Fresh and Mechanical Properties of Fly Ash-GGBS Based Geopolymer Concrete: Effect of NaOH Molarity, Slag Content and Curing Condition

Abstract

The manufacture of ordinary Portland cement (OPC) releases ~1 ton of CO2 per ton of cement produced, which is encouraging the construction industry to seek low-carbon binders. One of the most promising options is geopolymer concrete where industrial by-products are used as a complete replacement of cement. The present paper deals with an experimental investigation on M30 grade geopolymer concrete using low calcium (Class F) fly ash and ground granulated blast furnace slag (GGBS) as source material and sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solutions as alkaline activators. In this experimental program we investigated two series of mixes. The first series of binders consisted of 100 % fly ash and the NaOH concentration ranged from 8 M to 16 M. In second series, fly ash was replaced by GGBS 10-50% by mass and NaOH concentration was maintained constant 12M . All the mixtures were cured at room temperature and in oven at 60OC for 24 h. The ratio of Na2SiO3/NaOH was 2.0. The test results showed that the compressive strength of fly-ash-only concrete increased continuously with the increase of NaOH molarity, increasing by about 62% from 8 M to 16 M, and that oven curing improved the strength at each molarity. The addition of GGBS gave a much greater improvement, the 28-day ambient cured strength almost doubling from about 30 N/mm2 (fly ash alone, 12 M) to 61.42 N/mm2 at 30 % GGBS, which was the optimum replacement level. Above 30% GGBS the strength dropped back (due to rapid early stiffening). Most importantly, the combined mixes achieved nearly the same 28-day strength under the ambient curing as under the oven curing, which implies that the combination of fly ash-GGBS does not require any heat curing. The split tensile strength ranged from 7-8% of the compressive strength, and the split tensile and flexural strengths were linearly correlated to the compressive strength (R2 ~ 0.95 and 0.93). The results indicate that fly ash-GGBS geopolymer concrete is a viable ambient-curing and eco-friendly alternative for OPC concrete for structural-grade applications.

Introduction

The study focuses on geopolymer concrete (GPC) as an environmentally friendly alternative to Ordinary Portland Cement (OPC). OPC production is highly energy- and carbon-intensive, releasing nearly one tonne of CO? per tonne of cement. Meanwhile, industrial by-products such as fly ash and Ground Granulated Blast Furnace Slag (GGBS) are generated in large quantities and often disposed of in landfills, causing environmental pollution. Geopolymer concrete addresses both issues by utilizing these industrial wastes as binders, reducing cement consumption and lowering CO? emissions by up to 80%.

Geopolymers are formed by activating alumino-silicate materials like fly ash with alkaline solutions, creating a strong three-dimensional network that binds aggregates without Portland cement. They offer advantages such as high early strength, low shrinkage and creep, and excellent resistance to sulphates, acids, fire, and high temperatures. However, 100% fly ash geopolymer concrete develops strength slowly at room temperature and usually requires heat curing (60–90°C). Adding GGBS, which contains high calcium, overcomes this limitation by forming additional binding gels (C-S-H and C-A-S-H), enabling good strength under ambient curing.

Previous studies showed that compressive strength depends on factors such as NaOH concentration, curing temperature, and GGBS content, but comprehensive guidance on the combined effects of these variables remains limited. This study aims to fill that gap by designing an M30-grade geopolymer concrete, investigating the influence of NaOH molarity (8–16 M), varying GGBS replacement levels (10–50%), comparing ambient and oven curing, and establishing relationships between compressive, tensile, and flexural strengths.

Materials and Methodology

• Materials: Low-calcium Class F fly ash, GGBS, river sand, crushed granite aggregates, sodium hydroxide (NaOH), sodium silicate, and potable water.
• Mix Design: Binder content was fixed at 450 kg/m³, with an alkaline liquid-to-binder ratio of 0.50 and a sodium silicate-to-NaOH ratio of 2:1. Fly ash was partially replaced by GGBS in proportions ranging from 10% to 50%.
• Experimental Program: Ten mixes were tested. Fly ash-only mixes with five NaOH concentrations were evaluated for compressive strength, while fly ash-GGBS blends were tested for slump, compressive strength, split tensile strength, and flexural strength under both ambient and oven curing conditions.

Results

Workability:

• Increasing GGBS content reduced slump from 147 mm to 90 mm, indicating lower workability.
• The reduction was due to the angular shape of GGBS particles and their faster reaction with the alkaline activator.
• Despite this decrease, all mixes remained workable.

Compressive Strength:

• Higher NaOH molarity consistently increased compressive strength.
• Under ambient curing, 28-day strength increased from 22.33 MPa (8 M) to 36.18 MPa (16 M).
• Under oven curing, strength increased from 25.09 MPa to 39.93 MPa.
• Higher NaOH concentration improved the dissolution of silica and alumina in fly ash, producing more geopolymer gel (N-A-S-H).
• Oven curing accelerated geopolymerization, producing 10–12% higher 28-day strength and significantly improving early-age strength.
• However, the 100% fly ash mix at 12 M achieved only 30.23 MPa under ambient curing, below the M30 target strength (38.25 MPa), confirming that fly ash alone is insufficient for structural-grade concrete without heat curing.

Conclusion

An M30-grade geopolymer concrete was developed using fly ash and GGBS as the complete binder, and its fresh and mechanical properties were studied as functions of NaOH molarity (8-16 M), GGBS replacement level (10-50%) and curing condition (ambient versus 60 °C oven). The following conclusions are drawn: 1) The compressive strength of 100% fly ash geopolymer concrete increased steadily with NaOH concentration. The 28-day ambient-cured strength rose from 22.33 N/mm² at 8 M to 36.18 N/mm² at 16 M (about 62%), and the oven-cured strength from 25.09 to 39.93 N/mm², because a stronger alkaline solution dissolves more reactive silica and alumina from the ash. 2) Oven curing at 60 °C benefited the fly-ash-only mixes at every molarity, especially at early age: 7-day strengths reached 64-66% of the 28-day value under heat against only 50-54% under ambient conditions. Fly-ash-only concrete therefore remains dependent on heat curing, and at 12 M it could not reach the M30 target mean strength under ambient curing alone. 3) Partially replacing fly ash with GGBS raised the 28-day ambient strength dramatically, from about 30 N/mm² to as much as 61.42 N/mm². Strength peaked at 30% replacement (mix 70FA30GS) and declined at higher slag levels because rapid early stiffening hindered gel development and compaction. A 70:30 fly ash to GGBS ratio at 12 M is therefore the recommended optimum. 4) For the blended mixes, heat curing mainly accelerated early strength; the 28-day strengths under ambient and oven curing were nearly equal (within about 2% for the best mixes). Fly ash-GGBS geopolymer concrete can thus achieve well above M30 strength under ordinary ambient curing, removing the principal practical barrier to site use of geopolymer concrete. 5) Split tensile strength ranged from 2.99 to 4.79 N/mm² and stayed within 7-8% of the corresponding compressive strength for all mixes and both curing regimes. Flexural strength ranged from 4.15 to 6.53 N/mm², with the 20-30% GGBS mixes performing best. 6) Both secondary strengths correlated linearly with compressive strength (fsp = 0.082fc - 0.37, R² = 0.95; fr = 0.104fc - 0.14, R² = 0.93), providing simple tools for preliminary design. 7) Workability fell as GGBS content rose, with slump dropping from 147 mm at 10% slag to 90 mm at 50% slag, owing to the angular shape and early reactivity of the slag particles; all mixes nonetheless remained placeable. Overall, the study demonstrates that two industrial by-products - fly ash and GGBS - can together fully replace Portland cement in a structural-grade concrete that cures at ambient temperature, offering a practical route to lower-carbon construction while diverting waste from landfills. Future work should extend the programme to long-term durability (chloride and sulphate exposure, carbonation), elevated-temperature performance, microstructural characterisation (SEM/XRD) and full-scale structural elements.

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Copyright © 2026 Rana Kunal, Jainender Sharma, Ishan Tank. 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.