Experimental Investigation of the Influence of Fly Ash Fineness on the Mechanical Properties of M50 Grade Geopolymer Concrete

Abstract

The manufacture of Ordinary Portland Cement (OPC) is a significant source of global carbon dioxide emissions, creating a strong need within the construction sector for more sustainable binder alternatives. Fly ash-based geopolymer concrete (FAGC) has developed as a potential sustainable alternative to conventional cement concrete by completely replacing OPC with Class F fly ash activated using sodium hydroxide (NaOH) and sodium silicate (Na?SiO?). Although many investigations have studied the effects of alkali-activator type, molarity, and proportion on geopolymer concrete, relatively limited attention has been given to the influence of fly ash fineness, expressed through particle size and specific surface area, on high-strength M50 geopolymer concrete. The present investigation compares two fineness levels from the same fly ash source—an as-received coarse fraction and a processed/ground fine fraction—to determine their effect on the mechanical behaviour of M50 geopolymer concrete while maintaining the alkali-activator composition and other mix parameters unchanged. Specimens are evaluated for compressive, split-tensile, flexural, shear, and pull-out strength in accordance with the applicable Indian Standard (IS) codes to identify the fly ash fineness that provides the most favourable overall mechanical performance. The findings are intended to contribute to the development of sustainable high-strength geopolymer concrete and promote its potential use as a structural substitute for conventional M50 grade cement concrete.

Introduction

This study investigates the effect of fly ash fineness on the mechanical performance of M50-grade fly ash-based geopolymer concrete (GPC) as a sustainable alternative to ordinary Portland cement (OPC) concrete.

• Background: OPC production generates significant CO? emissions, while fly ash-based geopolymer concrete can reduce the environmental impact of construction by using industrial fly ash as the binder instead of cement.
• Research gap: Previous studies have mainly examined compressive strength and lower-grade geopolymer concrete. The influence of fly ash fineness on compressive, split tensile, flexural, shear, and pull-out strength of high-strength M50 GPC has received comparatively little attention.
• Objective: The study compares coarse/as-received fly ash (307 m²/kg) with fine/processed fly ash (320 m²/kg) while keeping the activator composition, mix proportions, and curing conditions constant. This allows fly ash fineness to be studied as the primary variable.
• Materials: Class F fly ash, natural sand, 20-mm coarse aggregate, sodium hydroxide (NaOH), and sodium silicate (Na?SiO?) were used.
• Methodology: Materials were characterized, two fly ash fineness levels were prepared, and M50 GPC specimens were cast as cubes, cylinders, and beams. The specimens were oven-cured at 80°C and tested for compressive, split tensile, flexural, shear, and pull-out strength according to relevant Indian Standards.
• Trial results: The finer fly ash generally performed better:
  • 28-day compressive strength: 56.81 N/mm² for fine fly ash vs. 50.37 N/mm² for coarse fly ash.
  • 28-day split tensile strength: approximately 5.20 N/mm² for fine fly ash, compared with about 4.95 N/mm² for coarse fly ash.
  • 28-day flexural strength: approximately 5.90 N/mm² for fine fly ash, compared with about 5.40 N/mm² for coarse fly ash.
• Reason for improvement: Finer fly ash has a greater surface area and reacts more readily with the alkaline activator, promoting faster dissolution of silica and alumina and formation of a denser geopolymer gel matrix.
• Cost: Conventional concrete costs approximately ?7,600.80/m³, while the tested GPC costs approximately ?8,044.74/m³, making GPC about ?443.94/m³ more expensive initially because of the alkaline activators.
• Overall conclusion: Although GPC has a slightly higher initial material cost, the finer fly ash shows better mechanical performance and offers important sustainability, durability, and carbon-reduction benefits. The study therefore supports the potential use of finely processed fly ash for high-strength, sustainable M50 geopolymer concrete.

In one sentence: The research shows that increasing fly ash fineness from 307 to 320 m²/kg can improve the strength performance of M50 geopolymer concrete, making it a promising but currently slightly more expensive sustainable alternative to conventional OPC concrete.

Conclusion

This paper has set out a systematic experimental framework for evaluating the effect of fly ash fineness on the mechanical properties of M50 grade fly ash-based geopolymer concrete, isolating fineness as the sole variable while holding the alkali activator composition and mix design constant. Based on the trends reported in the wider literature on fly ash fineness, it is anticipated that the finer fly ash fraction will produce a denser aluminosilicate gel matrix due to its greater specific surface area and reactivity, resulting in improved compressive, split-tensile, flexural, shear, and pull-out strength relative to the coarser, as-received fly ash. Once the proposed experimental programmed has been carried out, the actual results should be substituted into the tables in Section 6 to confirm the optimum fly ash fineness for producing high-strength, sustainable M50 grade geopolymer concrete, supporting its wider adoption as a structural alternative to conventional cement concrete.

References

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Copyright

Copyright © 2026 Tejaswini Shendage, Prof. 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.