ISCESTI-2025
International Conference on Innovations and Sustainability in Civil
Engineering: Shaping Tomorrow’s Infrastructure
14-16 May 2025
Department of Civil Engineering, NIT Patna, Bihar, India
Effect of Curing conditions on mechanical properties of fly ash and GGBFS based
geopolymer concrete
Shakaldev Sah1 & Dr. Govind Mohan 2
Department of Civil Engineering
National Institute of Technology, Patna, India
*E-mail: shakaldevs.pg24.ce@nitp.ac.in
Abstract:
As sustainable construction practices gain prominence, geopolymer concrete presents a viable solution for reducing the
environmental impact of the construction industry while maintaining or even improving the performance of concrete structures.
This abstract underscore the significance of geopolymer concrete as a promising alternative to conventional concrete, highlighting
its composition, properties, production process, and potential contributions to sustainable development. Compared to traditional
concrete, geopolymer concrete exhibits several advantages. It typically has lower carbon dioxide emissions during production due
to reduced or eliminated cement usage, making it an environmentally responsible choice. Geopolymer concrete also demonstrates
excellent resistance to various forms of deterioration, such as chemical attack and high temperatures, which can lead to enhanced
durability and longer service life of structures.
In this paper, the mechanical properties of geopolymer concrete were studied. Research into geopolymer concrete continues to
explore its mechanical properties like compressive strength, flexural tensile strength, and split tensile strength under ambient and
temperature curing. Solution of alkaline activator Na2SiO3 with 12M concentration of molarity, activator-to-binder ratio of 0.5,
Na2SiO3/NaOH ratio of 2.50, and two curing regimes viz., ambient curing, and heat curing at 90°C for 6 h were employed. Testing
of several strengths done on fly ash based geopolymer concrete by replacing fly ash with 10%, 20% and 30% GGBFS. After the
testing is perform, we find desired strength when oven curing is done at mix 30% GGBFS and 70% Fly Ash, strength getting
increased. With increase in GGBFS content mechanical properties of GPC showed better enhanced strength in oven curing as well
as ambient curing. Better strength was found in oven curing in comparison to ambient curing.
Keywords: Fly ash, GGBFS, Geopolymer Concrete (GPC), Alkaline activator (ALA)
1. Introduction
One of the materials that is utilized extensively worldwide is concrete. The main binder used to make concrete is
ordinary Portland cement (OPC). Due to the necessity of developing infrastructure facilities, the demand for concrete
is growing daily[1]. Ordinary Portland cement (OPC) production accounts for around 10% of carbon dioxide
emissions into the atmosphere. Due to the high demand for construction across a number of industries, including
buildings, transportation, dams, tunnels, sewage, etc., an alternative material binder that can replace OPC is
desperately needed for a cleaner and more sustainable building industry[2]. A coal byproduct from thermal power
plants is fly ash. Alumina and silica are also abundant in it. Geopolymer concrete is completely devoid of cement.
Alkaline solution serves as an activator and fly ash as a binder in geopolymers. To create alumina silicate gel, fly ash
and alkaline activator go through a geopolymerization process. Sodium hydroxide (NaOH) and sodium silicate (Na
Sio) are combined to create the alkaline solution used in this investigation[3]. The source material with a high
Alumina–Silica content, such as fly ash (FA), GGBFS, and the alkali activator, which is made up of sodium silicate
and sodium hydroxide solutions, are the two main components of fly ash and GGBFS based geopolymer concrete.
Alumina silicate gel is created when an alkaline solution reacts with a raw material that is high in silica (Si) and
aluminium (Al), such as fly ash, or GGBFS[4]. As may be seen from the discussion above, India has not done any in-
depth research on geopolymer concrete. Depletion of natural resources, including limestone, the primary component
used to make cement and, ultimately, concrete, is another issue facing India. Examining geopolymer concrete, which
is the type of concrete that contains no cement, in depth becomes crucial in this case[5]. Two crucial factors that are
used in the design of concrete components are compressive strength and splitting tensile strength. Three techniques—
direct tension testing, modulus of rupture testing, and split cylinder testing—can be used to evaluate the tensile strength
of concrete. The split cylinder test is well known for being easier to use and for yielding accurate results under uniform
stress[6].
ISCESTI-2025
International Conference on Innovations and Sustainability in Civil
Engineering: Shaping Tomorrow’s Infrastructure
14-16 May 2025
Department of Civil Engineering, NIT Patna, Bihar, India
2. Methodology
2.1. Materials
2.1.1. Alkaline activators increase the reactivity or performance of a material
in an alkaline (basic) environment. Alkaline activators are commonly
used in various applications, including cement and concrete production,
geopolymer synthesis, and the activation of certain industrial
waste materials. ALA is the mixture of Sodium Hydroxide (NaOH) &
Sodium Silicate (Na2SiO3). NaOH as an alkali activator significantly
reduces the setting time, and improves early strength. Na2SiO3 is used
as a setting accelerator and applied in the form of silicate mineral paint
to enhance waterproofing and improves durability by
geopolymerization process. Figure 1.Alkaline activator
2.1.2. Ground granulated blast furnace slag (GGBFS) is a by-product of
iron in blast-furnace. It mainly consists of silicate and aluminosilicate
of melted calcium. Ground slag is a supplementary cementitious
product that can be used as a filler during manufacturing of cement
related products. It increases the strength and durability of concrete
structure. It reduces voids and permeability & shrinkage cracks in
concrete. It improves long-term strength, lowers the heat of hydration,
decreases permeability, and increases durability.
Figure 2.GGBFS
2.1.3. Fly ash is primarily composed of silicon dioxide (SiO2), aluminum
oxide (Al2O3), iron oxide (Fe2O3), calcium oxide (CaO), and
various trace elements. One of the primary applications of fly ash is in
the production of concrete. It can be used as a cement replacement or
supplementary cementitious material (SCM) to improve the strength,
durability, and workability of concrete. When mixed with alkaline
activators, fly ash can also be used in the production of geopolymer
concrete, an eco-friendly alternative to traditional Portland cement-
based concrete[7].
Figure 3.Fly ash
2.2. Mix proportioning
Coarse and fine aggregates made up 60 to 70% of the mix's weight. The activator-to-binder ratio remained
consistent at 0.5[8] for every combination. All mixes had NaOH molarities of 12 M, and the weight ratio of
Na2SiO3 to NaOH was 2.5. The mix proportions of the GPC mixes utilized in this investigation are listed in Table
1,2&3. Ratio of GPC mix is (fly ash: sand: gravel: 1:1.6:3) as similar as M20 cement concrete[9] mix (Cement:
sand: gravel:1:1.5:3).
ISCESTI-2025
International Conference on Innovations and Sustainability in Civil
Engineering: Shaping Tomorrow’s Infrastructure
14-16 May 2025
Department of Civil Engineering, NIT Patna, Bihar, India
Table 1. Mix details of F90G10
Materials Amounts
1. Coarse aggregate (gravel average dia. 20mm) 85.04 kg
2. Fine aggregate (sand) 70.88 kg
3. Fly ash 42.48 kg
4. Alkaline activator 26 kg
5. GGBS 4.72 Kg
6. Coarse aggregate (gravel average dia. 10 mm) 56.7 Kg
Table 2. Mix details of F80G20
Materials Amounts
1. Coarse aggregate (gravel average dia. 20mm) 85.04 kg
2. Fine aggregate (sand) 70.88 kg
3. Fly ash 37.76 kg
4. Alkaline activator 26 kg
5. GGBS 9.44 Kg
6. Coarse aggregate (gravel average dia. 10 mm) 56.57 Kg
Table 3. Mix details of F70G30
Materials Amounts
1. Coarse aggregate (gravel average dia. 20mm) 85.04 kg
2. Fine aggregate (sand) 70.88 kg
3. Fly ash 33.04 kg
4. Alkaline activator 26 kg
5. GGBS 14.16Kg
6. Coarse aggregate (gravel average dia. 10 mm) 56.57 Kg
2.3. Mixing
The mixing method was accomplished in accordance with the recommendations made by earlier studies on
geopolymer concrete. Na2SiO3 and NaOH solution were mixed for 30 minutes to start the procedure. The fly ash
was then added to the mixer along with the coarse aggregate and sand, and the mixture was stirred for four to six
minutes. After that, the dry materials combination was combined with the alkaline solution and stirred for a further
four minutes. In 1th, 2nd And 3rd Mixing 270ml to 300 ml Admixture, which is 1% of total weight[10] of Alkaline
solution.
ISCESTI-2025
International Conference on Innovations and Sustainability in Civil
Engineering: Shaping Tomorrow’s Infrastructure
14-16 May 2025
Department of Civil Engineering, NIT Patna, Bihar, India
2.4. Casting
Three distinct sizes and shapes of concrete examples were made for
testing. Beam specimens measuring 500 mm in length, 100 mm in
width, and 100 mm in height (500 × 100 × 100 mm) were cast.
Cylindrical specimens were made according to conventional testing
measurements, measuring 150 mm in diameter and 300 mm in length.
In addition, 150 × 150 × 150 mm cubical specimens were cast, with
150 mm on each side (as shown in fig 4). These samples were used
to evaluate the concrete's mechanical qualities.
Figure 4.Casted sample
2.5. Curing
Two curing treatments were used in this experimental study. After a one-day delay, the first method of curing was
heat (oven) curing at 90°C for six hours (as shown in figure 5). The time between casting the specimens into the
moulds and unmoulding them was known as the delay time. Before being moved to the curing room or curing
oven, the delay period allows the concrete to complete its initial setting time. At room temperature, the other curing
condition was maintained. After 26 hours of heat curing, all GPC specimens were removed from the oven and
allowed to cure at room temperature (as shown in figure 6) prior to testing.
Figure 5. Oven curing in Muffle Furnace Figure 6.Ambient curing
ISCESTI-2025
International Conference on Innovations and Sustainability in Civil
Engineering: Shaping Tomorrow’s Infrastructure
14-16 May 2025
Department of Civil Engineering, NIT Patna, Bihar, India
3. Results
All the mixes were tested for compressive strength, split tensile strength and flexural tensile strength after 28 days of casting,
As per Indian standards[11]. Strength was reported as of the average value of three specimens.
Table 4.Testing of cube
Compressive Strength (MPa) Compressive Strength (MPa)
S.No. Sample Type (Oven Curing) (Ambient Curing)
1 F90G10 20.37 17.77
2 F80G20 33.85 20.37
3 F70G30 37.78 22.22
Table 5: Testing of cylinder
Split Tensile Test (MPa) Split Tensile Test (MPa)
S.No. Sample Type (Oven Curing) (Ambient Curing)
1 F90G10 2.36 2.09
2 F80G20 3.33 2.25
3 F70G30 3.44 2.78
Table 6: Testing of beam
Flexural Tensile Strength (MPa) Flexural Tensile Strength (MPa)
S.No. Sample Type (Oven Curing) (Ambient Curing)
1 F90G10 6.13 5.15
4 F80G20 6.17 6.03
7 F70G30 7.08 6.74
Table 7: Testing of cylinder
Compressive Strength (MPa) Compressive Strength (MPa)
S.No. Sample Type (Oven Curing) (Ambient Curing)
1 F90G10 24.72 13.01
2 F80G20 25.91 16.69
3 F70G30 26.42 17.82
ISCESTI-2025
International Conference on Innovations and Sustainability in Civil
Engineering: Shaping Tomorrow’s Infrastructure
14-16 May 2025
Department of Civil Engineering, NIT Patna, Bihar, India
4. Discussion
4.1. Compressive Strength
Copressive Strength (MPa)
The test results showed that, in comparison to ambient-cured
specimens, oven-cured Geopolymer Concrete (GPC) specimens 40
exhibited noticeably higher compressive strengths. The 30
compressive strengths for mix designs F90G10, F80G20, and 20
F70G30 were 20.37 MPa, 33.85 MPa, and 37.78 MPa for the 10
oven-cured samples, respectively (As shown in figure 7). Mix 0
F70G30 showed the highest compressive strength of any of these, Sample F90G10 F80G20 F70G30
demonstrating how well oven curing improves the mechanical Type
qualities of GPC. The findings unequivocally demonstrate that
raising the curing temperature has a beneficial impact on the S.No. 1 2 3
strength development of geopolymer concrete, particularly for Oven Curing Ambient Curing
grade mixes like F70G30.
Figure 7. Variation of compressive strength
4.2. Split tensile strength Split Tensile Strength (MPa)
It is evident from the split tensile strength data that specimens of 4
oven-cured Geopolymer Concrete (GPC) are stronger than those 3
that are ambient-cured. While the equivalent ambient-cured
2
specimens displayed lower values of 2.09 MPa, 2.25 MPa, and
2.78 MPa, the oven-cured mix designs F90G10, F80G20, and 1
F70G30 recorded strengths of 2.36 MPa, 3.33 MPa, and 3.44 MPa 0
(as shown in fig 8), respectively. Mix A3 showed the maximum Sample F90G10 F80G20 F70G30
tensile strength of all the mixes under both curing settings, Type
demonstrating the advantageous effects of oven curing.
S.No. 1 2 3
Interestingly, mix A3 had 30% GGBFS, suggesting that this
percentage greatly improves the tensile qualities of GPC, Oven Curing Ambient Curing
particularly when cured at high temperatures.
Figure 8. variation in split tensile strength
4.3. Flexural Tensile strength
Flexural Tensile Strength(MPa)
The findings of the flexural tensile strength test showed that
specimens of Geopolymer Concrete (GPC) that had been oven- 8
cured were stronger than those that had been cured at room 6
temperature. The equivalent ambient-cured specimens displayed 4
somewhat lower values of 5.15 MPa, 6.03 MPa, and 6.74 MPa, 2
whereas the oven-cured samples had flexural strengths of 6.13
0
MPa, 6.17 MPa, and 7.08 MPa for mix designs F90G10, F80G20,
Sample F90G10 F80G20 F70G30
and F70G30 respectively. In both curing circumstances, mix A3 Type
had the highest flexural strength of any of the mixes. Because of
better geopolymerization and matrix densification, a higher S.No. 1 4 7
percentage of GGBFS considerably improves the flexural
performance of fly ash-based GPC, especially when oven-cured. Oven Curing Ambient Curing
This mix comprised 30% GGBFS.
Figure 9. variation in flexural tensile strength
ISCESTI-2025
International Conference on Innovations and Sustainability in Civil
Engineering: Shaping Tomorrow’s Infrastructure
14-16 May 2025
Department of Civil Engineering, NIT Patna, Bihar, India
5.Conclusion
A discernible increase in mechanical strength was noted in fly ash-based geopolymer concrete when the amount of
Ground Granulated Blast Furnace Slag (GGBFS) was increased to 10%, 20%, and 30%. The concrete's compressive
strength and general durability are greatly increased by the inclusion of GGBFS, Mechanical properties of GPC
increases. Oven cured GPC specimen shows results in high compression & tension strengths than ambient
cured specimens. which speeds up the reaction process and creates a denser, more cohesive matrix. The examples
showed greater strength values as the GGBFS content rose, demonstrating that adding GGBFS to fly ash-based
geopolymer concrete is a practical method of improving the material's mechanical performance.
6.Future Scope
According to this study, adding more Ground Granulated Blast Furnace Slag (GGBFS) to Geopolymer Concrete
(GPC) improves the material's compressive strength. To completely comprehend the connection between GGBFS
composition and strength development over time, more research is necessary. The behaviour of GPC with different
GGBFS ratios under various loading scenarios, curing conditions, and environmental exposures can be investigated
in future studies. A thorough grasp of GPC performance can also be obtained by researching additional mechanical
and durability characteristics, including as flexural strength, tensile strength, shrinkage, and resistance to chemical
assault. The internal alterations that contribute to strength increase can also be explained by microstructural research.
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