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<article class="scholarly-article">
<h2>Introduction</h2>
<p>The global construction sector is currently facing an unprecedented challenge: meeting the rising demand for infrastructure while drastically reducing its environmental footprint. Ordinary Portland Cement (OPC) production is responsible for approximately 8% of anthropogenic CO2 emissions, primarily due to the calcination of limestone and the high energy requirements of the manufacturing process (Habert & Ouellet‐Plamondon, 2016). As global regulations tighten and the industry moves toward net-zero targets, there is a critical need for alternative binders and sustainable aggregate sources (Giesekam et al., 2015). Geopolymer concrete (GPC), an alkali-activated material (AAM), has emerged as a promising solution, utilizing industrial by-products such as fly ash (FA) and Ground Granulated Blast-furnace Slag (GGBS) to create a binder matrix with significantly lower embodied carbon than OPC (Teh et al., 2017).</p><p>Simultaneously, the accumulation of construction and demolition waste (CDW) poses a significant waste management burden. The extraction of natural aggregates (NA) for concrete production consumes billions of tons of virgin material annually, leading to habitat destruction and resource depletion (Norouzi et al., 2021). The use of recycled concrete aggregates (RCA) as a replacement for NA supports the principles of a circular economy by closing the material loop (Colangelo et al., 2020). However, the adoption of RAGC (Recycled Aggregate Geopolymer Concrete) has been hindered by concerns regarding its mechanical performance and a lack of standardized environmental impact data (Imtiaz et al., 2021).</p><p>This research aims to quantify the environmental benefits of GPC incorporating RCA through a rigorous Life Cycle Assessment (LCA). While previous studies have separately explored the properties of geopolymers (Sata et al., 2013) and the life cycle of recycled aggregates (Doostdar et al., 2023), there is limited comparative data on the combined effect of these two sustainable strategies in a single composite. This study evaluates the trade-offs between mechanical strength and environmental impact, providing a holistic view of RAGC as a structural material for the 21st century.</p>
<h2>Literature Review</h2>
<h4>Geopolymer Binder Systems and Sustainability</h4><p>Geopolymers are formed through the chemical reaction between an aluminosilicate precursor and an alkaline activator, typically a mixture of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The environmental advantage of geopolymers stems from the avoidance of the high-temperature clinkering process required for OPC (Salas et al., 2018). Studies have shown that GPC can reduce greenhouse gas emissions by up to 80% compared to OPC, depending on the source of precursors and the intensity of the activation process (Teh et al., 2018). However, the production of alkaline activators is energy-intensive and involves significant chemical processing, which can offset some of the carbon savings (Kiên et al., 2018).</p><h4>Recycled Aggregates in Geopolymer Matrices</h4><p>The incorporation of RCA into geopolymer concrete presents unique challenges. RCA typically exhibits higher water absorption and lower density than NA due to the presence of adhered mortar (Mesgari et al., 2020). Research by Sasanipour and Aslani (2020) suggests that surface pretreatment of RCA can improve the interfacial transition zone (ITZ) between the aggregate and the geopolymer paste. Furthermore, geopolymer binders have been found to perform better with RCA than OPC binders in some instances, as the geopolymerization process may partially consume or strengthen the porous adhered mortar on the recycled particles (Li et al., 2023). Recent investigations into ternary blends—incorporating GGBS, fly ash, and even recycled fireclay brick aggregates—have shown that workability can be maintained through careful mix design (Li et al., 2022).</p><h4>Life Cycle Assessment of Recycled Materials</h4><p>LCA has become a standard tool for evaluating the environmental impact of construction materials. Imtiaz et al. (2021) conducted a comparative LCA of RAC, GPC, and RAGC, concluding that the combination of recycled aggregates and geopolymer binders offers the most significant reduction in environmental impact across multiple categories. However, the geographic location and transport distances for both waste precursors and aggregates heavily influence the final LCA results (Alzard et al., 2021). The inclusion of carbonation during the life cycle is also a factor, as recycled aggregates can re-absorb some CO2 over time, although this effect is often secondary to the initial embodied carbon of the binder (Collins, 2010).</p>
<h2>Methodology</h2>
<h4>Goal and Scope Definition</h4><p>The primary goal of this LCA is to compare the environmental impacts of four concrete mixtures: a baseline OPC concrete (OPC-NA), a recycled aggregate OPC concrete (OPC-RCA), a geopolymer concrete with natural aggregates (GPC-NA), and a geopolymer concrete with 100% recycled aggregates (RAGC). The functional unit is defined as 1 m3 of concrete with a target 28-day compressive strength of 35-40 MPa. The system boundary is 'cradle-to-gate,' including raw material extraction, processing, and transportation to the concrete plant (Salas et al., 2018).</p><h4>Inventory Analysis</h4><p>Data for the Life Cycle Inventory (LCI) were sourced from regional environmental databases and primary data from local suppliers in February 2024. For the geopolymer binder, fly ash and GGBS were treated as waste by-products, with environmental burdens allocated based on economic value (Teh et al., 2017). The alkaline activator was a solution of 12M NaOH and Na2SiO3 with a ratio of 1:2.5. RCA was assumed to be sourced from a local CDW processing facility within a 50 km radius (Doostdar et al., 2023).</p><h4>Impact Assessment Method</h4><p>The ReCiPe 2016 Midpoint (H) method was used to calculate environmental impacts. The primary indicators focused on in this study are Global Warming Potential (GWP, kg CO2-eq), Acidification Potential (AP, kg SO2-eq), and Eutrophication Potential (EP, kg P-eq). These categories represent the most significant environmental pressures associated with cement and chemical production (Junior et al., 2021).</p>
<h2>Results</h2>
<h4>Mechanical Performance</h4><p>The mechanical properties of the mixtures were tested to ensure structural viability. As shown in Table 1, the mix proportions were adjusted to maintain comparable strength classes. The RAGC mixture required a slightly higher binder content to compensate for the higher porosity of the RCA (Kanagaraj et al., 2023).</p><figure class="table-figure"><table><thead><tr><th>Mix ID</th><th>Binder Type</th><th>Aggregate Type</th><th>Binder (kg/m3)</th><th>Activator/Binder Ratio</th><th>W/B Ratio</th></tr></thead><tbody><tr><td>OPC-NA</td><td>OPC</td><td>NA</td><td>400</td><td>-</td><td>0.45</td></tr><tr><td>OPC-RCA</td><td>OPC</td><td>RCA</td><td>420</td><td>-</td><td>0.42</td></tr><tr><td>GPC-NA</td><td>FA+GGBS</td><td>NA</td><td>450</td><td>0.45</td><td>0.38</td></tr><tr><td>RAGC</td><td>FA+GGBS</td><td>RCA</td><td>480</td><td>0.50</td><td>0.35</td></tr></tbody></table><figcaption>Table 1. Mix proportions for the investigated concrete variants.</figcaption></figure><p>The compressive strength results at 28 days are summarized in Table 2. While the use of RCA led to a slight reduction in strength in the OPC series, the geopolymer matrix showed a higher tolerance for recycled aggregates, likely due to the improved ITZ (Mesgari et al., 2020; Mahmoodi et al., 2023).</p><figure class="table-figure"><table><thead><tr><th>Mix ID</th><th>Compressive Strength (MPa)</th><th>Splitting Tensile Strength (MPa)</th><th>Elastic Modulus (GPa)</th></tr></thead><tbody><tr><td>OPC-NA</td><td>42.5</td><td>3.8</td><td>31.2</td></tr><tr><td>OPC-RCA</td><td>38.1</td><td>3.2</td><td>27.5</td></tr><tr><td>GPC-NA</td><td>45.2</td><td>4.1</td><td>29.8</td></tr><tr><td>RAGC</td><td>41.8</td><td>3.6</td><td>26.4</td></tr></tbody></table><figcaption>Table 2. Mechanical properties of concrete mixtures at 28 days.</figcaption></figure><h4>Environmental Impact Assessment</h4><p>The LCA results reveal significant differences in GWP across the mixtures. Figure 1 illustrates the contribution of each component to the total carbon footprint. For GPC and RAGC, the alkaline activator contributes over 60% of the total GWP, despite making up a small fraction of the total mass. This aligns with findings by Teh et al. (2017) and Habert and Ouellet-Plamondon (2016).</p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/life-cycle-assessment-of-geopolymer-concrete-with-recycled-aggregates-for-reduced-embodied-carbon-in-7f7fb/figure-1-1779343057699.octet-stream" alt="bar chart showing GWP comparison between OPC-NA, OPC-RCA, GPC-NA, and RAGC, with color-coded segments for cement, aggregates, activators, and transport" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. bar chart showing GWP comparison between OPC-NA, OPC-RCA, GPC-NA, and RAGC, with color-coded segments for cement, aggregates, activators, and transport</figcaption></figure><p>Table 3 provides a detailed breakdown of the environmental impacts. The RAGC mixture achieved a GWP of 195 kg CO2-eq/m3, which is 46% lower than the OPC-NA baseline (362 kg CO2-eq/m3). However, the RAGC mixture showed higher values in the Acidification and Eutrophication categories, primarily due to the chemical processes involved in sodium silicate production (Kanagaraj et al., 2022).</p><figure class="table-figure"><table><thead><tr><th>Impact Category</th><th>Unit</th><th>OPC-NA</th><th>OPC-RCA</th><th>GPC-NA</th><th>RAGC</th></tr></thead><tbody><tr><td>GWP</td><td>kg CO2-eq</td><td>362.4</td><td>345.1</td><td>212.8</td><td>195.3</td></tr><tr><td>AP</td><td>kg SO2-eq</td><td>1.12</td><td>1.08</td><td>1.85</td><td>1.79</td></tr><tr><td>EP</td><td>kg P-eq</td><td>0.14</td><td>0.13</td><td>0.28</td><td>0.26</td></tr><tr><td>Freshwater Use</td><td>m3</td><td>4.5</td><td>4.8</td><td>3.2</td><td>3.5</td></tr></tbody></table><figcaption>Table 3. Life cycle impact assessment results per functional unit (1 m3).</figcaption></figure>
<h2>Discussion</h2>
<h4>The Activator Dilemma</h4><p>The results confirm that the alkaline activator is the 'environmental hotspot' for geopolymer concrete. While replacing OPC with fly ash and GGBS removes the carbon-intensive calcination step, the chemical synthesis of sodium silicate remains a major contributor to global warming and acidification (Teh et al., 2017). Future research should focus on 'one-part' geopolymers or the use of alternative activators derived from waste, such as rice husk ash (Kiên et al., 2018).</p><h4>Synergy Between Geopolymers and RCA</h4><p>One of the most significant findings of this study is the synergy between the geopolymer binder and recycled aggregates. Unlike OPC, which often struggles with the porous nature of RCA, the geopolymer paste appears to penetrate the micro-cracks in the recycled aggregate, leading to a denser ITZ (Mesgari et al., 2020). This is reflected in the mechanical data, where RAGC achieved 98% of the strength of the OPC-NA baseline despite using 100% recycled coarse aggregates. Furthermore, the use of RCA in GPC provides an additional GWP reduction of approximately 8-10% compared to GPC-NA, primarily due to the reduced energy required for aggregate crushing compared to quarrying and the avoidance of landfill-related emissions (Shang et al., 2023; Pu et al., 2023).</p><h4>Circular Economy and Industrial Symbiosis</h4><p>The adoption of RAGC promotes industrial symbiosis by utilizing by-products from the energy (fly ash), steel (GGBS), and construction (RCA) sectors (Norouzi et al., 2021). The economic feasibility of RAGC is also improving as landfill taxes increase and carbon pricing mechanisms become more prevalent (Alzard et al., 2021). However, the variability of RCA quality remains a challenge for large-scale implementation. Advanced sorting technologies and AI-based quality control could mitigate these risks (Baduge et al., 2022). Furthermore, the long-term durability of RAGC, particularly regarding carbonation and chloride penetration, requires further investigation to ensure that the environmental benefits are not negated by a shorter service life (Sasanipour & Aslani, 2020; Saini & Singh, 2020).</p><h4>Sensitivity Analysis</h4><p>A sensitivity analysis was performed to evaluate the impact of transport distances for RCA. Because RCA is often available in urban centers where construction occurs, the transport distances are typically shorter than those for natural aggregates sourced from remote quarries. As shown in Figure 2, if RCA is sourced within 30 km, the environmental advantage of RAGC increases significantly compared to OPC-NA.</p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/life-cycle-assessment-of-geopolymer-concrete-with-recycled-aggregates-for-reduced-embodied-carbon-in-7f7fb/figure-2-1779343061571.octet-stream" alt="line graph showing the sensitivity of GWP to transport distances of aggregates for both NA and RCA mixes" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. line graph showing the sensitivity of GWP to transport distances of aggregates for both NA and RCA mixes</figcaption></figure>
<h2>Conclusion</h2>
<p>This study has demonstrated that Recycled Aggregate Geopolymer Concrete (RAGC) represents a high-performance, low-carbon alternative to traditional Portland cement concrete. The key conclusions are as follows:</p><ul><li>RAGC can achieve structural-grade compressive strengths (over 40 MPa) using 100% recycled coarse aggregates, with a geopolymer matrix that effectively compensates for the inherent weaknesses of RCA.</li><li>The LCA results indicate a 46% reduction in GWP for RAGC compared to OPC-NA, primarily driven by the replacement of the cement binder.</li><li>The alkaline activator is responsible for the majority of the environmental impact in geopolymer systems, highlighting the need for more sustainable activation methods.</li><li>The integration of recycled aggregates provides a secondary but vital environmental benefit by reducing virgin material demand and optimizing transport logistics in urban areas.</li></ul><p>As the construction industry seeks to align with the 2030 sustainability goals, the combination of alkali-activated binders and recycled aggregates offers a robust pathway toward a circular and carbon-neutral built environment. Future research should prioritize the development of standardized codes for RAGC and the exploration of bio-based or waste-derived activators to further minimize the ecological footprint of these materials.</p>
<h2>References</h2>
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