Nano-cellulose and biochar reinforced fly ash–ggbs based geopolymer concrete: a sustainable pathway to enhanced strength and durabilityABSTRACT
This research aims to explore the mechanical and durability properties of geopolymer concrete (GPC) with varying content of fly ash, Ground Granulated Blast Furnace Slag (GGBS), biochar, and nano-cellulose fibres under different curing conditions. GPC, compared to traditional concrete, showed superior strength properties, particularly in mixture containing optimized GGBS and biochar. The maximum 28 day compressive strength of 61.12 MPa was recorded in the mixture containing 22% fly ash, 72% GGBS, 6% biochar, and 0.6% nano-cellulose fibres. The mixture also exhibited good workability and enhanced tensile and flexural strengths. Tests of durability such as Rapid Chloride Permeability Test (RCPT), acid and sulphate resistance, and carbonation depth indicated that additions of biochar and nano-fibres greatly alleviated permeability and strength loss. Nano-cellulose fibres have also improved matrix compaction, minimizing carbonation depth and enhancing durability in aggressive exposures. ANOVA has ensured that there were significant workability and compressive strength differences, whereas RCPT differences were statistically insignificant, indicating time-dependent variability in chloride resistance. In general, the incorporation of biochar and nano-cellulose fibres into GGBS rich GPC enhances mechanical strength and durability, justifying its viability as a green replacement for ordinary Portland cement (OPC) for long-term infrastructure construction under extreme environmental conditions.
Keywords:
Geopolymer concrete; Biochar; Nano-cellulose fibres; GGBS; Micro analysis
1. INTRODUCTION
Geopolymer concrete is an alkali-activated binder system based on industrial by-products like fly ash and GGBS as precursors, reacting with alkaline activators such as sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) [1]. In contrast to Ordinary Portland Cement (OPC), which releases around 0.9 to 1.0 tonnes of CO2 per tonne, GPC can minimize carbon emission by up to 80% depending on the mixture and curing. GPC creates a hard alumino-silicate structure (N-A-S-H gel), and in calcium-rich systems (with GGBS), also produces C-A-S-H gel, enhancing mechanical properties [2]. Compressive strength of geopolymer mixtures is between 40 MPa and more than 80 MPa at 28 days, varying with curing conditions. Fly ash-based GPC typically requires elevated curing (60–80 °C for 24 hours), while GGBS-rich mixes can achieve 50–60 MPa under ambient curing within 28 days, due to the latent hydraulic reactivity of calcium phases [3]. GGBS is a calcium-bearing by-product which increases early-age strength in GPC. It reacts quickly with alkaline activators, producing dense C-A-S-H gel that enhances the geopolymeric network [4, 5]. Substitution of fly ash with GGBS in ratios up to 75% raises compressive strength considerably [6]. For example, a mixture of 25% fly ash and 75% GGBS has attained compressive strength up to 56 MPa at 28 days under ambient temperature [7]. In addition, GGBS enhances initial setting time (in 60–90 minutes), workability, and bonding characteristics [3, 6, 8]. High GGBS content (>80%) could result in quicker setting and lower workability because of accelerated reaction kinetics. Optimal proportions (50–75% GGBS) present the optimum combination of strength, setting characteristics, and durability [4, 9].
Biochar, which is made through pyrolysis of biomass at 400–600 °C, is a high-surface-area (>300 m2/g) porous, carbonaceous material with irregular particle size (1–50 μm). In GPC, biochar behaves as a micro-filler, enhancing particle packing, internal curing, and resistance to cracking [10]. At 2–8% by weight of binder addition, biochar can boost compressive strength by 5–15%. A blend with 6% biochar (by weight of binder) has demonstrated compressive strength of 61.12 MPa at 28 days, against 56 MPa without biochar [11]. Biochar’s high porosity enables retention of moisture in the matrix, which facilitates ongoing geopolymerization, particularly under ambient curing [12]. Dosages of more than 8% can decrease strength due to dispersion problems and porosity increase. Durability properties are also enhanced by biochar. RCPT values in GPC with biochar are decreased from >1500 Coulombs (in pure GPC) to <750 Coulombs, showing very low chloride permeability. Sulphate and acid resistance also increase, with strength loss in 5% H2SO4 decreased from 6.5% to <4.5% over 90 days [10, 13]. Nano-cellulose fibres are bio-sourced high tensile strength reinforcement materials (up to 2 GPa), aspect ratio >100, and diameters in the range of 5–50 nm [9, 14]. In GPC, mechanical and durability performance is improved by NCFs due to crack bridging, matrix densification, and interfacial bonding [4, 15]. Incorporation of 0.2–0.6% by binder weight of NCFs boosts flexural strength by 15–25% and split tensile strength by up to 20%. A blend with 0.6% NCF developed a flexural strength of 7.2 MPa and tensile strength of 5.8 MPa, whereas the control mixture had 5.5 MPa and 4.6 MPa respectively [16]. SEM indicates that NCFs are well-dispersed and bridging microcracks, encouraging a denser gel phase. In addition, NCFs decrease the depth of carbonation and chloride ion penetration as a result of decreased pore connectivity. Depth of carbonation after 90 days fell from 53 mm (in OPC concrete) to 40 mm in the optimized GPC mixture with NCFs, which reflects better resistance to CO2 ingress [17]. GPC exhibits excellent strength and greater durability in harsh conditions. The compressive strengths vary from 40 to 80 MPa, with improved split tensile (4–6 MPa) and flexural strengths (6–10 MPa) when GGBS, biochar, and NCFs are optimized [14, 18]. Durability parameters includes RCPT values which decreased from 2500–3000 Coulombs in OPC to <1000 Coulombs in GPC. Acid resistance test strength loss in 5% HCl after 90 days is 5.77% in OPC, compared to 4.35% in optimized GPC [19]. Sulfate attack the GPC maintains >90% of strength after 90-day immersion in 5% Na2SO4 [20]. Carbonation depth reduced from 53 mm in OPC to 39–42 mm in GPC with biochar and NCFs. Water absorption is decreased to <4.5%, and porosity is greatly reduced because of the synergistic action of biochar (filler) and NCFs (bridging matrix gaps) [21].
SEM analysis indicates that GPC blends containing GGBS and biochar have a more compact and denser microstructure than OPC. The matrix of OPC contains porous areas with visible calcium hydroxide platelets, whereas GPC has interlocking N-A-S-H and C-A-S-H gels [22]. The addition of biochar particles assists in the nucleation sites and pore refinement. NCFs are seen in SEM as thin fibrils dispersed within the matrix, spanning microcracks and strengthening interfacial regions [23]. SEM analysis verifies the diminution of portlandite and the prevalence of amorphous alumino-silicate gels in GPC. GGBS-rich mixes possess weak crystalline peaks of calcium silicates and zeolite phases [24, 25]. EDS spectra verify high Ca/Si and Si/Al ratios, which suggest the existence of both N-A-S-H and C-A-S-H gel phases [21, 26]. Nanomaterials are increasingly incorporated into geopolymer concrete (GPC) within a dosage range of 0.25–5% by binder weight, depending on their type and function [27]. Nano-silica (1–4%) is the most widely used, enhancing compressive strength, densifying the matrix, and reducing porosity [28]. Nano-alumina (1–2%) accelerates geopolymerization and improves tensile and flexural strength, while nano-titania (1–4%) contributes to durability and imparts photocatalytic properties [29]. Nano-clay (1–3%) improves rheology and microstructure, whereas nano-zinc oxide (0.25–0.75%) enhances strength in 3D-printed GPC [30]. Carbon-based nanomaterials such as graphene or CNTs, used at <1%, significantly improve toughness, fracture resistance, and durability, supporting sustainable high-performance GPC [31]. The advantages of GPC are improved strength, durability, or sustainability benefits of specific geopolymer formulations and nano-material incorporations. Limitations which includes issues like workability reduction, optimal dosage constraints, curing requirements, and cost considerations. Applicability in terms of field implementation, environmental conditions, and compatibility with existing construction practices. Gaps in existing research is, for geopolymer concrete, there is not a universally accepted methodology for mixture design when using multi-component additives such as biochar and nano-cellulose fibres. Empirical or trial-and-error methods are mostly used in existing practices, with results offering inconsistent performance. The effects of biochar and nano-cellulose on workability, setting time, and flow properties are not well understood, particularly for high-performance and self-consolidating geopolymer concretes. Limited long-term evidence is available of the performance for GPC in field aggressive conditions. There is no adequate microstructural examination of the interfacial bonds between nano-cellulose fibres, biochar particles, and the geopolymer matrix.
2. MATERIALS AND METHODS
Regular Portland Cement (OPC) 43 Grade is a fine grey powder with particle size <75 microns, a specific gravity of 3.15, and a surface area of 2258 cm2/g [32]. It is resistant to minimal volume expansion (1 mm) and is chemically constituted by 62.27% CaO, 21.24% SiO2, 3.82% Al2O3, and 2.56% Fe2O3 to ensure strength, durability, and thermal stability [33]. M-sand is a grey, angular aggregate with a specific gravity of 2.58 and bulk density of 2.73 g/cc. Its low moisture content (0.35%) and water absorption (1.0%) improve workability. Fineness modulus 1.48 and a maximum particle size of 1.15 mm qualify it for smooth finishes.
Natural coarse aggregates with 20 mm nominal size have a specific gravity of 2.77, are of low water absorption (0.5%), and possess a crushing value of 18.15%, making it suitable for structural uses. GGBS, being an industrial byproduct, has a specific gravity of 2.9, fineness of 428 m2/kg, and is rich in CaO (38.15%), SiO2 (32.64%), and Al2O3 (14.32%) at the chemical level, enhancing pozzolanic activity and durability [34].
Fly ash is spherical in shape, dark grey in color, has a specific gravity of 2.21, and contains high SiO2 (42.62%) content. The low bulk density (915 kg/m3) and high fineness (321 m2/kg) of fly ash make it suitable for improving the cohesiveness of concrete [35]. Biochar contains a specific gravity of 1.33and surface area of 185 m2/g and is carbon-rich (85.15%) content. It is useful in lightweight, sustainable concrete. Nano-cellulose fibres possess a specific gravity of 1.55 and are high in surface area (198 m2/g), offering nanoscale reinforcement, enhancing toughness, and reducing shrinkage. Combined, these materials present sustainable and high-performance options for next-generation concrete applications. Figure 1 presents the materials utilized in this study. Figures 2 and 3 presents the grading of fine and coarse aggregate.
Materials used in this research. (a) Cement; (b) Fine Aggregate; (c) Coarse Aggregate; (d) GGBS; (e) Flyash; (f) Biochar; (g) Nano Cellose Fibres; (h) Casting.
3. METHODOLOGY
The experimental approach employed in this research sought to examine the role of biochar and nano-cellulose fibres (NCF) on the strength, durability, and microstructure of geopolymer concrete (GPC). Low-calcium Class F fly ash and GGBS were the main binder materials that were selected based on their high aluminosilicate composition and synergistic reactivity. 12M concentration of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) were utilized as the alkaline activators, blended in a weight proportion of 2.5:1. Preparations were made for a control mixture with Ordinary Portland Cement (OPC), a geopolymer mixture cured at 60°C for 24 hours (GC1 to CG 14), and the other under ambient conditions (GC 15 to GC28). The mix proportion was kept constant with an activator-to-binder ratio of 0.45. Cube specimens (150 mm), cylindrical specimens (150 mm × 300 mm), and beams (100 mm × 100 mm × 500 mm) were cast. The compressive strength was tested as per IS 516 at 7, 28, and 90 days. Split tensile strength tests were carried out on cylindrical specimens according to IS 5816, while flexural strength tests according to IS 516 were performed using third-point loading on beam specimens.
Durability tests involved various tests. Acid resistance was tested by soaking 28-day-cured specimens in 5% HCl solution for 90 days, with regular measurements of mass loss and residual strength. Sulphate resistance was determined through immersion of specimens in 5% sodium sulphate (Na2SO4) solution for 90 days, followed by strength and dimensional change testing. Carbonation test was conducted employing phenolphthalein indicator after 90 days of exposing the samples to 4% CO2 at 25°C and 60% RH, with carbonation depth determined through color change. Microstructural analysis was conducted employing Scanning Electron Microscopy (SEM). The approach was developed to determine the viability of using biochar and NCF in GPC for improved performance and sustainability in construction. Table 1 show the material propositions of various mix.
4. RESULTS AND DISCUSSION
4.1. Workability test
Workability was determined using slump cone and compaction factor tests. The control mix (CC) showed the maximum slump (108 mm) and compaction factor (0.900), which reflects better flowability due to the availability of ordinary Portland cement and best water-to-cement ratio. Geopolymer concrete (GPC) mixes showed a decrease in workability with an increase in the ratio of GGBS and admixtures. Slump reduced continuously from GC1 (99 mm) to GC5 (92 mm), in proportion to higher GGBS content, which is finer and more reactive and hence places greater requirements on the mixing water and internal cohesion. The use of biochar further lowered slump and compaction result as a result of its porous, high-surface-area nature, which creates greater water absorption and inter-particle friction. The incorporation of nano-cellulose fibres (NCFs) had the greatest influence, especially with more than 0.4% contents. For example, GC14 (1% NCF) manifested a slump of 75 mm and compaction factor of 0.625, an indication of serious loss in workability. This unusual decrease is linked to fibre tangles, rising matrix viscosity, and reduced free water. These findings indicate that while mechanical performance enhances with the use of GGBS, biochar, and NCF, fresh-state mix behavior becomes stiff and requires optimization of admixtures, e.g., superplasticizers, to ensure pragmatic placement properties. Figures 4 and 5 presents the slump and compaction factor test results.
4.2. Compressive strength test
Compressive strength test results show a noticeable improvement in mechanical performance of GPC with higher content of GGBS and strategic use of additives. The control concrete attained 46.83 MPa after 28 days, while GC5 (100% GGBS) recorded a higher strength of 54.77 MPa when under hot curing. This increase is due to increased calcium content in GGBS, which increases the geopolymerization reaction and assists in the development of denser C-A-S-H gels. The addition of biochar (GC6–GC9) added another gain in strength, GC8 (6% biochar) achieving 57.89 MPa. Biochar is likely a micro-filler and internal curing agent, enhancing pore structure and matrix densification. The best performance was achieved in the mixes with NCFs, especially at 0.6% content. GC12 (hot-cured) and GC26 (ambient-cured) attained peak strengths of 60.31 MPa and 61.12 MPa, respectively, and it is clear that NCFs increase crack-bridging capacity and interfacial transition zone (ITZ) reinforcement. At above 0.6% NCF, marginal variation or loss of strength was noticed, which may be caused by fibre agglomeration or poor dispersion. These results validate that a mixture of 22% flyash, 72% GGBS, 6% biochar, and 0.6% NCF provides an optimized matrix with enhanced compressive properties in both curing conditions. Figure 6 illustrates the compressive strength test.
4.3. Split tensile strength test
The tensile strength data from the split tests show similar trends to the compressive strength results, indicating the effect of binder composition and additives upon resistance to tensile stress. The control concrete reached 2.86 MPa after 28 days, while geopolymer mixtures with high GGBS content showed noticeable improvements, with GC4 at 75% GGBS reaching 3.47 MPa. GGBS is the cause of increased matrix continuity and early-age strength caused by improved gel development. The addition of biochar (up to 6%) enhanced tensile strength by internal curing and being a rigid filler. GC8 had 3.54 MPa, higher than GC1 (2.77 MPa) and the control mix. More improvement was seen with the addition of NCFs, with GC12 and GC26 (0.6% NCF each) achieving 3.69 MPa and 3.74 MPa, respectively. The fibril structure offered by NCFs most probably aided enhanced tensile ability through energy dissipation and crack arresting. Tensile strength remained constant or decreased marginally for greater fibre contents, suggesting an optimum dosage level of around 0.6%. These findings validate the efficacy of hybrid geopolymer composites in enhancing tensile performance through fibre reinforcement and microstructural improvements. Figure 7 indicates split tensile strength test.
4.4. Flexural strength test
Flexural strength behavior is in close agreement with tensile and compressive strength behavior, validating the advantages of GGBS, biochar, and NCF in terms of improving structural performance. The control concrete reached a flexural strength of 4.23 MPa at 28 days, whereas GC4 (75% GGBS) and GC5 (100% GGBS) registered 5.12 MPa and 4.95 MPa, respectively. These improvements are due to the development of a finer matrix and enhanced C-A-S-H bonding network in GGBS-rich systems. Addition of biochar also enhanced flexural performance with its function in altering the pore structure and enhancing interfacial bonding. GC8 (6% biochar) attained 5.23 MPa, which signifies peak biochar performance. The maximum flexural strengths were realized in mixes with 0.6% NCF, namely GC12 (5.45 MPa) and GC26 (5.52 MPa), under hot and ambient curing conditions, respectively. NCFs assist in mechanical interlocking and provide enhanced toughness, ductility, and resistance to cracking, which are vital for flexural performance. Strength increased up to 0.6% and plateaued at higher percentages, with only slight decreases or insignificant increases due to fibre clustering or insufficient dispersion. This result validates that the synergy of optimized binder chemistry and nano-reinforcements can achieve geopolymer composites that have greater flexural strength compared to typical OPC-based concretes. Figure 8 illustrates the flexural strength test.
4.5. RCPT test
RCPT results evidently show the enhanced durability of geopolymer concrete (GPC) compared to conventional concrete (CC). The control mix (CC) revealed high charge passed values of 2309, 2133, and 2054 Coulombs at 28, 56, and 90 days, respectively, reflecting moderate chloride permeability. However, GPC mixes containing GGBS indicated much lower values, with GC5 (100% GGBS) registering only 887 C at 90 days, reflecting low to very low permeability. The addition of biochar further improved performance; GC8 (6% biochar) obtained 853 C at 90 days owing to its internal curing and filler effect advantages. The improvement was most striking with the addition of nano-cellulose fibre (NCF). GC12 and GC26, both containing 0.6% NCF, attained the lowest values of 749 C and 681 C at 90 days, reflecting extremely low permeability. NCFs enhance matrix densification and crack-bridging, which impede ionic transport. Yet, increasing fibre content above 0.6% proportionally enhanced permeability slightly, presumably because of fibre agglomeration. Ambient-cured mixtures had comparable performance to hot-cured counterparts, indicating the technical viability of preserving high durability without thermal curing. In general, the best combination of 22% fly ash, 72% GGBS, 6% biochar, and 0.6% NCF considerably outperformed OPC-based concrete in chloride resistance. Figure 9 illustrates the RCPT.
4.6. Sulphate attack test
The sulphate resistance of geopolymer concrete (GPC) mixtures was better than that of traditional concrete (CC) since it is indicated by the strength loss percentage with time. CC had strength loss at 28, 56, and 90 days amounting to 4.40%, 4.33%, and 4.29%, respectively, implying poor chemical durability. GPC mixtures with GGBS exhibited dramatic improvements. GC4 (25% Flyash + 75% GGBS) experienced just 3.45% loss at 90 days, whereas GC5 (100% GGBS) experienced 3.61%, reflecting the beneficial influence of GGBS in the mitigation of sulphate attack susceptibility. Addition of biochar (GC6–GC9) reduced the loss further due to pore refinement and pozzolanic effect. Optimal performance was realized in mixes incorporating nano-cellulose fibres (NCF). GC12 and GC26 with 0.6% NCF showed the lowest loss of 3.14% and 3.07% at 90 days, respectively. NCFs are believed to increase sulphate resistance by enhancing microstructural density and crack-bridging ability. Strength loss was marginally higher at NCF contents above 0.6% possibly due to fibre agglomeration and inferior dispersion. Ambient-cured mixes showed similar results to hot-cured specimens, confirming the efficacy of the optimized composition under normal conditions. In general, the highest resistance to sulphate attack degradation was achieved by the hybrid GPC mixture of 22% flyash, 72% GGBS, 6% biochar, and 0.6% NCF. Figure 10 is the loss of strength after sulphate attack test.
4.7. Acid resistance test
The sulphuric acid resistance of geopolymer concrete (GPC) was much better than that of ordinary concrete (CC), as evident from diminished weight and strength loss with time. CC showed 4.65% weight loss and 6.92% strength loss at the age of 90 days, while the GPC mixes with GGBS had a distinct improvement. GC4 (25% Flyash + 75% GGBS) had 3.87% weight loss and 5.75% strength loss, indicating improved chemical resistance. The incorporation of biochar (GC6–GC9) also minimized degradation due to high carbon content and pore-blocking ability. GC8 (6% biochar) registered 3.77% weight and 5.61% strength loss at 90 days. The optimum performance was experienced with nano-cellulose fibre (NCF) additions. GC12 and GC26 (0.6% NCF) recorded the minimum losses of 3.58%–3.51% in weight and 5.32%–5.22% in strength due to matrix densification, enhanced fibre-matrix bonding, and control of internal micro-cracks. But higher fibre content above 0.6% exhibited decreasing returns owing to fibre clumping. Room-temperature-cured blends exhibited equivalent resistance to hot-cured GPC, pointing towards the efficiency of ternary systems optimized under room temperatures. In general, the hybrid system containing 22% flyash, 72% GGBS, 6% biochar, and 0.6% NCF exhibited optimal resistance to sulphuric acid attack, surpassing all control and single-blend systems. Figures 11 and 12 illustrates the weight loss and strength loss after acid attack test.
4.8. Carbonation test
Carbonation depth tests at 90 days indicated that geopolymer concrete (GPC) had greater resistance against carbonation than conventional concrete (CC). CC mix registered a carbonation depth of 53 mm, while optimized GPC mixes showed considerably lower depths. Pure flyash-based mixes (GC1, GC15) showed increased carbonation (54–55 mm) due to lesser calcium content and lower alkalinity. The addition of GGBS, however, significantly improved carbonation resistance. GC4 (75% GGBS + 25% Flyash) and GC5 (100% GGBS) yielded 43 mm and 45 mm, respectively. Biochar addition (GC6–GC9) enhanced performance further through micro-filler effects and matrix densification, with the result for GC8 being 42 mm. The incorporation of nano-cellulose fibre (NCF) produced the lowest carbonation depths because of enhanced crack resistance and matrix refinement. GC12 and GC26 with 0.6% NCF had minimum carbonation depths of 40 mm and 39 mm, respectively. Depths increased slightly at higher fibre contents, presumably due to agglomeration effects and less workability. The best mix of 22% flyash, 72% GGBS, 6% biochar, and 0.6% NCF had the highest carbonation resistance, even in ambient curing conditions. These findings validate the application of superior GPC systems in high CO2 exposure environments, surpassing conventional OPC concrete in terms of durability parameters. Figure 13 illustrates the depth of carbonation.
4.9. SEM
Scanning Electron Microscopy (SEM) was employed on three sample mixes—CC (ordinary concrete), GC12 (room-temperature) cured geopolymer with nano-cellulose fibres and biochar, and GC26 (identical to GC12 but at room temperature cure)—to study the microstructural features controlling acid resistance behavior.
The SEM microstructure of CC mix demonstrated microcracked and porous matrix, with prominent calcium hydroxide (Ca(OH)2) platelets distributed in the structure. Such Ca(OH)2 crystals are recognized to be highly reactive with acidic environments, resulting in soluble salts that leach away, causing loss of mass and strength. The loose packing and incomplete hydration also played a role in the poor acid resistance exhibited (5.77% strength loss after 90 days in HCl).
GC12 - Hot Cured Geopolymer Concrete (22% FA, 72% GGBS, 6% BC, 0.6% NCF) had much denser and finer microstructure with far fewer pores and microcracks than CC. The SEM photograph contained a well-crystallized geopolymer matrix with abundant calcium-alumino-silicate hydrate (C-A-S-H) and sodium-alumino-silicate hydrate (N-A-S-H) gels. Incorporation of 6% biochar seemed to have added to internal curing and pore structure refinement, whereas the nano-cellulose fibres (NCFs) improved interfacial bonding as well as bridged microcracks. These properties combined account for the enhanced acid resistance (4.43% loss in strength in HCl) of GC12.
SEM characterization of GC26 revealed a slightly more open matrix than GC12, but still much denser than regular concrete. The geopolymeric gel phases were well developed even without thermal activation, suggesting successful ambient geopolymerization. Densification was encouraged by the biochar and nano-cellulose additives, although some unreacted fly ash particles were observable, which could be responsible for the somewhat lower performance compared to GC12 (4.35% strength loss in HCl). Nevertheless, the microstructure continued to show good integrity, verifying that ambient curing can yield a strong geopolymer matrix when the mix is optimal.
The SEM test unequivocally confirms the performance results: higher microstructure density, more effective polymeric gel formation, and successful filler/fibre functionality in GC12 and GC26 translate to improved resistance to hydrochloric acid. Interestingly, GC26’s approach to parity with GC12 demonstrates the success of ambient-cured geopolymer concrete as a sustainable, energy-conscious option without sacrificing durability. Figures 14, 15 and 16, indicates the SEM photographs of mix CC, CG12 and CG 26.
5. ANOVA
5.1. Workability
ANOVA comparison of the workability tests, including the slump cone and compaction factor values of 29 samples, indicated that the differences between the two test methods are statistically significant. The computed F-value (3479.046) is far greater than the critical F-value (4.012973), and the nearly zero p-value (4.2E-52) indicates that the difference observed is extremely significant. This suggests that the two groups do not have equal means and that the slump test and compaction factor test are testing different aspects of workability. The slump cone test revealed a larger mean value (90.07 mm) with higher variability (variance = 66.50), indicating its sensitivity to mix consistency changes. Contrarily, the compaction factor test yielded a lower mean (0.75) and less variance (0.0046), which points towards consistency in results and usability for more rigid mixes. These results confirm the symbiotic application of both tests in accurately measuring workability properties in various concrete mix designs. ANOVA of workability tests is presented in Table 2.
5.2. Compressive strength
The ANOVA analysis of compressive strength on 28, 56, and 90 days show a statistically significant difference in time. With the calculated value of F = 159.8952, which is significantly higher than F-critical = 3.105157, and p-value = 2.3E-29 (much lower than 0.05), the test proves that the differences in compressive strength between curing ages are very significant. The average compressive strength dropped from 1011.27 MPa at 28 days to 959.96 MPa at 56 days, and then to 920.21 MPa at 90 days, representing a marginal loss of strength with age. This trend could be due to slow hydration reactions or long-term shrinkage effects. Nonetheless, the 28-day strength can still serve as a good predictor of optimal performance for the test concrete mixes. The variation also diminishes with time, indicating more uniform strength development at subsequent curing times. These findings are crucial for the optimization of curing times and performance expectations in real-world applications. ANOVA of compressive strength tests is presented in Table 3.
5.3. RCPT
ANOVA test of Rapid Chloride Penetration Test (RCPT) results at 7, 14, and 28 days fails to demonstrate any statistically significant difference between the groups. The F-value calculated is 0.7594, which is significantly lower than the F-critical value of 3.105157, while the p-value of 0.4711 is significantly higher than 0.05. This demonstrates that the differences in RCPT values over time are not significant at the 95% confidence level. Although mean RCPT values do rise from 36.76 at 7 days to 56.04 at 28 days—indicating some resistance development against chloride penetration—the variability within each group is too great to determine a significant trend statistically. Large within-group variability, particularly at 14 and 28 days, suggests variability in performance between mixes or test conditions. The results indicate that RCPT values become better with curing, but the variations are not significant enough to make definitive conclusions without additional controlled testing. ANOVA of RCPT is presented in Table 4.
5. CONCLUSION
Geopolymer concrete (GPC) outperformed conventional concrete (CC) in both mechanical and durability properties.
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Optimal performance was achieved with 22% fly ash and 72% GGBS, yielding significant gains in compressive, tensile, and flexural strength.
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Biochar (6%) improved strength, densified the matrix, reduced permeability, and enhanced resistance to acid, sulphate, carbonation, and chloride ingress.
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Nano-cellulose fibres (0.6%) further enhanced mechanical strength and durability, with mix GC26 achieving the highest compressive strength (61.12 MPa), very low RCPT (681 C), and superior resistance to degradation.
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SEM analysis confirmed denser, refined microstructures in GC12 (hot-cured) and GC26 (ambient-cured), correlating with reduced Ca(OH)2 and higher geopolymer gel formation.
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ANOVA indicated significant variations in workability and compressive strength (p < 0.05), while RCPT results showed no significant variation (p > 0.05).
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Future research should focus on long-term stability, field-scale applications, optimal blend design, and LCA-based sustainability assessments for biochar- and nano-fibre-reinforced GPC. Lab tests typically evaluate short- to medium-term performance (28, 56 and 90 days). Real-world structures are exposed to variable loads, environmental cycles, and chemical attacks over decades. Long-term studies and predictive modeling are needed to ensure durability under field conditions.
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