Linear Low-Density Polyethylene (LLDPE) is a type of plastic with a high percentage of solid waste and is hardly degradable. The characteristics of LLDPE such as high toughness and low density have the potential to be an alternative aggregate substitute for applications in non-structural mortar composites. The purpose of this study was to determine the suitable formulation of LLDPE aggregate substitution against silica sand for the application of non-structural adhesive mortar composites. Vinyl Acetate/Ethylene (VAE) surfactant (1.2 wt%) was used to improve the interface between LLDPE and the cement matrix. Mechanical testing; XRD and FTIR characterization; density measurements; compression, adhesion, and thermogravimetric test; and morphological observation were carried out in this experiment. The Results showed that LLDPE substitution decreased physical properties and accelerated thermal degradation. Cracks occurred on higher amount of LLDPE composition due to big difference in polarity. Suitable LLDPE substitution composition for adhesive mortar is up to 50%.
Research Article
Mechanical and Thermal Properties of Non-structural Adhesive Mortar using Linear Low Density Polyethylene (LLDPE) Aggregate Substitution with Vinyl Acetate/Ethylene (VAE) Interface
https://doi.org/10.21203/rs.3.rs-3234362/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 19 Dec, 2023
Version 1
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cement composites
linear low density polyethylene
vinyl acetate/ethylene
interface
mechanical properties
thermal properties
Massive growth of construction is often followed by problems with environmental impacts such as increased waste, reduced or damaged vegetation, dust pollution, noise pollution, and over-exploitation of nature [1]. Natural aggregates in the form of minerals and rocks are one of the most widely used non-renewable resources, especially in the manufacture of concrete composites or cement-based building materials. So that, the excessive exploitation can quickly deplete these mineral reserves [2]. Until 2060, it is predicted that the world's consumption of natural minerals, especially sand, gravel, and rocks for construction, will be 55 gigatons annually, so a concept of sustainable development is needed by reducing consumption of these natural minerals [3, 4].
Waste material management is also important to achieve a sustainable development. Residual materials consist of various types, such as household waste, waste that can be degraded, to those that are difficult or cannot be degraded. Low density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) contributed to about 90.62% solid waste weighing around 49 million tons in 2019, of the total production of various types of plastic, which was around 460 million tons, that was dominated by leftover packaging [5]. Korenkova & Sidorenko [6] explained that new material alternatives that are durable or not easy to be degraded can be applied in various composites, such as in cement-based composites manufacturing which have the greatest uptake in the world of construction. Various variations of these alternative materials can be sourced from renewable materials, nanomaterials, even to waste materials. The implementation of this material is also diverse, such as being used as a filler material, binder material, and additive, which in essence aims to increase the durability of a composite for construction. Plastic-based materials in this case have the potential to be used as an alternative substitute material because they are durable and not easy to degrade.
Research conducted by Naik et al. [7], Albano et al. [8], Frigione [9], Jaivignesh & Sofi [10], Badache et al. [11]; Lee et al. [12], İpek et al. [13], Shiuly et al. [14], and Alrshoudi et al. [15]; demonstrated the potential for using plastic aggregate as a substitute for aggregate in the cement matrix, but various constraints occurred in the cohesion of the concrete composite which affected the physical and mechanical properties of the composite. There is a tendency to decrease in compressive strength as a compensation for the increase of flexibility from plastic aggregate substitution. In addition, the decrease in compressive strength is also caused by the poor condition of the interface between the plastic and the cement matrix due to the difference in polarity between the two materials. Plastic is a material that has high hydrophobicity compared to the cement matrix which tends to be hydrophilic. One of the efforts made to improve the interface between the plastic aggregate and the cement matrix is by pre-treating the plastic aggregate with chemical immersion. Naik et al. [7] research used hypochlorite and sodium hydroxide solutions for HDPE pre-treatment, while Lee et al. [12] research used calcium hypochlorite and peroxide solutions for PET aggregates.
In this study, the substitution of silica sand aggregate with LLDPE aggregate was used. LLDPE is a copolymer of ethylene with α-olefins such as: 1-butane, 1-hexane, 1-octane, and 4-methyl-1-pentene with short branched chains which have a density of around 0.919–0.925 g/cm3, slightly higher of LDPE is around 0.910 g/cm3 [16, 17]. LLDPE has a lower crystalline point (41% Xtal) than HDPE (60–80% Xtal) because it has short branch chains with controlled branching patterns and length [18, 19]. The melting points of LLDPE and HDPE do not differ much in the range of ~ 135°C [20, 21]. The greater the LLDPE content, the higher amount of water is required in the formulation because, the hydrophobicity present in the formulation increases significantly, so that more water is needed to be able to wet the entire mortar matrix and aggregate to achieve a good consistency and workable mortar dough [22, 23, 24, 25, 26]. To achieve a workable mix, a minimum ratio required for water and cement content is 0.35 (for structural concrete) and 0.5 (for mortar) [27, 28]. As a reference, the w/c ratio used in mortar with the addition of up to ~ 12.5% plastic mixed aggregate is 0.5 [29], and an increase in the w/c ratio can potentially slow down curing time because by increasing the amount of water, the cement particles will have farther distances between each other so that the formation of hydrate bonds will be slower [30, 31].
In this study, to increase efficiency and optimize the process, the plastic aggregate was not pre-treated. Vinyl acetate/ethylene (VAE) copolymer-based surfactant was added to the formulation to improve interfacial conditions. Kozlov et al. [32] explained that the addition of VAE was able to react in a hydrophilic environment with hydrophobic aggregates. Zarrouki et al. [33] explained the use of VAE depending on the phase it has. If VAE is in semicrystalline form, it can produce films that are widely used in the packaging industry as a coating material. If VAE is in an amorphous form with elastomer-like properties, it can be used as an additive in paint and mortar products, the glue industry, and building material thickeners. Zhu et al. [34] stated that by using redispersible polymer (RP) powder was able to increase the adhesion in cement mortar. This is due to the interaction between ions from the cement. Tang et al. [35] stated that the ratio between ethylene and vinyl acetate monomers varies (100:0 to 0:100) causing the polarity of this VAE to range very wide from low to high, so it also has the potential to be used as an interface for various materials that have different polarity. Therefore, in order to optimize the superior characteristics of plastic-based materials, in this study the development of non-structural adhesive mortar composites was carried out using VAE surfactants.
Camacho et al. [36] explained that the critical point for the characteristics of the mortar no longer depends on the compressive strength of the mortar. Mortar products with high adhesion are widely used in construction applications such as adhesives for concrete bricks, autoclave aerated concrete (AAC) and aerated lightweight concrete (ALC) lightweight bricks; ceramic and natural stone adhesives; as well as concrete joint adhesive. Even though they are non-structural in nature, these products still need to use aggregates to create the required hardness and strength according to their designation. Since the critical point of mortar with high adhesion lies in its tensile strength and flexibility, plastic-based aggregate substitution is expected to be a potential for the realization of cement-based composite products that are more environmentally friendly in accordance with the mission to create more sustainable construction developments.
In this study, we tried to substitute silica sand aggregate with un-treated LLDPE granules. By adjusting the amount of LLDPE aggregates in the formulation we shall achieve an optimum formulation based on its physical properties. because of its use as an adhesive mortar, the critical mechanical property is tensile strength. There is a lack of study in combining VAE as a surfactant to improve mortar interface using un-treated hydrophobic aggregates and preserve its physical properties, therefore this research needs to be done to provide insight and empirical evidence regarding the optimization of non-structural adhesive mortar formulations.
The research process was carried out to obtain a mortar formula prototype using LLDPE aggregate size ~ 0.6 mm as a substitution aggregate for natural sand aggregate (similar size) which is commonly used in non-structural mortar formulations. Aggregate substitution is carried out until it reaches the optimum volume fraction percentage so that it complies with the tensile strength standard in various commercial mortar product applications such as: commercial tile adhesive (SNI 8831–1:2019) of ≥ 0.5 N/mm2 and commercial autoclaved lightweight brick adhesive (Mortindo) of ≥ 0.3 N/mm2. The research process is shown in the flowchart in Fig. 1.
2.1 Materials
The materials used in this study were portland cement (OPC) type I (MP–Ultramix G), CaCO3 calcium carbonate mesh 400 (B–400, Kurnia Artha Pertiwi), dry natural silica sand with a size of ~ 0.6 mm (Karya Djoni), LLDPE granules with a size ~ 0.6 mm (Etilenas), ceramic tile with a water absorption percentage of ≤ 3% (Kuda Laut), epoxy glue (Avian), K–250 instant concrete substrate with ~ 2.4 mm screening aggregates (Global Union), poly (vinyl acetate)-ethylene (VAE) (Wacker Vinnapas eco 5044N), methyl cellulose (MC) (Dow Walocel MW 40000), and starch ether (SE) (Avebe Opagel FP6).
2.2 Manufacturing of Mortar Composite Prototype
Mortar was manufactured with different concentration formulations of LLDPE to silica sand ranging from 0–100%. The raw material formulation of mortar for each composition variable is shown in Table 1. Additives are added in constant amounts for each variable where VAE is at ~ 1.2 wt% and MC and SE are ~ 0.3 wt% and ~ 0.05 wt% respectively [37]. The use of water is added differently in each formulation to obtain the appropriate workability so that it can be applied for compression and adhesion testing [24, 25, 26]. Table 2 describes the composition of water added to the mortar formulation for each variable. The manufacture of composite mortar is carried out in three ways which adopt SNI 8837–1:2019 as stated below:
| Sample Code | Materials | Additives | Total Weight | ||||||
|---|---|---|---|---|---|---|---|---|---|
| OPC Type 1 | CaCO3 | Silica Sand 0.6 | LLDPE 0.6 | VAE | MC | SE | |||
| gram | gram | gram | gram | wt% | gram | gram | gram | gram | |
| LLDPE-Si 0% | 3500.00 | 1000.00 | 5500.00 | - | - | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 10% | 3500.00 | 1000.00 | 4950.00 | 550.00 | 5.42 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 20% | 3500.00 | 1000.00 | 4400.00 | 1100.00 | 10.83 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 30% | 3500.00 | 1000.00 | 3850.00 | 1650.00 | 16.25 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 40% | 3500.00 | 1000.00 | 3300.00 | 2200.00 | 21.66 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 50% | 3500.00 | 1000.00 | 2750.00 | 2750.00 | 27.08 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 60% | 3500.00 | 1000.00 | 2200.00 | 3300.00 | 32.49 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 70% | 3500.00 | 1000.00 | 1650.00 | 3850.00 | 37.91 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 80% | 3500.00 | 1000.00 | 1100.00 | 4400.00 | 43.32 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 90% | 3500.00 | 1000.00 | 550.00 | 4950.00 | 48.74 | 120.00 | 31.25 | 5.00 | 10156.25 |
| LLDPE-Si 100% | 3500.00 | 1000.00 | - | 5500.00 | 54.15 | 120.00 | 31.25 | 5.00 | 10156.25 |
| Sampel Code | Water Amount (ml/kg) | w/c ratio |
|---|---|---|
| LLDPE-Si 0% | 200 | 0.57 |
| LLDPE-Si 10% | 250 | 0.71 |
| LLDPE-Si 20% | 300 | 0.86 |
| LLDPE-Si 30% | 350 | 1.00 |
| LLDPE-Si 40% | 400 | 1.14 |
| LLDPE-Si 50% | 450 | 1.29 |
| LLDPE-Si 60% | 500 | 1.43 |
| LLDPE-Si 70% | 600 | 1.71 |
| LLDPE-Si 80% | 700 | 2.00 |
| LLDPE-Si 90% | 800 | 2.29 |
| LLDPE-Si 100% | 900 | 2.57 |
-
Dry mixed mortar in the form of powder (not yet hydrated) for characterization such as XRD, FTIR, and thermal testing (TGA).
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Hydrated mortar was made into concrete cubes with a size of 50×50×50 mm3 which were used for analysis of mechanical properties such as: density, moisture content, compressive strength, and morphology (FE-SEM).
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Mortar is applied as an adhesive that were used for adhesion test and determine the ability of the mortar as an adhesive for various desired substrates.
2.3 Preliminary Characterization
2.3.1 Mortar Composition Characterization Using X-Ray Diffraction [38]
Sample specimens were pulverized to a particle size of ≤ 3.4 µm. The specimens were ground using a ball mill in a dry process using a 0.05 wt% diethylene glycol (DEG) grinding agent to reduce excessive heat due to friction during the grinding process. The powder from these sample specimens was analysed using XRD with Cu Kα radiation at a wavelength of λ = 0.154 nm. The sample is scanned between 10–65° with a high-speed detector. The step-size used is around 0.019°/step. The voltage and power of the X-Ray tube used are 40 kV and 40 mA.
2.3.2 Polymers Characterization Using Fourier-Transform Infrared Spectroscopy [39, 40]
FTIR characterization was used to determine the chemical bonds found in various polymer specimens used in this study, consist of LLDPE, VAE, MC, and SE. This polymer characterization is used in the early stages as confirmation that the polymer used is in accordance with its characteristics and applications. Hydrophilic polymer powders (the VAE, MC, and SE specimens used in this study) were formed into thin film layers through a dissolution process to become a gel which was then dried in an oven at 40°C to obtain a thin film layer (1–1.5 µm). LLDPE granules were melted at around 260°C to also obtain a desirable thin film layer. This thin film layer were analysed with a resolution of 32 cm-1 at a wavenumber of 450–4000 cm-1 with a scan speed of 0.2.
2.4 Mechanical Properties Analysis
2.4.1 Compressive Test
Compressive testing was carried out based on SNI 03–6825–2002 (Indonesian testing method for portland cement mortar for civil works). Homogeneous mortar mix was poured into steel moulds with dimensions of 50×50×50 mm3. Moulding using 2 stage of layering was prepared to minimize trapped air. Each layer must be compacted 32 times with 4 rotations in the configuration and must be started no later than 2.5 minutes after mixing was carried out.
The upper surface of the test cube mould was leveled and left to set overnight (24 hours) before being removed from the mould and cured in a bath filled with clean water. The curing time was determined periodically until the 28th day. The samples for the compressive strength test in this study were taken in triplicate at the 7th and 28th day. The compressive strength test results can be calculated by Eq. 1.
\(K= \frac{{P}_{max}}{A}\) Eq. 1
where K is the compressive strength of the tile adhesive (Kg/cm2), Pmax is the maximum compressive force on the test tool (N), and A is the cross-sectional area of the test piece (mm2).
2.4.2 Measurement of Density
Density measurements were carried out based on SNI 03–6825–2002 using sample specimens with dimensions similar to the sample specimens used for the compressive strength test, namely 50×50×50 mm3. The sample specimens tested for density were the sample specimens that are already set and dry (not in the condition that they have just been lifted from the soaking process). Density can be calculated by Eq. 2.
\(\rho = \frac{m}{v}\) Eq. 2
where ρ is the density of the mortar (kg/m3), m is the mass of the sample (kg), and v is the volume of the sample (m3).
Oven-dry density measurements were carried out by drying the mortar samples using an oven at 100–110°C for 24 hours and then weighing them after cooling. The calculation of oven-dry density is also carried out by Eq. 2.
2.4.3 Adhesion Test
The adhesion test is carried out by adopting a method based on SNI 8837-1:2019; Botas et al. [41], Botas et al. [42], and Santos et al. [43]. The tensile strength test was preceded by making a K–250 concrete substrate with a size of 50×50×50 mm3. The tile substrate used is ceramic with a thickness of 7–10 mm with dimensions of 50×50 mm2 with a back profile pattern having a maximum depth of 0.25 mm. The appearance of concrete and ceramics can be seen in Fig. 2. The mortar is applied to the concrete substrate using a trowel until it forms a jagged line. The curing time for this process is 27 days. After 27 days, attach the dolly to the tile surface with a high adhesive glue (such as epoxy glue) and leave it for 24 hours. The tensile strength test was carried out using a pull-off device on the 28th day. Tensile strength is calculated by Eq. 3.
\({A}_{s}=\frac{L}{A}\) Eq. 3
where As is the individual adhesion (N/mm2), L is the total tensile force of the pull-off tool (N), and A is the surface area of the ceramic (mm2). The tensile strength value each time is determined by taking the average of 5 measurements. Figure 3 shows how the tensile strength test works.
2.4.4 Measurement of Water Absorption on Substrates Related to Adhesion Test
Tests for water absorption were carried out on the two main substrates used in the adhesive strength test, namely concrete cubes 50×50×50 mm3 and ceramic tiles 50×50 mm2. Initially, the samples were dried at 60°C for 24 hours, then weighed. The samples were then immersed in water for 24 hours at room temperature, then also weighed. The difference in weights determined the percentage of water absorption according to Eq. 4 where msaturated is the mass of the sample after immersion and mdry is the mass of the sample before immersion.
\(Water absorption=\left[\frac{{m}_{saturated}-{m}_{dry}}{{m}_{dry}}\right]\times 100\%\) Eq. 4
2.5 Mortar Characterization
2.5.1 Transition Zone and Microstructural Analysis Using Digital Microscope and Field Emission – Scanning Electron Microscope (FE-SEM) for Selected Variables [44]
Observation of the surface morphology of the mortar samples was carried out by digital microscope imaging of samples with dimensions of 10×10×10 mm3. The magnification used was 200×. The characterization of the transition zone in this mortar sample was carried out using FE-SEM with the back-scattered electron method with an intensity of 20 keV. Sample preparation was carried out using the method in the study of Nemati [45] where the sample was sliced thin with a thickness of 5 mm and dimensions of 25×25 mm2. The sample surfaces were polished with a carbide grinder and fixed with a glass slide with a diameter of 25 mm, then analysed with FE-SEM. The magnification used in FE-SEM observations is 5,000× and 20,000×.
2.5.2 Thermal Characterization of Selected Variables Using Thermogravimetric Analysis (TGA) [46]
Composite sample with a mass of about 50 mg was placed into an alumina crucible which had a volume of 100 mm3. Samples were heated simultaneously with a temperature range of 25°C to 1000°C. The temperature change was adjusted at around 5°C per minute in the test chamber conditions with argon to avoid potential oxidation. The gas flow rate is maintained at approximately 40 mL per minute.
2.6 Data Analysis
Statistical tests were carried out on the effect of differences in the composition of LLDPE aggregate (0% – 100%), on density and oven-dry density, difference in weight between densities, compressive strength, and mortar tensile strength. The statistical test method used was a one-factor completely randomized design (CRD). Statistical analysis of density, oven-dry density, and compressive strength was carried out with three repetitions, while statistical analysis of adhesive strength was carried out with five repetitions.
Data analysis for the effect of differences in the composition of the LLDPE aggregate on density, oven-dry density, difference in weight between densities, compressive strength, and adhesive strength was carried out using Microsoft Excel 2020 with one-factor complete randomized design (CRD) method and Duncan's post-test. Other data is tabulated in the form of tables, graphs, and images using Microsoft Excel 2020, Origin 2023, and Match! 3.
3.1 Raw Materials Preliminary Characterization
Crystalline raw materials (OPC type I, calcium carbonate, and silica sand) were characterized using XRD. Polymer raw materials (LLDPE, VAE, MC, and SE) were characterized using FTIR in Figs. 4 and 5. Diffractogram of OPC in Fig. 4a shows the dominance of Ca and Si elements starting from an angle of 29° – 40° at 2θ which is marked with an identical peak of alite (Ca3SiO5) at an angle of 29.5°; 41.4°; and 51.9° and belite (Ca2SiO4) at peaks of 32.3° and 32.7°. Alite and belite are the dominant compounds commonly found in cement, followed by other compounds in smaller amounts, such as calcium sulfate (CaSO4), silicon oxide (SiO2), and calcium silicon oxide (CaSiO3) [47, 48].
Diffractogram of calcium carbonate in Fig. 4b shows calcite dominance starting from an angle of 23° – 65° at 2θ which is marked by the highest peak at an angle of 29.5° and followed by other peaks at 23.1°; 36.1°; 39.5°; 43.3°; 47.7°; 48.7°; 57.6°; and 65°. The results of this analysis are similar to the databases recorded in Boudaira et al. [49] and RRUFFa [50]. Diffractogram of silica sand in Fig. 4c shows the dominance of silica (SiO2) starting from an angle of 20° – 80° at 2θ which is marked by the highest peak at an angle of 26.8° and followed by other peaks at 21.0°; 36.7°; 39.7°; 40.5°; 42.7°; 45.2°; 46.1°; 50.4°; 53.9°; 60.3°; 68.1°; 68.6°; 73.8°; and 76.1°. The results of this analysis are similar to the databases recorded in Hashemi et al. [51] and RRUFFb [52].
The FTIR spectra of LLDPE in Fig. 5a shows the typical spectra of polyethylene copolymers which are divided into olefins and ethylene which focus on the dominance of C–H bonds, where the C–H stretching bands are shown at wave numbers 2915.12 and 2848.22 cm-1, C–H bending at wave numbers 1471.70 and 1463.77 cm-1, and C–H out-of-plane bending at wave numbers 729.48 and 718.50 cm-1. The test results are similar to the database recorded in Ismail et al. [53]. The C–H (hydrocarbon) bond is the most important bond in the synthesis of polyethylene and its various types, which are the simplest structure of a polymer, so the catalytic oxidation of C–H is important to determine the bond strength of the polyethylene itself which can later determine the direction which polyethylene will be formed according to the density and chain branching [54, 55].
The FTIR spectra of VAE in Fig. 5b shows the typical spectra of the vinyl acetate/ethylene copolymer which is divided into ethylene groups and vinyl acetate groups, where the peak of the ester group of this copolymer is at wave number 1731.90; 1232.23; 1029.01; and 538.66 cm-1; with a C = O carbonyl group indicated by a wave number of 1731.90 cm-1 and an O–H hydroxyl group indicated by a wave number of 3693.33 cm-1. The ethylene group is indicated by the wave number 2928.45; 2848.22; 1428.14; 1371.54; and 711.70 cm-1. The results of this analysis are similar to the databases recorded in Ismail et al. [53] and Bidsorkhi et al. [56].
FTIR spectra of MC in Fig. 5c shows the spectra of the hydroxyl group (O-H) at wave number 3432.85 cm-1, the C-H group at wave number 2901.07 cm-1, the deformed C-H group at wave number 1463.81 and 1372.96 cm-1, the glycosidic group (C-O-C) at wave number 1063.08 cm-1, and the C = C group at wave number 945.22 cm-1. The results of this analysis are similar to the database recorded in Nadour et al. [57]. FTIR characterization of SE in Fig. 6d shows the spectra of the hydroxyl group (O–H) at wave number 3326.13 cm-1, the C–H group at wave number 2927.53 cm-1, the C–H group is deformed at wave number 1414, 69 and 1372.64 cm-1, the glycosidic group (C–O–C) at wave number 1015.00 cm-1, the O–CH3 group at wave number 940.88 cm-1, and the –COO group (which arises from starch methylation reaction) at wave numbers 1731.06 and 1595.11 cm-1. The results of this analysis are similar to the database recorded in Yaacob et al. [58]. MC or SE are derived from plant products, namely cellulose and starch, which go through a methylation process in the form of etherification of their hydroxyl groups with various reactive substances [59, 60, 61, 62]. The results of the preliminary characterization indicated that the raw materials were in accordance with the standards and previous studies.
3.2 Measurement of Density and Water Absorption
Figure 6 shows the results of the density measurements. In general, the density of LLDPE (938 kg/m3) is lower than that of silica sand (~ 2650 kg/m3 [63]) so that the increase of LLDPE content that replaces silica sand aggregate will decrease the density of the mortar. The density of 0% control specimen (without LLDPE aggregate) was 1893.33 kg/m3 while the density of 100% control (without silica sand aggregate) was 986.67 kg/m3. Conventional mortar density reference (without aggregate substitution), corresponds to the density range of 1450–2000 kg/m3 recorded in Newman & Owens [64]. The oven-dry density of the 0% control specimen was 1600.00 kg/m3, while the 100% control specimen was 613.33 kg/m3. The oven-dry density reference of conventional mortar (without aggregate substitution) corresponds to the density range of 1200–2000 kg/m3 recorded also in Newman & Owens [64]. As in the bulk density test, while the lighter aggregate is added, the lower the oven-dry density will become [65].
The weight percentage difference between bulk density and oven-dry density describes the amount of water lost when heated (~ 105°C). The weight percentage difference in the 0% control specimen was 15.49%, while in the 100% control specimen was 37.84%. In specimens with an aggregate composition of 10% − 40% LLDPE the weight difference is relatively stable as proofed by the absence of a significant difference from the statistical test results where the hypothesis is not rejected which can be seen from the similarity of the superscript alphabet notation above the data values in Fig. 6. This shows that the additives which was added such as VAE copolymer, was still able to maintain interfacial conditions similar to 0% control specimen. The results of subsequent analysis showed that the more addition of LLDPE aggregate composition caused an increase in the weight difference between bulk density and oven-dry density. This shows that the higher the LLDPE content, the mortar composite will experience a reduction in its ability to bind water, which is due to its increased hydrophobicity which results in reduced mortar cohesiveness. A decrease in water retention will result in a higher potential for water bleeding, so that it will affect the curing process which will also have an impact on the mechanical properties of the mortar [66, 67, 68, 69]. This phenomenon is also proven from the results of the compression test and adhesive test in sub-Chaps. 3.3 and 3.4.
3.3 Compression Test
The range of compressive strength data resulting from the test is 7.85–94.80 kg/cm2 for 7 days of age and 11.59–106.93 kg/cm3 for 28 years of age day (Fig. 7). The compressive strength test results showed a decrease of up to 91.7% and 89.2% in mortar specimens with 100% LLDPE aggregate aged 7 and 28 days. A drastic decrease in compressive strength occurred in 10% (5.42 wt%) LLDPE specimen of 40.8% (7 days of age) and 41.6% (28 days of age). This phenomenon reflects two things. First, the density and mechanical properties of the LLDPE aggregate substituted for silica sand have lower density and weaker strength; secondly, the polarity of the aggregate certainly affects the overall cohesiveness of the mortar composite, even though a fixed amount of 1.2 wt% VAE has been added as a polymer to improve the interfacial conditions.
The addition of additives such as VAE, MC, and SE have the potential to slow down the hydration of cement (C–S–H formation) caused by slowing down the mobility of ions in the mortar [70, 71]. From this statement it can be concluded that there is a distance between the cement particles, which may be due to the addition of aggregate or water molecules which can potentially reduce the compressive strength due to the slowing down of the formation of hydrate bonds in the concrete curing process. However, because the addition of additives in the formulation is constant, the main contributor that results in a decrease in compressive strength can be said to come from LLDPE aggregate substitution. This is supported by the crack pattern in Fig. 8 where the higher concentration of LLDPE added increases the ductility of the composite. In the control mortar specimen with 0% LLDPE, it was seen that the mortar was cracked because it was basically brittle, and the higher the addition of LLDPE, the more ductile the specimen would be with the ability to maintain a more thorough shape and deformation because ductility will usually help improve the distribution of loads to the entire mass of the composite [72, 73].
The phenomenon of a significant decrease in compressive strength (~ 42–90% in mortar aged 28 days) is in line with various previous studies indicating that the difference in strength of LLDPE aggregate and silica sand plays a role in reducing this compressive strength. Naek et al. [7] explained a decrease in compressive strength (day 28) of ~ 45% in concrete with the use of HDPE aggregate soaked in NaOH with a size of ~ 1 mm by 2 wt%. Albano et al. [8] explained that a decrease in compressive strength (day 28) of ~ 64% in concrete with the use of ~ 10 mm size PET was up to 20%. Badache et al. [11] stated that there was a decrease in compressive strength of up to 57% with the use of up to 60% recycled HDPE. İpek et al. [13], stated that there was a decrease in compressive strength of 65.7% and 70.3% when using LDPE aggregate with an aggregate cement (a/c) ratio of 7 and 6. Alrshoudi et al. [15] stated that there was a decrease in compressive strength of up to 61, 2%; 69.2%; and 74.8% in the use of recycled PP aggregate with a composition of 30%, 40% and 50%.
3.4 Adhesion Test
The adhesive test was carried out using a substrate that has low water absorption, so that the resulting tensile strength value is expected to reflect the real tensile strength of the mortar composite. A concrete cube substrate with dimensions of 50×50×50 mm3 has an average water absorption capacity of 6.92%, and a ceramic substrate (homogeneous tile) with dimensions of 50×50 mm2 has an average water absorption capacity of 0.15%. The compressive strength of the concrete cube substrate is in the range of ≥ 250 kg/cm2 according to concrete standards commonly used for floors or walls.
Figure 9 shows the results of the tensile strength test on mortar specimens starting from a sample of 0–100% LLDPE aggregate composition in order to replace silica sand aggregate. The 0% specimen showed a tensile strength of 0.71 N/mm2 and decreased to 0.05 N/mm2 at 100% LLDPE composition. The decrease in tensile strength along with the addition of LLDPE indicates a decrease in optimization in the interfacial zone which is affected by the difference in polarity between the aggregate and the matrix. This was compensated for by adding VAE at a fixed dose of ~ 1.2% to the specimen to improve interface. To meet the tensile strength criteria of high adhesive mortar such as tile adhesive (≥ 0.5 N/mm2) LLDPE can be added up to 10%, and to meet the tensile strength criteria of standard adhesive mortar such as light brick and wall plaster (≥ 0.3 N/mm2), the substitution of LLDPE can be added up to 50%. The addition of LLDPE above 50% will reduce the tensile strength so that it no longer meets the standards of mortar applications in general. This can actually be compensated for by adding more VAE to the formulation, but of course this greatly affected the increase in manufacturing costs, so that it is no longer feasible [37].
The decrease of tensile strength phenomenon in this study is in line with several previous studies using plastic as a substitute for aggregate in mortar composites. Albano et al. [8] explained that adding 20 wt% of ~ 10 mm PET reduced the tensile strength by up to ~ 32% (w/c 0.5) and ~ 37% (w/c 0.6). Frigione [9] explained that the addition of 0.1–5 mm PET as much as 1 wt% could reduce the tensile strength by up to ~ 2.4%. Jaivignesh & Sofi [10] explained that the addition of various types of plastic with a size of 1 mm up to 25% of the total aggregate could reduce the tensile strength by up to 10–24%. Shiuly et al. [14] explained that the addition of 1–5 mm PET as much as 40% of the total aggregate could reduce the adhesive strength by up to 48%.
Figure 10 shows the peel off pattern in the adhesive test where the higher amount of LLDPE added, the more difficult the mortar specimens to stick to the surface of homogenous tiles, which already have very low water absorption. In the 0% control specimen, the mortar is able to stick and adhere well to the concrete substrate and homogenous tiles surface. In contrast, in the 100% control sample, there was no longer any mortar attached to the homogenous tiles. This shows that the condition of the mortar specimen with higher LLDPE content becomes very hydrophobic, so that the performance of VAE as a polymer is no longer optimal to improve the interface (can be seen in the FE-SEM image in Fig. 14d). The role of VAE is important in maintaining the tensile strength of the mortar, especially for mortars that are applied thinly, where the tensile strength is directly related to the ability of the polymer to bridge microcracks in the composite, and increases elasticity thereby reducing the potential for deformation due to shear-force [74]. Peng et al. [71] explained that the mechanism of action of polymers in cement composites consist of the absorption of polymer particles on the surface of cement particles. Then the formation of layers occurred by the addition of a small amount of water because it is used for hydration. At last, occurred an interconnection between the polymer layers and cement hydration. This should reduce the porosity of the cement matrix, thereby increasing mechanical properties; whereby decreasing porosity could inhibit the formation of impurity substances such as sulfate and chlorine ions, which of course also increased toughness [75].
3.5 Characterization for the Selected Composition Variable
Based on the adhesive test results, the selected compositions were samples with LLDPE aggregate concentrations of 0, 10, 50 and 100%. This is done to reflect each category, where 0 and 100% represent positive and negative controls, 10% represent criteria for adhesive strength ≥ 0.5 N/mm2, and 50% represent adhesive strength criteria ≥ 0.3 N/mm2 (see Fig. 10). Subsequent tests included XRD/FTIR characterization, morphological analysis, and thermal analysis.
3.5.1 XRD/FTIR Characterization of Selected Mortar Composite
Figure 11 shows the diffractogram of selected samples at compositions of 0, 10, 50, and 100% LLDPE aggregate. In Fig. 11a the typical peak of calcite is at an angle of 29.5° (the peak with dominant intensity) and is followed by other peaks at 36.1°; 39.6°; 43.3°; 47.7°; 48.8°; 56.8°; as well as 57.7°. The typical peak of silica is at an angle of 26.7°; which is in accordance with the data recorded in İpekci et al. [76] and RRUFFb [52]. Figure 12d shows the diffractogram with 100% LLDPE aggregate composition, which is characterized by typical spectra in the range of 21.7° and 23.9°; which is consistent with the data recorded by Alsaygh et al. [77] and Marinkovic et al. [78]. The phenomenon that occurs is the addition of more and more LLDPE aggregate composition (which means reducing the composition of silica sand aggregate), characterized by a decrease in silica intensity (in the range of ~ 26.7°) and an increase in LLDPE intensity (in the range of ~ 21.7° and 23.9 °). The new LLDPE peaks are starting to be visible in Fig. 12b (10% aggregate composition) because there was no addition of LLDPE to the 0% control.
The FTIR spectra of LLDPE has a distinctive spectrum on the C–H bond which is the main bond in a polyethylene polymer, especially in the range of wave numbers 2800 and 2900 cm− 1 as in Zhang et al. [16] and Ismail et al. [53]. The addition of LLDPE as an aggregate can be seen in the FTIR spectra of mortar specimens in Fig. 12. The higher the LLDPE composition in a specimen, resulting the sharper LLDPE spectral intensity at wave numbers ~ 2800 and ~ 2900 cm− 1 (Fig. 12b, c, and d). In Fig. 12a, no LLDPE spectral peaks were detected because LLDPE aggregate substitution had not been added. The other bonds have relatively the same intensity in each of the existing specimens.
3.5.2 Morphology and Microstructure Observation of Selected Mortar Composite
Figure 13 shows digital microscope images of selected mortar samples with a magnification of 200×. In Fig. 14a silica is the dominant aggregate in the mortar which is characterized by darker colored and translucent crystals. This also applies to Fig. 14b with the composition of 10% LLDPE aggregate marked with white lumps. In samples with an aggregate composition of 0 and 10% LLDPE, the matrix was seen to be denser and less sandy, which indicated that the bonds and cohesion in the composite were well formed. The large addition of LLDPE aggregate (50%) as shown in Fig. 13c begins to show a sandier matrix, because of the higher amount of hydrophobic material. Although the addition of 1.2% VAE uniformly in all formulation supposed to compensate the polarity difference there still has a significant decrease of mechanical properties even though it is still included in application acceptance according to tensile strength standards. Sample with 100% LLDPE aggregate composition in Fig. 13d shows that LLDPE aggregates are dominantly scattered and have a sandy texture because the cohesiveness of the composite is not optimal due to the inability of VAE added as a good interfacial formation.
Figure 14 shows the FE-SEM images of mortar samples, with selected compositions and different magnifications. Figure 14a is a FE-SEM image of a sample with 0% LLDPE aggregate composition showing optimal hydration. This is indicated by the dominant presence of C–S–H in the form of shape like cauliflower [79]; which is able to bond well with silica aggregate (Fig. 14a2). At this point it is assumed that ettringite in the 0% sample has dissolved before the formation of C–S–H occurs. This is in line with a good hydration process where ettringite will form in a short time (initial reaction), then dissolves and begins to form C–S–H as in Žemlička et al. [80]; Moranville-Regourd & Kamali-Bernard [81] and Rego et al. [82]. Figure 15b is a FE-SEM image of a sample with an aggregate composition of 10% LLDPE showing optimal hydration with C–S–H dominance. Not much different from the 0% sample, this 10% sample has good cohesiveness, as seen from very minimal cracking at higher magnifications (Fig. 14b2).
A significant phenomenon occurred in samples with 50% aggregate composition (Fig. 14c) where it was seen that the hydration of the cement began to be disturbed by the large number of hydrophobic aggregates. This can be seen from the presence of a significant amount of needle-shaped ettringite [83] and microscopic cracks that are clearly visible, although this can still be compensated by the reaction of the surrounding cement and VAE (Fig. 14c2). Figure 14d with the composition of 100% LLDPE aggregate shows poor hydration. The FE-SEM image in Fig. 14d1 shows that the C–S–H bonds formed are almost non-existent and unable to bind the LLDPE aggregate. This is supported by a larger magnification image in Fig. 14d2 where cracks occur because there is no bond between the aggregates. The results of this analysis strengthen the result of the mechanical properties test in the previous sub-chapters (Sub-Chaps. 3.3 and 3.4) where the higher amount of hydrophobic aggregate added, the worse the interface shall be, even though polymer has been added, due to the inability of the polymer to form bonds, absorb water for hydration, and fill the pores in the composites [71, 74, 75].
Figure 15 shows the EDS spectra of the sample with the selected composition. The greater the addition of the LLDPE aggregate composition, the lower the Si elements and the more increase the C elements in the specimens. This is a contribution from the increase in hydrocarbon bonds (C–H) due to the increase in polyethylene groups in specimen. The increase in O element is due to the increase in hydrocarbon bonds which is a contribution from the increase in the amount of water used for hydration of the cement so that it becomes a more workable mortar. Elemental Ca has a relatively uniform amount in all formulation variations of the samples. The EDS results are in line with and support the FTIR and XRD characterization results in Sub-Chap. 3.5.1.
3.5.3 Thermogravimetric Analysis
Table 3 and Fig. 16 show the results of the thermogravimetric analysis of specimens with the selected compositions of 0, 10, 50 and 100%. In all samples, a 1% weight loss began to occur at a temperature in the range of 325–351°C where at this point the free water had completely evaporated and dehydration of C–S–H occurred. The phenomenon occurs in the temperature range of 400–550°C where the higher LLDPE composition contained in the specimen, the greater weight loss occurs at the lower degradation temperature as well (Fig. 16b, c, and d). This is because the higher composition of LLDPE, consist of higher C–H bonds in the sample, which are the main bonds in LLDPE. The increase in C–H can be reflected by increasing the intensity of the carbon element in the EDS data in Fig. 15. C–H dehydration also occurs from the contribution of the use of 1.2 wt% VAE polymer which also has C–H bonds, one of which can be seen in the histogram in Fig. 16a as a control without LLDPE composition at a temperature range of 464–500°C.
| Specimen | Temperature Range (°C) | Degraded Components |
|---|---|---|
| Mortar + LLDPE aggregate | ≥ 105 | water evaporation |
| 325–400 | volatile compound evaporation | |
| C–S–H started to degrade | ||
| 400–550 | C–H degradation | |
| 600–750 | CaCO3 decarbonization | |
| 750–1000 | stable phase below crystalline degradation temperature |
The final degradation step is the decarbonation of CaCO3 where all test specimens experience a relatively uniform decarbonization in the range of ~ 650°C which is contributed by the composition of CaCO3 which is approximately the same amount, either as a filler material, or as one of the materials in the manufacture of OPC. At temperatures above 750–1000°C the weight loss is relatively stable because it is still below the degradation temperature of the remaining crystalline aggregates, so from the histogram the specimens which have the highest silica content (Fig. 16a) will experience the least weight loss (~ 15% up to 1000°C), less than those with little or no silica content (Fig. 16d). The results of this thermal test are in line with the thermal tests of mortar composites conducted by Alcaron-Ruiz et al. [84], DeJong & Ulm [85], Zhao et al. [86], and Pradhan et al. [87] where the decarbonation point of cement and calcium carbonates exist in the range of 600°C to 900°C. Thermal analysis can be used as a reference for the application of mortar with LLDPE aggregate according to its designation and fire resistance ability.
3.6 Summary of Research Results
Substitution of silica sand aggregate with LLDPE aggregate has the potential to be used as an alternative formulation of environmentally friendly mortar composites. The characteristics of LLDPE which is weaker than silica sand will affect the mechanical properties of the mortar composite. This is reflected in the results of the compressive test in the study, where by substituting silica sand aggregate with 10% LLDPE aggregate, the compressive strength was reduced by ~ 41% and then gradually decreased to ~ 90% in samples with 100% LLDPE aggregate composition.
The higher composition of the LLDPE aggregate in the composite had an impact on decreasing the density of the composite (either bulk density or oven-dry density). The weight difference between the two densities, which indicates the ability to hold water (water retention), is quite stable in the range of 15–16% in samples with LLDPE composition of 0, 10, 20, 30, and 40%, above that composition the water bleeding has increased due to the increased hydrophobicity of the aggregates. The success of the composite in retaining water up to 40% LLDPE composition substitution is a contribution from the ability of the VAE copolymer to maintain the interface between materials of different polarity, especially LLDPE and the entire composite. VAE is a copolymer that has a wide polarity range because it is formed from vinyl acetate groups which tend to be hydrophilic and ethylene which tend to be hydrophobic.
The different polarity between LLDPE and the overall composite matrix also affected the mechanical properties of the mortar composite. This is also reflected in the results of the tensile test on the mortar composite which decreased with the addition of the LLDPE composition. The addition of 1.2 wt% VAE copolymer was able to improve interfacial conditions in composites with LLDPE aggregate substitution of up to 50% of silica sand aggregates based on mortar acceptance standards according to SNI 8837–1:2019 and commercial mortars (≥ 0.5 N/mm2 for tile adhesives and ≥ 0.3 N/mm2 for light brick adhesives).
The greater the composition of the LLDPE aggregate in the mortar composite, the slower the hydration reaction to cement can be. Morphologically this can be proven from the remaining ettringite in samples with high LLDPE composition (50%). In samples with 100% LLDPE composition, hydration is not optimal, as seen from the inability of the matrix to surround and wrap the LLDPE aggregate resulting in very clear cracks appearing in the FE-SEM image.
The higher the use of LLDPE in the composite, the easier the sample will be degraded at lower temperatures, however other materials such as cement and calcium carbonate that remain, degrade quite stably at a temperature range according to their characteristics (700–900°C). Of course, this thermal degradation can be used to determine the proper application of mortar composites with LLDPE aggregate substitution.
The addition of LLDPE aggregate as a substitute for silica sand aggregate has an effect on the mechanical properties of the mortar. The density of LLDPE which is much lower than silica sand provides the advantage of better load-bearing ability when applied, and of course has the potential to reduce structural loads. The lower the hardness of LLDPE than silica sand, significantly affects the decrease in the compressive strength of the mortar (~ 42–90%) and depends to the composition used. The difference in polarity between the LLDPE aggregate and the cement matrix affects the tensile strength of the mortar where the 10% LLDPE composition is the maximum composition to meet the tile adhesive criteria (tensile strength ≥ 0.5 N/mm2), while the 50% LLDPE composition is the maximum composition to meet the adhesive criteria lightweight brick-laying (tensile strength ≥ 0.3 N/mm2).
The use of VAE surfactant (~ 1.2 wt%) acts to maintain the interfacial conditions between hydrophobic aggregate (LLDPE) and hydrophilic matrix (cement). Good and improved interfacial conditions are proven to be able to maintain mortar cohesiveness. However, due to the fixed amount surfactant in each composition variable, at a certain point, VAE is no longer able to maintain the composite interface, which is reflected by the occurrence of cracks in the composite (≥ 50%). Based on the results of this study, the substitution of LLDPE aggregate for silica sand has the potential to be used as an alternative to an effective mortar formulation so that, besides being applicable, this formulation is also able to reduce the impact of environmental pollution by plastic waste. For the use of mortars with high adhesion properties (such as tile adhesive products) the recommended formulation is LLDPE aggregate substitution up to 10% composition, while for mortars with moderate adhesion properties (such as light brick adhesive products) the recommended formulation is up to 50% LLDPE aggregate composition.
5. Acknowledgement
This study was conducted by the support dan research facility of Integrated Laboratory of Biomasses and Bioproducts (iLab), Research Center for Biomasses and Bioproducts, Badan Riset dan Inovasi Nasional (BRIN). All researchers would also like to thank PT Wacker Chemicals Indonesia and PT Pintu Mas Mulia Kimia for their outstanding support of raw materials and additives. Finally, we would like to thank all research assistants and laboratory instrument operators who have supported the success of this research.
6. Research Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
7. Author contribution statement
AH conceived and conducted the experiments, prepare research draft, figure, and tables. ALJ supervised the research and carried out research draft. IB conceived, designed, and supervised the research. S and BS supervised the research and proof read the draft results. MS carried out research draft and did statistical analysis. S; DP; N; and FA assisted in field works and instrumental operation. JAL and MAK assisted in laboratory works, processing raw data, proof read the research draft.
8. Disclosure Statement
No potential conflict of interest was reported by the authors.
9. Data availability statement
The data that support the findings of this study are available from the authors but restrictions apply to the availability of these data, which were used under license from the Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Indonesia and Research Center for Biomasses and Bioproducts, Badan Riset dan Inovasi Nasional for the current study, and so are not publicly available. Data are, however, available from the authors upon reasonable request.
10. ORCID
Albert Hendriko 
https://orcid.org/0000-0002-6688-3155
Ariadne Lakshmidevi Juwono https://orcid.org/0000-0002-2068-9402
Ismail Budiman https://orcid.org/0000-0002-8990-1950
Subyakto https://orcid.org/0000-0002-6009-9220
Bambang Soegijono https://orcid.org/0000-0002-5190-7943
Deni Purnomo https://orcid.org/0000-0002-5360-8736
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No competing interests reported.
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Graphical Abstract
