Feasibility study of compressed bricks utilizing rubber glove manufacturing residue

The demand of the rubber glove had increased significantly since early of the year 2020 due to the COVID-19 virus breakout especially in the medical and healthcare sector. Rubber Glove Manufacturing Residue (RGMR) is the by-product from the effluent treatment by coagulation process in the manufacturing procedure. Non-recyclable RGMR are disposed to landfill after treated. This research is aimed to reuse the RGMR in cemented interlocking bricks to address the natural resources depletion issues and greenhouse gases released from the burnt brick production. The comparison in terms of physical and mechanical properties was conducted in between RGMR and crumb rubber interlocking bricks (RIB). RGMR and crumb rubber were used as partial fine aggregate replacement by 10% while fly ash was used as a cement replacement material by 56%. Four mixes were tested, and the interlocking bricks were categorized as medium weight bricks. However, only control mix and RGMR-2 mix fulfilled the loadbearing masonry requirement of a minimum of 13.8 MPa. RGMR-2 utilizing 10% fine aggregate replacement using RGMR without cement replacement. As for the mixes utilizing fly ash and fine aggregate replacement, RGMR-1 mix and RIB mix show compressive strength of 12.15 MPa and 11.64 MPa respectively. Besides that, the use of RGMR reduced the water absorption compared to RIB mix due to the larger particle size of crumb rubber. The water absorption rate for RIB mix has exceeded the limit of water absorption for the medium weight bricks of 240 kg/m3. The finding indicates that the RGMR exhibited a better performance as compared to the crumb rubber.


Introduction
The usage of rubber gloves in the medical and healthcare industry as well as domestic use have become an essential due to the worldwide emergence of acquired immune deficiency syndrome (AIDS) and the events of virus breakouts such as the H1N1, SARS, MERS, and the most recent COVID-19. Malaysia is the world-leading rubber gloves producer, contributing about 63% of the world demand which was close to 300 billion in the year 2019. Rubber glove production company is dealing with a large amount of rubber glove manufacturing residue (RGMR) generated from the wastewater during the glove production process. Recycle and reuse of RGMR have been a common practice worldwide but the limitation on the technology causes most of the RGMR to end up in landfill.
Waste rubber products such as waste rubber tires, waste rubber gloves, and waste rubber condoms have been used as fillers in asphalt and Portland cement concrete [1]- [3], as fine aggregate in bricks [4], as material in polymer blend [5] and etc. In addition, natural rubber latex also has been used in mortar and has shown a reduction in water absorption and improvement in durability [6]. Later, Jose & Kasthurba [7] used natural rubber latex (NRL) in laterite soil-cement blocks and found that a reduction in water absorption of 6.8% was observed with the addition of 5% NRL while compressive strength is increased about 20% with the addition of 2 to 3% NRL. However, a reduction in compressive strength was noticed when the percentage of NRL is beyond 3%.
Rubber is hydrophobic in nature. Some researchers found that utilizing crumb rubber in brick production leads to the reduction in compressive strength due to this nature of the rubber that engulfs the air around its surface that formed concentrated pressure. Fly ash was introduced to overcome the shortcoming due to the slower hydration process as compared to cement which decreased the hydration heat and thus increasing the strength [8]- [10].
Al-Fakih et. al. [11] used crumb rubber as fine aggregate replacement in the interlocking bricks and found that the compressive strength and flexural strength of the interlocking brick are 18.03 MPa and 2.23 MPa, respectively, which can be classified as load bearing masonry unit based on ASTM C90 requirement. However, the water absorption of the bricks was not mentioned in this publication. Thus, this experimental program is intended to compare the performance of RGMR with the crumb rubber in the rubber based interlocking bricks. The compressed interlocking bricks of size 250 mm x125 mm x105 mm were developed by mixing the river sand, rubber material, cement and fly ash. The rubber materials used were crumb rubber and RGMR. The mix for crumb rubber interlocking brick (RIB) was referred to the publication by Al-fakih et. al. [11].

Materials
The binder material used is Type 1 Ordinary Portland Cement (OPC), which fulfill the requirement stated in ASTM C150. Besides the cement used as a binder, Class F fly ash was used as a cement replacement material (CRM) to partially replace the volume of cement. Local washed river sand with the range of sizing 0.3 -1.2 mm was chosen as the fine aggregate of the bricks accordance with ASTM C33. Sieve analysis was conducted for the river sand to determine the size distribution. The fineness modulus of the sand was 2.98, which is within the recommended range (2.3-3.1).
Since RGMR are the waste obtained from the manufacturing plants, laboratory testing was carried out to check the amount of heavy metal. The results for the laboratory test for chemical properties is shown in Table 1. The raw RGMR material received was in dried granular size, which ranges from 20 mm -80 mm. As the raw RGMR received is too large to be used as fine aggregates for the manufacturing of bricks, the raw RGMR material has been crushed and ground into powder form using FRITSCH Mortar Grinder and the particles size was controlled to be smaller than 500micron by sieving process. The density of the powder produced was tested using Gas Pycnometer, where Helium gas was used for the analysis. The density of the RGMR Powder is 1764 kg/m 3 with a standard deviation of 0.9 kg/m 3 .
Scanning Electron Microscope (SEM) was done to determine the RGMR particle shape and surface morphology. Figure 1 shows the SEM image of RGMR and it is observed that the surface of RGMR particles is uneven and rough, visually similar to the surface of crushed granite aggregate. Sieve analysis was done on the RGMR powder to determine the size distribution of the material. Based on the sieve analysis, the fineness modulus of the sand was calculated to be 0.84, which means that the particles are very fine compared to the sand (2.98). The size of the crumb rubber used were ranges from 600micron to 2.36 mm. The comparison of the fine aggregates used for the bricks is tabulated in Table 2.

Mix design
The interlocking bricks were produced by mixing cement, fly ash, sand, RGMR, and water. The cement was partially replaced by fly ash while the sand was partially replaced by the crumb rubber or RGMR powder. The percentage of cement and fine aggregate replacement were based on publication by Al-fakih et al. [11], which adopting 56% of fly ash as cement replacement and 10% of crumb rubber as fine aggregate replacement and labeled as RIB mix. The mix ratio of cement: sand was 1:2 while the maximum water content was 10% of the total weight. RGMR-1 mix contained RGMR with fly ash as cement replacement while RGMR-2 contained only RGMR with 100% cement content. The mix design is tabulated in Table 3. The RGMR and crumb rubber are rubber material that are less reactive and weak in water absorption hence reduced the adhesion with cement paste. Therefore, during the mixing stage, RGMR and the crumb rubber was mixed homogeneously with sand to ensure the even distribution of the materials. The volume of water used in the mixes ranged from 9-10% of the total weight. The amount of water used is crucial to the bricks production as the bricks produced might collapsed after compression if the excessive water is added. After the fine aggregate was thoroughly mixed, cement and fly ash was added into the mixer. The RGMR powder exhibited a hydrophobic feature to the dry mix when the water was initially added. Thus, the water was added slowly in the beginning stage to obtain the homogeneous mix. Then, the mixture was loaded into the steel mold of the interlocking machine of size 250 mm x 125 mm x 105 mm (length x width x height) and were compressed at 17 MPa.

Testing
Mechanical properties of the interlocking bricks were tested under compressive strength and modulus of rupture while physical properties were tested for water absorption, size measurement and efflorescence.
Compression test of the interlocking bricks were tested accordance with ASTM C140-11a. A total number of 5 specimens were being tested for each of the mix design. The bricks were tested with an automated compressive machine with a capacity of 2000 kN with rate of 0.5 N/mm 2 per second. Modulus of rupture, which is known as the flexural test was conducted accordance with ASTM C67-02c whereby the brick is loaded at the midspan and simply supported with 200 mm span.
Water absorption was tested accordance to ASTM C140-11a. Three samples were tested and data for dry weight, saturated weight, and the immersed weight were recorded. The size measurement test is important to ensure the consistency in the shape of the bricks produced due to the interlocking features of the brick. The size measurement test was carried out accordance with ASTM C67-02c. Vernier Caliper was used to measure the length, width, height, and the web thickness of the bricks. Efflorescence test was tested accordance with ASTM C67-02c. Efflorescence is defined as the crystalline deposit, usually white, of watersoluble compounds on the surface of masonry bricks.

Results and discussions
The results of compression test are tabulated in Table 4. The average compressive strength of the control mix is 24.83 MPa, while the mix RGMR-2 is 13.94 MPa. Only these two mixes fulfill the minimum compressive requirement for loadbearing concrete masonry unit based on ASTM C90, which is 13.8 MPa. The replacement of fine aggregate by 10% volume with RGMR Powder had significantly reduced the strength of the interlocking bricks by 44%. Meanwhile, the RGMR Mix and RIB Mix are categorized as non-load-bearing concrete masonry as the minimum compressive requirement for the non-load-bearing bricks based on ASTM C129 is 4.14 MPa. Comparing between RGMR and RIB mix, the compressive strength of the mixes are 12.15 MPa and 11.64 MPa respectively. The two mixes were cast for the same binder ratio and the same amount of fine aggregate replacement. The RGMR mix exhibits a better performance in terms of compressive strength. The strength of RGMR mix is higher because the particles size of the RGMR powder is relatively smaller than the crumb rubber, hence the voids in the mix were filled with the finer RGMR powder, resulted in evenly distributed stress between the bricks particles. Table 4 shows that the flexural strength of all the mix various from 2.30 MPa -2.98 MPa, with the Control mix exhibited the highest flexural strength. Other mixes with rubber based are similar. By comparing the flexural strength of Control mix with other 3 mixes which used 10% of fine aggregate replacement, the strength had reduced 20-23%. This is due to the reduction of load-bearing particles and the weak bonding between the particles. Although the rubber-based mixes do not perceive a significant different, RGMR shows slightly better improvement than the crumb rubber. The rough surface and the smaller particle size of the RGMR powder contributed to the flexural strength by enhancing the interlocking between the particles and reduced the pores. Table 4 shows that the usage of RGMR and crumb rubber reduced the compressive and flexural strength of the bricks. Similar results were obtained by Ankush Thakur et al. [12] and E. Sodupe-Ortega et al. [13] which indicated that high percentage of crumb rubber results in lower compressive strength of the sample. The results shows that the usage of RGMR and crumb rubber as partial of the fine aggregate replacement are significant on the compressive strength than the flexural strength. Generally, the flexural strength is much lower than the compressive strength, which indicated that the brick can be loaded in compression but not to withstand bending load. However, flexural strength is not a requirement for concrete masonry unit accordance with ASTM C140. It can be concluded that flexural strength is correlated with compressive strength. The SEM imaging was conducted to facilitate the interpretation of the compressive strength results. It was observed that image under 3000x magnification shows a densified mix with inclusion of fly ash (Fig 2.(d)) compared to the mix without fly ash (Fig 2.(c)). Whereas it can be seen that RIB Mix (Fig 2.(b)) shows evenly distributed voids compared to the RGMR Mix (Fig 2.(c)) with larger voids. This is because the crumb rubber used for the RIB mix is having a larger particle with the fineness modulus of (0.92) compared to the RGMR Powder (0.84). The crumb rubber particle can be seen clearly on the image of RIB Mix, where the dark area (bottom left) shows the crumb rubber used. A large rubber particle creates a large weak zone on compression as the crumb rubber behaves in elastic properties during compression, resulting in high deflection on the crumb rubber area, hence the sample will be more easily to be failed in compression. Using smaller rubber particle should improves the compressive strength of the interlocking bricks.
From Figure 2. (c), it is observed that the RGMR Powder is not fully coated with the C-S-H formed by the hydration process of fly ash and cement. This is due to the hydrophobic properties of the RGMR Powder which repels water and less reactive. The hydrophobic characteristic can be seen during the mixing process. Although the surface of the RGMR Powder is rough, due to the natural properties of the RGMR Powder which is hydrophobic, the bonding does not improve along with the surface roughness. This finding matches the compressive strength decrement when comparing the Control Mix and the RGMR-2 Mix, where the inclusion of 10% RGMR Powder as a fine aggregate replacement significantly reduces the strength of the interlocking bricks by 44%.
From the SEM image of RGMR-1 Mix (Fig 2.(c)), there are a lot of rounded particles in the mix, representing that there are lots of unreacted fly ash. The reactivity speed of fly ash is slower than the cement. As the sample was tested at 28 days, this might be indicating that the fly ash needs a longer period for all the fly ash to be hydrated. The unreacted fly ash acts as a filler instead of contributing to the formation of C-S-H gel. The unreacted fly ash material causes the strength of the RGMR-1 (with fly ash) mix to be lower compared to the RGMR-2 mix (without fly ash). It can be concluded that the compressive strength of the interlocking bricks is significantly reduced due to the hydrophobic properties the RGMR powder and crumb rubber. The used of fly ash as 56% of cement replacement, reduces the compressive strength by only 11%. Meanwhile, for comparison of the rubber-based material, the RGMR powder has a better performance compared to crumb rubber as the particle size of RGMR Powder (<500micron) is much smaller than the crumb rubber particles size (600micron-2.36 mm). The fine particles able to fill up the void area between the sand particles, hence a denser particle distribution in volume and much better materials interlocking force of the mix could be achieved. Table 5 shows the density and water absorption of the interlocking bricks. Based on the classification by ASTM C90 and ASTM C129, all four mixes with the oven-dry density ranges between 1780 kg/m 3 to 1991.85 kg/m 3 fall under the category of medium weight masonry bricks. It is found that the used of fly ash in the mixes significantly reduced the density of the interlocking bricks by 99.1 kg/m 3 . This is due to the reason that the density of fly ash (1800 kg/m 3 ) is 42% lower than the cement (3120 kg/m 3 ). In addition, the used of crumb rubber as a fine aggregate replacement had significantly reduced the density of the interlocking bricks. The used of RGMR powder as a fine aggregate replacement does not reduce the density significantly compared to the crumb rubber. This is because the fine particles of the RGMR powder filled the void area between the sand particles, created a densified brick that is having more solids particles and less air entrapped in the interlocking bricks. Moreover, the density of the RGMR Powder (1764 kg/m 3 ) is higher compared to the density of crumb rubber (950 kg/m 3 ).
The water absorption test based on ASTM C90 mentioned that medium weight density bricks should have water absorption lower than 240 kg/m 3 . Table 5 shows that the water absorption for RIB mix (306 kg/m 3 ) exceeded the maximum water absorption limit for medium weight masonry. The other three mixes, which are the Control mix, RGMR-1 mix, and RGMR-2 mix are within the acceptable range. Although crumb rubber with hydrophobic properties should improve the water absorption, RIB bricks have larger void that enhances the water absorption compared to all the other mixes due to the larger particle size of crumb (>600micron). As for the RGMR powder, other than the hydrophobic properties, the fine particles of the RGMR Powder play an important role to fill up the void area in the bricks, hence reducing the water absorption.
The inner dimension of the interlocking bricks mold for the interlocking bricks production is 250 mm x 125 mm x 100 mm. Based on ASTM C90, the measured dimension of the specimen shall not differ more than 3.2 mm comparing to the specified standard dimension. Table 6 shows the size measurement results. All dimensions have less than one mm differences compared to typical dimension. Thus, all the dimension of bricks produced is within the allowable range.  Efflorescence is the crystalline deposit which usually white in colour, in the form of water-soluble compounds on the surface of masonry bricks. The lime-based compounds, such as carbonate are the major factor that causing the efflorescence. The calcium hydroxide (Ca(OH) 2 ) from the water are slowly dissolved and reacted with the carbon dioxide in the surrounding air to produce the calcium carbonate (CaCO 3 ) [14]. The efflorescence test was conducted in accordance with ASTM C67-02c, by examining the existence of white precipitate on the surface of bricks. The white precipitate which is the salt deposit was observed.
It was observed from Figure 3 that all the mixes exhibit little differences on the 25.4 mm depth submerged with water from the bottom of the bricks. Figure 3 below shows the comparison between two bricks with the same mix, where the bricks on the left were kept dry without contacting any water in the drying room while the bricks at right were partially immersed in water to a depth of 25.4 mm. All the samples that submerged with water are showing a light white line precipitated on their surfaces, compared with the dry samples which were found to have no salts deposits. The exposure to water and air increased the growth of white precipitates on the surface due to the CaCO 3 from the cement and lime [14]. There is no specific level of acceptance on the efflorescence and practically depends on the acceptance of the client and users. Therefore, cement content can be the subject matter in the future research to reduce the efflorescence on the bricks.

Conclusions
The use of the RGMR interlocking bricks can overcome the environmental issues created from the production of conventional bricks and at the same time utilizing the residue from rubber glove manufacturing, hence improve the sustainability of the building material. The interlocking bricks production release lesser carbon dioxide and waste compared to conventional masonry burnt bricks and able to accelerate the speed of masonry wall construction due to its key and lock feature that ease the construction process. In addition, the use of RGMR Powder in the interlocking bricks reduced the density of bricks and natural resources needed. This is a value-added to the rubber glove production industry in terms of solid waste management.
The physical properties test, it can be concluded that all the bricks are uniform in dimension, hence the bricks produced can be easily laid with the interlocking features. All four mixes are considered as medium weight bricks. The use of 56% fly ash as the cement replacement materials reduced the compressive strength of the interlocking bricks by 1.79 MPa or 13% but flexural strength increased by 0.03 MPa or 1.3%. In addition, the density of interlocking bricks reduced by 99.1 kg/m 3 or 5% since fly ash has lower density compared to cement.
The use of RGMR and crumb rubber as fine aggregate replacement materials are hydrophobic which significantly reduced the strength of the interlocking bricks. The use of 10% RGMR Powder reduced the compressive strength by 44% while for the RIB mix utilizing crumb rubber has reduction in compressive strength of about 53%. The performance of RGMR Powder is slightly better than the crumb rubber due to the finer particle size and the uneven surface of the particles. Besides, the RGMR Powder enhanced the water absorption performance. Meanwhile, RIB mix that utilizing crumb has the highest water absorption due to the larger pores in the brick.
As the paper focuses on comparing the physical and mechanical properties of interlocking bricks utilizing RGMR Powder and crumb rubber, the optimized percentage of fine aggregate replacement by RGMR Powder has not been carried out. Further research on the optimization of RGMR Powder can be carried out to maximize the usage of the RGMR Powder. Besides, due to the poor bonding between the RGMR powder particles and the binder materials, research can be done on the surface treatment of the particles using chemical coating method.