Waste Management 80 (2018) 112–118

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Waste Management

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Recycling waste plastics in developing countries: Use of low-densitypolyethylene water sachets to form plastic bonded sand blocks

https://doi.org/10.1016/j.wasman.2018.09.0030956-053X/� 2018 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Department of Civil and Environmental Engineering,Imperial College London, South Kensington, London SW7 2BU, UK.

E-mail address: c.cheeseman@imperial.ac.uk (C. Cheeseman).

Alexander Kumi-Larbi Jnr a, Danladi Yunana a, Pierre Kamsouloumb, Mike Webster b, David C. Wilson a,Christopher Cheeseman a,⇑a Environmental and Water Resource Engineering Section, Department of Civil and Environmental Engineering, Imperial College London, SW7 2AZ, UKbWaste Aid UK, Wye, Kent, UK

a r t i c l e i n f o

Article history:Received 15 January 2018Revised 14 August 2018Accepted 2 September 2018

Keywords:LDPESandWater sachetsWaste plasticsCommunity-drivenWaste management

a b s t r a c t

In many developing countries low-density polyethylene (LDPE) sheets, bags and water sachets are amajor waste problem because local collection and recycling systems do not exist. As a result, LDPE hasno value and is dumped causing aesthetic, environmental and public health issues. A relatively simpletechnology has been developed in the Cameroon that produces LDPE-bonded sand blocks and pavers.The application of this technology is an example of a community-driven waste management initiativethat has potential to impact on the global plastics waste crisis because it can transform waste LDPEand other readily available types of plastics into a valuable local resource. In this research, waste LDPEwater sachets have been melted and mixed with sand to form LDPE-bonded sand blocks. The effect ofsand particle size and sand to plastic ratio on density, the compressive strength and water adsorptionare reported. Optimum samples have been further characterised for flexural strength and thermal con-ductivity. LDPE-bonded sand is a strong, tough material with compressive strengths up to �27 MPa whenproduced under optimum processing conditions. The density and compressive strength increase as theparticle size of the sand decreases. The potential for using this simple technology and the materials it pro-duces to transform LDPE plastic waste management in developing countries is discussed.

� 2018 Elsevier Ltd. All rights reserved.

1. Introduction Plastic waste has become so ubiquitous that it is now a serious

Developing countries (DCs) typically have inadequate solidwaste management, with low waste collection rates, disposal pri-marily by dumping and limited outlets for reusing potentially recy-clable materials (Wilson et al., 2015). However, waste materials inDCs can provide livelihoods to a highly entrepreneurial informalsector (Wilson et al., 2006). The management of wastes, and partic-ularly waste plastics, has become a high profile, environmental andpublic health issue. Recycling infrastructure for these materialsoften does not exist in DCs, and as a result, waste plastics have lit-tle or no value, resulting in uncontrolled disposal as shown inFig. 1. Dumping into waterways has severe adverse effects on localcommunities. Waste plastics are not only unsightly, but they blockurban drainage systems and sewers, causing flash floods, as well asproviding a fertile breeding ground for mosquitos and other water-borne diseases.

threat to marine ecosystems and biota. It has been estimated thatbetween 4.8 and 12.7 million metric tonnes of plastic waste wasadded to the oceans in 2010 (Jambeck et al., 2015). Oceans aredownstream from waterways, 60–80% of marine litter is plasticand poor waste management in DCs is a major cause and contrib-utor to plastics in the oceans (Grantham Institute, 2016).

Despite the low biodegradability of plastics and the associatedpotential for long-term adverse environmental impacts, single-use polyethylene drinking water sachets such as those shown inFig. 2 are very widely used throughout much of Africa. These areused in enormous numbers because water sold in sachets hashigher quality than the local tap water. As a result, water sachetuse has increased to such an extent that they are now amajor envi-ronmental issue in many parts of Africa, as reported for the Accra-Tema Metropolitan Area in Ghana (Stoler et al., 2012). Uncon-trolled and indiscriminate dumping of plastics into water bodiesis very common in DCs because there is often no local recyclinginfrastructure. It is estimated that 15–40% of waste plastic isdumped into water bodies and this contributes to the estimated5.25 trillion pieces of plastic debris currently in the oceans(Crawford and Quinn, 2017; Sebille et al., 2016).

Fig. 3. Typical block paver made in the Cameroon using LDPE and sand.Fig. 1. Photographs showing waste plastics including water sachets blocking urbandrainage in Ghana.

Fig. 2. Single-use LDPE drinking water sachets that are widely used in Africa.

A. Kumi-Larbi Jnr et al. /Waste Management 80 (2018) 112–118 113

Previous research has reported on the re-use of waste plasticsas construction materials in developing countries. Polyethyleneterephthalate (PET) bottles filled with sand or earth have mechan-ical properties suitable for use in walls and in slab construction(Mansour and Ali, 2015). Plastic bottles can also be filled with plas-tic food wrappers to form eco-bricks (Lenkiewicz and Webster,2017). Lightweight concrete has been produced by using wasteplastics as aggregate (Gu and Ozbakkaloglu, 2016; Ismail and Al-Hashmi, 2008). Plastic coated aggregates have been used to formasphalt and this allows a 10% reduction in bitumen usage(Vasudevan et al., 2012). Plastic fibres have been used in concreteto provide a cost-effective, corrosion resistant reinforcementoption (Gu and Ozbakkaloglu, 2016). PET fibres have also beenused to improve the compressive strength and energy absorptioncapacity of soils (Consoli et al., 2002).

Plastic-bonded sand paver blocks as shown in Fig. 3 were firstproduced using waste plastics in the Cameroon by Pierre Kam-souloum in 2006. This has now become a leading example of acommunity-driven waste management initiative that has had animpact on local communities and local waste management(Lenkiewicz andWebster, 2017). By turning wastes into potentiallyvaluable resources it also has the potential to contribute to solvingthe global waste crisis. However, the manufacturing process andthe mechanical properties of the materials formed have not previ-ously been reported in the scientific literature to date and thesetechnologies will benefit from the type of laboratory-based sys-tematic research reported in this paper (Wilson and Webster,2018). The aim of this research was therefore to optimise the

production process at laboratory scale and determine the proper-ties of these materials in order to provide guidance to those work-ing in the field on the key production parameters that determineperformance. In this work the effect of the sand particle size andthe sand to plastic ratio in LDPE-bonded sand is reported. The opti-mum samples have been further characterised for stress-strainbehaviour during loading to failure in bending. The thermal con-ductivity of samples is also reported.

2. Materials and methods

LDPE water sachets from Ghana were used in these experiments(Space Poly Product Limited, Ghana). LDPE is a thermoplastic thatcan be moulded and remoulded repeatedly when heated. It is ahighly flexible material because it contains numerous side chainsthat increase the distance between the main C-C chains, reducedpacking and intermolecular attraction. It typically has a densityin the range of 0.91–0.94 g�cm�3.

Commercially available silica sand with a particle density of2.65 g�cm�3 was used as an inert filler. This was dried and usedas-received and also sieved to give four different size fractions withparticle sizes (d) in mm of d < 0.5, 0.50 < d < 1.00, 1.00 < d < 2.36and 2.36 < d < 4.75. These four size fractions were assumed to haveaverage particle sizes of 0.25 mm, 0.75 mm, 1.68 mm and3.55 mm.

The mix designs of LDPE bonded sand samples used to investi-gate the effects of sand particle size and sand to plastic ratio areshown in Table 1. The process flow diagram used to produceLDPE-bonded sand samples is shown in Fig. 4. The water sachetswere first heated in a saucepan on a hotplate (Jenway 1000 serieshotplate) with the temperature of the mix measured using aninfrared thermometer (RayTemp 3). The water sachets softenedbetween 110 and 150 �C and when they had an appropriate consis-tency the sand was added. The sample were then continuouslymixed until a homogenous blend of sand and LDPE had formed.The mix was then cast into three gang 50 � 50 � 50 mm cube steelmoulds that had been coated with a silicone-based release spray.The moulds were pre-heated to approximately 100 �C as thisallowed the hot samples to be compacted and formed into shapebefore cooling to room temperature.

The density of samples was determined using the Stable MicroSystems (SMS) industrial specific gravity balance (model SG/30).The water absorption of samples was determined after 24 himmersion in distilled water at room temperature, as describedin ASTM D570.

Table 1Systematic variation in mix design of samples to investigate the effect of sand particle sizes, sand to plastic ratios, and water absorption on the mechanical properties of LBS.

Experiment Sand to plastic ratio by weight Sand proportion by wt. % Sand particle size (d) Number of test samples

Effect of sand particle size 3 75.0 Ungraded 63 75.0 d < 500 mm 63 75.0 0.5 mm < d < 1.0 mm 63 75.0 1.0 mm < d < 2.4 mm 63 75.0 2.4 mm < d < 4.7 mm 6

Effect of sand to LDPE ratio 1 50.0 d < 500 mm 42 66.7 d < 500 mm 43 75.0 d < 500 mm 44 80.0 d < 500 mm 45 83.3 d < 500 mm 46 85.7 d < 500 mm 4

Investigating the effect of water absorption 1 50.0 d < 500 mm 32 66.7 d < 500 mm 33 75.0 d < 500 mm 34 80.0 d < 500 mm 35 83.3 d < 500 mm 36 85.7 d < 500 mm 3

Compressive stress-strain 3 75.0 d < 500 mm 4

Flexural stress-strain 3 75.0 d < 500 mm 3

Thermal properties 3 75.0 d < 500 mm 3

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The compressive strength of at least three samples of each mixwas determined using a loading rate of 0.3 MPa/s (Automax 5 test-ing machine). Selected samples were further tested in compressionusing a testing machine that allowed the compressive modulus tobe determined (Zwick model 1474). Test samples were subjectedto compressive loads at a strain rate of 10 mm/min to obtainload-compression data. The initial set of experiments investigatedthe effect of sand particle size on the compressive strength of sam-ples containing 75% by weight addition of sand. The sand particlesize that gave the highest compressive strength in these sampleswas then used to determine the effect of varying the sand to LDPEratio.

Three-point flexural tests were conducted in accordance withASTM C580-02 (Zwick 1474 testing machine) using a crossheadspeed of 10 mm/min on samples tested on a 29.7 mm span. Threespecimens of each type of sample were tested.

Thermal conductivity tests were completed on the50 � 50 � 50 mm cube samples using a Hot Disk M1 analyser(Thermal 18 Instruments Ltd). This is a non-destructive test basedon the transient plane source technique according to ISO 22007-2:2015. The hot disk sensor was sandwiched between the surfacesof two block samples. The sensor acts as a heat source and recordstemperature as a function of time in minute Kelvin resolution(Ashraf, 2016).

Polished LBS samples were impregnated with florescent resinand observed using optical microscopy (Olympus, BX 51TRF).

3. Results

Fig. 5 shows the effect of sand particle size on the density andcompressive strength of LDPE-bonded sand blocks containing75% by weight of sand. The theoretical density was calculated fromthe density of sand and LDPE using the rule of mixtures. There is anegative linear correlation between the sand particle sizes anddensity which range from 1.70 g�cm�3 to 1.77 g�cm�3. The use oflarger sand particle sizes decreased the compressive strength andincreased porosity and the samples containing sand particles withd < 500 mm achieved the highest compressive strengths.

The effect of sand to plastic ratio on the density and compres-sive strength of LDPE-bonded sand blocks is shown in Fig. 6. Highersand additions reduce porosity and increase density, which rangedfrom 1.46 g�cm�3 to 1.91 g�cm�3. Increasing the amount of sandincreased the compressive strength for additions up to 75 wt%. Thiswas the sand addition that produced the maximum compressivestrength of 27.3 MPa.

Fig. 7 shows the compressive stress-strain behaviour of thesamples with maximum compressive strength. The graph showsvisco-elastic behaviour similar to that of asphalt. Initial uniaxialcompressive loading produces a linear stress-strain relationship.Further loading causes a shear plane to form, as shown in Fig. 8.The shear plane is caused by friction between the sand grains asthey slide against each other. At this point, the sample behavesas a frictional solid as the stress strain curve flattens out afterreaching the peak friction coefficient. The curved section of thegraph shows pure shear as the peak friction coefficient equalsthe frictional angle. The linear descent in the graph is due to vari-ation in the friction angle which reduces the coefficient of friction.Further loading only resulted in increments in strain (Leon et al.,2016). The flexural to compressive strength ratio of the optimisedLDPE-bonded sand blocks was 0.53.

Two failure modes were observed for LDPE-bonded sand sam-ples. Samples with sand proportions <75 wt% were more ductilewhen subjected to compressive loads. The sand significantlychanges the flow of the plastic under load by reducing the ductilityas observed in disposable cups and polypropylene composites(Mitchell et al., 2014). Increasing the sand addition reduces theductility of the material. LDPE-bonded sand blocks exhibited brittlebehaviour when tested in bending.

Plastic aggregates significantly reduce the thermal conductanceof concrete due to the low thermal conductivity of plastics(Iucolano et al., 2013). However, LDPE-bonded sand has higherthermal conductivity (1.72W/mK) than concrete or cement mor-tar. The thermal diffusivity and specific heat values are0.86 mm2/s and 2.0 MJ/m3 K respectively. This was unexpected,but can be explained by the reduced porosity observed in theLDPE-bonded sand samples. A reduction in air voids is reportedto reduce the rate of heat transfer in asphalt (Hassn et al., 2016).

Process Description:

1. Thermoplastic binder is cleaned and dried

2. Shredded thermoplastic binder is heated on hot plate

3. Sand is dried

4. Sand sieved and required amount measured

5. Melting thermoplastic is stirred to distribute the heat

6. Sieved sand is added while stirring continues

7. Homogenous plastic-bonded sand mortar is formed

8. Plastic bonded sand mortar is transferred into moulds and compacted

9. Samples are allowed to set and harden

10. Samples removed from moulds to produce plastic-bonded sand sample.

6

7

10 9 8

5

4 2

1 3

Fig. 4. Process flow diagram showing the method used to produce LDPE-sand composite materials.

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Fig. 9 shows an optical microscopic image of the polished opti-mised LDPE-bonded sand samples. This shows that the sand parti-cles are encapsulated and mechanically bonded into the plasticmatrix. The LDPE binder bonds the inert sand filler by filling sur-face irregularities and forming a strong inter-facial bond betweenthe filler and the matrix (Awaja et al., 2009). This produced thehigh strength of the optimised LDPE-bonded sand samples.

4. Discussion

Solid waste management should be considered an essential ser-vice on which modern society depends. Unfortunately, in manyparts of the world and particularly in DCs, waste management isnot given sufficient priority and does not receive the required levelof investment to deliver a sustainable waste management system.

As a result, more than two billion people worldwide do not have awaste collection service and the waste of more than three billionpeople is not managed correctly (Wilson et al., 2015). As the pop-ulation in many DCs increases the situation is likely to becomeworse. When there is inadequate waste management local com-munities will dump or burn waste. This impacts on public health,the environment and the local economy. Poor waste managementalso contributes to global environmental issues such as climatechange and air pollution. Dumping waste plastics because thereis no alternative outlet available blocks drains causing floodingand produces stagnant water in which insects can breed. Plasticsdisposal in water-ways is a major source of marine plastic andburning plastics causes air pollution that adversely impacts onpublic health.

It is now increasingly recognised that community-based wastemanagement initiatives have a key role in addressing waste

Fig. 5. Effect of sand particle size on the compressive strength of LDPE-bonded sandblocks containing 75 wt% sand: (a) density data and (b) compressive strength data.The error bars represent the standard deviation in the measured parameter.

Fig. 6. Effect of varying sand proportion on the compressive strengths of LDPEbonded sand: (a) density data and (b) compressive strength data. The error barsrepresent the standard deviation of the compressive strengths of the test samples.

Fig. 7. Compressive stress-strain curves of optimized LDPE bonded sand samplesshowing the viscoelastic behavior similar to asphalt under compression.

116 A. Kumi-Larbi Jnr et al. /Waste Management 80 (2018) 112–118

management problems in many parts of the world and particularlyin DCs (Lenkiewicz and Webster, 2017). This type of initiative isparticularly important in communities where local authorities donot have the resources to provide adequate waste managementservices. It is also clear that these types of initiative can providesustainable livelihoods through developing innovative productsmade by processing wastes. Plastic waste LDPE is a typical exampleof a problem that has potential to be transformed into a solution toproblems using the type of processing reported in this research.

LDPE-bonded sand is a material that has potential to be used ina wide range of applications and it is already used in blocks forroad and pavements in the Cameroon. Optimised LDPE-bondedsand has a density of 1.76 g�cm�3 and a compressive strength of27.3 MPa and this is significantly higher than the widely usedsandcrete mortars and is comparable to C20/25 concrete whichhas typical densities ranging from 2.24 g�cm�3 to 2.40 g�cm�3

(Kivrak et al., 2006). After shear failure under compressive loading,LDPE-bonded sand can still resist more than 30% of the maximum

a b

Compressive Force

Fig. 8. LDPE bonded sand samples before (a) and after compressive test and (b) showing the development of the shear plane in samples.

Fig. 9. Optical Microscopic image of a polished surface of LDPE-bonded sandshowing the encapsulation of sand grains (white) within the LDPE matrix.

A. Kumi-Larbi Jnr et al. /Waste Management 80 (2018) 112–118 117

failure load, suggesting LDPE-bonded sand could improve buildingsafety because of this visco-elastic behaviour.

The experiments show that the mechanical properties of LDPE-bonded sand are influenced by the sand particle size and sand toplastic ratio. The reduced compressive strengths and densitiesobserved with increasing sand particle size is due to lattice struc-tural changes occurring in the plastic-sand matrix, as shown inthe schematic diagram shown in Fig. 10a. Larger sand particle sizereduces the compressive strength of LDPE-bonded sand by increas-ing porosity and for a given volume, the binder thickness encapsu-lating the sand reduces as the surface area of the sand increases. Tocompensate for this phenomenon in normal Portland cement con-crete, a higher water/admixture is used (Mehdipour and Khayat,2017). In LDPE-bonded sand, larger air voids are formed and thesereduce the compressive strength.

Fig. 10b is a schematic diagram showing the variation in themicrostructure of LDPE-bonded sand samples with increasing

sand addition. Samples with sand proportions of 75 wt% have suf-ficient volume of LDPE binder to encapsulate the sand particlesand achieve optimum compressive strength. Excess LDPE binderis present in samples with sand additions below 75 wt%. Thecompressive strength of these samples is mainly dependent onthe LDPE content with little contribution from the interfacialbond between the sand and LDPE. Increasing the sand additionreduces sample porosity. However, sand additions above 75 wt%reduce the compressive strength because the LDPE binder volumeis not sufficient to properly coat and bind the sand grainstogether.

5. Conclusions

LDPE-bonded sand is a resource efficient material that cantransform waste LDPE into a valuable local resource. Water sachetsmade from LDPE are a problem because there are often very lim-ited recycling options for this material and they have an adverseimpact on public health and the environment. LDPE water sachetsand other sources can be used to form LDPE-bonded sand. Thisrequires simple processing and produces a durable, relatively light-weight material. No water is required in the production process.The particle size of the sand, compaction method and cooling rateare critical to attain optimum properties. The sand addition toachieve the maximum compressive strength was �75 wt%. Thecompressive strength of optimum LDPE-bonded sand is compara-ble to C20/25 concrete and greater than typical Portland cementsandcrete. LDPE-bonded sand has properties suitable for use in arange of applications. It is currently used to form paving blocksfor hard standing areas and pavements. It has potential to be usedin roofing tiles and partitioning walls. LDPE-bonded sand behavesas a viscoelastic material similar to asphalt in compression. It failsin shear, but LDPE-bonded sand samples retain at least 30% of loadafter failure. The production of LDPE-bonded sand can have majorsocial, public health and environmental benefits. By transformingwaste plastics into a valuable resource this simple technologyhas potential to generate local employment, clean-up the environ-ment, produce new construction materials and significantly reducethe amount of waste LDPE entering the oceans.

a)

b)

Sand grains

Sand grains

Air pores

Air pores

LDPE binder

LDPE binder

LDPE binder

LDPE binder

Sand grains

Sand grains

Air pores

Air pores

Fig. 10. (a) Lattice structural changes in LDPE bonded sand showing the increasing porosity and decrease in binder volume around the sand grains as sand particle sizeincreases; (b) Variation in the microstructure of LDPE bonded sand showing the reduction in binder paste volume and thickness encapsulating the sand grains as sandproportions increase.

118 A. Kumi-Larbi Jnr et al. /Waste Management 80 (2018) 112–118

Acknowledgements

The authors would like to thank Dr Luc Vandeperre, AndrewMorris and Les Clarke for their assistance in conducting somemechanical tests and the optical microscopy. Zoomlion Ghana Lim-ited is also acknowledged for sponsoring this project and supplyingLDPE plastic water sachets.

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