Effect of Alkali Activators on Mechanical and Durability Properties of Fly Ash based Geopolymer Concrete

Abstract

The production of ordinary Portland cement (OPC) is one of the major sources of carbon dioxide emissions in the construction industry, creating a need for more sustainable building materials. Fly ash-based geopolymer concrete (GPC) has emerged as a promising alternative because it can reduce environmental impact while providing good engineering performance. This study examines how different proportions of sodium hydroxide (NaOH) and sodium silicate (Na?SiO?) affect the strength and durability of GPC compared with conventional OPC concrete. Initial trials using NaOH:Na?SiO? ratios of 1:0.5 and 0.5:1 were conducted to identify a suitable curing condition. The main investigation was then carried out using geopolymer mixes with activator ratios of 1:1 and 0.5:1, all oven-cured at 80°C, with OPC concrete used as the control mix. Compressive, split tensile, flexural, shear, and pull-out strengths were evaluated using Indian Standard methods, while durability was assessed through water penetration and X-ray diffraction (XRD) tests. The results showed that heat curing significantly improved early-age strength, especially for the 0.5:1 mix. Although OPC concrete achieved slightly higher 28-day strength, the geopolymer mixes showed comparable mechanical performance, lower water penetration, and clear evidence of successful geopolymerization.

Introduction

The text presents an experimental study on fly ash-based geopolymer concrete (FAGPC) as a sustainable alternative to conventional ordinary Portland cement (OPC) concrete. The main focus is to determine how the NaOH concentration, Na?SiO?/NaOH ratio, and curing temperature influence the mechanical and durability performance of geopolymer concrete.

Background and Research Gap

OPC production contributes significantly to global CO? emissions. Using fly ash, a by-product of coal-fired power plants, as the main binder in geopolymer concrete can reduce the environmental impact of concrete while maintaining structural performance.

The performance of FAGPC depends strongly on alkaline activators, particularly:

• Sodium hydroxide (NaOH)
• Sodium silicate (Na?SiO?)

Their concentration and ratio affect geopolymerization, gel formation, curing, and strength development. Although many previous studies have investigated geopolymer concrete, most have focused mainly on compressive strength. Fewer studies have simultaneously examined split tensile, flexural, shear, pull-out, and durability properties under identical experimental conditions.

The study therefore aims to systematically compare geopolymer concrete with conventional OPC concrete and identify an effective activator ratio and curing condition for structural applications.

Materials

The major materials used were fly ash, NaOH, sodium silicate, natural sand, and crushed coarse aggregate.

Important properties include:

• Fly ash: 55.90% SiO?, 27.80% Al?O?, and only 3.95% CaO.
• The combined SiO? and Al?O? content is 83.7%, classifying the fly ash as a low-calcium Class F fly ash, which is suitable for geopolymerization.
• NaOH: 98.8% purity.
• Sodium silicate: 14.7% Na?O, 29.4% SiO?, and 55.9% water.
• Coarse aggregate: 20 mm maximum size with good strength and abrasion characteristics.
• Fine aggregate: natural sand with a fineness modulus of 3.0.

Methodology

The experimental procedure involved:

1. Reviewing previous research to establish the research gap.
2. Collecting and characterizing all raw materials.
3. Preparing geopolymer mixes using different NaOH–Na?SiO? ratios and NaOH molarities.
4. Initially testing mixes with ratios of 1:0.5 and 0.5:1.
5. Oven-curing the preliminary specimens at 80°C and evaluating their 7-day compressive strength.
6. Selecting a suitable curing condition based on the preliminary results.
7. Casting cubes, cylinders, and beams for the main investigation.
8. Testing the specimens for:
   • Compressive strength
   • Split tensile strength
   • Flexural strength
   • Shear strength
   • Pull-out strength
   • Durability
9. Comparing the geopolymer concrete results with conventional OPC concrete.

The tests were conducted according to relevant Bureau of Indian Standards, including IS 5816 and IS 516.

Results and Discussion

The results were divided into two stages:

• Stage 1: Determination of the most suitable curing condition by comparing 7-day compressive strength of geopolymer mixes with different activator ratios at 80°C.
• Stage 2: Comparison of OPC concrete with geopolymer mixes having NaOH:Na?SiO? ratios of 1:1 and 0.5:1 under the selected curing condition.

The study evaluates how activator ratio and curing temperature influence different mechanical properties and durability. The analysis is intended to identify the geopolymer mixture that can provide performance comparable to conventional concrete.

Overall Conclusion

Conclusion

This study investigated the influence of NaOH?SiO? alkali activator ratios on the mechanical and durability performance of fly ash-based geopolymer concrete (GPC) and compared its behavior with that of conventional ordinary Portland cement (OPC) concrete. The results showed that heat curing at 80°C significantly enhanced the early-age strength of the geopolymer mixes, with the 0.5:1 activator ratio exhibiting the greatest improvement. Although OPC concrete achieved the highest overall mechanical strength, the GPC (1:1) mix developed compressive, split tensile, flexural, shear, and pull-out strengths that were very close to those of conventional concrete, indicating its suitability for structural applications. In terms of durability, geopolymer concrete demonstrated superior resistance to water penetration and improved performance under aggressive environmental conditions. XRD analysis confirmed the formation of an amorphous aluminosilicate gel, which plays a key role in the development of both strength and durability in the geopolymer matrix. Considering the overall balance between mechanical performance and durability, the 1:1 activator ratio was found to be the most suitable for structural applications, while the 0.5:1 mix is recommended for environments requiring enhanced durability. Overall, fly ash-based geopolymer concrete can be considered a promising low-carbon alternative to conventional concrete, offering comparable structural performance along with improved environmental sustainability.

References

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Copyright

Copyright © 2026 Abhishek Vijay Jadhav, V. M. Bogar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.