The influence of plastic waste on the properties of lime-based mortars

Abstract

The increasing consumption of plastics, and the threat of environmental impact of their disposal, renders the management of their waste of interest. In the present study, an effort has been made to investigate the influence of waste polyvinyl chloride (PVC) and polypropylene (PP) of various origin in lime-based mortars, applied in the restoration of traditional structures. Nine mortar compositions (based on lime and natural pozzolan) were designed, manufactured and tested, in which natural aggregates were partially substituted by waste plastics (in proportions of 12.5% and 25% v/v of aggregates). According to the research results, shrinkage deformations were reduced in the short term in all modified compositions, porosity showed slight changes, while apparent specific gravity and capillary absorption index were decreased. Mechanical properties were enhanced in the case of the PVC and the lower PP flakes addition (12.5% v/v), while compressive strength was reduced around 5%–25% in the higher proportion (25% v/v). Generally, it could be asserted that the 12.5% v/v proportion of waste plastics offered benefit, and in particular PVC addition was beneficial in terms of physico-mechanical properties, rendering the exploitation of plastic waste in lime-based systems feasible.

Graphical abstract

Keywords

plastic waste / lime mortars / pozzolan / polypropylene / polyvinyl chloride

1 Introduction

The use of plastics ubiquitous in every-day life, due to their remarkable properties, including durability, strength, low density, impermeability, flexibility, and resistance to chemicals [1–5]. They are extensively used in a variety of applications (packaging, household, electronics, engineering, textiles, car components and furniture), with single use products and packaging predominant. For the year 2020, the largest end-uses of plastics were for packaging (40.5%), building and construction (20.4%), automotive industry (8.8%), electrical and electronics (6.2%) and household (4.3%) [4].

Approximately 40% of plastic products are garbage after less than a month [1–4]. Due to their extensive use, plastic waste is inevitable and found on land, in the sea and even inside humans and other animals (as microplastics). The leakage of plastics into the environment has a significant impact for current and future generations, and there is still a lack of knowledge about their long-term effects.

During the years 1950–2020, around 10.4 Gt of plastics were produced, while up to 2050 the total plastics’ production will be increase to 26 Gt [2,3]. In Europe, annual production currently varies from 55 to 65 Mt [4]. Nowadays, around 14%–18% of plastic waste is recycled, 24% is thermally treated (incineration, gasification, pyrolysis), while the rest is disposed in controlled or uncontrolled landfills or into the wider environment [2,3]. Europe shows the highest recycling rate; in the year 2020, 24.5 Mt of plastic waste was generated, 14% was recycled (providing 3.5 Mt of recyclates) and 50% was incinerated for energy recovery (emitting 30 Mt of CO₂) [1].

Regarding the polymer type, the greatest concerns involve polypropylene (PP), polyethylene (PE), high-density polyethylene (HDPE), polyvinyl chloride (PVC) and polyethylene terephthalate (PET) [4]. Examples of uses include: PP for packaging and for producing caps, wrappers, containers, pipes; PE for bags, trays, containers, food packaging; HDPE for toys, bottles, pipes, PVC for window frames, floor and wall coverings, pipes; PET for bottles [4].

The exploitation of waste plastics in the construction sector emerged during the last decade [6–8]. Relevant research has already shown the benefits arising from their application in mortars [7,9,10], concrete [8,11], asphalt mixtures [12], and tiles [13]. In all cases, the objective is to produce alternative, eco-friendly building materials with desirable properties, including durability, resistance to environmental factors (including humidity and chemical assets), low density (light weight mortars and concrete), thermal insulation, strength, and fresh state properties [14–17].

Considering that the construction sector is responsible for the 50% of carbon emissions, 30%–40% of natural resources consumption and 50% of total solid waste [17], the substitution of conventional raw materials by waste plastics (that would be otherwise disposed of in environment) seems to be a promising move toward a sustainable and circular economy.

According to literature [18–21], a proportion of 10 to 70% per volume of natural aggregates may be substituted by different types of plastic waste, including PP, PET, and PVC. Záleská et al. [18] concluded that the partial (10%–50%) substitution of conventional aggregates by various PP waste in concrete mixtures, resulted in lightweight materials showing insulating properties and maintaining mechanical properties for non-bearing applications. Al-Mansour et al. [19] partially (10%) substituted quartz sand by PP and polyamide pellets in cement-based mortars, resulting in an enhancement of the interfacial transition zone (ITZ) between aggregates and the paste, as well as density and strength decrease. Finally, Liguori et al. [20] tested mortars based on natural hydraulic lime (NHL) with waste plastic aggregates that showed a chemical interaction with the paste, enhancing their behavior in terms of thermal degradation and fire resistance. Reduction of the NHL mortars’ density was observed, followed by a porosity increase due to the morphology of the plastic aggregates and the creation of large pores (> 100 μm), as well as decrease of thermal conductivity [21].

Although there is an intense interest in the exploitation of plastics waste in cementitious products (mortars, concrete) [6–19], there is still limited research in lime-based mortars [20,21]. The latter case, seems to be a promising field of research, since sustainability criteria should apply to the restoration of historic and traditional structures, focusing on minimizing the environmental burden due to the consumption of conventional raw materials and applying the principles of cyclic economy [22]. To this extent, the exploitation of waste plastics in lime-based systems could lead to the development of alternative, cost effective and eco-friendly mortars, linked with the preservation of environment and natural resources. However, compatibility issues should be followed, especially in the case of historic structures, taking into account the type, properties and preservation state of the authentic building materials [23,24].

In this paper, an effort has been made to envisage the influence of various types of waste plastics (PP, PVC) in traditional mortars based on lime and natural pozzolan. Natural aggregates were partially substituted by four types of plastics waste, in proportions of 12.5% and 25%, volume to volume. The aim of the study was to identify whether this substitution is feasible in the case of lime-based mortars and investigate the role of waste plastics on the behavior of mortars, according to their type and proportion.

2 Materials and methods

During the experimental part of the study, nine mortar mixtures were produced, based on hydrated lime (powder) and natural pozzolan from the island of Milos (residue on 45 μm ≤ 10%). Their proportion (by mass) was 1:1, following former research work [23–25] and relevant literature on lime-based mortars [26,27]. In Tab.1, the characteristics of the binding agents are provided.

The aggregates were natural (from river deposits), siliceous with a 0–4 mm gradation (Tab.2). Four types of plastic waste partially substituted natural aggregates, including PVC flakes (recycled from pipes) and three types of PP flakes (from plastic crates, spacers for packaging and yogurt cups) (Fig.1). They were obtained from the Recycling Company Karatsialis Bros & CO, located in Thessaloniki, Greece, without having any further treatment. Their characteristics are presented in Tab.2.

In all cases, the Binder/Aggregate (B/A) ratio was 1/2. A reference mortar was manufactured (code number 1), while in the other compositions, natural sand was partially substituted by the waste plastics in proportions of 12.5% and 25% v/v. The constituents of the mortar mixtures (given in parts of weight) are presented in Tab.3.

To reduce water demand, a superplasticizer (polycarboxylate, sulfate free) was added in all mixes (1% w/w of the binding agents) [23–25], while the Water/Binder (W/B) ratio was adjusted for achieving workability (15 ± 1) cm (following EN1015-3 [28]). The production and curing of mortars were carried out according to EN1015-11 [29], resulting in 18 prismatic specimens (4 cm × 4 cm × 16 cm) of each composition.

28, 90, and 180 d after their production, the physico-mechanical characteristics of the mortars were determined, consisting of porosity, absorption, apparent specific gravity (RILEM CPC 11.3) [30], capillary absorption index (EN 1015-18: 2002) [31], dynamic modulus of elasticity (BS 1881-203:1986) [32], flexural and compressive strength (EN1015-11) [29]. The volume and mass changes of two specimens per composition (cured at (60% ± 2%) relative humidity (RH) and (20 ± 1) °C) were recorded (immediately after their demolding) for assessing their shrinkage deformations. Additionally, microstructure observation was performed with stereoscope (Leica Wild M10), assisted by image analysis (ProgRes). The physical characteristics were tested in two specimens per composition and the mechanical properties in three specimens (mean values were taken), appliedby load-controlled testing (at a stable loading rate of 0.5 kN/s).

All results were compared, in order to determine the impact of the different type and proportion of waste plastic particles on the performance of the tested mortar compositions.

3 Results and discussion

3.1 Water content

Regarding the water demand of the mixtures, as presented in Tab.3, it was generally observed that waste plastics had a small influence, slightly increasing the water content. The reference mortar (composite 1) showed a W/B ratio equal to 0.56 for achieving workability 14.8 cm, that was approximately maintained in all compositions. With waste plastics addition, a small increase of the water content was identified (3%–10%), followed by a respective increase of workability (1%–7%).

Comparing all mixtures, the highest W/B ratio was recorded in composition 3-1 (12.5% PP crates) and the lowest one (also with the highest workability) by 4-2 (25% PP spacers). It was generally observed that the higher amounts of plastics resulted in a lower water demand.

The water demand of the mixtures was in agreement with relevant findings from the literature, with the demand depending on the type, origin and particle size of waste plastics [32]. PP addition resulted in an increase of the water demand, especially when substituting more than 30% of the conventional aggregates [18]. This may be linked with the angular shape and sharp edges of the plastic particles [33]. In the case of recycled PET, for the same W/B ratio an increase of workability was attained [14,15]. According to Saikia and de Brito [34], the type of plastics may result in a small increase or decrease of the mixtures’ workability, depending on their individual characteristics. With a stable W/B ratio, PP aggregates slightly increase the slump value, while use of PET may reduce workability [34].

3.2 Physical properties

Regarding the shrinkage deformations of the compositions, determined through their volume and mass changes (Fig.2), the following remarks can be made.

Waste plastics addition reduced mass changes (around 10%–30%), compared to those of the reference mixture, up to the 20 d age (Fig.2(a)). The lowest changes were observed in composition 5-2 (PP cups, 25%) and series 3 (PP crates). Up to 60 d, the lowest reduction was shown in compositions 5-2 (PP cups, 25%) and 3-1 (PP crates, 12.5%), whereas series 2 (PVC) and 4 (PP spacers) had more intense changes.

Volume changes (Fig.2(b)) were considerable between the testing days, related either to volume increase (composites 4-2 and 5-1) or reduction (composite 1, 2-1, 2-2, 3-1, 3-2, 4-1, 5-2). The maximum volume increase was around 1%, while loss ranged from 1% to 4%, with the highest reduction being in 2-1 (PVC, 12.5%). In relation to the reference mixture (composite 1), the lowest changes were observed in series 5 (PP cups) and the highest in composition 2-1 (PVC, 12.5%). Generally, the addition of 25% PVC (2-2), PP crates flakes (series 3) and PP cups particles (series 5), enhanced the volume stability of the mixtures.

According to Tab.4, where the physical properties of the mortar compositions are presented, an increase of porosity and absorption was recorded from the ages of 28 to 90 d, probably due to shrinkage micro cracks formation during the specimens’ curing [35,36]. According to Papayianni and Stefanidou [27], the porosity values of lime-pozzolan mortars are time-dependent, while the water content, as well as the type, proportion and gradation of aggregates play a crucial role. However, porosity generally reduces with time, due to the slow process of pozzolanic reaction and carbonation. When mortars are exposed to dry conditions (RH ≤ 60%), as per EN1015-11, shrinkage cracks may appear [27]. Additionally, the pore size distribution is affected by the W/B ratio, since the development of large pores is favored by the increase of the water content [27,35].

At the age of 28 d, porosity ranged from 16.9% to 21.5%; at 90 d the range was from 25% to 27.6%; at 180 d it was from 21.5% to 28.9%. Compared to the reference mortar, the mortar with 12.5% addition of PVC (composites 2-1) showed a small decrease of porosity (up to 10%). Porosity was increased around 7% in the 25% PVC flakes addition (composites 2-2). The impact of PP showed changes of porosity according to its type, whether the PP was from spacers, cups, or crates. Generally, the lower values of change were recorded in compositions 4-1 (PP spacers, 12.5%) and 5-2 (PP cups, 25%) and the highest ones in 3-2 (PP crates, 25%). According to Záleská et al. [18], waste plastics addition results in a slight porosity increase, due to the increase of the aggregates gradation, leading to the development of voids in the structure of the specimens and especially in the ITZ. Additionally, the sharp shape of some plastic particles may induce greater porosity [33]. At the lower proportion of plastics (12.5%), porosity was maintained or decreased, while the 25% substitution led to values’ fluctuations according to the plastic type.

According to the microscopic findings presented in Subsection 3.4, the highest proportion of PVC, and the addition of PP flakes from crates, created angular voids in the matrix. Generally, the mean diameter of the plastic particles (ranging from 0.2 to 1 mm) played an important role, with the thickest ones creating larger voids. In all cases, an even distribution of the particles in the mortar mass was detected, as well as a firm ITZ.

Water absorption generally followed the porosity trend, with the lowest values being recorded in mixtures 4-1 and 5-2 and the highest ones in 3-2 and 4-2 (Tab.4). According to Saikia and de Brito [33], the water absorption of mortars modified with waste plastics is influenced by the type, proportion and grain size of plastics, as well as the W/B ratio of the mixtures. Generally, the lower water absorption of plastics compared to natural aggregates [11,18,33], results in the decrease of modified mortars absorption, while the porosity of the mixtures plays an important role. The increase of the plastic particles proportion from 12.5% to 25% led to an increase of absorption up to 15%, especially in the case of series 3 (PP crates) and 4 (PP spacers).

Apparent specific gravity on the other hand, was reduced in all modified mixtures (up to 15%), with the lowest values being observed in compositions 4-2 (PP spacers, 25%) and 3-2 (PP crates, 25%). As expected (due to the lower specific gravity of plastic wastes compared to natural aggregates), the highest proportion of waste plastics resulted in the apparent specific gravity decrease of mortar, in line with relevant Refs. [7–9,11,12].

The capillary absorption index (Tab.4), was significantly influenced by the addition of waste, with 28 d values ranging from 0.371 to 0.512 kg/m²·min^0.5 and 90 d ones from 0.214 to 0.615 kg/m²·min^0.5. Compared to the reference mortar, series 5 (PP cups) had the lowest capillary absorption, showing a reduction up to 18% at 28 d and up to 57% at 90 d, with the lowest porosity and absorption values.

Generally, in lime-based mortars, the higher water content leads to increase of porosity and shrinkage deformations, due to water evaporation [27,35]. According to relevant studies in cementitious materials, the substitution of natural aggregates by waste plastics, results in the creation of large pores (> 100 μm), due to the gradation and angular shape of the plastic particles [18,33,34]. On the other hand, the slightly higher water demand, as in the case of series 3, may also favor the formation of large pores in the matrix [33]. The low water absorption of plastic particles compared to natural aggregates, may result in the water absorption decrease of the mortars [11], while the reduction of capillary pores significantly reduces capillary absorption index [33].

3.3 Mechanical properties

Mechanical properties were significantly influenced by the type and proportion of plastics waste, as presented in Fig.3–Fig.7, showing in all cases a remarkable positive development from the age of 28 to 90 d.

Dynamic modulus of elasticity presented fluctuations among the compositions and their age (Fig.3), with values to range from 12 to 15.5 GPa at 28 d, 14 to 17.7 GPa at 90 d and 13 to 17.2 GPa at 180 d. Generally, the 90 d values where the highest, probably due to microcracks formed in the structure of the specimens during further curing. This was also observed in former research works [22,36,37], rendering the 90 d values of dynamic modulus of elasticity more representative in the case of lime-based mortars. At 90 d the higher moduli (≥ 16 GPa) were recorded in the reference mortar and in the mortars with lower proportion of waste plastics (12.5% v/v). The highest value was observed in composition 2-1 (PVC, 12.5%) and the lowest in 4-2 (PP spacers, 25%). The increase of plastics proportion from 12.5% to 25%, resulted in a reduction of dynamic modulus of elasticity of around 10%–15%.

The modulus of elasticity decrease due to waste PP, has also been reported in Refs. [18,34,38–40]. It could be related with the low modulus of waste plastics compared to conventional aggregates (PP has a modulus of 685 MPa), as well as the differences of the wave velocity in the plastic particles and the paste [18,34,39]. Additionally, the formation of cracks and voids around the plastic particles due to the evaporation of excess water and low packing due to their size and shape, plays an important role [18,34,39]. In the case of PET application in cement mortars, Marzouk et al. [40] identified a decrease of modulus of elasticity up to 50%.

Flexural strength significantly increased from 28 to 90 d in all mixtures (around 15%–55%); however, 180 d values were reduced in most cases, following the same trend as dynamic modulus of elasticity (Fig.4). This observation has also been also testified in previous research work [36] and may also be correlated with the creation of microcracks during curing. At 90 d, which seems to be the most representative testing age, the highest values (> 4 MPa) were recorded in series 2 (PVC addition) and in the reference mortar. In the other modified mortars, 90 d strength ranged from 2.5 to 3.2 MPa, while the increase of plastic waste from 12.5% to 25% v/v reduced strength by around 10%–20% (with high reductions in the case of series 4).

The reduction of flexural strength due to the increase of waste plastics amount has been also observed and reported in Refs. [14,16,34,35], referring to a decrease up to 50%. As in the case of the modulus of elasticity, the decrease may be strongly influenced by the unstable ITZ between the plastic particles and the matrix, due to the smooth surfaces of the former and the loss of the excess water [34]. According to Saikia and de Brito [34], during the tensile splitting test, PP particles were detached from the cement matrix, due to poor bonding.

A significant observation made during the flexural strength testing was that in the case of PP addition (especially in the higher proportion), the two parts of the specimens were not detached after failure. It seemed that the PP particles bridged the cracks, preventing the brittle failure of the specimens. Saikia and de Brito [33,34] also stated that specimens containing flaky-shaped PET particles, did not split into two fractions when tested for tensile strength, since the flakier granules restrained the split parts. As a result, the post-crack behavior of the specimens was significantly improved [34].

In Fig.5, the fracture pattern of the specimens modified with PP particles (series 4 and 5) after flexural strength testing is presented. In both series, it was observed that the presence, as well as the increased proportion of PP particles, enhanced the post-failure state of the specimens, with PP flakes interconnecting the splitting parts.

Compressive strength showed a gradual increase in all cases from 28 to 180 d, varying between approximately 100% and 140% (Fig.6). This enhancement is closely related to the low strength development of lime-based mortars and is in accordance with relevant Refs. [23–26,36,41–43], indicating the importance of measuring long-term mechanical properties. The strength development rate of the modified mortars, in the case of PVC addition and the lowest proportion of PP particles, followed the same trend as that of the reference mixture (Fig.6).

Higher 180 d strength values (≥ 14 MPa) were recorded in the reference mortar, series 2 (PVC addition), as well as in all modified compositions with the lower proportion (12.5%) of PP. In all cases, the increase of waste plastic content from 12.5% to 25% v/v reduced strength by around 5%–20%. The lowest reduction was observed in series 2 (PVC) and the highest in series 5 (PP flakes from cups).

The compressive strength reduction due to the increasing ratio of waste plastics has also been reported in the literature, regardless of the plastics type [14,18,34,38–40]. According to Kou et al. [38], the substitution of aggregates by waste PVC in proportions in the range 5%–45%, resulted in a compressive strength decrease approximately in the range 10%–50%. In the case of waste PET, proportions in the range 5%–20% reduced strength around 10%–25% [15], while proportions in the range 30%–50% decreased values 15%–33% [14].

Generally, it could be asserted that the partial substitution of natural aggregates by waste plastics in lime-pozzolan mortars significantly influences their mechanical characteristics. The addition of recycled PVC flakes enhances strength (both flexural and compressive) compared to that of the reference mortar, at both proportions (12.5% and 25% v/v), with the lower one being more effective. In the case of the different types of PP (crates for fruits, spacers for packaging and yogurt cups), the lower amount (12.5% v/v) seems to be more beneficial in terms of mechanical properties.

The reduction of strength due to the increase of the waste plastics proportion in the matrix, has also been reported in Refs. [14,18,38–40]. According to Záleská et al. [18], this feature could be related to the lower strength level of plastic aggregates compared to the conventional ones, as well as their hydrophobic nature resulting in a less stable interface with the matrix. Kou et al. [38] also reported an unstable ITZ between waste plastic particles and the paste, due to crack and pore formation. Cracks and pores could be attributed to the evaporation of excess water from the saturated natural aggregates, the coarser particle size of plastic particles (leading to reduced packing), as well as their lower elastic modulus compared to that of conventional aggregates [40]. The latter could explain the decrease of the dynamic modulus of elasticity values observed in this study, especially in the case of PP addition.

Generally, the adhesion of the waste plastic particles with the paste [18,38–40], seems to be a key element of the modified mixtures’ performances, resulting in lower strength levels. Although in the case of cementitious materials a strength reduction of 10%–50% is to be expected [14,18,34,38–40], in lime-pozzolan mortars the addition of PVC slightly enhanced strength. On the other hand, for PP addition, depending on the PP type and proportion, a strength reduction up to 20% was observed. It seems that the partial substitution of natural aggregates by waste plastics, could be feasible in lime-based mortars for the production of alternative, eco-friendly materials for specific applications (repair or reconstruction of traditional structures).

3.4 Microstructure

Regarding the microstructure of the mortar mixtures (Fig.8), series 2 (PVC) presents a stable structure with a good connection between the paste and the PVC particles that were fully incorporated in the matrix. In the highest proportion of PVC particles (Fig.8(c)), angular voids from the particles’ detachment were recorded. In the case of PP flakes coming from crates (Fig.8(d) and Fig.8(e)), an even distribution of the particles was observed, forming a stable ITZ with the paste, even in the higher proportion. The larger mean diameter of the specific PP particles (~1 mm), compared to all others, made them dominant in the matrix, especially in the higher proportion (Fig.8(e)). In the rest of the PP modified compositions, the same conclusions may be reached regarding even distribution of the flakes and a stable interface with the paste.

Generally, the loose connection of the waste plastics particles with the matrix, testified in cement mixtures in Refs. [18,38–40], has not been observed in the tested lime-based mortars. On the contrary, a stable ITZ was noticed, without the formation of pores or micro-cracks. Safi et al. [14] also identified a stable adhesion between the plastic particles and the cement matrix, attributing the lower level of mechanical strength to other factors (i.e., the lower strength of the plastic particles themselves).

It can be therefore stated that the strength decrease recorded with the higher proportion of PP particles could be linked with other parameters, related to the characteristics of the particles themselves (gradation, thickness, mechanical properties). Saikia and de Brito [34] stated that after performing tensile splitting test, most of the PP particles of the specimens did not fail but were debonded from the cement matrix, evidencing the low bonding between the waste plastic particles and the matrix. In this study and taking into account the improved post-failure condition of the PP modified specimens (interconnection of the splitting parts after flexural testing), it is asserted that materials with flake-shaped plastic waste may not fail during splitting test, with the plastic acting as a bonding agent of the matrix.

It is therefore concluded that it is not only the type and proportion of the waste plastics particles that play important roles on the properties of lime-based mortars but also their shape and size. Rounded particles provide a homogeneity in the matrix, while flake-shaped ones may improve the fracture brittleness during flexural strength testing, as in the case of fiber reinforced composite systems. According to Aminul Haque et al. [44], fibers influence the failure modes of cement systems, having a minor impact on the first cracking strength but playing a crucial role on the composites’ post-cracking behavior. In addition, fibers arrest the range and dispersion of cracks, providing a more effective mechanical performance [44]. They may also provide adequate post-cracking, load-carrying capacity, due to the stress transfer across the first crack and its redistribution, which could be defined by the toughness parameters [45]. These parameters may reflect the crack bridging effect of fibers [45], defining the area under the load-deflection curve.

To this extent, waste plastic flakes could act as adjusters of lime-based mortars’ residual strength, controlling the deflection rate of the cracked specimens and improving their overall performance. However, focused research is needed in order to clarify their specific parametric influence.

4 Conclusions

The exploitation of plastics waste in the constructional sector, to achieve energy and natural resources savings and preservation of the environment, could be also applied in repair works of traditional structures (due to the high amounts of raw materials needed). The partial substitution of natural aggregates by waste PVC and PP particles was therefore assessed during the present study, resulting in the following conclusions.

1) The water demand of the mixtures was generally slightly influenced by the type and proportion of waste plastics.

2) Regarding shrinkage deformations, mass changes were reduced in the short term (up to the 20 d age) in all modified compositions. Compared to the reference composition, the addition of PP flakes coming from yogurt cups showed the lowest mass and volume changes.

3) Porosity was decreased with the addition of waste PVC and was slightly increased with the various types of PP, while apparent specific gravity decreased in all cases, resulting in lightweight materials. The capillary absorption index was reduced in most cases, with the lowest values being recorded in the higher proportion of PP originating from yogurt cups.

4) Mechanical characteristics were influenced by the type and proportion of plastic waste, showing in all cases a remarkable development from the age of 28 to 90 d. Dynamic modulus of elasticity and flexural strength showed the highest values at 90 d, probably due to the further formation of microcracks. The highest values were observed in the lower proportion of waste plastics (12.5% v/v) and especially in the PVC addition, while the highest proportion of PP flakes improved the post-failure state of the specimens by interconnecting the splitting parts. Compressive strength, showed an increase from 28 to 180 d, varying from 100% to 140%, with the highest final strength being recorded in the PVC addition and in all modified compositions with a lower proportion of plastics. In all cases, the increase of waste plastics from 12.5% to 25% v/v reduced strength by 5%–20% at all tested ages.

5) The microstructure of the modified compositions, presents an even distribution of the waste plastic particles, as well as a stable ITZ with the paste.

Generally, it could be stated that the 12.5% v/v substitution of natural aggregates by waste plastics was beneficial in the case of lime-pozzolan mortars, and in particular that PVC addition was beneficial in terms of physico-mechanical properties. Nevertheless, the larger proportion of PP flakes improved the fracture brittleness of the mortar specimens during flexural strength testing. It seems that the exploitation of waste plastics in lime-based systems is feasible and could lead to sustainable, eco-friendly materials for specific applications, taking into account the application requirements and the individual properties of the waste plastic particles. However, focused research is needed in order to investigate their overall impact on lime-based systems, as well as their effectiveness for specific applications.

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