Abstract
The sodium hydroxide (NaOH) molarity in Self-compacting geopolymer concrete (SCGC) is essential for activating the precursor and aggregate to develop strength, workability, and microstructure. In this study, SCGC mixes prepared with 50% fly ash (FA) and 50% ground granulated blast furnace slag (GGBS) to investigate fresh and hardened properties with NaOH molarities (M) ranges from 8 to 16, the ratio of Na2SiO3 to NaOH kept constant at 2.5 and ratio of alkaline solution to the binder at 0.45 with 2% SP for the polymerization process. SCGC workability studies indicate the NaOH concentration increased, the slump flow decreased, and 14 M was the optimum molarity. The compressive, split tensile, and flexural strength results showed 39.4 MPa, 4.72 MPa, and 5.91 MPa at 28 days. The C–S–H gel enhanced the strength qualities studied from Fourier transform infrared spectroscopy. The scanning electron microscope showed microstructural densification of the entire system, which improved with the NaOH concentration, and the strength increased with the degree of polymerization and polycondensation. Hence, based on workability, the optimized NaOH concentration is 14 M with binder contents of FA (50%) and GGBS (50%). This study helps to improve the microstructure and strength properties with potential cost implications of SCGC.
1 Introduction
The construction industry has played a critical role in developing economic activity, promoting development, and creating jobs [1, 2]. However, the production of construction materials affects natural resources and is responsible for many environmental problems [3]. The emission from building sectors is the reason for a quarter of global carbon dioxide (CO2) [4, 5]. On the other hand, the rapid population proliferation may increase the requirement for urbanization and improved infrastructure, which increases the consumption of cement and concrete. Currently, 4500 MT of concrete is consumed globally every year [6]. Therefore, the construction industry should use alternate raw materials to produce concrete to ensure sustainability. Industrial by-products in concrete can provide optimal strength and durability while minimizing environmental impacts [7]. Cement is more utilized on earth next to water and is the third primary origin of atmospheric pollutants, such as SOx, NOx, CO, particulate matter, and volatile organic compounds that lead to greenhouse gas emissions [8]. These pollutants can contribute to the formation of smog, acid rain, and respiratory issues, and they can have harmful consequences on the surroundings [9]. So, there is an immediate necessity for alternate materials to cement and conventional concrete to maintain sustainability [10].
Geopolymer concrete (GPC) has been accepted as a substitute for conventional concrete and an environment-friendly binder in the construction sector [11]. GPC is synthesized with high alumina and silica-based industrial remains including as coal ash, slag, metakaolin, high calcium wood ash, and waste glass powder. This innovative concrete lowers CO2 emission during preparation and decreases consumption of natural resources [11–13]. GPC is a promising sustainable solution since it has great compressive strength (CS) and durability [14]. GPC has a high-viscosity nature that sometimes leads to improper compaction and failure. The development of Self-compacting Geopolymer concrete (SCGC) would lead to an alternative solution to this problem [15]. SCGC is a unique innovation with geopolymer and SCC properties in the concrete industry [16]. SCGC can flow and compact under its weight in the absence of any outside shaking or manual force by uniformly filling all the voids and interstices, resulting in a uniform and homogeneous mix [17]. This results in improved structural integrity and increased durability of the concrete. SCGC provides improved concrete quality, shorter construction time, and better workability [18]. Also, this technology helps to manage the problems of inadequate consolidation [19]. In addition, the production of SCGC is from waste materials that contribute to sustainable practices by cutting down the environmental impact of waste disposal. For instance, thermal power plant waste, iron industry slag, alkaline binders, and SP are used in SCGC [20]. Fly ash (FA) is the main precursor of the SCGC, and a fine powdery material by-product of thermal power plants mainly composed of spherical glass-like particles with pozzolanic properties. Worldwide, FA production was around 600 to 700 million tons annually [21]. India produced 200 to 250 million tons from 202 thermal power plants [22]. The large quantity of FA can be used in GPC to develop the world's construction industry because it consists primarily of reactive silicon dioxide, aluminum oxide, and other minor constituents that are the primary source of pozzolan utilized to create cost-effective and ecologically friendly geopolymer concrete mixtures [23]. Also, FA helps to achieve the desired workability and flowability by reducing the yield stress and viscosity of the concrete mix [24]. Ground granulated blast furnace slag (GGBS) is a by-product obtained during the iron-making process in blast furnaces in the steel industry [25]. Iron ore is processed in a blast furnace to produce molten iron and generates a mixture of non-metallic materials, including slag. GGBS has pozzolanic and latent hydraulic qualities, which means it can react with water, calcium hydroxide, and calcium silicate gel to create more cementitious compounds when used in GPC [26].
The inclusion of superplasticizers (SP) enhances its self-compacting characteristics. Furthermore, SP can act as high-range water reducer and helps to enhance the workability of the SCGC. There are commercially available poly-carboxylic ether polymers-based SPs such as MasterGlenium SKY 8233, MasterGlenium ACE 30, and Sulphonated Naphthalene Polymers based SP - Conplast SP550 [27]. FA and GGBS-based SCGC possess pozzolanic and hydraulic properties that are essential for the geopolymerization process and create a strong and durable binder [28]. These combinations of FA and GGBS as binder materials with appropriate SP can facilitate the necessary flowability and filling ability to self-compact without vibration [29]. Also, it generates less heat during the geopolymerization process compared to conventional cement-based SCC [30]. This reduced heat of hydration can mitigate the risk of thermal cracking, particularly in massive concrete structures with the environmental benefits of waste reduction. Consequently, the CS of SCGC based on FA and GGBS can be comparable to or even higher than that of conventional cement-based concrete, depending on the mix design and curing conditions [31]. In addition, alkaline activators are essential components in the production of SCGC and have key role in the geopolymerization process, which involves the reaction between source materials and the alkaline activators to form a three-dimensional polymeric network [32]. NaOH and Na2SiO3 are the two alkaline activators of GPC [33]. Alkaline activators facilitate the polycondensation of oligomers and form a compact microstructure [34].
2 Research significance
NaOH molarity increase can lead to higher CS in SCGC because higher molarity provides more alkaline ions, leading to a more efficient polymerization reaction and better bonding between the aluminosilicate particles [35]. At the same time, fresh properties are negatively impacted with higher NaOH molarity in SCGC fresh properties. However, NaOH molarity optimization to obtain suitable workability and strength is crucial to achieving the desired balance between workability, strength, durability, cost-effectiveness, and environmental impact. On the other hand, researchers conducted a few studies with similar materials were produced different results, shown in Fig. 1(a–c), which challenge to optimize and implement in construction field. Also, influence of NaOH molarity in microstructural level is not recorded and, hence the recommendation of optimum NaOH molarity with self-compacting performance in GPC is still difficult.
Literature comparison of slump flow and V-funnel of SCGC with different NaOH molarities
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
Literature comparison of L-box and compressive strength of SCGC with different NaOH molarities
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
Literature comparison of split tensile strength and flexural strength of SCGC with different NaOH molarities
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
As shown in Table 1, previous literature has investigated the effects of different NaOH molarities and SCGC properties. As a result, the current state of knowledge on SCGC, particularly concerning the optimization of NaOH molarity, is inadequate, which poses challenges in decision-making regarding its application. Without a thorough understanding of how varying NaOH molarities affect the properties and microstructure of SCGC, stakeholders in the construction industry may face difficulties in selecting appropriate mix designs for specific applications. The present work aims to investigate the workability and hardened characteristics of SCGC on different NaOH molarity from 8 M to 16 M using FA and GGBS mixed proportions of FA (50%) and GGBS (50%) to optimize the NaOH molarity with microstructural behavior such as crystalline structures, various bonding, morphological properties, and elemental composition to address this novel research gap.
SCGC properties and microstructural effects of different NaOH molarity in SCGC
| S. No | Precursors | NaOH molarities | Fresh properties | Hardened properties | Microstructural effects | Ref |
| 1. | FA | 8 M, 10 M, 12 M, 14 M, 16 M |
|
| – | [36] |
| 2. | FA | 10 M, 12 M, 14 M |
|
| – | [37] |
| 3. | GGBS | 10 M, 12 M, 14 M |
| – | – | [38] |
| 4. | GGBS | 8 M, 10 M, 12 M, 14 M |
| – | – | [39] |
| 5. | GGBS | 8 M, 10 M, 12 M, 14 M |
|
| – | [40] |
| 6. | GGBS | 8 M, 10 M, 12 M, 14 M |
|
| – | [41] |
| 7. | FA, GGBS | 8 M, 10 M, 12 M |
|
| – | [42] |
| 8. | FA, GGBS | 12 M, 14 M, 16 M |
|
| – | [43] |
| 9. | FA, GGBS and Alccofine | 10 M, 12 M, 14 M |
|
|
| [44] |
3 Materials
Figure 2 illustrates a research process designed to determine how NaOH molarity affects SCGC. Following a critical review of the literature regarding SCGC, material properties, and characterization techniques, it discusses the knowledge gaps indicated in the literature. In the next step, SCGC samples will be prepared with varying NaOH molarities and their fresh and mechanical properties tested. Microstructural studies of the SCGC samples were also conducted with the help of XRD, FTIR, SEM, and EDAX. The conclusions and limitations of the study were arrived at by comparing the experimental results with the relationship between NaOH molarity and SCGC properties.
Workflow for optimization of NaOH molarity in SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
3.1 Fly ash and GGBS
FA is an industrial byproduct that can be used as a feedstock for SCGC and act as an alternative to traditional cement after process and activation. FA (class F) was aggregated from Tuticorin thermal power station. It has color grey with fine powder and a relative density of 2.1 [45]. GGBS is a source material in SCGC from the by-product of the iron and steel industry and was collected from the JSW plant in Madurai. The color was white, and the relative density of GGBS was 2.9. The composition of FA and GGBS was obtained using XRF (Bruker S8 Tiger, Germany) and tabulated in Table 2. It shows that SiO2 and Al2O3 are major compositions of FA, which are present at 63.16% and 29.69% respectively. The major concentrations of GGBS are CaO, SiO2 Al2O3, and MgO at 37.37%, 35.14%, 16.81%, and 7.24% respectively. Elemental compositions of FA and GGBS also noticed the aluminum and silicate characters that are essential for SCGC according to ASTM C618 and IS:12089-1987 [46, 47].
Chemical compositions of FA and GGBS
| S. No | Chemical compositions | FA | GGBS |
| 1 | SiO2 | 63.16 | 35.14 |
| 2 | Al2O3 | 29.69 | 16.81 |
| 3 | K2O | 1.85 | 0.41 |
| 4 | TiO2 | 1.60 | 0.85 |
| 5 | CaO | 1.41 | 37.37 |
| 6 | MgO | 0.99 | 7.24 |
| 7 | P2O5 | 0.97 | – |
| 8 | Fe2O3 | 0.35 | 0.07 |
| 9 | Na2O | 0.21 | 0.24 |
| 10 | SO3 | 0.16 | 1.65 |
| 11 | PbO | 0.02 | – |
| 12 | BaO | – | 0.10 |
| 13 | MnO | – | 0.07 |
| 14 | Cl | – | 0.03 |
The spherical with smooth morphology of FA depicted in Fig. 3 (A&B) and the elemental composition of FA in Fig. 4, which are done by ZEISS-EVO 18, Japan equipped with EDX. The FA consisted mostly of glassy and hollow with cenospheres, similar to those reported by Davidovits. The essential elements are present in the FA, such as Si (22%), Al (15.3%), and Ca (0.7%), which can contribute to the binding properties of SCGC.
(A&B) scanning electron micrograph of FA
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
Elemental compositions of FA
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
GGBS have irregular shapes and rough surfaces, as illustrated in Fig. 5, with their elemental composition detailed in Fig. 6. The microstructural investigation was employed using SEM to analyze the morphological features of raw GGBS powder sample, which revealed that GGBS substances were sharp and angular (uneven) in shape. The major elements of GGBS are Ca (22.5%), Si (12%), and Al (7.4%), which act as an alternative to cement by supplying pozzolans in SCGC.
Scanning electron microscope images of GGBS
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GGBS compositions
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Figure 7 shows that the FA and the GGBS were determined using the XRD method. The FA is a complex mixture of fine particles and components with a crystalline structure, such as quartz and mullite, and the amorphous structure and phases of the GGBS.
XRD of FA and GGBS
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3.2 Alkaline activating solution
Alkaline activators are one of the major ingredients of SCGC mix as it undergoes geopolymerization and provides binding properties to it. NaOH and Na2SiO3 solutions were used as alkaline activators. Sodium hydroxide pellets with 98% purity mixed with water and Na2SiO3 solution were poured into the prepared NaOH solution after 24 h. It is common practice to allow the prepared alkaline solution to cool and mature for 24 h before use, to reduce the heat generated during the mixing process of SCGC. The polymerization process starts after mixing the Na2SiO3 solutions that release heat and it needs one hour to cool before adding in the dry mixture. The sodium silicate to sodium hydroxide ratio was kept constant at 2.5 with NaOH molarity of 8 M–16 M in this study.
3.3 Aggregates
Manufacture Sand (M-Sand) was collected from the nearby quarry from Srivilliputhur. M-Sand with a relative density of 2.73 and an average size of the particle of 2.65 was used and graded by zone II as per Indian standard 383-1970 [48]. In this study, a 12.5 mm particle-size coarse aggregate was used and obtained from a local quarry. The fineness modulus of coarse aggregate is 7.16 and the specific gravity of coarse aggregate was carried out as per Indian standard:2386-1968 (Part-III) [49] and obtained as 2.83. SCGC requires a lesser quantity of coarse aggregate and a higher amount of fine aggregates to improve the viscosity and neglect segregation.
3.4 Superplasticizer (SP)
Sufficient workability of fresh concrete was obtained by incorporating commercially available SP. MasterGlenium SKY 8233 (MGS) was used for the study obtained from Astra Chemicals. It is comprised of ether-based polycarboxylic which provides higher performance, durability, workability and strength with a water-reducing character. Table 3 shows the characters of MGS.
Characters of SP
| Properties | SP |
| Brand name | PCE |
| Specific gravity | 1.09 at 25 °C |
| pH | ≥6.6 |
| Chloride ions | <0.25% |
4 Methods
4.1 Mix proportion
In this experimental work, five different mixtures of SCGC were prepared with binder contents such as FA and GGBS at a mixture dosage of 450 kg m−3. The study objectives are to analyze the influence of various NaOH concentrations in fresh and hardness behavior of SCGC. The NaOH concentrations were kept at eight to sixteen and the alkali solution ratio to the binder was at a constant value of 0.45 and the ratio of Na2SiO3 to NaOH of 2.5 was considered as constant value. Further, 12% of additional water and 2% SP with binder weight added to achieve desired flowability of SCGC properties. The SCGC mix specimen details are tabulated in Table 4.
The mix ratio of FA and SCGC based on GGBS
| Mix ID | FA (kg m−3) | GGBS (kg m−3) | Fine aggregate (kg m−3) | Coarse aggregate (kg m−3) | NaOH (kg m−3) | % of solids | % of distilled water | Molarity | Na2SiO3 (kg m−3) | SP (%) | Extra water (%) |
| M1 | 225 | 225 | 961.08 | 786.33 | 57.86 | 25.5 | 74.5 | 8 M | 144.66 | 2% | 12% |
| M2 | 225 | 225 | 961.08 | 786.33 | 57.86 | 30.6 | 69.4 | 10 M | 144.66 | 2% | 12% |
| M3 | 225 | 225 | 961.08 | 786.33 | 57.86 | 35.4 | 64.6 | 12 M | 144.66 | 2% | 12% |
| M4 | 225 | 225 | 961.08 | 786.33 | 57.86 | 40 | 60 | 14 M | 144.66 | 2% | 12% |
| M5 | 225 | 225 | 961.08 | 786.33 | 57.86 | 44.3 | 55.7 | 16 M | 144.66 | 2% | 12% |
4.2 Specimen preparation and casting
FA, GGBS, fine and coarse aggregates were dry mix blended for 2.5 min in pan mixer (100-L capacity). Subsequently, alkaline solution, SP and water (12%) were blended for 3 min of wet mixing. Again, the fresh SCGC was blended for a further two to three minutes to ensure the consistency of the mixture. SCGC underwent for slump flow test, T50cm slump flow test, V-funnel test, and L-box test to evaluate the segregation prevention. The SCGC was cast into the cube, cylinder, and prism molds without any compaction to fill the voids of the molds by self-weight. The specimens were left at ambient conditions for 24 h after being poured into the molds. After this initial curing period, it was stored for the 7 th and 28th day for hardened properties testing. Figure 8 shows the preparation of SCGC.
Preparation of SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
4.3 Fresh SCGC properties
The fresh concrete properties of FA and GGBS in SCGCs evaluated using slump flow, T50cm slump flow, L-box, and V-funnel test to assessment of filling ability.
This test was performed to assess the flow of fresh SCGC. T50cm is the time needed to reach 50 cm for its final spread of concrete after the removal of the slump cone which indicates the flowability for the workability and viscosity of SCGC. The L-box test measures heights at the horizontal elements and the vertical sections of concrete at the faces. The V-funnel test evaluates the flow time of concrete through a V-shaped funnel. The measurement of the flow time through a V-shaped funnel provides information about its viscosity and flowability. Figure 9 shows the tests conducted in the laboratory as per EFNARC 2002 guidelines [50].
Tests on the flowability of fresh SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
4.4 Mechanical properties
The FA and GGBS-based SCGCs were evaluated by Compressive strength (CS), Split Tensile Strength (STS), and Flexural Strength (FS) under ambient curing for 7 and 28 days. Cube specimens (100 × 100 × 100 mm) with aspect ratio 1 are used and tested, as per Indian standard 516:1959 [51]. The STS, also known as the indirect tensile test, is a method for determining the STS of concrete. The Cylinder specimens (100 × 200 mm) with aspect ratio 2 were used and tested as per Indian standard 5816:1999 for the STS [52]. FS testing, also called modulus of rupture testing, was performed to measure FS and material stiffness. Prismatic specimens (100 × 100 × 500 mm) were used and tested as per Indian standard 516:1959 for the FS test [51]. The tests for the mechanical properties were conducted using a Universal Testing Machine with 400 kN capacity.
4.5 Microstructural studies
4.5.1 X-ray diffraction (XRD)
XRD is analytical method for determining crystallographic structure of SCGCs with molarities of 8–16 M [51]. In X-RD, an X-ray beam is directed at a sample, and the X-rays interact with the atoms in the sample material. The interaction causes the X-rays to scatter in different directions. The scattered X-rays produce a diffraction pattern that contains information about the atomic distances and arrangement in the SCGC. The structural and crystalline nature of the SCGC studied using X-ray diffraction instrument (Bruker D8 advance ECO, Germany), and correlated with the XRD patterns which were recorded in the range of 10–80°.
4.5.2 Fourier transform infrared (FTIR)
FTIR is a powerful analytical technique for identifying and characterizing organic and inorganic materials based on their molecular vibrations [52]. FTIR spectrometer (IRTracer-100, Japan) helps to acquire in the range of 4,000–400 cm−1 [54]. It is used in various scientific and industrial fields for SCGC analysis, identification and quality control. In FTIR spectroscopy, infrared light is passed through a sample, and the interactions between the light and the sample's molecules cause absorption or transmission of specific frequencies of infrared radiation [53]. The resulting spectrum is a unique fingerprint of the molecular vibrations present in the SCGC.
4.5.3 Scanning electron microscopy (SEM) and energy dispersive X-ray analysis
SEM imaging technique is used for studying the surface morphology and microstructure of SCGCs with molarities of 8–16, to study the properties of materials with high resolution up to 2 µm. EDAX analytical technique is used for determining the elemental composition of SCGCs with molarities of 8–16. In this technique, the sample surface is scanned with an electron beam, and it interacts with atoms in the sample, characteristic X-rays are emitted [54]. These X-rays have specific energies that correspond to the elements present in the SCGCs.
5 Results and discussion
5.1 Slump flow
The flowability from different molarities of FA-GGBS-based SCGC mixtures was measured, and the results are shown in Table 5. Figure 10 expresses the variations in slump flows at different sodium hydroxide molarities. According to the EFNARC guidelines, SCGC has a good filling capacity when the slump flow values vary between 650 and 800 mm [50].
Fresh properties of SCGC mixes
| Mix ID | Molarity | Slump flow (mm) | T50cm Slump flow (S) | V-funnel (S) | L-box |
| M1 | 8 M | 784 | 2.5 | 7.3 | 0.96 |
| M2 | 10 M | 742 | 3.1 | 8.2 | 0.92 |
| M3 | 12 M | 716 | 3.8 | 9.4 | 0.89 |
| M4 | 14 M | 685 | 4.3 | 10.3 | 0.87 |
| M5 | 16 M | 639 | 5.8 | 12.4 | 0.79 |
Slump flow rate
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
The slump flow values for SCGC mixes based on FA and GGBS varied between 639 and 784 mm. The maximum yield value was measured at 8 M and the lowest at 16 M. The yield value decreased, when the NaOH molarity rose from 8 to 16 M. It was found that with the increase of NaOH concentration, the settling value gradually decreased, which could be caused by viscosity nature of the alkaline activator solution.
5.2 T50cm slump flow
T50cm diameter of different SCGC mixes based on FA and GGBS was recorded in Table 5. The flow time variations for T50cm showed in Fig. 11. According to EFNARC guidelines T50cm flow allowable time should be between 2 and 5 s [50]. T50cm flow time for SCGC blends based on FA and GGBS ranges from 2.5 to 5.8 s. The maximum and minimum T50cm settling flow rate was evaluated at 8, 10, 12, 14 and 16 M. Also, it was found that the T50cm settling flow dimension increased with the NaOH molarity concentration. In addition, it is found that T50cm slump yield gradually increased the flowability with rise of NaOH molarity.
T50 cm Slump flow rate
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5.3 V-funnel flow
The fresh SCGC mixtures based on FA and GGBS prepared SCGC were evaluated using V- funnel test and the results are shown in Table 5. Figure 12 shows the variations of the different SCGC mixtures obtained from V-funnel test. Allowable value for V-funnel test time should be varied between 6 and 12 s [50]. The V-funnel test values for SCGC mixtures based on fly ash and GGBS ranged from 7.3 to 12.4 s. As NaOH molarity increases, the filling ability decreases, leading to a longer V-funnel time.
V-funnel flow rate
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5.4 L-box flow
The L-box test evaluated the passing ability of SCGC mixtures between the spacing of 41 ± 1 mm. The ratio for the passing ability criteria must be higher than 0.8 in accordance with EFNARC guidelines [50]. The values for permeability are shown in Table 4 and Fig. 13.
L-box flow rate
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The results show that regardless of the NaOH molarity, all SCGC mixtures with FA and GGBS fulfil the requirements. The L-box flow rate ratio decreases dramatically with increase in the NaOH molarity, as shown in Table 5 and it varied from 0.96 (8 M) to 0.79 (16 M). It was found that the workability decreased significantly as the NaOH molarity increased.
5.5 Compressive strength
CS test results for various NaOH molarity are listed in Table 6, and Fig. 14 depicts the influence of CS with NaOH molarity of FA and GGBS-based SCGC at 7 and 28 days of curing periods. The observations showed that increase of NaOH molarity has positive effect on the FA and GGBS based SCGC.
Hardened properties of SCGC
| Mix ID | Molarity | CS (MPa) | STS (MPa) | FS (MPa) | |||
| 7 days | 28 days | 7 days | 28 days | 7 days | 28 days | ||
| M1 | 8 M | 23.8 | 33.4 | 2.85 | 4.05 | 3.58 | 5.01 |
| M2 | 10 M | 25.3 | 35.8 | 3.03 | 4.36 | 3.82 | 5.37 |
| M3 | 12 M | 27.2 | 37.6 | 3.28 | 4.58 | 4.09 | 5.64 |
| M4 | 14 M | 29.6 | 39.4 | 3.55 | 4.72 | 4.44 | 5.91 |
| M5 | 16 M | 24.5 | 34.2 | 2.94 | 4.1 | 3.67 | 5.13 |
CS of SCGC with NaOH molarities
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
The maximum CS is achieved when the NaOH molarity from 14 M. After 28 days, SCGC with 14 M indicates more CS 39.4 MPa (10.52%) higher than with all other NaOH molarity. Hence, the optimum molarity was found to be 14 M for FA and GGBS-based SCGC.
5.6 Split tensile strength
The STS test results of SCGC after 7 and 28 days are shown in Fig. 15 and Table 6. The FA and GGBS-based SCGC attained higher strength when the molar NaOH concentration was from eight to sixteen. After 28 days, the combination of FA and GGBS-based SCGC at 14 M showed the highest STS of 4.72 MPa. The STS was enhanced by 9.47% for 14 M compared to all other NaOH molarity. Hence, the optimum molarity is 14 M for the mixture of FA and GGBS based on SCGC.
STS of SCGC with NaOH molarities
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
5.7 Flexural strength
FS test results for SCGC 7 and 28 days after casting are shown in Fig. 16 and Table 6. The SCGC based on FA and GGBS becomes stronger when the molar NaOH is from eight to sixteen. After 28 days, SCGC with 14 M showed the highest FS of 5.91 MPa, the enhancement of FS was by 10.60% compared to all other NaOH molarity. Hence, the optimum molarity was obtained as 14 M for SCGC.
FS of SCGC with NaOH molarities
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
5.8 Microstructural studies
5.8.1 XRD analysis
XRD analysis was used to investigate the mineralogical properties of FA and GGBS based on different NaOH molarities of SCGC after four weeks of ambient curing. The study of X-ray diffraction analysis of different peaks (Fig. 17) for SCGC based on different NaOH molarities for the crystalline phases includes quartz, mullite, gypsum, A-S–H (alkali-silica hydrate), and C-A-S–H (calcium alumina silicate hydrate). For FA and GGBS based on different NaOH molarities of SCGC, it is also observed that only the glassy components react and therefore the primary crystalline peak consists of quartz (SiO2) and mullite (3Al2O3–SiO2). The primary strong peak positions of quartz (SiO2) and mullite (3Al2O3–SiO2), quartz peaks typically appear around 2θ-values of 26.51°, 27.63°, 28.39°, 42.22° and 59.81° and mullite peaks occur around 2θ-values of 23.54°, 33.72°, 35.31°, 39.28° and 50.70° [55]. In addition, the secondary crystalline peaks for A-S–H (alkali-silica hydrate) and C-A-S–H (calcium-alumina-silicate hydrate) are typically associated with sharp diffraction peaks in the XRD patterns. The secondary peak position of A-S–H (alkali silica hydrate) and C-A-S–H (calcium alumina silicate hydrate), A-S–H peaks typically appear around 2θ values of 29.61° and 30.74° and C-A-S–H peaks appear around 2θ values of 31.21° and 31.76° [56, 57]. Finally, the presence of gypsum in X-ray diffraction (XRD) patterns typically shows up as broad, diffuse features rather than sharp diffraction peaks. The FA and GGBS based on 14 NaOH molarities of SCGC clearly showed strong peaks in quartz (SiO2) and mullite (3Al2O3–SiO2). The primary polymerization products in the SCGC process are generally amorphous in nature. The presence of strong peaks in quartz and mullite indicates the coexistence of both crystalline and amorphous phases in the geopolymer matrix. The amorphous phases, especially A-S–H and C-A-S–H, play an important role in binding the geopolymer particles together and contribute to the overall performance of the concrete [58].
XRD pattern of various NaOH molarities
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
5.8.2 FTIR analysis
Figures 18–22. FTIR spectra of SCGCs with different NaOH molarities to understand the chemical composition and bonding in the materials. The strong signals of SCGC in the range of 3468.01–3990.72 cm−1 can be detected, which is attributed to O–H [59]. This indicates the presence of water molecules or hydroxyl groups in the sample. N–H stretching vibrations involving N–H bonds generally occur at lower wave numbers, typically in the range of 3338.78–3537.45 cm−1. The O–H stretching vibrations are related to water or hydroxyl groups rather than N–H bonds [60]. The presence of bands at 2767.85 and 3379.29 cm−1 in the FTIR spectra of SCGCs with NaOH molarities ranging from 8 M to 16 M indicates the presence of C–H (carbon-hydrogen) groups. The interpretation of these bands as C–H groups is consistent with the likely source of organic molecules from the SP used in the concrete mix [59].
FTIR spectrum of 8 NaOH molarity
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FTIR spectrum of 10 NaOH molarity
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FTIR spectrum of 12 NaOH molarity
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
FTIR spectrum of 14 NaOH molarity
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
FTIR spectrum of 16 NaOH molarity
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
The bands at 2628.98–2767.85 cm−1 are also due to the stretching and bending vibrations of the O–H (acid) bonds exhibited by the water molecules bound to the SCGC network [61]. The efflorescence phenomenon resulting from the geopolymerization process between unreacted Na+ and the surrounding CO2 is associated with the splitting of absorption bands in the ranges of 2119.77–2389.80 cm−1, 1406.11 to 1994.40 cm−1, and 671.23 to 1165 cm−1 observed at 8M–16 M for C≡N, C=O, and C–C, respectively [62]. Overall, these results suggest that 14 M of SCGC functional groups can form and characterize bonding, possibly related to silica and alumina compounds that undergo polymerization during the geopolymerization process.
5.8.3 SEM with EDAX
The SEM and EDAX were performed to study microstructural behavior change and elemental composition of various NaOH molarity in FA and GGBS-based SCGCs of ambient cured specimens. Figs. 23–27 represent SEM images of SCGC mixes with 8–16 M NaOH molarity at different resolutions. The micrographs of SCGCs show compact and dense structures between binder and coarse particles that claim good mechanical strength of SCGC. However, micrographs show that the increase of NaOH molarity leads to denser particles and a strong pozzolanic-binder matrix in 14 M seems a denser microstructure on the surface of the aggregates. Also, few voids and microspheres were seen due to the absorption of air and FA presence as shown in Fig. 23. Silica (Si), Calcium (Ca), Alumina (Al), and Sodium (Na) are major elements [63]. Figures 28–32 show the EDAX spectrum of SCGC of NaOH molarities from 8 to 16 M. The main peak is due to silica in FA and GGBS existence and its reaction with the alkaline activator.
SEM image of 8 molarity of NaOH-based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
From Fig. 23 (a, b), it can be seen that the SCGC specimen contains numerous FA particles and GGBS that are attributed to unreacted particles. Due to ambient curing, the polymerization reaction in 8 M NaOH-based SCGC is delayed.
From Fig. 24 (c, d), the SCGC specimen shows the dense particles with lower FA and GGBS particles, which indicates the molarity increment can improve the reaction of polymerization. Furthermore, it gains better compressive strength than SCGC with 8 M. It can be seen that SCGC specimen with 10 M contains few pores indicating air entrapped in the mixture.
SEM image of 10 molarity of NaOH based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
The SCGC specimen with 12 M shows denser particles compared to the 8 and 10 molarities, resulting in greater strength due to higher polymer gel formation, as shown in Fig. 25 (e, f). However, the aggregates influenced the polymer matrix with higher NaOH molarity.
SEM image of 12 molarity of NaOH based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
The SEM image of the 14 NaOH molarity blended SCGC shows the geopolymerization process has developed the formation of denser particles without any unreacted aggregates that deliver higher compressive strength than any other SCGC specimens in Fig. 26 (g, h).
SEM image of 14 molarity of NaOH based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
From Fig. 27 (i), the SCGC specimen of 16 M of NaOH contains few unreacted FA particles and GGBS polymerization reaction delay, which delivers less compressive strength than 14 M. However, Fig. 27 (j) saw better polymeric reactions than 8 M and 10 M.
SEM image of 16 molarity of NaOH-based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
Figure 28 shows the EDX images of SCGC with 8 M specimen that indicates the elements with weight percentages such as Si (15.7%), Na (7.2%), Al (6.7%), Ca (6.4%), Fe (2.4%) and Mg (1.6%) with minor elements [64].
EDX spectrum of 8 molarity of NaOH-based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
Figure 29 shows the EDX images of the SCGC specimen with 10 M, highlighting the elements with their respective weight percentages: Si (15.2%), Ca (8.5%), Na (6%), Al (6.4%), Fe (1.9%), and Mg (1.7%), along with minor elements. Na distribution indicates the geopolymer gel location supplied from the alkaline activators [65].
EDX Spectrum of 10 molarity of NaOH based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
EDX spectrum of 12 molarity SCGC shows the various elements Si (14.7%), Ca (10.4%), Na (7.5%), Al (6.5%), Fe (1.7%) and Mg (1.5%) with minor elements in Fig. 30. Ca and Na weight percentages increasing gradually from 8 to 12 M and due to the GGBS and variation of NaOH molarity [66].
EDX Spectrum of 12 molarity of NaOH based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
From Fig. 31 the EDX spectrum of 14 molarity SCGC appear the various elements Si (16.4%), Ca (7.8%), Na (7.6%), Al (7%), Fe (1.4%) and Mg (1.5%) with minor elements. The minor changes in the elemental distribution from 12 M such as Na weight percentages show slight increment variation of NaOH molarity [67].
EDX spectrum of 14 molarity of NaOH based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
The 16 molarity SCGC spectrum from EDX revealed weight percentages of Si (15.9%), Ca (8.1%), Na (9.0%), Al (6%), Fe (1.3%) and Mg (1.5%) with minor elements from Fig. 32. The 16 NaOH molarity reflects in the spectrum by Na (9%) than any other SCGCs. The elemental analysis conforms various molarity of SCGCs compared with presented major elements in each SCGC from 8 to 16 M [68].
EDX image of 16 molarity of NaOH based SCGC
Citation: International Review of Applied Sciences and Engineering 2024; 10.1556/1848.2024.00867
6 Conclusion
Overall, this study concluded that polymerization degree is crucial for the strength and quality of geopolymer concrete. Also, it is very important to maintain geopolymer with self-compacting characteristics, such as better workability and flowability. It may be reduced due to NaOH concentrations and binder materials. It is essential to optimize the NaOH concentrations with binder materials such as FA and GGBS of SCGC. The performance of FA and GGBS-based SCGC with different molarities (8–16 M) in properties and behavior of fresh, hardened, and microstructural characteristics was evaluated to determine the optimum NaOH molarity. The following conclusions can be drawn from experimental investigation,
The workability is a fundamental property that influences the quality, and strength of for SCGCs. SCGC workability shows better result at 8 M NaOH concentration and slightly reduced up to 16 M NaOH concentration. The mix with NaOH of 14 M produced the optimum workability for SCGCs. Moreover, it improves as the amount of FA used as a partial substitute for GGBS.
The mechanical properties of SCGC are most important because they influence the performance and durability of concrete in various applications. The GGBS replacement up to 50% of FA in SCGC provides better mechanical properties. The SCGC prepared with 8 M of NaOH leads minimum CS but it increases from 8 M to 14 M in SCGCs. Further, it decreased when the molarity of NaOH concentration was increased to 16 M, which clearly explains the early strength attained at 8 M and rapidly developed from 10 to 14 M.
Microstructural characterization in SCGC is crucial for understanding the material's behavior, performance, and durability. It involves analyzing the internal structure of the concrete at the microscale, often using techniques like SEM, XRD, and FTIR. Also, NaOH molarity is a crucial aspect that influences the structural behavior of SCGCs. The results of microstructure analysis of SCGC from XRD, FTIR, and SEM equipped with EDAX indicate an improvement of polymerization reactions with FA-GGBS-based SCGC with NaOH molarity increment up to 14 molarity. XRD analysis showed the presence of minerals that enhanced the CS. Also, SCGC revealed a C, N-(A)-S-H gel formation with NaOH molarity increment.
FTIR spectrum clearly shows the various vibrations and absorption bands of O–H bonds, N–H bonds, C–H, O–H (acid), C≡N, C=O, and C–C that enhanced the strength qualities. SEM analyses indicate that overall increment of NaOH concentration provides the polymerization degree and polycondensation that enhanced the particle density of SCGC and increased the strength. The significant elements such as Si, Ca, Al, Na and Fe noted in the EDAX study of SCGC mixes with different NaOH molarities. Hence, this study concluded that the optimized concentration of NaOH as 14 M and binder contents such as 50% FA, 50% GGBS with 2% SP (MasterGlenium SKY 8233) provided optimum CS as 39.4 MPa, STS as 4.72 MPa, and FS as 5.91 MPa.
7 Limitations and future scope
The limitations of this study include precursor, and filler material quality variations, depending on specific curing conditions, and the impact of SP type and dosage. Furthermore, comprehensive research is needed to control the connection between NaOH molarity and SCGC properties due to the absence of cost-benefit analysis and difficulties in transforming the process to industrial production. Future studies may explore the durability of SCGC, such as its ability to withstand cracking, acid erosion, and freeze-thaw cycles, while also performing a comprehensive evaluation of the costs and benefits to determine the economic and environmental effects of various NaOH concentrations. Additional research is needed to evaluate how SCGC production affects the environment and to compare it with traditional concrete to showcase possible advantages and disadvantages.
Author contribution
Mr. Vigneshkumar A*: Investigation, methodology framing, experimental setup, writing- original draft.
Dr. C. Freeda Christy: Supervision, methodology suggestions, writing - reviewing, and editing.
Dr. M. Muthukannan: Supervision and reviewing.
Dr. U. Johnson Alengaram: Methodology suggestions and reviewing.
Acknowledgments
Author Vigneshkumar A is grateful and thanks the International Research Centre (IRC), Kalasalingam Academy of Research and Education (KARE) for providing a University Research Fellowship (URF) and Instrumental research facilities.
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