fillers and binders. In addition to increasing durability and strength, there is also a need for better salt resistant cement-based materials for offshore applications, such as support columns for bridges, wind turbines, and coastal barriers. Many of these structures are comprised of high-strength reinforced metal alloys, rock, and concrete materials to deflect ocean wave impacts (Burks et al., 2010, 2011;
Middleton et al., 2013; Håkansson et al., 2015; Middleton et al., 2015; Waters et al., 2017; Saburow et al., 2017; Shao et al., 2000; Wang et al., 2008; Escudie et al., 2006; Chuang et al., 2017; Chen et al., 2006). The material composition of the concrete structures is a mixture of ordinary Portland cement (OPC) with sand and stone aggregate. OPC is the primary binding paste of aggregated concrete structures that are heavily used in marine and shore environments because of their versatility, high strengths, and chemical stability under extreme conditions (Yang, 2017; Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995; Gjørv et al., 1979). Unfortunately, although cement based materials are often durable and strong, they are not impenetrable, and in marine environments they can suffer from aggressive exposure to ocean seawater mechanical impact and the corrosive effects of saltwater (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995). Corrosion and deterioration of many concrete structures can be attributed to construction quality and design parameters. Large standard concrete structures can have open pores and voids, ranging from 11 to 18% by volume (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995). When these cement structures are continuously exposed to saltwater environments, corrosion, frictional wearing, and aging will occur. In some cases, within several years or less in service, signs of extreme deterioration ranging from surface pitting, large cracks, spalling, and structural compromises will appear (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995). Although the degree and modes of damage will vary in concrete structures, their failures can be catastrophic and very costly. While never before investigated, it is possible that the addition of FPMC waste from PCCC cores could improve the properties of cementitious materials in saltwater environments. The goal of this study 2
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Fig. 3. a–f. ACCC cut rods (a), and crushed ACCC chips (b), mixture of virgin carbon and glass fiber bundles (1:1 wt %) (c), and ground PFP (d) with magnified closeups in e) and f).
was therefore to assess the potential for reclaimed recycled conductor core hybrid FPMC materials to be used in cementitious materials for coastal offshore applications. We therefore tested OPC with and without various FPMC admixtures before and after accelerated saltwater aging to determine the effects of the admixtures on the compressive strength and aging resistance of the cement.
HV polymer core composite material used in this study was manufactured by CTC Global, and is comprised of a unidirectional hybrid carbon fiber (T700) wrapped with a surrounding glass fiber (ECR glass) shell, coated with an epoxy based thermoset resin (Fig. 1b). A total of five different waste material admixtures were considered in this study: conductor core crushed rod chips, conductor core particle FPMC powder, virgin carbon fiber bundles, virgin glass fiber bundles, and virgin carbon/glass fiber bundle mixture. The virgin carbon (Toray 700) and glass fibers (E-glass) were purchased from Epoxy World and the conductor admixtures were manufactured from conductor core rods, which had not been used in service. The fine particle fiber-powder (PFP) was collected during the cutting of the conductor core rods using a wet saw before their subsequent crushing. Fig. 3a–d shows the different fiber and FPMC admixtures used in this study. The magnified views in Fig. 3e–f show small particles in the PFP admixture. Table 1 shows the various admixtures and their specifications.
2. Experimental materials and methods Improving the performance of cementitious materials is often done by additives known as admixtures, which can alter the performance of the mix for a specific application. In this study, the cement was used as the matrix, while different conductor core FPMC materials were used as admixtures. 2.1. Fibers (admixture) Admixtures were produced from conductor core rods and virgin carbon and glass fibers with varying shapes and sizes (Fig. 3a–d). The 3
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Chopped: stranded glass fiber Chopped: stranded glass and carbon fiber (1:1 ratio)
Chopped: bundled stranded carbon fiber
Table 2 Chemical composition of typical OPC (wt%) Quickrete Portland cement, type I/ II [From manufacture].
Powder: glass, carbon, resin Neat: Ordinary Portland cement FPMC type
CaO
SiO2
Fe2O3
Al2O3
Trace Amounts
66
21
3
6
< 4%
Table 3 Imitated seawater composition. Sodium Chloride
NaCl
24.53 g/L
Magnesium Chloride Sodium Sulfate Calcium Chloride Potassium Chloride Sodium Bicarbonate Potassium Bromide Boric Acid Strontium Chloride Sodium Fluoride Water
MgCl2 Na2SO4 CaCl2 KCL NaHCO3 KBr H3BO3 SrCl2 NaF H2O
5.20 g/L 4.09 g/L 1.16 g/L 0.695 g/L 0.201 g/L 0.101 g/L 0.027 g/L 0.0025 g/L 0.003 g/L 988.968 g/L Total: 1025 g/L
2.2. Cement (matrix) OPC is a finely ground powder material that is mixed with water before setting. Sand and cement are mixed to create mortar, while mixing aggregates and sand with cement results in concrete. Mixtures are specified to meet required characteristics and cement binder is typically not used alone. For example, concrete often includes 60–80% of course or fine aggregate admixtures such as sand and gravel (Aïtcin, 1998; Dvorkin et al., 2006). Curing, or hardening, of OPC comes from heat-hydration, which is a chemical reaction of the cement compounds, and is commonly conducted in several stages (Aïtcin, 1998; Dvorkin et al., 2006; Becker et al., 2019). The raw chemical compounds of cement are calcium oxide, silica, alumina, along with trace amounts of iron oxide and other common calcium derivatives of limestone, clays, and shale. Calcium oxides consist of largely tri-calcium and di-calcium silicate clinkers from gypsum and calcium oxide derivatives with trace amounts of aluminum and ferrous oxides (Aïtcin, 1998; Dvorkin et al., 2006). The compounds and ratios for OPC are listed in Table 2. After approximately 2–3 days, the hydration (curing) process is complete; however the cement will continue to cure as long as water and non-hydrated compounds are present. In some cases, the curing process can take many years before the cement reaches maximum compression strength, but it is common to test strengths after 7–56 days of curing (Aïtcin, 1998; Becker et al., 2019). Water is an important component in cement manufacturing. Too much water will reduce strength, and too little can reduce workability thus creating incomplete formation within a mold.
Crushed: HV conductor core
Virgin glass fiber Particle fiber powder Ordinary Portland cement Admixture
Table 1 FPMC admixtures.
Chips
Virgin glass/carbon fiber
Virgin carbon fiber
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2.3. Sample preparation The various FPMC admixtures were added to the cement at 6.0 wt %. The cement and admixtures were thoroughly dry mixed with a handscoop and rotary mixing paddle for homogeneity and consistency before adding water. The water ratio was kept constant at 33.33% of the combined total weight of all constituents, as cement paste mixture workability decreases with the addition of the FPMC admixtures. The cement paste without (neat) and with admixtures was poured into 50 mm3 molds and allowed to pre-cure for 24 h according to the ASTM C109/C109 M standard (Norma, 1999). Afterwards, the cubes were submerged in clean tap water for 56-days at ∼23 °C before compression tests or saltwater aging experiments were performed. A total of 36 OPC specimens were created. 18 (3 of each type of admixture) were tested 4
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Fig. 4. Compression strengths of virgin OPC with and without FPMC admixtures.
MPa). This may be due to glass fibers providing hydration and pozzolanic reactive properties, which consequently correlate to beneficial performance in cementitious materials (Gjørv et al., 1979; Norma, 1999; Chung et al., 2002; Ogi et al., 2005). The compression strength of the virgin carbon/glass fiber admixture was an average of the individual carbon fiber/cement and glass fiber/cement composites. Without further analysis is not clear to why the strength of OPC with particle fiber material was higher than for the cement with the other FPMC admixtures. However, the relative size effect, voids/air gaps and dispersion within the cement matrix likely affect the mechanical strengths as larger FPMC materials have much lower homogeneity compared to smaller particles (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016; Halamickova et al., 1995; Gjørv et al., 1979; Aïtcin, 1998). Thus, the smaller the admixture particle size, the more evenly it will distribute throughout the cement paste matrix. This may account for the lower average compression strengths seen in the OPC samples with chips, virgin glass/carbon fibers and virgin carbon fiber admixtures. Optical microscope images of the sample surfaces after the compression tests are shown in Fig. 5a–f. The fracture surfaces of the neat cement compared to the cement with admixtures are visibly different. Crack initiation and final rupture of cement is dependent on the existing voids and micro-gaps within the solid matrix (Aïtcin, 1998; Dvorkin et al., 2006; Becker et al., 2019; Norma, 1999; Chung et al., 2002; Ogi et al., 2005). However, in Fig. 5 e, the existence of larger gaps and voids within the cement matrix is limited in the admixture with PFP, which may contribute to the retained compressive strengths. There is therefore a high probability of even particle fiber material dispersion throughout the cement, which can mitigate strength loss through increasing homogenous mixing as shown in Fig. 5b. ACCC chips and virgin carbon fiber admixtures performed much worse than neat OPC and cement with PFP admixture, and this is supported visually by Fig. 5c, d, where the virgin carbon fibers and polymer resin bound chips acted as crack initiators. Additionally, the presence of fiber clumping was seen in the virgin carbon fiber admixture specimens, which contributed to the weak mechanical bonding between the cement and admixture, and consequently it performed the poorest of all the specimens tested. Rockwell hardness results for the samples from all six admixtures are shown in Fig. 6, with each data point consisting of 9 test indentations. The neat OPC sample had a hardness of 84.1, and all admixture samples except for the ACCC chips exhibited an increase in hardness. The addition of PFP admixtures increased the hardness the most to 93.5, an increase of over 11% over the neat cement, while the chips
for compression strengths and hardness as virgin samples. The remaining 18 (3 of each type) were exposed to salt-water aging and subsequently tested for compression as aged samples. 2.4. Saltwater aging Salt-aging was performed to evaluate the resistance of the various cement mixtures to accelerated sulfate chemical attack caused by seawater and marine/coastal environments (Costa et al., 2002; Yamini et al., 2019; Luznik et al., 2013; Narayan et al., 2016). After curing for 56 days in clean tap water at 23 °C, the manufactured cement specimens were placed in a 3% simulated seawater solution at 80 °C for 30 days. The seawater solution contained 24.53 g/L of sodium chloride, as well as other compounds commonly found in ocean water, as detailed in Table 3. 2.5. Compression and hardness testing Virgin and aged OPC samples with and without FPMC and fiber admixtures were subjected to compression tests using an Instron hydraulic testing system until failure. The compression tests were conducted according to the specifications listed in the ASTM Standard C109/C109 M (Norma, 1999; Chung et al., 2002; Ogi et al., 2005). After the testing, optical microscope images of the broken sample surfaces were taken. The virgin cement samples with the admixtures were also tested for their hardness using a Rockwell hardness tester with a half inch diameter carbide ball (HR15Y). The crosshead speed was 5.08 cm/ min. 3. Experimental results and discussion 3.1. Virgin test results The compressive strengths of OPC with various admixtures are shown in Fig. 4. All cement/admixture composites exhibited lower strengths than the neat Portland cement, which had an average strength of 74 MPa. A much lower average strength can be observed for the carbon fiber/OPC combination compared to the glass fiber/cement and carbon/glass/cement combinations. The strength of the PFP/cement composite exhibited the highest compression strength values of all the admixture samples. The ACCC chips and virgin carbon fiber cement mixtures performed the worst. The virgin glass fiber/cement samples had an average compression strength of 53 MPa, while strength of the virgin carbon fiber/cement composite was significantly lower (33 5
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Fig. 5. a–f. Optical microscopy images of 50X neat (a), 40X PFP (b), 10X chips (c), 10X virgin carbon/glass fiber (d), 10X virgin glass fiber (e), 10X virgin carbon fiber (f). Embedded carbon fiber bundles, glass fiber bundles, and composite core chips are denoted by red, yellow, and blue arrows, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
decreased hardness to 83.3, a decrease of 0.95%. Scatter was smallest for the particle fiber powder sample, but greatest for the chips.
compression strengths after aging. The recycled composite core chips and virgin glass/carbon fiber admixture samples also performed poorly compared to OPC with PFP. Virgin glass/carbon fiber and chips lost ∼12% and ∼24% of their compression strengths after aging, respectively. The glass/carbon/cement composite again performed near the average of the glass/cement and carbon/cement samples. The reduction in strength is possibly due to the internal ingression of salt species such as sodium chloride, which can damage the cementitious matrix (Aïtcin, 1998; Dvorkin et al., 2006). The addition of PFP admixture, however,
3.2. Salt-aged test results The compression strength results from the samples after salt aging are shown in Fig. 7. While neat cement lost approximately 30% compression strength after aging, the cement with the PFP admixture lost only ∼6% compression strength after aging (Fig. 7), leading to similar 6
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Fig. 6. Rockwell HR15Y hardness values for OPC mixtures.
Fig. 7. Compression strengths of OPC/admixture composites after saltwater aging.
may reduce or delay these deleterious effects. This is further investigated in Section 4 by using molecular dynamics simulations.
with the FPMC. 4.1. Methods
3.3. SEM image analysis
The MD models were generated in Materials Studio using built-in library structures to generate nanoscale OPC and PFP admixture particles, and results were analyzed using previously utilized methods (Bleszynski et al., 2016). Because the experimental PFP admixture contains small fragments of glass and carbon fibers and epoxy, nanoscale cylinders of glass and carbon fibers were created using Materials Studio Nanostructure Module using pre-constructed carbon and glass compounds. For simplicity the epoxy was simulated in the models as bisphenol-A (BPA) molecules, a major constituent of epoxy resins. Two nanoscale OPC models were created: standard density OPC (neat) and OPC with the PFP admixture. As previously shown in Fig. 3e, f, the PFP admixture consisted of microscopic glass, carbon, and epoxy particulates. The OPC was created using nanoscale particles of calcium oxide, silicon dioxide, ferric oxide, and aluminum oxide. The individual nanoparticles and components included in the MD models are shown in Figs. 9a–e and 10 a–c. These particles were placed in a 78 Å x 78 Å x 78 Å lattice, or box, using Amorphous Cell in a wt% equal to the typical
Following compression testing, SEM image analysis was conducted on the remaining PFP virgin and saltwater aged sample fragments (Fig. 8a–c). SEM analysis shows clear fracturing of the carbon fiber and micro-fracturing of the OPC in the virgin sample (Fig. 8a). By contrast, the saltwater aged sample (Fig. 8b, c) shows protruding carbon fibers with no significant micro-fracturing in either the fibers or the OPC. 4. Molecular dynamics simulations and discussion To better understand and assess how cement with and without FPMC and fiber admixtures would withstand ocean environments, a series of Molecular Dynamics (MD) diffusion models were created. The diffusion of water through nanoscale OPC was investigated to assess how rapidly water, or seawater, would diffuse using a Mean Squared Displacement (MSD) analysis. This was done in order to quantify the interaction between cement molecules and the compounds associated 7
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composition of OPC, Table 1. Forcite dynamics at NVT (a constant number of molecules, volume, and temperature) and Mean Squared Displacement analysis was then conducted to assess the diffusion of water molecules through the different cement mixtures. The two models, neat OPC and OPC with particle fiber powder are shown in Fig. 11. The overall MD methodology and build process is shown in Fig. 12. 4.2. Molecular dynamics diffusion analysis The diffusion analysis was conducted using Forcite analysis to generate the mean squared displacement (MSD) data according to the formula below, where N is the number of diffusive atoms in the model and D is the diffusion coefficient (Bleszynski et al., 2016; Kalinichev et al., 2007).
D=
1 d lim 6Na t dt
Na
[ri (t )
ri (0)]2
i=1
Mean squared displacement is a technique used in studying the dynamics of molecule collisions and observing the trajectory of any given molecule as it randomly wanders erratically through an available volume. Although there is no directed motion, a molecule will not remain indefinitely close to its initial position and will wander depending on factors such as particle or atom charge, material density, and temperature. This mechanism is relevant to transport processes in materials, most notably in diffusion (Bleszynski et al., 2016). 4.3. Diffusion results Fig. 13 illustrates the diffusion of water through OPC with and without PFP admixture. The MD simulations indicate that the MSD of water through neat OPC is several times higher than in OPC with the PFP admixture. The dynamic diffusion model can be used to explain the higher degradation of the compression strength of the neat cement samples in the salt-aging experiments, as water will carry salt ions through the cement network. The lower rate of water diffusion observed in the PFP/ cement model could be beneficial for cement used in offshore environments, as seawater will diffuse through the material at a decreased rate. Neat cement, which has large pores and voids, exhibits faster water diffusion. The addition of PFP admixtures to OPC decreased porosity, which reduced the rate and transport of salt ions. This could decrease the chances of chloride and sulfide attacks within the cement structure (Kalinichev et al., 2007; Chen et al., 2008). Magnesium sulfate reacts with the constituent calcium oxides in OPC forming calcium sulfate and magnesium hydroxide precipitates, which causes deleterious leaching effects and ultimately failure of the overall structure (Chen et al., 2008). The addition of PFP admixture therefore promotes durability by delaying the time of aging by chloride or sulfide attacks, giving the material a longer lifespan in seawater. 5. Concluding remarks Considering the various FPMC admixture/cement combinations considered in this study, the 6 % PFP/composite was most advantageous in terms of retaining compression strength after exposure to accelerated salt-water aging. By contrast all other admixtures types and neat OPC lost significant compression strength after aging. The addition of PFP to cement also significantly improved hardness by over 11%, compared to neat OPC. This is beneficial for applications where increased durability and wear resistance is critical. The retained properties of the OPC / PFP admixture after aging in ocean salt-water supports the notion of recycling FPMC waste into cementitious materials for off shore and other corrosive environments. As long as the recycled FPMC waste is finely ground, the new OPC
Fig. 8. a–c. SEM images of PFP admixture samples showing protruding carbon fibers; (a) virgin at 10 μm, (b) saltwater aged at 10 μm, and (c) saltwater aged at 50 μm.
8
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Fig. 9. a–e. Molecular Dynamics nanoparticles used in the OPC diffusion simulation from top left to bottom right: aluminum oxide (a), ferric oxide (b), water (c), silicon dioxide (d), calcium oxide (e).
Fig. 10. a–c. Primary nanoparticles used in Molecular Dynamics to represent the FPMC admixture material from left to right: bisphenol-A (epoxy) (a), carbon fiber (b), and glass fiber (c).
Fig. 11. a, b. Molecular dynamics of OPC without (a) and with (b) nanoscale particle fiber powder admixture elements.
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preliminary data analysis Maciej Kumosa – aquired funding, graduate and postdoctoral advising and supervision, experimental and numerical data analysis, major help with the rewriting and editing the final draft, help with revising the reviewed manuscript and some help with responding to reviewers 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. Acknowledgments This work was funded by the National Science Foundation I/UCRC Center for Novel High Voltage Materials and Structures under #IIP 1362135 and by the NSF Grant Opportunities for Academic Liaison with Industry program under #CMMI-123252. The authors would also like to thank Dr. Jacob John, NNF at the University of Nebraska for his assistance with this research, and Dr. Joseph Hoffman for providing
Fig. 12. Build process and methodology for MD diffusion analysis of water through nanoscale OPC with and without PFP admixture.
Fig. 13. MSD diffusion of water through neat OPC vs. OPC with PFP.
editing support
composite material could have favorable properties for use in marine or coastal environments due to its lower diffusion rate of water, as shown in the molecular dynamics simulations. Therefore, it has been determined in this study that hybrid carbon/ glass polymer ACCC waste, which has either reached its end-of-service life or can no longer be used for its original purpose, could be successfully recycled as an admixture for OPC materials, thereby improving their properties. If properly processed, recycling FPMC waste into cementitious materials could be a valuable and environmentally responsible alternative to disposal in landfills.
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