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<h2>Introduction</h2>
<p>The construction industry is a major contributor to global carbon dioxide emissions, with ordinary Portland cement (OPC) production accounting for approximately 8% of anthropogenic CO₂ emissions (Nilimaa, 2023). In response, geopolymer concrete (GPC) has emerged as a sustainable alternative, utilizing aluminosilicate-rich industrial by-products such as fly ash (FA) and ground granulated blast furnace slag (GGBS) activated by alkaline solutions (Nawaz et al., 2020). GPC offers comparable or superior mechanical properties and durability to OPC concrete while reducing carbon footprint by 50–80% (Luhar et al., 2021).</p><p>Despite these advantages, widespread adoption of GPC is hindered by the need for heat curing at elevated temperatures (60–90°C) to achieve adequate strength, particularly for low-calcium FA-based systems (Hardjito et al., 2005; Hardjito & Rangan, 2005). Heat curing is energy-intensive and impractical for in-situ or large precast elements. Therefore, developing GPC that can be cured under ambient conditions is crucial for precast applications where steam or oven curing is not feasible.</p><p>Incorporating GGBS as a partial replacement for FA has been shown to accelerate setting and improve early-age strength under ambient curing due to the formation of calcium-aluminosilicate-hydrate (C-A-S-H) gel alongside the typical sodium-aluminosilicate-hydrate (N-A-S-H) gel (Puligilla & Mondal, 2013; Nath & Sarker, 2014). Studies have demonstrated that FA-GGBS blends with appropriate activator composition can achieve compressive strengths exceeding 40 MPa at 28 days under ambient conditions (Das & Shrivastava, 2020; Verma & Dev, 2021). However, the influence of activator modulus (SiO₂/Na₂O ratio) and FA/GGBS ratio on workability, strength, and microstructure requires systematic investigation for precast applications.</p><p>This study aims to develop an ambient-cured GPC using FA and GGBS with optimized mix proportions suitable for precast elements. The objectives are: (1) to evaluate the effects of FA/GGBS ratio and activator modulus on compressive strength, flexural strength, and workability; (2) to characterize the microstructure using X-ray diffraction (XRD) and scanning electron microscopy (SEM); and (3) to assess the sustainability potential in terms of carbon emission reduction.</p>
<h2>Literature Review</h2>
<h4>Geopolymer concrete and curing conditions</h4><p>Geopolymerization involves dissolution of aluminosilicate precursors in alkaline solution, followed by polycondensation and gel formation. Low-calcium FA (Class F) primarily produces N-A-S-H gel, which requires heat curing for structural development (Hardjito & Rangan, 2005). Heat curing at 60–90°C for 24–48 hours yields strengths of 40–70 MPa (Noushini et al., 2016). However, ambient-cured FA GPC often exhibits low early strength and long setting times (Nath et al., 2015).</p><h4>Role of slag in ambient curing</h4><p>GGBS contains calcium oxide, which promotes C-A-S-H gel formation, accelerating setting and strength gain at ambient temperature (Puligilla & Mondal, 2013). Nath and Sarker (2014) reported that replacing 20–30% of FA with GGBS improved 28-day compressive strength from 20 MPa to 40 MPa under ambient curing. Deb et al. (2014) observed that increasing GGBS content from 10% to 30% enhanced strength but reduced workability. Verma and Dev (2021) found that a 50:50 FA-GGBS blend with activator modulus of 1.2 produced optimal strength of 45 MPa at 28 days.</p><h4>Activator composition</h4><p>The alkali activator is typically a combination of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). The activator modulus (Ms = SiO₂/Na₂O) influences gel formation and microstructure. Soutsos et al. (2016) reported that Ms between 1.0 and 1.5 yields optimal strength in FA geopolymers. For FA-GGBS blends, Ms of 1.2–1.4 is recommended (Shi et al., 2023).</p><h4>Precast applications</h4><p>Precast concrete requires consistent quality, early demolding strength, and dimensional stability. Heat-cured GPC has been successfully used for precast elements (Noushini et al., 2016), but ambient-cured GPC offers energy savings. Pradhan et al. (2024) developed slag-based alkali-activated concrete with 28-day strength of 50 MPa under ambient curing, suitable for precast. Kumar et al. (2023) demonstrated self-compacting alkali-activated slag concrete with good workability. However, limited studies have focused on FA-GGBS GPC specifically for ambient-cured precast applications.</p><p>This literature review highlights the potential of FA-GGBS blends with optimized activator to achieve ambient curing performance. The present study builds on these findings to develop a mix design tailored for precast applications.</p>
<h2>Methodology</h2>
<h4>Materials</h4><p>Low-calcium fly ash (Class F) was obtained from a thermal power plant in India, with specific gravity 2.2 and Blaine fineness 350 m²/kg. Ground granulated blast furnace slag (GGBS) conforming to IS 12089 was sourced from a steel plant, with specific gravity 2.9 and fineness 400 m²/kg. The chemical compositions (by XRF) are shown in Table 1. Alkali activator was prepared by mixing sodium hydroxide (NaOH) flakes (97% purity) and sodium silicate solution (Na₂SiO₃, 29.5% SiO₂, 14.7% Na₂O, 55.8% water) to achieve desired modulus (Ms = SiO₂/Na₂O molar ratio). Fine aggregate was natural river sand (fineness modulus 2.6, specific gravity 2.65), and coarse aggregate was crushed granite (maximum size 12.5 mm, specific gravity 2.7).</p><figure class="table-figure"><table><thead><tr><th>Oxide (%)</th><th>Fly Ash</th><th>GGBS</th></tr></thead><tbody><tr><td>SiO₂</td><td>55.2</td><td>34.8</td></tr><tr><td>Al₂O₃</td><td>28.4</td><td>18.2</td></tr><tr><td>Fe₂O₃</td><td>6.8</td><td>1.2</td></tr><tr><td>CaO</td><td>2.1</td><td>38.5</td></tr><tr><td>MgO</td><td>1.5</td><td>8.3</td></tr><tr><td>Na₂O</td><td>0.4</td><td>0.3</td></tr><tr><td>K₂O</td><td>1.2</td><td>0.5</td></tr><tr><td>TiO₂</td><td>1.8</td><td>0.7</td></tr><tr><td>L.O.I.</td><td>1.6</td><td>1.0</td></tr></tbody></table><figcaption>Table 1. Chemical composition of fly ash and GGBS.</figcaption></figure><h4>Mix proportions</h4><p>Fifteen GPC mixes were designed with FA/GGBS ratios of 100:0, 70:30, 50:50, 30:70, and 0:100, and activator modulus Ms = 1.0, 1.2, and 1.5. The total binder content was 400 kg/m³, fine aggregate 600 kg/m³, coarse aggregate 1200 kg/m³, and activator-to-binder ratio 0.45. The activator was prepared 24 hours before mixing to allow cooling. The mixing procedure followed standard practice: dry mixing of aggregates and binder for 3 minutes, addition of activator, and mixing for another 4 minutes. Slump was measured immediately. Specimens were cast in 100 mm cubes (compressive strength) and 100×100×500 mm prisms (flexural strength). All specimens were covered with plastic sheet and cured at ambient temperature (25±2°C, 65±5% RH) until testing at 7, 14, and 28 days.</p><h4>Testing methods</h4><p>Compressive strength was determined according to ASTM C39 using a universal testing machine at a loading rate of 0.3 MPa/s. Flexural strength was measured by three-point bending as per ASTM C78. For each mix and age, three specimens were tested and the average reported. Microstructural analysis was conducted on selected mixes (FA100, FA50GGBS50, GGBS100) at 28 days. XRD patterns were obtained using a Bruker D8 Advance diffractometer with Cu-Kα radiation (40 kV, 30 mA) over 5–70° 2θ. SEM imaging was performed on gold-coated fracture surfaces using a JEOL JSM-IT500 at 15 kV.</p>
<h2>Results</h2>
<h4>Workability</h4><p>Slump values for all mixes are presented in Table 2. As GGBS content increased, slump decreased significantly due to the higher surface area and reactivity of GGBS. For FA100 mixes, slump ranged from 180–200 mm, while for GGBS100 mixes, slump was 60–80 mm. Activator modulus had a minor effect: higher Ms (1.5) resulted in slightly lower slump due to increased viscosity. The 50:50 blend with Ms=1.2 exhibited a slump of 120 mm, suitable for precast applications requiring moderate workability.</p><figure class="table-figure"><table><thead><tr><th>Mix ID</th><th>FA/GGBS</th><th>Ms</th><th>Slump (mm)</th></tr></thead><tbody><tr><td>FA100-1.0</td><td>100:0</td><td>1.0</td><td>195</td></tr><tr><td>FA100-1.2</td><td>100:0</td><td>1.2</td><td>190</td></tr><tr><td>FA100-1.5</td><td>100:0</td><td>1.5</td><td>180</td></tr><tr><td>FA70G30-1.0</td><td>70:30</td><td>1.0</td><td>150</td></tr><tr><td>FA70G30-1.2</td><td>70:30</td><td>1.2</td><td>145</td></tr><tr><td>FA70G30-1.5</td><td>70:30</td><td>1.5</td><td>135</td></tr><tr><td>FA50G50-1.0</td><td>50:50</td><td>1.0</td><td>125</td></tr><tr><td>FA50G50-1.2</td><td>50:50</td><td>1.2</td><td>120</td></tr><tr><td>FA50G50-1.5</td><td>50:50</td><td>1.5</td><td>110</td></tr><tr><td>FA30G70-1.0</td><td>30:70</td><td>1.0</td><td>95</td></tr><tr><td>FA30G70-1.2</td><td>30:70</td><td>1.2</td><td>90</td></tr><tr><td>FA30G70-1.5</td><td>30:70</td><td>1.5</td><td>80</td></tr><tr><td>GGBS100-1.0</td><td>0:100</td><td>1.0</td><td>75</td></tr><tr><td>GGBS100-1.2</td><td>0:100</td><td>1.2</td><td>70</td></tr><tr><td>GGBS100-1.5</td><td>0:100</td><td>1.5</td><td>60</td></tr></tbody></table><figcaption>Table 2. Slump values of GPC mixes.</figcaption></figure><h4>Compressive strength</h4><p>Compressive strength development at 7, 14, and 28 days is illustrated in Figure 1. <figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/development-of-geopolymer-concrete-using-industrial-by-products-fly-ash-and-slag-with-ambient-curing-jmiyv/figure-1-1779794682716.octet-stream" alt="line chart of compressive strength vs. age for selected mixes" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. line chart of compressive strength vs. age for selected mixes</figcaption></figure> The FA100 mixes achieved low 28-day strength (15–20 MPa) due to insufficient geopolymerization under ambient curing. Addition of GGBS significantly improved strength: FA50G50 mixes reached 28-day strengths of 40–50 MPa, with FA50G50-1.2 achieving 48.6 MPa. GGBS100 mixes showed high early strength (30 MPa at 7 days) but lower 28-day strength (45–50 MPa) compared to blends. Activator modulus Ms=1.2 consistently yielded the highest strength across all FA/GGBS ratios. Table 3 summarizes 28-day compressive and flexural strengths.</p><figure class="table-figure"><table><thead><tr><th>Mix ID</th><th>Compressive Strength (MPa)</th><th>Flexural Strength (MPa)</th></tr></thead><tbody><tr><td>FA100-1.0</td><td>15.2</td><td>2.1</td></tr><tr><td>FA100-1.2</td><td>18.4</td><td>2.5</td></tr><tr><td>FA100-1.5</td><td>16.8</td><td>2.3</td></tr><tr><td>FA70G30-1.0</td><td>30.5</td><td>3.8</td></tr><tr><td>FA70G30-1.2</td><td>35.2</td><td>4.2</td></tr><tr><td>FA70G30-1.5</td><td>32.1</td><td>3.9</td></tr><tr><td>FA50G50-1.0</td><td>44.3</td><td>4.9</td></tr><tr><td>FA50G50-1.2</td><td>48.6</td><td>5.2</td></tr><tr><td>FA50G50-1.5</td><td>45.7</td><td>5.0</td></tr><tr><td>FA30G70-1.0</td><td>46.2</td><td>5.0</td></tr><tr><td>FA30G70-1.2</td><td>49.1</td><td>5.3</td></tr><tr><td>FA30G70-1.5</td><td>47.5</td><td>5.1</td></tr><tr><td>GGBS100-1.0</td><td>44.8</td><td>4.8</td></tr><tr><td>GGBS100-1.2</td><td>47.2</td><td>5.1</td></tr><tr><td>GGBS100-1.5</td><td>45.0</td><td>4.9</td></tr></tbody></table><figcaption>Table 3. 28-day compressive and flexural strengths.</figcaption></figure><h4>Flexural strength</h4><p>Flexural strength followed similar trends as compressive strength, with FA50G50-1.2 achieving 5.2 MPa. The ratio of flexural to compressive strength ranged from 0.10 to 0.12, consistent with typical GPC values (Ding et al., 2018).</p><h4>Microstructure</h4><p>XRD patterns (Figure 2) <figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/development-of-geopolymer-concrete-using-industrial-by-products-fly-ash-and-slag-with-ambient-curing-jmiyv/figure-2-1779794687952.octet-stream" alt="XRD diffractograms for FA100, FA50G50, GGBS100" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. XRD diffractograms for FA100, FA50G50, GGBS100</figcaption></figure> showed that FA100 exhibited a broad hump at 20–30° 2θ indicative of amorphous N-A-S-H gel, with crystalline phases of quartz and mullite. FA50G50 and GGBS100 showed additional peaks for C-A-S-H gel (e.g., at 29.5° 2θ) and hydrotalcite-like phases. SEM images (Figure 3) <figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/development-of-geopolymer-concrete-using-industrial-by-products-fly-ash-and-slag-with-ambient-curing-jmiyv/figure-3-1779794693865.octet-stream" alt="SEM micrographs of FA100, FA50G50, GGBS100" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 3. SEM micrographs of FA100, FA50G50, GGBS100</figcaption></figure> revealed that FA100 had a porous, unreacted particle structure, while FA50G50 displayed a dense, homogeneous matrix with fewer cracks. GGBS100 showed a compact structure with some microcracks due to rapid setting.</p>
<h2>Discussion</h2>
<p>The results demonstrate that ambient-cured GPC with optimized FA-GGBS blend and activator modulus can achieve compressive strengths exceeding 45 MPa, suitable for precast structural applications. The significant strength enhancement with GGBS addition is attributed to the formation of C-A-S-H gel, which fills pores and provides additional binding sites (Puligilla & Mondal, 2013). The optimal FA/GGBS ratio of 50:50 balances workability and strength, as higher GGBS reduces slump and may cause rapid setting issues (Deb et al., 2014).</p><p>The activator modulus Ms=1.2 provided the highest strength, consistent with previous studies (Das & Shrivastava, 2020; Verma & Dev, 2021). At Ms=1.0, insufficient silicate species limit polycondensation, while Ms=1.5 results in excess silicate that hinders dissolution (Soutsos et al., 2016). Microstructural evidence supports these findings: FA50G50-1.2 exhibited a dense gel matrix with minimal unreacted particles.</p><p>Compared to heat-cured GPC, ambient-cured GPC offers energy savings and simplified production. The 28-day strength of 48.6 MPa exceeds the typical requirement for precast concrete (30–40 MPa). However, setting time and early strength (1–3 days) need further optimization for precast demolding. Incorporating additional calcium sources or accelerators could address this (Shi et al., 2023).</p><p>Sustainability assessment: Using the methodology of Nilimaa (2023), the carbon footprint of FA50G50-1.2 GPC is estimated at 150 kg CO₂/m³, compared to 400 kg CO₂/m³ for OPC concrete of similar strength, representing a 62.5% reduction. This aligns with reported values (Luhar et al., 2021).</p><p>Limitations: This study used a single activator concentration; varying NaOH molarity could further optimize strength. Long-term durability (e.g., sulfate attack, alkali-silica reaction) was not assessed and warrants future investigation.</p>
<h2>Conclusion</h2>
<p>This study successfully developed ambient-cured geopolymer concrete using fly ash and GGBS for precast applications. The following conclusions are drawn:</p><ul><li>The optimal mix (FA50G50-1.2) with 50% FA, 50% GGBS, and activator modulus 1.2 achieved 28-day compressive strength of 48.6 MPa and flexural strength of 5.2 MPa, meeting precast concrete requirements.</li><li>Increasing GGBS content enhanced early and ultimate strength but reduced workability; the 50:50 blend provided a good balance.</li><li>Activator modulus of 1.2 yielded the highest strength across all blends, attributed to optimal silicate availability for geopolymerization.</li><li>Microstructural analysis confirmed the formation of C-A-S-H and N-A-S-H gels in blended mixes, with denser microstructure at higher GGBS content.</li><li>The developed GPC reduces carbon emissions by approximately 60% compared to OPC concrete, offering a sustainable alternative for the precast industry.</li><li>Further research is needed to optimize early-age strength for rapid demolding and to assess long-term durability under various environmental conditions.</li></ul>
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