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Article

Use of E-Waste in Metakaolin Blended Cement Concrete for Sustainable Construction

by
Thirumalini Selvaraj
1,
Shanmugapriya T
1,*,
Senthil Kumar Kaliyavaradhan
2,
Kunal Kakria
1 and
Ravi Chandra Malladi
1
1
School of Civil Engineering, Vellore Institute of Technology, Vellore 632014, India
2
CSIR-Structural Engineering Research Centre, Taramani, Chennai 600113, India
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(24), 16661; https://doi.org/10.3390/su142416661
Submission received: 18 October 2022 / Revised: 29 November 2022 / Accepted: 29 November 2022 / Published: 13 December 2022
(This article belongs to the Section Waste and Recycling)
Browse Figures
Figure 1
<p>Collection and segregation of e-waste to metallic and non-metallic components.</p> "> Figure 2
<p>Particle size distribution of fine aggregates NS and NMP.</p> "> Figure 3
<p>XRD Mineralogical composition of metakaolin.</p> "> Figure 4
<p>FE-SEM images of metakaolin and non-metallic portion.</p> "> Figure 5
<p>Compressive strength of all the specimen at 7 and 28 days.</p> "> Figure 6
<p>Split tensile strength of different mix specimen at 28 days.</p> "> Figure 7
<p>Flexural strength of all mix specimen at 28 days.</p> "> Figure 8
<p>Water absorption of engineered mix specimen at 7 and 28 days.</p> "> Figure 9
<p>Sorptivity of all specimens at 28 days.</p> "> Figure 10
<p>Rapid chloride penetration for all the mixes at 28 days.</p> "> Figure 11
<p>Mineralogical identification of samples REF, M10N0, M10N10 using XRD.</p> "> Figure 12
<p>FE-SEM images of the samples (<b>a</b>) REF (<b>b</b>) M0N10 (<b>c</b>) M10N0 (<b>d</b>) M10N10.</p> "> Figure 13
<p>(<b>a</b>,<b>c</b>,<b>e</b>,<b>g</b>) FE-SEM Images (<b>b</b>,<b>d</b>,<b>f</b>,<b>h</b>) EDS Spectrums for REF, M0N10, M10N0 and M10N10.</p> ">
Review Reports Versions Notes

Abstract

:
This paper investigates the use of non-metallic portion (NMP) reclaimed from e-waste (i.e., waste printed circuit board—PCB) as replacement of natural sand in the blended cement concrete by using Metakaolin (MK) as supplementary cementitious material for its effect on the mechanical, durability, microstructural, and mineralogical properties of concrete. It was found that the blended mixes containing NMP and MK outperformed the control mix. With the addition of 10% NMP and 10% MK, the maximum compressive strength was obtained, with the splitting tensile and flexural strength following the same trend. The performance of the mixes was lowered above 10% replacement levels, although it was still better than the control mixture. When compared to other mixes, 10% NMP and 10% MK concrete had the lowest sorptivity and water absorption values, as well as the highest resistance to chloride-ion penetration. FESEM was used to confirm the results, and then XRD was used to determine the elemental classification. This study lays the groundwork for a long-term strategy for utilising NMP and MK as extremely effective concrete additives.

1. Introduction

Concrete is mainly composed of cement, with fine and coarse aggregates as the main ingredients. Cement is the world’s most prevalently and excessively used binder, contributing to a 65% share in global warming due to its production. Cement production releases tonnes of hazardous and toxic gases in the air with a 5–7% share in CO2 emissions [1]. Considering the ill effects of cement uses and concerns relating to environmental and sustainability demands for alternative pozzolanic materials, there have been questions on the safety and durability of the new engineered materials [2]. Supplementary cementitious materials (SCM) like fly ash, blast furnace slag, micro silica and metakaolin, etc., were widely adopted in concrete to replace the cement. River sand is best preferred for concrete in construction. Because of the rapid expansion of the building sector, sand utilization has escalated, resulting in a shortage of river sand. In addition, river sand extraction disrupts the ecological equilibrium [3].
The disposal of industrial waste has become a global challenge as a result of urbanisation and technological advancements. Consequently, the identification of sand substitute has had a significant impact on the construction industry. Different materials like M-sand, washed bottom ash, quarry dust, sheet glass and construction demolition waste, etc., are used as partial replacement to natural sand [3,4,5]. On international e-waste day, the waste electrical and electronic equipment forum (WEEE) predicted that 57.4 M tonnes of e-waste will be generated in 2021, a 21% increase from 2014, with 70 M tonnes projected by 2030 [6]. As stated by Sejal Mehta in 2019, 3.2 M tonnes of e-waste were produced from India, trailing only China and the United States [7]. Outdated devices and new technology necessitate the purchase of new, upgraded devices, resulting in a considerable quantity of waste printed circuit board (PCB) [8]. An appreciable percentage of e-waste is found to end up in landfills or incinerators, which in turn emits harmful fumes [9]. PCB account for 3% of all electrical and electronic waste, and their high amount of dangerous heavy metals and brominated flame retardants has drawn the attention of researchers [10,11]. Precious metals, epoxy resin, glass fibres, ceramics, and polymers are all used in PCB [12].
Due to the large volumes, feasibility of separation, and high economic worth of PCB, they are first processed for metal recovery. Several separation methods are used to purify and segregate materials based on their physical properties: gravity separation [13], electronic separation [14], triboelectric separation [15], and reverse flotation [16]. Non-metallic portions from PCB are often considered as waste and neglected; however, there have been studies to reuse these non-metals sustainably. Non-metals have been tried to mould electronic components [17], modify asphalts [18] and produce phenolic compounds [19]. Studies by Senthil Kumar and Baskar and Li et al. explored the potential to use non-metals as fillers in paints, adhesives, polyester composites and building materials [20,21]. This non-metallic component as a possible alternative for fine aggregate in blended cement concrete has not been the subject of adequate investigation.
Metakaolin (MK) is made by calcination of kaolin clay at 600–800 °C to remove the water present between the crystalline layers, which are then ground to the desired fineness level [22,23]. An optimum amount of 10–20% metakaolin replacement to binder in concrete enhanced the properties like cement hydration and pozzolanic activity [24,25]. The addition of MK had an effect on fresh and hardened properties as well as in increasing durability aspects like chloride induced corrosion and carbonation depth reduction [26]. Moreover, gas permeability and water absorption are also decreased with the addition of MK [27]. MK addition has increased the packing density of concrete, thus enhancing the strength and resistance to aggressive agents of the engineered mortar [28]. The chloride ion penetration was a lowered total charge, passing by 87% in mixes with MK as compared to conventional mixes [29]. The self-compacting concrete with MK replacement from 5–15% by volume to binder was evaluated for its mechanical and durability properties. A mix with 10% MK was reported to provide good fresh and hardened properties while reducing the energy consumption and CO2 emission [30]. The resistance of conventional concrete doped with MK towards sulphate attack was studied by Al-Akhras, who found an indirect relationship between MK and sulphate attack, also concluding the optimum performance at 10 to 15% addition of MK in concrete [31].
Based on previous studies, NMP was widely used as an additive in preparing different composites to evaluate its effects on mechanical properties, whilst little research was carried out in construction for its large-scale applications. A research gap exists for using NMP as a fine aggregate replacement in concrete, with MK as a cement substitute for evaluating durability and analytical studies. This study aims to find a sustainable and eco-friendly way to recycle NMP on a large scale by using it in concrete, reducing the impact on the environment. Hence in this study, MK and NMP as cement and fine aggregate replacements at different levels were considered, then optimum composition was identified using the mechanical performance of concrete. The durability studies like sorptivity, water absorption, RCPT, and analytical techniques like XRD, FESEM were performed on the optimum mix. This study offers an innovative solution by laying a roadmap for using NMP and MK in concrete and possible use in construction activities.

2. Materials and Methods

2.1. Recovery of Non-Metallic Powder from e-Waste

E-waste generated from printed circuit boards (PCB) is segregated in a two-step process outlined as waste collection followed by segregation of metallic and non-metallic components as shown in Figure 1. Collection comprises of selecting various sources of e-waste ranging from TV sets, mobiles and similar electronic devices after dismantling and components like plastic, glass and metals are removed to ease recycling. Collected e-waste was sent to a rotary crusher to break and crush into finer particles which are further separated in a screening chamber to remover bigger particles. Finely crushed powder was passed through an eddy-current separator to bifurcate metallic and non-metallic particles. Due to their significant economic worth, metallic fractions were shipped to foundries for reuse whilst non-metallic powder (NMP) is disposed in landfills.

2.2. Materials

Ordinary Portland cement (OPC) of 53 grade—conforming to Bureau of Indian standards code IS:12269-2013 [32]—was considered in all concrete mixes. Physical properties of the cement according to Indian standards were presented in Table 1. Well graded angular aggregates with a maximum size of 12.5 mm and natural river sand (NS) free from grits and organics (passing 4.75 mm sieve confirming to Indian standards IS: 383-2002 [33]) were used as coarse and fine aggregates. Metakaolin (MK) complying with ASTM C618-03 (2010) [34] with specific gravity 2.52 was used as supplementary cementitious material (SCM) for replacing cement ranging 5–15% by weight in modified mixes. Processed non-metallic powder (NMP) produced from Victory Recyclers, Tamilnadu, India was used as partial replacement to fine aggregates at a ratio of 5–15%. Chemical composition of cement, metakaolin and NMP were obtained by X-ray fluorescence spectroscopy (Rigaku ZSX Primus II; WDXRF; capability Be4-U92) and are presented in Table 2. Particle size distribution and physical properties of the aggregates are presented in Figure 2 and Table 3. High range water reducing admixture (Conplast SP430) with specific gravity 1.2 confirming to IS: 9103-1999 [35] was used as super plasticizer.

2.3. Mix Design

A conventional mix of M30 grade concrete capable of achieving characteristic compressive strength of 30 MPa after 28 days of curing (conforming to Indian standard code IS: 10262-2009 [36]) was considered as a reference mix. Partial amounts of metakaolin and non-metallic particles ranging from 5–15% were replaced with cement for binder and natural sand for fine aggregate in different combinations, as shown in Table 4. Super plasticizer dosage was optimized and fixed to 4.212 lit/m3 for all the mixes. Pan type mixer was used to get a good uniform mixture and transferred to moulds with proper compaction using tamping and rod and vibratory table for one minute. After 24 h, cast cubes, beams and cylinders were demoulded and cured in potable water at ambient temperature of 28 ± 2 °C for 7 and 28 days. Several 100 mm cube moulds were used to cast and perform compressive strength, Sorptivity, and water absorption. Cylinders with 100 mm diameter and 200 mm depth were cast to perform splitting tensile strength tests and rapid chloride penetration (RCPT) tests. Beams of 100 × 100 × 500 mm were used for flexural strength. A group of three specimens were cast for each test.

2.4. Mechanical Performance

Mechanical performance of the different concrete mixes was accessed by testing specimen for compression, flexure and split tensile strength conforming to Indian standard IS: 516-1959 [37]. A low-capacity compressive testing machine (50 tonne capacity) was used for compression and split tensile with a uniform loading rate of 2.5 kN/s. Beams were tested for flexure in a flexural testing machine with 100 kN capacity.

2.5. Durability Performance

2.5.1. Water Absorption

Water absorption for the cubes was evaluated according to ASTM C642 (2013) [38]. Cast concrete cubes were oven-dried at 105 °C until they achieved a consistent mass, then the dry weight of the samples was recorded as W1. Later samples were completely immersed in water for 48 h at 21 °C, and their weight was recorded as W2. Water absorption was calculated using (1).
Water   absorption   % = W 2 −   W 1 W 1 × 100

2.5.2. Rapid Chloride Penetration Test (RCPT)

Rapid chloride penetration test (RCPT) was performed according to ASTM C1202 (2012) [39]. The total charge passed for the first six hours was calculated by relating current passed with time, which indicates chloride penetration in concrete.

2.5.3. Sorptivity

The rate of absorption of water (sorptivity) was performed on cube specimens as per ASTM C1585-13 (2013) [40]. After curing for 28 days, specimens were kept in an environmental chamber at a temperature of 50 ± 2 °C with 80 ± 3% relative humidity for 72 h. Later, the peripheral surface of the specimen was sealed with a non-absorbent coating and kept in a channel with the water level not exceeding 5 mm from datum. Surface water was cleaned with a damp cloth and the weight of the sample was taken at a time period of 60 min.

2.6. Analytical Techniques

2.6.1. X-ray Diffraction (XRD)

Mineralogical composition of metakaolin and concrete samples are identified by X-ray Diffraction using a Bruker D8 advance diffractometer equipped with Ni filter. Measuring parameters: 2.2 kW Cu Kα radiation, λ = 1.5405 A°, 0 to 90° 2θ with a goniometer speed of 0.01° 2θ s−1. A small portion of the binder rich fraction of the sample after the compressive test was collected from the specimen by eliminating coarse aggregate, and then crushed to a fine powder; later, it was sieved through a 63-micron sieve to perform XRD analysis.

2.6.2. Scanning Electron Microscopy with EDS

Field Emission Scanning Electron Microscopy (FE-SEM) with Energy Dispersive Spectroscopy (EDS) investigations were carried out using Thermo Fisher FEI-Quanta 250 FEG, to access the morphological characteristics of the raw materials and 28 days of hydrated concrete specimens. Small chunks of samples from specimens after the compression test were collected and dehydrated, then sputter coated with gold to prevent electron charge. Spot and sectional EDS analysis were carried on different locations to identify the composition in elemental level.

3. Results and Discussion

3.1. Characterization of Raw Materials

The presence of siliceous and argillaceous compounds abundantly in metakaolin evidenced from XRF makes it more suitable for pozzolanic activity and acts as good filler material [41]. XRD results confirm the rich presence of silica and alumina in metakaolin in the form of Quartz, and majorly as clay minerals Kaolinite and Nacrite as shown in Figure 3. Figure 4 shows the microstructural images of MK and NMP using FE-SEM and confirms the presence of very fine micro level angular shaped particles in metakaolin. NMP microstructure confirms the existence of a fibrous particulate fraction in the shape of small rod-like structures, which helps in achieving good flexural strength. A previous study conducted by same authors (Kakria et al. [42]) on leachate studies of NMP confirmed that the heavy metal concentration was under the permissible limits after comparing with regulations given in EN 12457-2 (2002) [43].

3.2. Mechanical Performance

3.2.1. Compressive Strength

Figure 5 presents the compressive strength of all the specimens with different mixes using MK and NMP, including reference numbers. Individual addition of MK to 10% in mix M10N0 has increased the compressive strength by about 16% and declined for higher replacements. The presence of siliceous and argillaceous content in the MK acted as pozzolan and activated the formation of calcium silicate hydrates (CSH) by reacting with calcium hydroxide [44]; the fine particle size of MK tends to fill the interstitial spaces in the concrete and made concrete denser [45]. This results in the development of load-bearing phases of cement with greater compressive strength [44]. Similarly, the addition of NMP as replacement to the fine aggregate increased the compressive strength by about 22.7% with 10% replacement level in mix M0N10. NMP consists of more fine and fibrous particles, and acts as an effective filler material that enhances the strength parameter in concrete [46]. There was a drop in the compressive strength beyond 15% addition of NMP for mix M0N15 due to the poor bonding of NMP fibres in the mix [42]. Further, the combined addition of MK and NMP leads to the improved efficacy of compressive performance, and the maximum compressive strength was found in mix M10N10 with 48.4 N/mm2, which is 39.8% more than the reference mix. Further addition of MK and NMP results in a drop in compressive strength due to more water demand, resulting in no further reaction between MK and calcium hydroxide [47,48].

3.2.2. Splitting Tensile Strength

Figure 6 shows the splitting tensile strength of the various mix specimens. Individual incorporation of MK has increased split tensile strength to 3.31 N/mm2 and 3.62 N/mm2 for 5 and 10% replacement levels, which is majorly observed due to the filler effect. Whereas, the sole addition of NMP as fine aggregate replacement to 5 and 10% has increased the strength to 3.56 N/mm2 and 3.67 N/mm2, respectively. Hence, the interaction of NMP, which is fibrous particles with concrete, acted as a crack arrester with a stress transfer mechanism, yielding a superior performance [49].
In the absence of MK, the optimum NMP content for maximum strength was about 10% by volume of natural sand, which agrees with the optimum NMP content as filler obtained by Mou et al. [46]. Similarly, the graphical results reveal that the splitting tensile strength generally increases with the combined addition of MK and NMP compared with sole addition. The combined addition of 10% MK and 10% NMP increases the splitting tensile strength by 27% after 28 days. The improved strength characteristics are attributed to the better binding of aggregates and a smooth interfacial transition zone (ITZ), which helps in reducing molecular-structural deformities while ensuring a consistent mix. The addition of NMP, a fibrous particle, acts as a crack arrestor and prevents a chain reaction for crack propagation [50].

3.2.3. Flexural Strength

The flexural performance of the engineered mixes along with references is shown in Figure 7. The individual addition of 10% MK shows a minor 1.81% improvement in the flexural strength values as compared to reference; on the other hand, individual addition of 10% NMP shows a 4.5% improvement in the flexural performance. The average value of flexural performance for optimum mixes was registered as 5.86 N/mm2 at 28 days, and that of reference was found to be 4.96 N/mm2. The maximum flexural strength was exhibited in mix M10N10 with an 18.15% improvement in the strength values as compared to the reference mix. The enhancement in the flexural properties is majorly attributed to the bonding of fibres in NMP to the cement matrix. FESEM imaging of NMP in Figure 4 shows the presence of the needle-like fibrous material confirming it enhancing its flexural strength. The NMP fibrous particles act as crack arrestors and prevent the spreading of surface and interstitial cracks, thus improving its flexural properties, which is consistent with the results of Zheng et al. [51].

3.3. Durability Performance

Durability studies were performed to ensure the adequate life cycle performance of the designed mix. The samples were evaluated through water absorption after 7 and 28 days, Sorptivity and total charge test after 28 days.

3.3.1. Water Absorption

The water absorption of the controlled samples was higher than the engineered mixes; the independent addition, as well as combined addition of MK and NMP, is found to reduce the water absorption compared with reference mix as shown in Figure 8. The mixed incorporation of MK and NMP mixes resulted in a significant reduction in total water absorption values. From Figure 8, the maximum reduction in water absorption was found to be in mix M10N10 by a substantial 52% and 75% with respect to the controlled mix REF for 7 and 28 days, respectively. The water absorption had values lower than the recommendations given by Concrete Society [52] pertaining to good concrete. This decrease in water absorption is due to the formation of a compounded and dense microstructure due to enhanced quantities of hydration products by MK and filling of voids by NMP, as reported in mechanical performance and seen in FESEM images (Section 3.4.2). Specimens with lower water absorption show higher compressive strengths and vice versa. The filling of interstitial voids in concrete helps to reduce damages caused to the concrete due to freeze and thaw cycles, conforming to increased service life. Moreover, the water absorption is mainly dependent on the network of the capillary in the paste. Hence, the lower the interconnectivity of the capillaries in the paste, the lower the water absorption values [53]. The lower paste volume of the engineered mixes is attributed to smaller capillaries and therefore lower water absorption values, indicating the refinement of pore structure [54].

3.3.2. Sorptivity

Sorptivity is another indicator of voids used to validate the results observed in the water absorption test. Figure 9 shows the graph for the sorptivity coefficient at 28 days. The addition of MK has an inverse relation to the sorptivity values, with the lowest value at a 10% replacement level. Further, the reduction in the sorptivity of individual addition of NMP attains the lowest value at 10% addition, which may be assigned to the proper blending and better homogeneity of the concrete. The mixes with 15% MK and 5, 10 and 15% NMP reveal that an increase in the content of MK and NMP also increases the sorptivity values, being maximum at 15% addition. This is attributed to the larger grain size of NMP fibres than those of cement and MK particles. From Figure 5, the highest compressive strength was obtained for the mix M10N10, with the lowest value of sorptivity as compared with the individual addition of MK and NMP concrete samples. The sorptivity value was 2.14 mm/h1/2, whereas for the reference sample it was 5.42 mm/h1/2 which is lower by 60.5% than the control sample. It is concluded that 10% replacement of cement MK and 10% replacement of natural sand shows the optimal performance for compressive strength and sorptivity results.
Moreover, MK is known for its particle packing effect on the pore structure due to its wide presence over the samples, which are also responsible for lower sorptivity values. Sorptivity is also an indicator for compressive strength, as lower sorptivity pertaining to densely compacted pore structure shows higher compressive values. Mix REF shows a higher sorptivity value and hence lowers the compressive strength, whereas mix M10N10 and M15N10 show the lowest sorptivity values and higher compressive strengths.

3.3.3. Rapid Chloride Penetration (RCPT) Test

RCPT was conducted for all samples and the total charge passing in 6 h measuring chloride permeability is presented in Figure 10. The RCPT values were seen to have a huge dip as compared to the controlled mix which is referred to as very low, as suggested in ASTM C1202 (2012) [39]. The reference mix showed values in between low and very low criteria, and the values obtained by 10% replacement of NMP and MK and 10% NMP and 15% MK were found to be three times less than the controlled mix. On the addition of MK, the samples outperformed the controlled mix indicating a decrement in the chloride permeability.
The RCPT results are found to be contradicting other durability parameters like water absorption and sorptivity; for instance, mix M15N0 shows lower chloride ion penetration values, indicating a less porous structure and contradicting the values perceived by other tests. RCPT works on the principle of diffusion of chloride paths along with water capillaries, but sometimes chloride ions react with cement compounds synthesizing stable chloro-complex compounds like tricalcium-aluminates. The corrosion process is initiated due to large amounts of chloride. Calcium silicate hydrates present in the cement due to the pozzolanic reaction are increased along with tricalcium-aluminates, due to the large amounts of alumina present in MK. Therefore, mixes with MK in the absence of NMP tend to have a higher chloride binding tendency and lesser free chloride ions, as the increase in alumina (Al2O3) values decreases the total charge. Hence the mixes with large amounts of MK show increased resistance towards chloride ions, and a similar trend was also reported by Zhu and Bartos [55].

3.4. Analytical Studies

3.4.1. X-ray Diffraction (XRD)

XRD was conducted on the 28th day on reference mix REF, optimum mixes with MK and NMP, i.e., M10N0 and M10N10. Figure 11 shows the obtained XRD peaks for those mentioned samples. After considerable amounts of CSH formation in the reference mix, unreacted calcium hydroxide existed as portlandite, with the addition of metakaolin alumino silicate rich minerals like Kaolinite and Nacrite in MK that react with calcium hydroxide during the hydration due to pozzolanic reaction, and lead to the increased production of Calcium Silicate Hydrates (CSH) and Calcium Alumino Silicate Hydrates (CASH) which are presented as CS in samples M10N0 and M10N10, ultimately acting as load bearing phases [45]. This increased amount of CSH and CASH supports the better performance of the material in both mechanical and durability aspects. Little quantities of Ettringite formation are observed in all the samples, which may be formed during the cement hydration. Unreacted alumino silicates in the form of kaolinite are observed in M10N0, whereas in M10N10 this unreacted kaolinite reacted with calcium hydroxide to form CSA. The amount of quartz is also increased in the mixes M10N0 and M10N10, which act as a load bearing phase for the concrete. The addition of NMP has impacted on the hydrated phases of cement, which was evidenced in the FE-SEM images.

3.4.2. Microstructure and Morphological Studies

FE-SEM and EDS are performed for samples REF, M10N0, M0N10 and M10N10 to identify the morphological and microstructural characteristics of the specimens. Figure 12 shows the FE-SEM images for the mentioned specimens. In the reference mix, cement matrix with porous media is observed. Formation of CSH and undehydrated particles in the form of portlandite are also observed, as shown in Figure 12a. Figure 12c shows the dense formation of CSH when MK is added to the mix; there is no presence of unreacted portlandite due to pozzolanic action of MK which is evident in XRD in Section 3.4.1. Unreacted MK fills the porous stratum and acts as a filler material. Addition of both NMP and MK leads to the formation of very dense microstructure with the packing of NMP micro fibres with the CSH shown in Figure 12d, which helps in gaining a good filler effect and also helps in attaining the best performance in both mechanical and durability aspects. Figure 13c,g shows the embedded NMP micro fibrous material in the CSH matrix and forms a dense matrix which helps in improving the flexural strength of the concrete.
Higher magnification images show less voids are present in the micro-structure due to the micron size of MK in Figure 12c, filling the voids which is in accordance with the results of durability performance. The presence of acicular patches is visible in the structure with major elemental composition of oxygen, silica and alumina, which is consistent with the addition of MK in the mix. The compressive strength performance of the mixes with MK in absence of NMP is higher than the conventional mix, and sole addition of NMP mixes as MK improves the pozzolanic reaction; however, the flexural strength is lower as compared to the counterpart which is due to the binding of strands of NMP [56,57].
The optimum concentration of MK and NMP, as shown in Figure 12d and Figure 13g,h, has resulted to a dense structure. The elemental proportions for EDS spectrum are presented in Table 5. CSH and CSAH phases were identified in Figure 13a,e and clearly evidenced with elemental proportions of EDS in Figure 13b,f. The NMP microfibres are well integrated throughout the matrix with C-S-H gel formation. The volume of voids has dropped and there is a smooth foundation of aggregates and binder with the absence of any major cracks. The webbed NMP fibres helped to arrest the cracks and thus provide a better mechanical performance. The dense matrix and large amounts of hydration products are accountable for its improved performance, which is the load bearing phases of pozzolanic reaction. NMP has assisted in improving flexural and splitting tensile performance, whereas MK has assisted in improving the compressive properties. Moreover, MK, due to its smaller particle size, has assisted in pore filling and thus improved the durability and porosity of the structure.

4. Conclusions

Utilization of mineral admixture like metakaolin and e-waste in concrete helped in achieving a double-sided goal by reducing the CO2 emissions and reuse of the waste, which is a major aspect in the circular economy. Along with this, from the above-discussed findings the following conclusions can be derived for the tested concrete with MK and NMP.
The addition of NMP as a natural sand substitute positively impacts the strength of concrete mixes. The optimal replacement of natural sand with NMP was 10%, which gave the maximum strength compared to that of control mix. This finding may be connected to its micro-shape and size, together with the binding action of NMP microfibers.
Using MK as a 10% replacement for cement provided better performance on the strength and durability properties of the mixes. This is accounted for due to MK’s pore filling nature, and high level of Silica content leading to a higher production of CSH and CASH gels. The combined addition of NMP and MK is very effective in increasing the strength and durable performance of concrete mixes up to the replacement level of 10% MK with cement and 10% NMP with natural sand, but beyond that it is less effective, with M10N10 being the optimum. NMP in large quantities acts as an inhibitor to silica, preventing the formation of CASH gels.
The water absorption and sorptivity values decline and are lowest at 10% replacement level of NMP and 10% MK, but the drop in the water absorption and sorptivity of NMP and MK combined mixes is less than that of controlled controls samples. The water absorption and sorptivity values were decreased for the mix M10N10. Chloride ion permeability is substantially lower than in controlled mixes. According to ASTM C 1202–94 assessment criteria, all mixes had “very low” chloride permeability concretes, with fewer than 1000 coulombs of total charge flowing. FESEM and XRD results showed the development and changes at the microstructural level on varying content of NMP and MK, which assists the betterment of mechanical and durability properties. The elements present in raw materials lead to the transition to load-bearing minerals, governing the improvements.
In summary, this study paves an effective and economical way for reusing NMP waste to replace fine aggregates and MK as a replacement to cement, reducing the adverse effects of cement hydration. The use of e-waste in concrete covers the void left by the lack of natural sand, while simultaneously addressing the issue of waste disposal.

Author Contributions

S.T.: Conceptualization, Methodology, Funding acquisition, Validation; T.S.: Writing—Review and Editing, Supervision; S.K.K.: Resources; K.K.: Writing—Original Draft, Visualization; R.C.M.: Investigation and Visualization. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors wish to thank VIT for providing “VIT SEED GRANT” for carrying out this research work.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Collection and segregation of e-waste to metallic and non-metallic components.
Figure 1. Collection and segregation of e-waste to metallic and non-metallic components.
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Figure 2. Particle size distribution of fine aggregates NS and NMP.
Figure 2. Particle size distribution of fine aggregates NS and NMP.
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Figure 3. XRD Mineralogical composition of metakaolin.
Figure 3. XRD Mineralogical composition of metakaolin.
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Figure 4. FE-SEM images of metakaolin and non-metallic portion.
Figure 4. FE-SEM images of metakaolin and non-metallic portion.
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Figure 5. Compressive strength of all the specimen at 7 and 28 days.
Figure 5. Compressive strength of all the specimen at 7 and 28 days.
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Figure 6. Split tensile strength of different mix specimen at 28 days.
Figure 6. Split tensile strength of different mix specimen at 28 days.
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Figure 7. Flexural strength of all mix specimen at 28 days.
Figure 7. Flexural strength of all mix specimen at 28 days.
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Figure 8. Water absorption of engineered mix specimen at 7 and 28 days.
Figure 8. Water absorption of engineered mix specimen at 7 and 28 days.
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Figure 9. Sorptivity of all specimens at 28 days.
Figure 9. Sorptivity of all specimens at 28 days.
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Figure 10. Rapid chloride penetration for all the mixes at 28 days.
Figure 10. Rapid chloride penetration for all the mixes at 28 days.
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Figure 11. Mineralogical identification of samples REF, M10N0, M10N10 using XRD.
Figure 11. Mineralogical identification of samples REF, M10N0, M10N10 using XRD.
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Figure 12. FE-SEM images of the samples (a) REF (b) M0N10 (c) M10N0 (d) M10N10.
Figure 12. FE-SEM images of the samples (a) REF (b) M0N10 (c) M10N0 (d) M10N10.
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Figure 13. (a,c,e,g) FE-SEM Images (b,d,f,h) EDS Spectrums for REF, M0N10, M10N0 and M10N10.
Figure 13. (a,c,e,g) FE-SEM Images (b,d,f,h) EDS Spectrums for REF, M0N10, M10N0 and M10N10.
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Table
Table 1. Physical properties of cement according to Indian standards.
Table 1. Physical properties of cement according to Indian standards.
Physical PropertyIndian StandardOPC 53 Grade
ConsistencyIS 4031-4 (1998)35%
Initial settingIS 4031-5 (1998)41 min
Final settingIS 4031-5 (1998)605 min
Soundness IS 4031-3 (1998)1.2 mm
Specific gravityIS 4031-11 (1998)3.14
Table
Table 2. Oxide composition of cement and metakaolin.
Table 2. Oxide composition of cement and metakaolin.
Material/OxideCaOMgOSiO2Fe2O3Al2O3K2ONa2OLOI
OPC64.502.4020.803.355.600.510.122.72
Metakaolin0.110.0251.500.5845.200.910.101.58
NMP24.90.5752.20.9818.960.650.351.39
Table
Table 3. Physical properties of aggregates used in concrete.
Table 3. Physical properties of aggregates used in concrete.
Physical PropertyNatural SandNon-Metallic PowderCoarse Aggregate
Fineness modulus2.682.897.52
Specific gravity2.641.852.77
Water absorption (%)1.362.910.60
Bulk density (kg/m3)17238821820
Table
Table 4. Types of mixes and codes used for the study.
Table 4. Types of mixes and codes used for the study.
Specimen CodeMix Proportions (kg/m3)
CementMKFine AggregateNMPCoarse AggregateWaterSP
REF3510826011141584.212
M5N0333.4517.55826011141584.212
M10N0315.935.1826011141584.212
M15N0298.3552.65826011141584.212
M0N53510784.741.311141584.212
M5N5333.4517.55784.741.311141584.212
M10N5315.935.1784.741.311141584.212
M15N5298.3552.65784.741.311141584.212
M0N103510743.482.611141584.212
M5N10333.4517.55743.482.611141584.212
M10N10315.935.1743.482.611141584.212
M15N10298.3552.65743.482.611141584.212
M0N153510702.1123.911141584.212
M5N15333.4517.55702.1123.911141584.212
M10N15315.935.1702.1123.911141584.212
M15N15298.3552.65702.1123.911141584.212
Note: REF: Reference mix; M5, M10, and M15: 5%, 10% and 15% by mass replacement of cement with metakaolin by weight; N5, N10, and N15: 5%, 10%, and 15% by mass replacement of natural sand with non-metallic powder.
Table
Table 5. Elemental proportions for EDS spectrum.
Table 5. Elemental proportions for EDS spectrum.
SpectrumCaSiAlOKNa
Figure 13b31.6310.433.1254.82--
Figure 13d-28.707.7651.2710.811.46
Figure 13f28.9214.506.4650.13--
Figure 13h5.7023.087.2855.977.97-
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Selvaraj, T.; T, S.; Kaliyavaradhan, S.K.; Kakria, K.; Malladi, R.C. Use of E-Waste in Metakaolin Blended Cement Concrete for Sustainable Construction. Sustainability 2022, 14, 16661. https://doi.org/10.3390/su142416661

AMA Style

Selvaraj T, T S, Kaliyavaradhan SK, Kakria K, Malladi RC. Use of E-Waste in Metakaolin Blended Cement Concrete for Sustainable Construction. Sustainability. 2022; 14(24):16661. https://doi.org/10.3390/su142416661

Chicago/Turabian Style

Selvaraj, Thirumalini, Shanmugapriya T, Senthil Kumar Kaliyavaradhan, Kunal Kakria, and Ravi Chandra Malladi. 2022. "Use of E-Waste in Metakaolin Blended Cement Concrete for Sustainable Construction" Sustainability 14, no. 24: 16661. https://doi.org/10.3390/su142416661

APA Style

Selvaraj, T., T, S., Kaliyavaradhan, S. K., Kakria, K., & Malladi, R. C. (2022). Use of E-Waste in Metakaolin Blended Cement Concrete for Sustainable Construction. Sustainability, 14(24), 16661. https://doi.org/10.3390/su142416661

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