Life Cycle Carbon Emissions Savings of Replacing Concrete with Recycled Polycarbonate and Sand Composite

Abstract

Recent work demonstrated that 50:50 sand-recycled polycarbonate (rPC) composites have an average compressive strength of 71 MPa, which dramatically exceeds the average offered by commercial concrete (23.3–30.2 MPa). Due to the promising technical viability of replacing carbon-intensive concrete with recycled sand plastic composites, this study analyzes the cradle-to-gate environmental impacts with a life cycle assessment (LCA). Sand-to-plastic composites (50:50) in different sample sizes were fabricated and the electricity consumption monitored. Cumulative energy demand and IPCC global warming potential 100a were evaluated to quantify energy consumption and greenhouse gas emission associated with sand–plastic brick and two types of concrete, spanning the life cycle from raw material extraction to use phase. The results showed that at small sizes using Ontario grid electricity, the composites were more carbon-intensive than concrete, but as samples increased to standard brick–scale rPC composite bricks, they demonstrated significantly lower environmental impact, emitting 96% less CO₂/cm³ than sand–virgin PC (vPC) composite, 45% less than ordinary concrete, and 54% less than frost-resistant concrete. Energy sourcing has a significant influence on emissions. Sand–rPC composite achieves a 67–98% lower carbon footprint compared to sand–vPC composite and a 3–98% reduction compared to both types of concrete. Recycling global polycarbonate production for use in sand–rPC composites, though small compared to the total market, could annually displace approximately 26 Mt of concrete, saving 4.5–5.4 Mt of CO₂ emissions. The results showed that the twin problems of carbon emissions from concrete and poor plastic recycling could be partially solved with sand–rPC building material composites to replace concrete.

1. Introduction

Global greenhouse gas (GHG) emissions and carbon dioxide (CO₂) concentrations in the atmosphere continue to increase [1], which has destabilized the climate [2,3]. This has resulted in potentially irreversible negative repercussions for both the natural environment and the social and economic welfare of humanity [4,5]. Most importantly to humanity, climate change is already responsible for numerous deaths and potentially a billion future premature deaths spread over a period of very roughly one century [6,7]. To prevent these severe consequences, it is imperative to reduce carbon emissions [1,3] by a large collection of methods [8,9,10,11,12].

Although carbon emissions from energy use receive most of the attention, concrete manufacturing is the most detrimental material for the environment [13]. Portland cement, which makes up 10–15% of concrete, is responsible for 8% of global carbon emissions [14,15]. This is due to emissions from fossil fuel combustion to operate rotary kilns and the high temperatures required for the calcination of limestone, which results in 1.25 tons of CO₂ for every 1 ton of cement [13]. Traditional concrete is the most used material globally (30 billion metric tons/year) [16]. For perspective, if concrete were a country, it would be behind only the U.S. and China in carbon emissions [15,17,18]. Globally, the emissions from cement production continue to grow annually, reaching a new peak of 1.7 billion metric tons of CO₂ in 2021 [19]. Beyond CO₂ emissions, concrete uses 18% of global industrial water consumption and 9% of industrial water withdrawal each year [20].

A much more visual source of environmental degradation is plastic waste, as only 9% is recycled [21], and thus long-lived plastic has widespread negative environmental effects [22]. Although concrete’s environmental impact is largely unseen compared to ubiquitous plastic pollution [23], the problem of concrete is much more severe than plastic [18]. The vast amount of unrecycled plastic, however, presents an opportunity to use polymers as a replacement aggregate or fiber reinforcement in concrete [24]. To go a step further, one such approach has the potential to solve these two environmental problems simultaneously: finding a way to recycle long-lived waste plastics that pollute the environment and replace cement manufacturing. This has been proposed in the form of material substitutes in the experimental testing of plastic–sand bricks [25]. Several methods have been explored to manufacture plastic–sand brick composites and test them for compressive strength, tensile strength, efflorescence, thermal resistance, and water absorption [26,27]. This includes testing thermoplastics such as both low-density polyethylene (LDPE) and high-density polyethylene (HDPE), as well as polypropylene (PP), polystyrene (PS), and polyvinylchloride (PVC) [28]. Plastic–sand composites can achieve a compressive strength equivalent to the average for commercial concrete (from 23.3 to 30.2 MPa [29]), with the highest strength from common thermoplastics being 32.7 MPa for a 3:1 LDPE–sand material composite [30]. Most recently, a study with 50% polycarbonate (PC) and 50% sand composition yielded an average compressive strength of 71 MPa and a simultaneous increase in compressive stiffness more comparable to concrete [31].

This study produced samples according to ASTM D695 [32] and standard brick size to determine energy consumption. In this research, 50:50 sand-to-plastic composite charges were first prepared using an open-source recyclebot single-screw extruder [33,34]. The samples were formed using an open-source hot press inside custom molds [35]. ASTM D695 is a standard test method for evaluating the compressive properties of rigid plastics. Accordingly, each part was positioned lengthwise in an Instron 5980 series universal testing machine (Instron Corp., Norwood, MA, USA) and subjected to compressive loading at a strain rate of 1.3 mm/min until failure, in compliance with the guidelines of the standard. The scientific gap in the research lies in the limited studies addressing the environmental and scalability aspects of sand-recycled polycarbonate composites as sustainable alternatives to conventional construction materials. While prior research has investigated the mechanical properties of sand–plastic composites, this study uniquely examines their cradle-to-gate (from raw material extraction to use phase) environmental impacts and their feasibility at production scales comparable to traditional concrete. This work bridges this gap by evaluating the cumulative energy demand (CED) and IPCC global warming potential (GWP) over a 100-year horizon of sand–plastic bricks, providing critical insights into their viability as a replacement for carbon-intensive concrete.

2. Materials and Methods

2.1. Materials

The materials and sources used in this study are listed in

Table 1. List of materials.

Material	Small Sample, Average Amount [31]	Average Amount, Standard Construction Brick Size	Unit
Silica sand	80.4	811	g/batch
Recycled polycarbonate	50.4	854.75	g/batch
Energy Consumption
By hot press	3.61	3.61	kWh/batch
By plastic extruder	0.03	0.52	kWh/batch
Volume	8.19	1069.26	cm3/sample
No. of samples per batch	12	1	sample/batch

based on experimental setup.

The sand–plastic samples were fabricated using a combination of extrusion and compression molding techniques [31]. The current setup allows for a small-sample batch to make 12 samples suited for ASTM D695 and one standard-sized brick of sand–plastic composite. To ensure each sample maintained a homogeneous composition, the granulated rPC and silica sand were added to a single-screw extruder to produce uniform “pebbles” that were more compatible with the custom mold and compression molding process. This process was followed to maintain homogeneity, as the commercial rPC particles varied from 1 cm to 5 cm in length. Once these pebbles were extruded and cooled, they were prepared for integration with the hot press and compression molding.

Each of the pockets of the ASTM D695 custom mold was partially loaded with the prepared charges of homogeneous silica sand and rPC. The mold was placed into the hot press and preheated to soften the loaded pebbles. Once soft, the mold was removed and the plunger-style inserts added to each pocket before returning the assembly to the open-source scientific hot press [35] to compress the layer of softened material. This effectually ensured homogenization among pebbles, reduced any potential porosity in the final product, and ensured the charge conformed to the entire volume of the mold. This process was then repeated with additional layers of sand–plastic pebbles, softening, and pressing operations until the mold was fully loaded, and the material was densely compacted. Once complete, the mold was then allowed to cool within the hot press, after which the formed bricks were carefully removed.

This method, which combines extrusion and hot pressing, was created to improve material bonding and provide a more repeatable manufacturing process for developing sand–plastic samples.

2.2. Life Cycle Assessment

Life cycle assessment (LCA) is a widely recognized methodology for analyzing environmental impacts across a product’s entire life cycle [36,37,38,39,40]. According to ISO standards 14040 [41] and 14044 [42], an LCA consists of four key stages: goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and interpretation [42,43,44,45]. The analysis was conducted using openLCA (version 2.1.0) [46], an open-source software platform widely used for life cycle assessments. Data for key materials such as concrete and silica sand were obtained from open-access databases, which provide comprehensive LCI data specific to North America. For rPC, emissions and energy consumption data were sourced from peer-reviewed literature, ensuring traceability and accuracy in modeling the environmental impacts.

2.2.1. Goal and Scope

In the goal and scope phase, the study objectives were defined, along with the selection of system boundaries and the functional unit. The system boundary outlined which processes were included in the analysis. The study conducted a cradle-to-gate assessment, focusing on the environmental impacts from raw material extraction to the production of sand–plastic bricks and two types of concrete (ordinary and frost-resistant). To enable direct comparisons, the functional unit was defined as 1 cm³ of material. This choice facilitated normalization of data and allowed for a clear evaluation of environmental performance across the different materials [47]. The system boundary included processes related to material production, while excluding the use phase and end-of-life stages, as the focus was on the manufacturing impacts.

The analysis assumes primarily the Ontario electricity grid mix, characterized by a significant proportion of renewable and nuclear energy. This assumption has a critical influence on LCIA. Other assumptions include standard production conditions and consistent material compositions.

2.2.2. Life Cycle Inventory (LCI)

The LCI phase involves compiling data on inputs and outputs from unit processes. LCI data for concrete were sourced from the USLCI database and relevant literature [48,49,50,51,52,53]. Two types of concrete were considered: (i) standard concrete used in residential construction and (ii) frost-resistant concrete for bridge construction. Additionally, silica sand production was also modeled using data from existing studies, as it serves as the primary aggregate in sand–plastic composites. An open-loop recycling process was applied to the recycled polycarbonate used in manufacturing sand–plastic bricks. The recycling process for polycarbonate used in the study involved an open-loop system. Waste polycarbonate was collected and granulated into smaller particles before being extruded into uniform pebbles using a single-screw extruder. These pebbles were then mixed with silica sand and processed through a hot press to form sand–plastic composites. The process ensured material homogeneity and enhanced bonding between components. Data on energy consumption and emissions associated with this recycling process were derived from prior studies and incorporated into the LCA.

ISO standards 14040 and 14044 specify two methods for recycling in LCA. A closed-loop system is used when recycled material maintains its original properties and is reused in similar applications [44]. In contrast, an open-loop system is applied when the material’s properties change or are used in different applications [42,43]. Therefore, the environmental impacts of virgin polycarbonate (vPC) production were excluded for this study. The energy demand and emissions generated during the polycarbonate recycling process were considered, with data from existing literature [44,54,55,56,57,58,59,60].

North America was defined as the geographic region for all life cycle stages, as the proposed sand–plastic composites were manufactured in Ontario. Moreover, the total energy consumption was documented on-site during the manufacturing to identify the energy-intensive process.

2.2.3. Life Cycle Impact Assessment (LCIA)

The impact assessment phase focused on climate change and primary energy demand during raw material production. Primary energy demand, also called cumulative energy demand (CED), includes energy derived from fossil and non-fossil sources [36,61,62,63,64,65]. Primary energy represents the energy embedded in natural resources like coal, natural gas, biomass, and uranium, which must be converted into usable energy.

Climate change impacts were assessed by evaluating greenhouse gas (GHG) emissions contributing to global warming. The global warming potential (GWP) was used to compare the time-integrated radiative forcing of GHGs against that of carbon dioxide (CO₂) [66]. Commonly reported GHGs include CO₂, methane, ozone, and nitrous oxide. GWP results are expressed as CO₂ equivalents (CO₂ eq) over different periods. The use of GWP 100 in this study provided a standardized measure for assessing the long-term impacts of greenhouse gas emissions by evaluating their radiative forcing over a 100-year period [8,39,67,68,69,70,71,72,73,74,75]. This metric effectively captures the persistent effects of carbon dioxide and other long-lived greenhouse gases. The 100-year time horizon, however, has limitations, as it underestimates the impact of short-lived climate pollutants, such as methane, which has a GWP approximately 28–34 times that of CO₂ over 100 years, but its warming potential is much higher in the short term (over 20 years, it is 84–87 times more potent) [67,68,69,70,71,72,73,74,75].

In the context of this study, the choice of GWP 100 is particularly relevant for comparing sand-recycled polycarbonate composites to traditional concrete, as the emissions profiles of these materials are dominated by CO₂. The use of GWP 100 ensured that the study captured the cumulative climate impact of CO₂ emissions from energy-intensive processes such as cement production, polycarbonate recycling, and composite manufacturing. However, it is important to highlight that the study’s focus on CO₂-equivalents might underplay the potential contributions of other GHGs, particularly in scenarios where alternative raw materials or processes might emit higher levels of methane or nitrous oxide.

2.2.4. Interpretation

To further contextualize the results, a sensitivity analysis was conducted. This analysis evaluated the influence of production scale and energy sources on environmental performance. Scaling up production from small-batch ASTM D695 samples to standard brick sizes demonstrated a significant reduction in energy demand and emissions per unit. Additionally, scenarios with different electricity mixes—ranging from 100% coal to 100% solar energy—highlighted the importance of renewable energy adoption in reducing the environmental footprint of sand–plastic bricks.

Finally, the results were interpreted to provide actionable insights into the scalability and environmental viability of sand–plastic composites. Comparisons with ordinary and frost-resistant concrete revealed that the adoption of renewable energy and larger production scales could substantially enhance the sustainability of these composites.

3. Results

3.1. Ordinary and Frost-Resistant Concrete

The CED for ordinary concrete is calculated as 1.89 × 10⁻³ MJ/cm³, equivalent to 5.25 × 10⁻⁴ kWh/cm³. The majority of the energy demand, approximately 72.92%, is attributed to cement production, reflecting the energy-intensive nature of cement manufacturing. Concrete mixing accounts for 6.19% of the total CED, while the production of macadam, superplasticizer, and gravel each contributes less than 5% to the total energy demand.

In contrast, the CED for frost-resistant concrete is higher, at 0.00283 MJ/cm³, corresponding to 7.86 × 10⁻⁴ kWh/cm³. Like ordinary concrete, cement production is the dominant energy consumer, responsible for 71.67% of the total energy demand. The energy contribution from concrete mixing, however, is significantly higher for frost-resistant concrete, accounting for 13.64% of the total CED. The production of tarmacadam (2.64%), superplasticizer (1.10%), and gravel (0.17%) remain minor contributors, each accounting for less than 5% of the total energy demand. Figure 1 shows the breakdown of the two different kinds of concrete.

Regarding GHG emissions, ordinary concrete emits 4.2 × 10⁻⁴ kg CO₂ eq/cm³, with 99.99% of these emissions arising from cement production. Similarly, frost-resistant concrete has higher emissions, producing 5 × 10⁻¹ kg CO₂ eq/cm³, with cement production again responsible for around 74% of the total CO₂ emissions. The high proportion of emissions from cement production in both types of concrete underscores its significant environmental impact on the overall life cycle of these materials.

3.2. Sand–Plastic Brick

The CED of sand–plastic composite is 8.60 × 10⁻² MJ/cm³, equivalent to 2.49 × 10⁻² kWh/cm³. Most of this energy consumption, 93.78%, is attributable to the electricity required for brick production. PC recycling accounts for 4.73% of the total energy demand, while silica sand production contributes 1.49%.

The process of manufacturing these ASTM D695 composites consumed 3.63 kWh of electricity.

Table 2. Electricity consumption (Ontario grid mix) in different stages of sand–plastic brick fabrication.

Electricity Consumption	kWh
To reach and maintain the optimum extruder temperature	0.6
While adding material	1.98
While baking	1.27
While extruding rPC to mix the material	3 × 10−2
To reach and maintain the optimum hot press temperature	5.1 × 10−1

provides a breakdown of electricity consumption at various stages of the brickmaking process.

The sand–plastic composite has a carbon footprint (CF) of 1.87 × 10⁻³ kg CO₂ eq./cm³. Of this, 98.1% of emissions are associated with electricity use during brick production. Additionally, less than 1% of the total CF results from the PC recycling process, while silica sand production contributes around 1.5% of total emissions.

3.2.1. Case 1: Sensitivity Analysis of CED and CF per cm³ on Scalability for ASTM D695 Composites and Standard-Brick-Size Sand–rPC Composite Compared to the CED and CF of Ordinary and Frost-Resistant Concrete

Electricity Mix: Ontario Grid Mix

The scalability of sand–plastic composite production was evaluated in terms of environmental impact, focusing on differences between small-batch (ASTM D695 standard sample size) and large-batch (standard brick size) production. In

Table 3. Sensitivity analysis of CED and CF per cm³ for sand–plastic bricks as the number of ASTM D695 samples per batch increases from 12 to 121 and standard-brick-size samples per batch increases from 1 to 10 compared to the CED and CF per cm³ of ordinary and frost-resistant concrete.

Construction Material Type	No. of Samples per Batch	CED (kWh/cm3)	CF (kg CO2eq./cm3)
Ordinary concrete		5.20 × 10−4	4.20 × 10−4
Frost-resistant concrete		7.80 × 10−4	5.00 × 10−4
Sand–rPC composite (ASTM D695)	12	2.49 × 10−2	1.87 × 10−3
	36	8.53 × 10−3	6.60 × 10−4
	60	5.27 × 10−3	4.10 × 10−4
	84	3.86 × 10−3	3.10 × 10−4
	108	3.09 × 10−3	2.50 × 10−4
	121	2.79 × 10−3	2.30 × 10−4
Sand–rPC brick (standard brick size)	1	2.79 × 10−3	2.30 × 10−4
	5	1.97 × 10−4	1.83 × 10−5
	10	7.48 × 10−5	7.47 × 10−6

, ASTM D695 samples are denoted “composite” and standard-brick-size samples “SPB”. The sand–plastic composite, adhering to ASTM D695 standards, is produced in batches capable of yielding 12 individual samples, though the production facility has greater capacity. Additionally, a standard-brick-sized sand–plastic composite (SPB) was manufactured, approximately equivalent to producing 135 ASTM D695 composite samples.

The CED of sand–rPC composites demonstrates a significant dependence on production scale, which is a critical factor in determining the environmental feasibility of these materials. In small-scale production, such as ASTM D695 samples (12 samples per batch), the CED is relatively high, at 2.49 × 10⁻² kWh/cm³, primarily due to the frequent heating and processing cycles required. This value is approximately 47 times higher than the CED of ordinary concrete, which was calculated to be 5.25 × 10⁻⁴ kWh/cm³. Similarly, sand–rPC composite (12 samples per batch) has 31 times higher CED than frost-resistant concrete, with a CED of 7.86 × 10⁻⁴ kWh/cm. As production scales up, the CED of sand–rPC composites decreases substantially. For instance, increasing production from 12 samples to 108 ASTM D695 samples reduces the CED per sample by 87.6%, to 3.09 × 10⁻³ kWh/cm³. At standard-brick-size production (one sample per batch), the CED further decreases, but remains four and three times higher than that of ordinary and frost-resistant concrete, respectively. When production is scaled to 10 standard brick-sized samples, the CED of sand–rPC composites is reduced to 7.48 × 10⁻⁵ kWh/cm³, making it six times lower than the CED of ordinary concrete and nine times lower than the frost-resistant concrete.

The scalability of sand–rPC composites presents a distinct advantage over traditional concrete, particularly when considering the potential for large-scale manufacturing. Larger-scale production not only reduces per-unit energy demand but also positions sand–rPC composites as a viable alternative to concrete in construction applications. The findings suggest that transitioning from small-scale experimental setups to industrial-scale production could unlock significant environmental benefits, particularly in terms of reduced energy consumption. Achieving these benefits requires optimization of manufacturing processes to maximize energy efficiency and reduce environmental impacts. Enhancing the thermal efficiency of heating equipment, minimizing energy losses during extrusion and hot pressing, and streamlining material handling are critical steps toward achieving more sustainable production.

The CF of small-batch composite samples, produced in sets of 12, is significantly higher compared to larger batches due to the frequent heating and processing cycles required in small-scale production. When compared to traditional materials, the CF of 12 composite samples is 92% higher than ordinary concrete and 90% higher than frost-resistant concrete, making small-batch production less environmentally favorable.

Scaling up production to a standard brick size, which is equivalent to producing 135 samples, however, shows a substantial improvement in CF. The CF of SPB is approximately 64% lower than that of 12 composite samples, demonstrating the environmental advantage of larger-batch production. This reduction is achieved due to the reduced need for repetitive heating and mixing cycles, which improves the energy efficiency of the process and lowers CF. In fact, SPB emits 31% less CO₂ per cm³ than ordinary concrete and 56% less than frost-resistant concrete, making it a more sustainable alternative to both concrete types at this scale.

For sand–plastic composite to achieve a lower CF than traditional concrete, at least 84 samples need to be produced in a single batch, beyond which the CF begins to dip below that of ordinary concrete. This threshold highlights the critical role of production scale in making sand–plastic composite a viable, lower-impact alternative to conventional construction materials.

3.2.2. Case 2: Comparative Environmental Analysis Between Sand–rPC Composite (Made from Recycled and Virgin PC) and Concrete

This section evaluates the CF of sand–plastic composites made with vPC and rPC, focusing on the impact of scalability. The analysis considers the manufacturing processes within the Ontario electricity grid mix, which predominantly relies on low-carbon sources like hydroelectricity and other renewable energy technologies.

The CF of vPC-based sand–plastic composites is significantly higher than the rPC based composites. In the smallest batch (Composite_12), the CF of vPC composites is nearly three times that of rPC, highlighting the substantial environmental cost of using virgin polycarbonate over recycled polycarbonate in the same application. As production scales up 12 samples to 121 samples per batch, this difference remains pronounced: in Composite_121, the CF of vPC composites is around 18 times higher than that of rPC, while a 90% drop in CF can be observed with the increase in production rate of vPC composite from Composite_12 to SPB_10. When compared to traditional concrete, both Composite_121 (vPC) and SPB_10 (vPC) exhibit higher CF values, although scaled production helps narrow this gap. For Composite_121 (vPC), the CF is approximately 7.9 times that of ordinary concrete and about 5.2 times that of frost-resistant concrete, indicating that while CF reduction occurs with larger-batch production, this composite still has a significantly higher CF than concrete. These results are summarized in Figure 2.

3.2.3. Case 3: Sensitivity Analysis of Sand–rPC Composite Carbon Footprint Based on Electricity Source—100% Coal to 100% Solar Energy

These results, however, depend on the energy mix, as the production of rPC and production of sand–plastic composite (Composite_12) are highly energy-intensive, with total electricity consumption accounting for over 95% of the cumulative carbon footprint. To observe the impact of the electricity mix, Figure 3 shows a sensitivity of the extremes going from 100% sustainable solar photovoltaic-based energy production to 100% coal-fired electricity. The transition from a coal-based energy source to solar energy results in an approximate 99.9% reduction in carbon emissions, emphasizing the substantial environmental benefits of renewable energy. When analyzing a mixed scenario of 90% coal and 10% solar, the carbon footprint exhibits a modest improvement, reflecting a 10% reduction compared to the 100% coal scenario. Expanding the analysis to regional electricity mixes, the United States [76] exhibits a reduction of approximately 63% in carbon emissions compared to the 100% coal scenario. In Canada [77], and particularly in Ontario, the carbon footprint decreases even further, with an impressive reduction of about 95.4% compared to coal.

Furthermore, transitioning from the U.S. energy mix to that of Canada results in a decrease of approximately 64% in carbon emissions. The shift from the broader Canadian energy mix to Ontario’s specific energy profile leads to a decrease of around 67% in carbon emissions. This emphasizes the effectiveness of local energy strategies in Ontario, showcasing how provincial initiatives can further enhance sustainability outcomes. Transitioning from the Ontario energy mix to a scenario utilizing 100% solar electricity, however, results in an impressive reduction in carbon emissions, estimated at approximately 98%.

As can be seen in Figure 3, it is clear that manufacturing sand–rPC composites in the United States does not provide large carbon footprint advantages because of the relatively high carbon intensity of the grid. For manufacturing in the U.S., using 100% solar energy is preferred, which can be done because the costs of PV-generated electricity are now so much lower than grid electricity in some areas in the U.S. that even grid defection is economically viable [78]. Figure 3, however, also demonstrates that even without self-generation in specific states such as South Dakota (SD), where the share of renewable energy in electricity production is significantly higher, the carbon footprint of sand–plastic composite manufacturing can be notably reduced. In these states, the carbon footprint of sand–plastic composite production in SD is lowered by 92% compared to the U.S. grid mix. This highlights the critical role that the regional electricity mix plays in determining the environmental viability of sand–plastic composite manufacturing.

The CF of vPC-made sand–plastic composite is not significantly influenced by the electricity mix. The overall LCI for vPC production is based on data from the U.S. Life Cycle Inventory (USLCI) [79] and is not large enough to overcome the influence of PC production. The analysis showed that rPC-made composite emits approximately 91% to 100% less GHG compared to vPC-made composite, depending on the energy source.

Furthermore, when compared to ordinary concrete and frost-resistant concrete, the sand–plastic composite exhibits a 93% reduction in carbon footprint when 100% solar energy is used. For other cases, the CF of sand–plastic composite is 2 to 104 times higher than traditional concrete. In the previous section, it was pointed out that production scalability can decrease the CF significantly. Another strategy to reduce CF in the future could be replacing silica sand with beach sand or even waste stamp sand [80]. In that case, the emissions would be related only to the transportation of beach sand or stamp sand. Figure 3 shows that production of silica sand is responsible for 0% to 96% of total CF. For instance, the CF of sand–plastic composite (100% solar) is reduced to 1.57 × 10⁻⁶ kg CO₂ eq., which is around 99.7% less than the CF of ordinary and frost-resistant concrete.

4. Discussion

This study successfully demonstrated that the environmental properties of sand–rPC composites are more than adequate to replace concrete even with smaller production rates. To further emphasize this realistic potential, a standard dimensioned brick was produced using an alternative custom mold and the same routine as the ASTM D695 samples, as shown for the top surface (Figure 4). This larger product emphasizes the compatibility of this material with existing precast applications and encourages the substitution of sand–rPC composites in place of existing precast concrete products. As seen in Figure 4, although aesthetically pleasing and similar in appearance to marble, it is clear that the manufacturing process used does not currently achieve completely uniform dispersal of sand within the waste plastic composite, and thus is unlikely to have the optimal mechanical properties for the material. This inhomogeneity may still be tolerable, however, as the compressive strength already tested on smaller samples far exceeds those of the concrete used in standard commercial structures. In future work, the mechanical properties of larger samples must be retested to ensure that the compressive strength captured by the small samples and manufacturing method is scalable. As a result, using a test such as ASTM C109 [81] to validate a scaled sample with visible inhomogeneity against a more applicable standard tailored for cement shall be conducted next.

As stated in the previous section, the CF of the ASTM D695 sample is eight times higher than the CF of standard-sized sand–rPC composite. Considering this, the LCIA shows that the CF per cubic centimeter of standard-sized sand–rPC composite is 47% to 94% less than ordinary and frost-resistant concrete. The CF of the composite, however, is around 10 times higher when the electricity consumption is coal- or fossil-fuel based.

Each year, Canada produces over 3 million tons of plastic waste, with a mere 9% of this material being recycled. The majority ends up in landfills, waste-to-energy facilities, or is released into the environment [82]. A similar situation exists in the United States, where recycling rates are equally low [83]. The U.S. Department of Energy’s National Renewable Energy Laboratory revealed that the energy content of plastic waste sent to landfills could potentially meet 5% of the energy demands of the U.S. transportation sector or 5.5% of those for the industrial sector [84]. As of 2022, global production capacity for polycarbonate exceeded 7.1 million tons (Mt), with actual production reaching over 5 Mt. Notably, North America represented 14% of this global demand [85,86].

If the total annual production of PC were recycled and mixed with sand to replace conventional concrete, it could yield approximately 1.1 × 10¹³ cm³ of sand–rPC composite each year. In 2022 [87], global concrete consumption approached 30 Gt per year [50,88,89,90,91]. Given that the density of ordinary concrete is roughly 2.4 g/cm³, the volume of sand–plastic composite produced from recycled polycarbonate would displace approximately 26 Mt of concrete, which represents only about 0.1% of total concrete production.

Although this is only a small fraction of the total concrete, this replacement has significant implications for GHG emissions. By substituting ordinary concrete with sand–rPC composite, it is estimated that 4.5 Mt of CO₂ eq. could be saved. In comparison, if frost-resistant concrete were replaced, this figure could increase to approximately 5.4 Mt. To contextualize these savings, replacing 0.1% of global concrete production with sand–rPC composite could have environmental benefits equivalent to the carbon sequestration potential of approximately 204 to 245 million mature trees (or approximately 1 million to 2.5 million acres, as a forest typically has 100 to 200 trees per acre [92,93,94]. Mature trees absorb an average of 22 kg of CO₂ annually, making reforestation a common metric for comparing CO₂ reduction [95,96,97]. Additionally, these savings are also analogous to offsetting 4.5% to 5.4% of the global data center CO₂ emissions, which are known to be considerable in an increasingly digitalized world [98]. According to the U.S. Energy Information Administration, the national annual average CO₂ output rate for electricity generation in the United States is around 0.417 kg CO_{2 eq.} per kWh [99].

5. Limitations and Future Work

This study has several limitations that should be acknowledged. Future work must aim to refine the extrusion and compression molding process to guarantee the compositions are uniform or uniform within an acceptable and repeatable compressive strength. These efforts would focus on obtaining a sand–rPC composite that could potentially serve as a replacement for high-strength concrete, which has a compressive strength rating exceeding 70–80 MPa [100].

One significant limitation is the lack of comprehensive data on PC recycling, which restricted the ability to conduct a thorough water footprint analysis. Additionally, the experimental data for sand–rPC composite production were compared against commercial-scale data for concrete production. While this comparison provides valuable insights, it does not fully capture the complexities associated with scalability, which have only been briefly addressed in this study. It should, however, be pointed out that this study represents conservative overestimates of the environmental impacts of 50:50 PC–sand composites and that industrial scale manufacturing would result in further energy and emissions savings.

While performing the sensitivity analysis on the scalability of sand–rPC composites, the CED was not included in this study. This decision stemmed from the observation that the production of sand–rPC composites at its current experimental stage is predominantly electricity-intensive, with minimal contributions from other energy sources. Unlike concrete production, which involves significant energy contributions from both electricity and transportation, the current sand–rPC production process relies almost entirely on electricity. Since CED accounts for all energy sources with calorific value, including electricity, transportation, and fuel use, it would not fully capture the complexities of sand–rPC composite production at this early stage.

As sand–rPC composite production scales up to commercial levels, however, the inclusion of transportation and other energy inputs, such as material handling and logistics, will make CED a more relevant metric. At that point, a comprehensive analysis of CED could provide deeper insights into the environmental impacts of sand–rPC composites compared to traditional materials. This study, therefore, focused on the CF as the primary metric for sensitivity analysis, as it directly reflects the emissions associated with electricity use, which is currently the dominant energy input. Future work could expand on this by evaluating CED in commercial production systems to provide a more holistic assessment of energy demand across all stages of production.

It should also be pointed out that as manufacturers strive to achieve net-zero emissions, many are exploring using eco-friendly materials to replace traditional aggregates and cement in concrete production [49,52,101,102,103,104,105,106,107,108]. However, this study did not include a comparison between sand–rPC composite and these alternative concrete types. Future work could thus focus on conducting a comparative analysis between various types of eco-friendly concrete and sand–rPC composites to understand their environmental impacts better. Additionally, expanding the dataset on PC recycling would allow for more comprehensive assessments, including water footprint evaluations. Exploring these avenues could significantly contribute to understanding sustainable construction materials and their role in reducing overall environmental impact. Further, in the long term, sand–rPC composite can contribute to carbon sequestration compared to traditional concrete. While concrete emits significant CO₂ during production, only reabsorbing a small fraction through carbonation over time, sand–rPC composite embeds carbon-rich rPC directly into the material. rPC used in sand–rPC composite contains carbon that would otherwise be released into the atmosphere if incinerated or left to decompose in landfills. By incorporating these materials into sand–rPC composite, the carbon remains trapped within the brick for decades, effectively locking it away from the atmosphere.

6. Conclusions

Concrete production remains one of the largest contributors to global CO₂ emissions, while plastic recycling rates are critically low. This study demonstrates the potential of sand-recycled polycarbonate composites as a sustainable alternative to traditional concrete, addressing both the environmental challenges of carbon emissions and plastic waste. The results provide a comprehensive analysis of the environmental benefits of sand–rPC composites, particularly in terms of their reduced carbon footprint and energy demand at larger production scales.

The key findings include the following.

• Environmental Impact of Small-Scale Production:

• At small-sample scales, sand–rPC composites are 5 to 48 times more energy-intensive than ordinary and frost-resistant concrete, primarily due to the electricity-intensive nature of the production process.

• Environmental Benefits of Larger Scales:

• As production scales increase, the CF of sand–rPC composites decreases significantly, with 62% to 90% reductions observed in larger batch sizes.
• At standard brick sizes, sand–rPC composites emit:

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  96% less CO₂ per cm³ compared to sand–virgin polycarbonate composites.

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  45% less CO₂ than ordinary concrete.

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  54% less CO₂ than frost-resistant concrete.

• In contrast, sand–vPC composites emit 14 times more CO₂ than ordinary concrete and 1.5 times more than frost-resistant concrete, providing no environmental benefit.

• Role of Energy Sourcing:

• Transitioning from coal-based energy to solar energy reduces carbon emissions by 99.9%, while a mixed-energy scenario of 90% coal and 10% solar achieves a modest 10% reduction.
• Switching from the U.S. electricity mix to Canada’s grid achieves an 86.6% reduction in emissions, and Ontario’s low-carbon grid achieves a further 68% reduction.
• Using 100% solar energy in Ontario decreases emissions by 98%, emphasizing the importance of renewable energy in sustainable material production.

• Global Potential of Sand–rPC Composites:

• Recycling global polycarbonate production for sand–rPC composites could displace approximately 26 million tons of concrete annually, representing 0.1% of global concrete production.
• This substitution could result in CO₂ savings of 4.5 to 5.4 million tons annually, equivalent to the carbon sequestration potential of 204 to 245 million trees.

These findings underscore the critical importance of scaling up production and integrating renewable energy to maximize the sustainability of sand–rPC composites. Future work should focus on optimizing manufacturing processes, incorporating transportation and other energy elements into cumulative energy demand analysis and developing scalable production methods for industrial applications. With further refinement, sand–rPC composites can play a transformative role in mitigating the dual crises of plastic waste and carbon emissions, paving the way for more sustainable construction practices.