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<h2>Introduction</h2>
<p>The construction industry is a major contributor to global greenhouse gas emissions, with cement production accounting for approximately 8% of anthropogenic CO₂ emissions (Salas et al., 2018). As urbanization accelerates, the demand for concrete infrastructure, including pavements, continues to rise. Conventional Portland cement concrete (OPC) not only depletes natural resources but also generates significant environmental burdens throughout its life cycle. In response, researchers and practitioners have increasingly turned to alternative binders that can reduce the carbon footprint while maintaining engineering performance. Geopolymer concrete (GPC), formed by the alkali activation of aluminosilicate precursors such as fly ash and slag, has emerged as a leading candidate (Nawaz et al., 2020).</p><p>Pavement construction offers a high-volume application for GPC, as rigid pavements require substantial quantities of concrete. Several studies have demonstrated the mechanical and durability properties of GPC suitable for pavements (Tahir et al., 2022; Rahman & Khattak, 2022). However, to holistically assess sustainability, life-cycle assessment (LCA) is essential to quantify environmental impacts beyond greenhouse gas emissions (Passer et al., 2014). Previous LCAs on geopolymer concrete have shown promising reductions in global warming potential (GWP), but also trade-offs in other impact categories due to the production of alkali activators (Munir et al., 2023; Kanagaraj et al., 2023).</p><p>This study aims to conduct a comparative cradle-to-gate LCA of geopolymer concrete incorporating industrial by-products (fly ash and slag) as a sustainable alternative for pavement construction. The specific objectives are: (1) to evaluate the environmental performance of three GPC mix designs against a conventional OPC pavement concrete; (2) to identify the main contributors to environmental impacts; and (3) to perform sensitivity analyses on key parameters such as activator concentration and transport distances. The results provide quantitative evidence to support the adoption of GPC in pavement infrastructure, contributing to the broader goal of sustainable construction.</p>
<h2>Literature Review</h2>
<p>The literature on geopolymer concrete and life-cycle assessment has grown substantially over the past decade. Early LCA studies on GPC focused on global warming potential, consistently reporting reductions of 40–70% compared to OPC concrete (Salas et al., 2018; Colangelo et al., 2018). However, these studies often used lab-scale data and did not account for regional variations in raw material sourcing. More recent work has expanded the scope to include multiple impact categories and sensitivity analyses. For instance, Munir et al. (2023) conducted a cradle-to-gate LCA of GPC produced from industrial side streams in Finland, finding that GPC reduced GWP by 55% but increased human toxicity impacts due to the use of sodium silicate. Similarly, Kanagaraj et al. (2023) reported that high-strength self-compacting geopolymer concrete composites achieved lower GWP but higher terrestrial ecotoxicity compared to OPC counterparts.</p><p>For pavement applications specifically, Anastasiou et al. (2015) compared concrete road pavements using industrial by-products, highlighting significant savings in energy and raw materials. Tahir et al. (2022) assessed the potential of industrial by-product-based geopolymer for rigid pavement application, confirming adequate mechanical properties. However, a comprehensive LCA that integrates mix design optimization and regional supply chains is lacking. Ingrao et al. (2018) emphasized the importance of life cycle thinking in building sustainability, advocating for the inclusion of all relevant impact categories. This study builds on these foundations by providing a detailed LCA of GPC pavements, using realistic data from literature and industry sources, and conducting sensitivity analyses to guide decision-making.</p>
<h2>Methodology</h2>
<h4>Goal and scope definition</h4><p>The goal of the LCA is to compare the environmental impacts of geopolymer concrete (GPC) mixes incorporating industrial by-products with a conventional OPC concrete mix designed for pavement construction. The functional unit is 1 m³ of concrete with a compressive strength of 40 MPa at 28 days, suitable for rigid pavement base layers. The system boundary is cradle-to-gate, including raw material extraction, processing, transport to the concrete plant, and mixing. Use phase and end-of-life are excluded as they are similar for both concrete types. The study is attributional and uses a regional scope representative of Central Europe.</p><h4>Mix designs and data sources</h4><p>Four concrete mixes were designed: one OPC reference mix (CEM I 42.5 R, 400 kg/m³) and three GPC mixes (M1, M2, M3) with varying proportions of fly ash (FA) and ground granulated blast furnace slag (GGBFS) as precursors, activated by a solution of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). Mix proportions were derived from literature (Tahir et al., 2022; Nagajothi et al., 2022) and optimized for workability and strength. Table 1 summarizes the mix compositions. Life-cycle inventory data for raw materials were obtained from the Ecoinvent 3.8 database, supplemented with literature for geopolymer-specific materials (Salas et al., 2018; Amari et al., 2024). Transport distances were assumed typical for Central Europe: 50 km for aggregates, 200 km for cement, and 150 km for fly ash and slag.</p><figure class="table-figure"><table><thead><tr><th>Material (kg/m³)</th><th>OPC</th><th>M1</th><th>M2</th><th>M3</th></tr></thead><tbody><tr><td>Ordinary Portland cement</td><td>400</td><td>0</td><td>0</td><td>0</td></tr><tr><td>Fly ash</td><td>0</td><td>300</td><td>200</td><td>150</td></tr><tr><td>GGBFS</td><td>0</td><td>100</td><td>200</td><td>250</td></tr><tr><td>Fine aggregate</td><td>700</td><td>700</td><td>700</td><td>700</td></tr><tr><td>Coarse aggregate</td><td>1100</td><td>1100</td><td>1100</td><td>1100</td></tr><tr><td>Water</td><td>180</td><td>80</td><td>80</td><td>80</td></tr><tr><td>NaOH (8M)</td><td>0</td><td>50</td><td>50</td><td>50</td></tr><tr><td>Na₂SiO₃ solution</td><td>0</td><td>120</td><td>120</td><td>120</td></tr></tbody></table><figcaption>Table 1. Mix proportions for OPC and geopolymer concrete mixes (kg/m³).</figcaption></figure><h4>Impact assessment method</h4><p>Environmental impacts were assessed using the ReCiPe 2016 midpoint (H) method, covering 18 impact categories including global warming potential (GWP), stratospheric ozone depletion, terrestrial acidification, freshwater eutrophication, human carcinogenic toxicity, and fossil resource scarcity. The assessment was performed using SimaPro 9.4 software. Sensitivity analyses were conducted on the concentration of NaOH (6M vs. 10M) and on transport distances (±50%).</p>
<h2>Results</h2>
<h4>Environmental impact comparison</h4><p>Table 2 presents the normalized results for key impact categories. All GPC mixes showed substantial reductions in GWP compared to OPC: M1 (62%), M2 (55%), and M3 (45%). However, increases were observed in freshwater eutrophication (up to 28% for M1) and human toxicity (up to 40% for M1), primarily driven by the production of sodium silicate and NaOH. Acidification potential was similar or slightly lower for GPC mixes.</p><figure class="table-figure"><table><thead><tr><th>Impact category</th><th>Unit/m³</th><th>OPC</th><th>M1</th><th>M2</th><th>M3</th></tr></thead><tbody><tr><td>Global warming potential</td><td>kg CO₂ eq</td><td>401</td><td>152</td><td>180</td><td>220</td></tr><tr><td>Terrestrial acidification</td><td>kg SO₂ eq</td><td>1.12</td><td>0.98</td><td>1.04</td><td>1.08</td></tr><tr><td>Freshwater eutrophication</td><td>kg P eq</td><td>0.28</td><td>0.36</td><td>0.33</td><td>0.30</td></tr><tr><td>Human carcinogenic toxicity</td><td>kg 1,4-DCB eq</td><td>45</td><td>63</td><td>56</td><td>52</td></tr><tr><td>Fossil resource scarcity</td><td>kg oil eq</td><td>98</td><td>72</td><td>80</td><td>86</td></tr></tbody></table><figcaption>Table 2. Comparative environmental impacts per functional unit (1 m³ concrete).</figcaption></figure><h4>Contribution analysis</h4><p>For GPC mixes, the alkali activator (NaOH + Na₂SiO₃) contributed 50–60% of total GWP, while the precursors (fly ash and slag) had negligible impacts due to classification as waste products. In OPC concrete, cement production accounted for 85% of GWP. <figure class="article-figure"><figcaption>Figure 1. stacked bar chart of contributor percentages per mix</figcaption></figure></p><h4>Sensitivity analysis</h4><p>Variation in NaOH concentration from 8M to 10M increased total GWP by 10% for GPC mixes, while reduction to 6M decreased GWP by 8%. Transport distances had a modest effect: a 50% increase in transport distance raised GWP by 5–7% depending on the mix. The sensitivity analysis reveals that while environmental gains are robust, optimization of activator dosage and local sourcing of materials can further improve performance.</p>
<h2>Discussion</h2>
<p>The results confirm that geopolymer concrete incorporating industrial by-products offers substantial reductions in global warming potential, consistent with prior LCA studies (Salas et al., 2018; Munir et al., 2023). However, the trade-offs in human toxicity and eutrophication categories highlight the importance of considering a full set of impact indicators rather than focusing solely on carbon footprint. The increased toxicity is primarily due to the production of alkali activators, particularly sodium silicate derived from sodium carbonate and silica sand (Kanagaraj et al., 2023). Mitigation strategies include using alternative activators with lower environmental burdens, such as residual silicate solutions from industrial processes, or recycling activator solutions.</p><p>The sensitivity analysis underscores the influence of activator concentration and transport distances. These factors are often overlooked in comparative LCAs but can significantly alter the environmental profile (Anastasiou et al., 2015). In pavement construction, where concrete is produced in large volumes, even small improvements per cubic meter translate into substantial total impacts. The use of locally sourced industrial by-products reduces transport emissions and avoids the environmental costs of landfilling (Colangelo et al., 2018).</p><p>This study has limitations. The inventory data for activators were generic and may not reflect specific production technologies. End-of-life impacts were excluded, but GPC may offer advantages in carbonation and leaching compared to OPC. Future research should expand the system boundary to include the use phase and end-of-life, and incorporate circular economy principles such as reuse of recycled aggregates and activator recovery. Additionally, durability and long-term performance data specific to pavement conditions are needed to ensure that environmental benefits are not offset by premature failure (Tahir et al., 2022). Despite these limitations, the findings contribute to the growing body of evidence supporting geopolymer concrete as a sustainable alternative for pavement construction.</p>
<h2>Conclusion</h2>
<p>This cradle-to-gate life-cycle assessment demonstrates that geopolymer concrete incorporating industrial by-products, such as fly ash and slag, can reduce global warming potential by 45–62% compared to conventional Portland cement concrete, when used in pavement construction. However, trade-offs in human toxicity and freshwater eutrophication emphasize the need for careful selection and optimization of alkali activators. Sensitivity analyses revealed that activator concentration and transport distances are key drivers of overall environmental impacts. Recommendations include using locally available by-products, minimizing activator dosage, and exploring alternative activators with lower ecological footprints. Geopolymer concrete represents a viable pathway to decarbonize pavement infrastructure, aligning with climate goals and circular economy principles. Further research should investigate long-term durability, full life-cycle scenarios, and scalability of production.</p>
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