Introduction

The present scenario in the production of ordinary Portland cement (OPC) is a carbon intensive process that generates a substantial amount of greenhouse gas which accounts for approximately 8% of global CO2 emissions, significantly contributing to global climate change due to its greenhouse gas effects1,2. Concrete made from Portland cement is an extremely versatile building material that requires minimal upkeep throughout its lifespan. However, exposure to extreme temperatures can compromise its durability, raising concerns about its ability to withstand chemical assaults, particularly from alkaline and acid environments3,4,5,6. Furthermore, it has been claimed that the alkaline properties of ordinary Portland cement concrete (OPC) impact the sustainability and longevity of buildings in acidic surroundings7,8. During its lifetime, concrete is exposed to various severe conditions depending on its intended use and environmental factors. Researchers have highlighted that acid and alkaline environments, such as those containing sulfuric acid, hydrochloric acid, and magnesium and sodium sulfate, cause detrimental effects on concrete properties. These conditions substantially shorten the service life of concrete structures and negatively affect the environment9,10,11,12.

On the other hand, geopolymers are an alternative to Portland cement, which is made from alkali activated precursor materials. Geopolymers are inorganic materials rich in silicon and aluminium that react with strong alkaline activators, typically a combination of sodium or potassium hydroxide and sodium or potassium silicate13. Sodium hydroxide (SH) and sodium silicate (SS) are the most commonly used due to their superior performance, cost-effectiveness, and availability13,14. Geopolymer concrete (GPC) offers many environmental benefits along with outstanding mechanical properties and better long term durability properties. It is produced using aluminosilicate rich materials selected from industrial by-products as precursors. Incorporating these byproducts like fly ash (FA), ground granulated blast furnace slag (GGBS), silica fume (SF) and rice husk ash (RHA) into concrete reduces the demand for OPC, thereby promoting environmental sustainability15,16,17.

From the various literature, it is observed that significant improvements in the mechanical and long term durability properties of geopolymer concrete (GPC) with the use of various precursor materials, including fly ash, ground granulated blast furnace slag, silica fume, rice husk ash, and metakaolin15,16,17,18,19. Additionally, incorporating nanoparticles into geopolymer also significantly enhances strength and durability20,21. Given the increasing demand for GPC alongside conventional OPC concrete, assessing their resistance to acid and alkaline environments presents a promising area for the present research. Previous studies have indicated that GPCs exhibit superior durability characteristics compared to conventional OPC concrete22,23. Many researchers have reported that previous studies assessing the resilience of fly ash (FA) and slag based GPC against acid and alkaline environments have yielded relatively positive results24,25,26,27.

The performance of GPC is significantly influenced by the kinds and attributes of the raw materials, the curing conditions, and a few activator parameters, such as the mass ratio of the NaOH solution to the Na2SiO3 solution, the modulus of the NaOH solution, and the concentration of the NaOH solution26,28,29,30,31,32. Adding discrete fibers as reinforcement in GPC is a popular technique to increase the ductility of concrete without compromising its compressive strength33,34,35,36. The construction industry has been very interested in GPC Fiber Reinforced Concretes over the past decade since it has been witnessed that the addition of fiber enhances certain characteristics of ordinary GPC37. Researchers have reported that an optimum dosage of steel fibers in concrete is around 1%, which improves mechanical properties without adversely affecting other concrete properties32,38.

Embodied energy and carbon emissions of concrete are critical metrics in evaluating its environmental impact. These measures account for the total energy consumption and CO2 emissions from material extraction to construction39,40,41. Indirect transport has the greatest environmental impact, with raw material transport accounting for approximately 80% of the embodied energy and 79% of CO2 emissions41. Understanding these factors is essential for developing sustainable concrete solutions that minimise ecological footprints. Further, carbon emission equivalent (ECO2-e) in concrete assessments evaluates the environmental impact by quantifying CO2 emissions during production. Understanding these metrics is crucial for assessing sustainability and optimising eco-friendly concrete solutions. Researchers have chosen various parameters to estimate the CO2 equivalent (ECO2-e) more accurately, enhancing the precision of environmental impact assessments for concrete production42.

In this context, the present experimental investigation addresses the research gap by evaluating the long term durability and ecological aspects of geopolymer concrete with varying sodium hydroxide concentrations. It also provides a comparative assessment of cost, embodied energy and carbon emissions between geopolymer concrete and ordinary Portland cement of a similar grade. The research also aims to use industrial by products which promotes resource efficiency and waste utilisation. The main objective is to estimate the structural viability of GPC as a durable, low carbon alternative to OPC by optimising the mix ingredients and evaluating the performance of acid and alkali conditions. The findings aim to support the broader adoption of GPC in sustainable construction by demonstrating its potential to significantly reduce the carbon footprint while maintaining structural performance.

Significance of the study

This study is significant as it highlights the potential of geopolymer concrete to reduce carbon footprints and improve environmental sustainability in the construction industry. By utilising industrial by-products such as FA and GGBS, geopolymer concrete offers a viable alternative to conventional cement concrete. The comprehensive investigation into mix proportion optimisation and long-term durability demonstrates that geopolymer concrete not only enhances mechanical properties but also provides excellent resistance to acidic and alkaline environments. This research underscores the importance of geopolymer concrete in promoting sustainable construction practices and reducing embodied energy and carbon emissions.

Materials and methodology

Materials

In this section, all materials used in the present study are characterised for their basic properties. Detailed descriptions and specifications of each material, including their physical and chemical properties, are provided to ensure a comprehensive understanding of their roles and contributions to the concrete mix designs. This characterisation is crucial for evaluating the performance and sustainability of both the cement and geopolymer concretes investigated in this study.

The precursor materials used were Class F fly ash (FA) from KPTCL Bellary, and ground granulated blast furnace slag (GGBS) sourced from JSW Steel Plant, Thorangalu, Karnataka. The locally available ordinary Portland cement of 53 grade was used in the present study, in compliance with IS 26943. Table 1 provides the details of the physical properties and chemical composition of the materials used in this study.

Table 1 Properties of materials used in the study.
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The chemicals, sodium hydroxide (SH) and sodium silicate (SS) were procured from Om Sanvi Enterprise, Bangalore, India. For this study, the alkali activators were prepared by mixing sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) in the required quantity as per mix details. The mass ratio of sodium silicate solution to sodium hydroxide solution was maintained at 2.5 based on previous studies and literature10,19,21. Table 2 provides the properties of NaOH and Na2SiO3.

Table 2 Properties of alkaline activators.
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The M-sand utilised in this investigation was bought from the nearby quarry industry. The coarse aggregates of downsize 20 mm were obtained by pulverising hard rock stones locally market. Table 3 gives the physical characteristics of aggregates characterised as per IS 238644 and IS 38345. The particle size distribution of aggregates is presented in Fig. 1.

Table 3 Physical properties of aggregates.
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Fig. 1
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Particle size distribution of aggregates with limits.

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In this study, crimped steel fiber was selected for investigation. The steel fiber used in this study has a length of 30 mm, a diameter of 0.5 mm, and an aspect ratio of 60. In this investigation, 1% fiber by mass of the binder was utilised based on previous literature32,38. In this study, normal potable tap water was used for the production and curing of cement concrete, while deionised water was used for the production of geopolymer concrete to minimize the influence of external alkalis on the mix composition.

Mix design and Mix designation.

The concrete mix design was carried out according to IS 10262-200946 for cement concrete, with details presented in Table 4 for M40 grade concrete with a w/c ratio of 0.35 and geopolymer concrete mix design was selected based on previous studies. Several mixes were prepared with steel fibers (SF) and without steel fibers (NF) for both cement concrete and geopolymer concrete. For example, CCMS indicates a cement concrete mix (control concrete), while CCMSF represents cement concrete with steel fibers. Geopolymer concrete mixes are denoted as follows: M08SF for an 8 molar concentration of sodium hydroxide solution with steel fibers, M10SF, M12SF, and M14SF for 10, 12, and 14 molar concentrations, respectively. Similarly, M08NF denotes an 8-molar concentration of sodium hydroxide solution without steel fibers, with M10NF, M12NF, and M14NF for 10, 12, and 14 molar concentrations without steel fibers.

Table 4 Mix proportion for cement concrete and geopolymer concrete.
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Preparation of specimens

As presented in Table 4, the materials were proportioned according to the requirements for the preparation of required specimens and mixed in a laboratory mixer. The alkaline solution was prepared at least 24 h before mixing with the solid components and must be used within 36 h to avoid turning into a semi-solid state. The binder and aggregates were dry-mixed in the mixer for 2–3 min before gradually adding the premixed alkaline solution. Mixing then continued for an additional 4–6 min until a stable mix was achieved. The samples were cast into moulds and were then kept in a room with temperatures ranging between 25 ± 2 °C. The samples were demoulded after 24 h and left in the same room to cure until testing for geopolymer concrete specimens. However, the cement concrete specimens were cured in the normal water bath.

Tests on concrete

All the mixes were evaluated for slump in the fresh state according to the guidelines of IS 119947. The compressive strength of the hardened concrete was assessed as per IS 516–195948 at 7, 14, 28, 90, and 180 days. Additionally, all concrete mixes were subjected to long-term durability studies in strong acid and alkaline environments, including HCl, H2SO4, and Na2SO4 solutions. The solutions for these studies were prepared as defined in standards and previous research studies21,30,49,50. Specimens were selected after 28 days of curing to perform these durability studies. The deterioration of the specimens subjected to acid and alkaline environments was evaluated by assessing the percentage of weight loss and comparing their residual compressive strength (RCS). The performance of all mixes was evaluated by measuring weight loss and strength loss at 28, 90, and 180 days.

Results and discussion

Workability

Placing fresh geopolymer concrete in structural applications can be more challenging than placing conventional cement concrete of equivalent slump. This is primarily due to the high viscosity of the alkaline activators used, particularly the sodium silicate solution. The increased viscosity imparts higher cohesiveness to the geopolymer concrete, making it more resistant to flow. The results of this study, presented in Fig. 2, found that fresh geopolymer concrete was highly cohesive compared to cement concrete and became relatively stiffer as the molarity increased. This behaviour aligns with the findings in the literature, where elevated molarity decreases the fluidity and workability of the mixture10,19,21. Furthermore, the addition of steel fibers further intensified the reduction in workability. Fibers increase the surface area within the mix, which leads to additional water absorption, further reducing the flowability of the concrete. High fiber content in the mix can lead to a harsh, dry consistency, making it challenging to handle, place, and compact the concrete effectively32,38.

Fig. 2
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Effect of fiber and alkali activator on the workability of concretes.

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Compressive strength

Figure 3 demonstrates the impact of concrete age on the compressive strength of both cement concrete and geopolymer concrete (GPC), with varying concentrations (8, 10, 12, and 14 molars) of sodium hydroxide (SH) solution, both with and without fibers. The data clearly illustrates that as the molarity of sodium hydroxide increases, the strength of geopolymer concrete also increases. This trend aligns with previous research findings, indicating a positive correlation between the concentration of sodium hydroxide and the compressive strength of geopolymer concrete51. The maximum compressive strength at 28 days for 14 molar geopolymer concrete is 8.34% higher compared to cement concrete. Adding steel fibers to the mix further improves the strength by 9.11%. In geopolymer concrete, increasing the SH concentration from 8 to 14 molar results in a 6.38% increase in strength. Additionally, the inclusion of steel fibers in the geopolymer concrete leads to a further strength increase of up to 7.32%. Figure 4 shows that the rate of strength development in geopolymer concrete is comparable to that of cement concrete, with the strength gain rate being similar across all mixes. It is noteworthy that increasing the SH concentration in geopolymer concrete results in early strength gain due to the early surface hydrolysis of the precursor materials52,53. Geopolymer concrete with an 8 molar SH concentration exhibited better strength even after 90 days, likely due to the slow rate of reaction, which leads to strength gain at later ages. Furthermore, the data illustrates that the addition of fibers and the proper selection of SH concentration can increase concrete strength by up to 10%. This improvement is due to the fibers restricting the propagation of micro-cracks and the proper surface hydrolysis of precursor materials, thereby increasing the load-carrying capacity of the specimens52,53.

Fig. 3
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Compressive strength at different curing ages (MPa).

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Fig. 4
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Compressive strength growth rate compared to 28 days of strength.

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Long term durability tests

Resistance to H2SO4 environment

The weight loss of GPC samples after 180 days in H2SO4 solution ranged from 9.89% to 18.69%, whereas cement concrete cubes exhibited a higher weight loss of 20.59%. When the molarity increased from 8 M, the resistance improved by 13%, 32%, and 41.27%, respectively. Adding steel fibers (SF) in GPC further enhanced acid resistance in mixes by 1.35%, 4.18%, 9.43%, and 4.69% with respect to the concentration of SH, respectively. The mixes CCMS and CCMSF had residual compressive strength (RCS) of 63.1% and 69.18% respectively. The deterioration of RCS in H2SO4 decreased with higher molarity and the addition of fibers. In geopolymer concrete, the highest RCS recorded was 81.23% for M14SF with 1% SF at 180 days. Figure 5 shows a dramatic drop in compressive strength for all concrete types exposed to H2SO4, with a slower rate of reduction over time. The susceptibility of OPC concrete to acid attack can be attributed to the presence of C-S-H and Ca(OH)2 gel, which react with acid solutions, causing tensile stress and ultimately leading to cracks in the concrete matrix54. In contrast, the alumina and silica compounds in the GPC matrix react with Ca(OH)2 to form more stable C-A-S-H and N-A-S-H gels. These gels reduce acid diffusion, leading to a lower percentage of weight loss. Additionally, N-A-S-H and C-A-S-H gels have a high filler effect in the GPC pores, resulting in a denser matrix30. The superior performance of GPC in sulfuric acid solution compared to OPC concrete can be attributed to differences in reaction mechanisms and phase composition. The deterioration of GPC samples in an acidic environment occurs due to the disintegration of the geopolymer gel, damaging the aluminosilicate bonds and releasing silicic acid, resulting in a reduction in residual compressive strength50.

Fig. 5
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(a) Weight loss and (b) residual compressive strength under H2SO4 environment.

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Resistance to Na2SO4 environment

From Fig. 6 it is evident that the weight loss due to Na2SO4 in all the mixes, where GPC exhibited lower weight loss in Na2SO4 for instance a mix CCMS had an overall weight loss of 6.64% for immersion 180 days, and GPC with mix M14SF had only 2.19% of weight loss which is 70% lesser than OPC. However, there was a noticeable strength loss of about 22.2% in cement concrete whereas the strength loss in GPC specimens reduced as the concentration of SH increased from 8 to 14 M, along with the concentration and addition of fibre showed that the concrete performing very well in Na2SO4 environment even after 180 days. Researchers indicate that sodium sulfate attack progresses in stages: the outer skin expands and cracks the interior, which remains chemically unchanged. Continued immersion disintegrates the surface, allowing sulfate to react with and crack the interior hydration products55,56,57. However, in GPC strength loss is up to 16% which is a similar observation in the literature. The strength loss in geopolymer concrete, because of the crystallisation of sodium sulphate within the pore structure of concrete itself can be particularly damaging to concrete.

Fig. 6
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(a) Weight loss and (b) residual compressive strength under Na2SO4 environment.

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Resistance to HCL environment

Figure 7 shows the resistance to HCl, with weight loss for CCMSF and CCMS ranging from 10.88% to 12.73% at 180 days. GPC exhibited better resistance, with weight loss decreasing from 9.72% to 6.98% as SH concentration increased from 8 to 14 M. Additionally, the inclusion of SF further improved resistance, with M141SF experiencing a weight loss of 5.94%, which is 53.34% less than that of cement concrete. These findings align with previous research30. In samples exposed to HCl, GPC exhibited a residual compressive strength loss ranging from 4.63% to 8.9%, compared to 26.2% for OPC and 21.23% for CCMSF. GPC samples showed excellent resistance, with no interior deterioration after the exposure period. While substantial surface decay was observed in cement concrete, GPC showed only slight surface erosion. Overall, all samples exposed to H2SO4 solutions deteriorated more than those attacked by HCl solutions which is similar to findings from other researchers58.

Fig. 7
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(a) Weight loss and (b) residual compressive strength under HCL environment.

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Ecological analysis of concretes and Cost analysis

The embodied energy (EE) and embodied carbon emissions (ECO2e) of concrete in this case study were computed to assess their ecological impacts, in line with previous studies40 and as stated in various literature sources41. As illustrated in Fig. 8, a system boundary for concrete EE estimation used throughout the extraction, transportation, production, assembly, installation the product system during its production. The EE and ECO2e statistics for the components and the concrete were obtained from the literature40,41. The computations are presented in Table 5 for geopolymer concrete mix with 14 M concentration and normal concrete. The numerical figures presented are indicative and are derived from the current case study. The EE of GPC and OPC were 1597 and 3062 MJ/m3, respectively, indicating that GPC requires 47.84% less energy. Figure 9 clearly shows that the embodied energy of cement concrete is predominantly due to the cement, which accounts for nearly 82% of the total. In contrast, for geopolymer concrete, the alkaline activators account for approximately 47% of the embodied energy.

Fig. 8
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Flow chart for the system boundaries for the estimation of embodied energy and carbon emission for concrete.

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Table 5 Embodied energy for GPC and OPC.
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Fig. 9
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Embodied energy distribution of concrete ingredients.

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The ECO2e of GPC and OPC were 146.37 and 345.3 kgCO2e/m3, respectively, showing that GPC had 57.6% lower embodied CO2 emissions. Therefore, GPCs are more environmentally friendly. Figure 10 shows that embodied CO2 emissions from cement concrete are predominantly due to cement use, which accounts for nearly 95% of the total emissions. In contrast, for geopolymer concrete, alkalis account for approximately 72% of the CO2 emissions.

Fig. 10
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Embodied carbon dioxide emission of concrete ingredients.

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From Table 5, the approximate cost of GPC is Rs 7013/m3, whereas OPC costs Rs 5693/m3 based on the rates incurred during the laboratory study. According to the literature, the cost of OPC ranges from Rs 5700 to Rs 6300 per m359 and GPC costs around 7600 per m360. GPC is more expensive than OPC, with costs influenced by the source of materials and transportation60. In this research, alkalis were sourced from over 300 km away, increasing the cost per m3 of GPC. As GPC usage grows in the construction industry, the local availability of all ingredients is expected to improve, potentially reducing the cost per m3 of GPC. Figure 11 shows the cost distribution by ingredients for producing one m3 of GPC and OPC based on individual material costs. It is clear from the figure that the cost of cement accounts for around 51% of OPC’s total cost, whereas the cost of alkalis accounts for 54% of GPC’s total cost, even considering the long-distance procurement of alkalis.

Fig. 11
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Distribution of concrete cost per cubic meter by ingredients.

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The advantages of GPC over OPC in terms of embodied energy, embodied CO2 emissions and cost can be observed in Table 6 and expressed in terms of percentage advantage are presented in Fig. 12. Using the 28 day strength as a benchmark, the energy required to produce one MPa of concrete strength is 37.58 MJ/m3 for GPC, compared to 76.06 MJ/m3 for OPC. This value is almost 50% higher for OPC than for GPC. Thus, the energy required to produce one unit of strength in GPC is significantly lower than in OPC. The CO2e emitted to produce one MPa of strength is 3.44 kg/m3 for GPC, whereas it is 8.58 kg/m3 for OPC. Although GPC is approximately 25% more expensive than OPC due to the high cost of alkalis used in its production, it offers significantly greater early strengths and better durability, providing several construction advantages not accounted for in this cost comparison. Similarly, researchers have demonstrated that using geopolymer concrete can reduce global warming potential by over 60% compared to conventional cement concrete60.

Table 6 Comparison of GPC with OPC.
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Fig. 12
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Compression of advantages of GPC over OPC in percentage.

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Conclusion

This study presents key insights into the potential of geopolymer concrete for structural applications as a sustainable alternative to ordinary Portland cement concrete. The findings support the following conclusions:

  • GPC exhibited enhanced long-term durability, particularly under aggressive exposure conditions such as acid and sulfate environments, where it showed better performance than OPC. GPC demonstrated significantly lower weight loss and strength deterioration across all environments tested, highlighting its superior resistance to harsh conditions and underscoring its potential for structural longevity and performance.

  • While an increase in the concentration of sodium hydroxide molarity led to marginal strength improvements, the optimal mix with 14 M provided adequate mechanical properties suitable for structural use.

  • The early strength development of GPC can be an advantage in time sensitive structural applications, including precast elements and accelerated construction schedules.

  • Despite slightly higher initial costs due to alkali activators, GPC’s potential for local material sourcing and reduced lifecycle impacts enhances its feasibility in structural design.

  • GPC demonstrated a 47.84% reduction in embodied energy and a 57.62% reduction in CO2 emissions compared to OPC, aligning with sustainability goals.

  • With its lower environmental footprint, improved durability, and structural grade strength, GPC stands out as a viable and eco-efficient material for structural concrete applications.

From the present study, which provides the directions for future research which can explore large scale structural applications of GPC, including beams, columns, and precast components. Further experimental investigations may focus on different types of fiber with optimising them, hybrid fiber systems, chemical admixtures and the use of nano additives to enhance, fresh properties, mechanical properties, crack resistance and overall performance of GPC. Investigating the long term field performance, including leachability and corrosion resistance in real environments, is essential for wider adoption. Additionally, integrating cost analysis with region specific local materials and logistics can improve practical feasibility. Microstructural analysis and life cycle assessment would provide much better understanding into GPC’s behaviour, supporting its use in sustainable infrastructure development.