Enhancement of the Mechanical Properties of a Geopolymer Concrete Due to Chemical and Microstructural Interaction of the Binder Material

Many experimentation and researchers were made in the recent decades in concrete and advancement in the field of concrete technology were given a major focus. In the years of research, many new replacement materials were identified and further details studies over the materials were made to have a clear and better idea about the material. In this study, the binder which is to be cement basically is replaced with a combination of Alccofine-1203(A), Metakaolin (MK), and Ground Granulated Blast-furnace Slag (GGBS) in different ratios to arrive a geopolymer concrete mix. In addition, the study also focuses on the chemical interaction of the replacement material and its microstructure interaction. The liquid binder (l/b) ratios varied between 0.48 to 0.54, and about seven geopolymer concrete mixes were arrived and experimental investigation done for mechanical property. The maximum compressive strength obtained from the mix proportions G50M35A15 at an l/b ratio of 0.50 was taken as a mix design for the Geopolymer mix. The identified combination of Granulated Blast-furnace Slag, Metakaolin, Alccofine-1203 (GMA) is used for further study. In general, when finer particles were replaced or added to the material, the finer particle will reduce the voids and thereby increasing the strength of the concrete by developing a denser material combination. Experimentally, results proves that there is a significant hike in the mechanical behaviour of the concrete. The microstructure investigation is carried out through SEM analysis and energy dispersive X-ray analysis (EDAX) which indicated a good interlocking characteristics of the materials which inturn directly impacts the mechanical properties of the concrete.


Introduction
In the construction industry, cement has long been a preferred binder because of the ease with which it can be used and the abundance of the material. Because of the increasing population and urbanization, cement demand is rising. By 2030, India's cement consumption will reach above 550 million tonnes, but there will be a 230 million-tonne supply gap. In addition to using up to 5% of natural resources each year, cement production also contributes to climate change by emitting 1.35 billion tonnes of greenhouse gases every year [1][2][3]. Because of the rapid growth of the nation's infrastructure, cement use will be unavoidable. Construction companies must work together to identify low-impact concrete binding materials as part of their efforts to help the environment and meet future concrete demand. As an alternative of using OPC as a partial binding ingredient, several scientists investigated the use of lower carbon-emitting supplementary cementitious materials. During the twentieth century, industrialization evolved to fulfill basic requirements. Thermal power plant waste and waste from iron and steel mills, rice mills, and mines can be harmful to the environment if not properly managed.
Fly ash, GGBFS, and rice husk ash, as well as alccofine 1203 are examples of industrial by-products for which land is required for proper disposal. Disposal of industrial byproducts is out of the question for most companies. Industrial by-products can be used as an ingredient to green concrete in place of or in addition to cement. When industrial trash is deposited on surrounding fields, it pollutes the ecosystem and waterways. After becoming aware of the environmental consequences of generating solid waste and disposing of them casually, the researchers began employing industrial by-products as construction materials [4]. A French scientist Davidovits identified few chemical combination for synthesized geopolymer concrete in 1979 [5]. Materials made from alumina-silicate industrial waste can be transformed using Geopolymer technology into highvalue-added products that minimize pollution and safeguard our planet. The presence of an alkaline solution is required to manufacture an amorphous geopolymer binder from industrial by-products high in alumina such as alccofine 1203, GGBFS, fly ash, metakaolin, rice husk ash and silica fume. [6,7] The components of geopolymers (Si-O-Al) have tetrahedral connections are classified as inorganic polymers. Alkali activators and aluminum-silicate source materials can improve geopolymer concrete's efficiency. When cured at room temperature, geopolymer concrete containing 100 percent fly ash as a binder develops a low compressive strength [8].
According to Hardjito et al., temperature curing can considerably improve the compressive strength of fly ash-based geopolymer concrete [9]. To avoid this, [10] Pradip Nath and coworkers did a study on GGBFS to raise the maximum amount of calcium in the source of their material and speed up the geopolymerization processes; instead of fly ash used in site settings, geopolymer concrete cannot be used due to temperature curing limits. Alccofine 1203, a ternary blender, enhanced the pore filling and workability of geopolymer concrete. New geopolymer concrete can be made using waste by-products [11]. According to this study, scientists have created a new type of geopolymer concrete with better mechanical and microstructural qualities than traditional concrete. Industrial by-product-based developed geopolymer concrete may improve quality and reduce costs by replacing OPC concrete or mortar. This helps the environment and conserves resources. [12] Fly ash can be replaced with wollastonite and glass fiber reinforced steel to improve the overall durability and mechanical performance of ternary blended geopolymer concrete (GGBS). According to tests [13] on the strength and durability of geopolymers made from ground glass fiber and glass powder, GGF-based geopolymer mortar bars expanded lower than Portland cement mortar specimens. Natural stream conservation and construction sand supply reduction are two goals that can be achieved using M-Sand.
After reviewing the literature and work done by the previous researchers it is found that the research work on finding out the mechanical and microstructural properties of geopolymer concrete using (GGBS-Metakaolin and Alccofine-1203) GMA GPC with M Sand as a fine aggregate is limited, and an attempt has been made in this present research in order to expand the industry's use of geopolymer technology.

Ground Granulated Blast-Furnace Slag(GGBS)
GGBS the byproduct of iron manurafcturing is blasted and reduced to a lesser finner particle than cement and used as a industrial by product for repleacement of binder. The chemical composition and fineness of the grinding determine slag performance. IS: 12,089-1987 regulates the quality of slag (Specification for Granulated Slag for Manufacture of Portland Slag Cement). The color appears dull. GGBS has a consistency of 35.0, a specific gravity of 2.72, a loss on ignition of 1.50%, and a fineness (Blanes) value of 4550 cm 2 /g. The chemical ingredients of GGBS are listed in Table 1. Figures 1 and 2 depicts the SEM image and EDAX of GGBS, respectively which defines the material to be smooth and edged shape microstructure.

Metakaolin
A clay mineral which is anhyrrously calcined called as Metakaolin which is used in this study, has a specific gravity of 2.5 and a specific surface area of 20,000 cm 2 /gm. As seen in Table 2, the metakaolin contains a wide range of chemical components. A metakaolin SEM image is depicted in Fig. 3 and an EDAX image of metakaolin is shown in Fig. 4 corresponding infers a porous, angular shaped and platy particle.

Alccofine 1203
Alccofine 1203 is a low calcium silicate micro-fine material generated from blast furnace slag with excellent reactivity. These materials are finer and hence using as an binder material reduces the viod and implies in the increase in the strength of the concrete. Depending on the application, it can improve the properties of both fresh and cured concrete. The Alccofine employed in this investigation is designated as Alccofine1203, and it complies with IS:456-2000 [14] and IS:12,089-1987 [15], among other standards. According to the findings, the particles of alccofine were irregular in shape and had sharp edges. Alccofine has a specific surface area of 1204 cm 2 /gm and a specific gravity of 2.6, respectively. Table 3 lists the chemical components and physical attributes of Alccofine 1203 as well as their corresponding values. SEM Image of Alccofine 1203 is depicted in the Fig. 5. Figure 6 shows the EDAX image of Alccofine 1203.

M-Sand and Coarse Aggregate
M-Sand was used as fine aggregate and it is confirmed to zone II by a 4.75 mm IS sieve, and Coarse Aggregate, verified to zone II by a 20 mm IS sieve, with a fineness modulus of 6.89, was used (IS 393) [16]. When aggregates are employed, they are already surface saturated and dried; therefore, their properties have already been defined. Table 4 summarizes the characteristics of the aggregates.

Alkaline Activators
For a geopolymer structure similar to hard rock, an alkaline solution binds industrial waste materials, fine and coarse aggregates together. The alkaline solution was prepared a day ahead of time and stored in a refrigerator is just to avoid ionisation process. The alkaline solution was made of sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ). The sodium hydroxide molar concentration was left at 8 M (arrived by trail and error method). The mass proportion of SiO 2 to that of Na 2 O is taken as 2.28. The proportion of sodium silicate to sodium hydroxide was taken to be 2.5 when making the sodium silicate solution (SiO 2 with 32.42%, Na 2 O = 14.20%, and H 2 O = 53.06%). Before employing it in the polymerization process, the sodium hydroxide/sodium silicate solution was left for 24 h.

Manufacturing of Geopolymer Mortar
There is no codal provisions for designing geopolymer concrete, in this study, seven mortar mixes were made with varying binder ratios to investigate the influence of GGBFS, metakaolin, and Alccofine 1203 with different l/b (Liquid binder) ratios of 0.48, 0.50, 0.52, and 0.54 on the strength of the mortar and no superplastizer is used in this study. Table 5 depicts the mixed proportions. The Geopolymer concrete were cured by ambient curing method in which a moderate exposure condition is adopted. Where M is Metakaolin, G is GGBFS, A-Alccofine 1203; CA is Coarse aggregate, FA is Fine Aggregate, SS is Sodium Silicate, SH is Sodium Hydroxide, SP is Superplasticizer, H 2 O is Water.

Optimum Mix Values for Geopolymer Concrete
By incorporating the above mix design, concrete cubes were casted with different l/b ratios and twenty-eight days compressive strength of the cubes were experimentally investigated to identify the optimum mix value of the geopolymer concrete and graphically represented in Figs. 7, 8, 9 and 10.       15 there is a decrease in 3.98% than that of the compressive strength of the mix G 50 M 50 A 0 . It is observed and evidenced that Alccofine-1203 has improved the compressive strength significantly up to the first three mixes. When the amount of Metakaolin in the last mix is lowered, there is also a significant reduction in compressive strength. It is also evident from this graph that the mix G 35 M 50 A 15 has obtained the maximum compressive strength, and it is the optimum value. From Fig. 9, for the l/b ratio of 0.52 without Alcoffine120 the compressive strength for the twenty-eight days is about 31.08 N/mm 2 . From mixing the G 45 M 50 A 5 to G 50 M 35 A 15, the Alcoffine-120 has been added, and it is observed that for 1,7,14 and 28 days with an l/b ratio of 0.52, there is a significant improvement in compressive strength for all the mixes. For 28 days of compressive strength the mix G 45 M 50 A 5, there is an increase in 3.28%, for the mix G 40 M 50 A 10 there is an increase in 9.72%, for the mix G 35 M 50 A 15 there is an increase in 12.93%, for the mix G 50 M 45 A 5, there is an increase in 5.37%, for the mix G 50 M 40 A 10 there is a negligible increase of 0.06%, for the mix G 50 M 35 A 15, there is a decrease in 0.65% than that of the compressive strength of the mix G 50 M 50 A 0 . It is observed and evidenced that Alcoffine-1203 has improved the compressive strength significantly up to the first five mixes. When the amount of Metakaolin in the last mix is lowered, there is also a significant reduction in compressive strength. It is also evident from this graph that the mix G 35 M 50 A 15 has obtained the maximum compressive strength and is the optimum value.
From Fig. 10, for the l/b ratio of 0.54 without Alc-cofine1203 the compressive strength for the twenty-eight days is about 30.20 N/mm 2 . From mixing the G 45 M 50 A 5 to G 50 M 35 A 15, the Alccofine-1203 has been added, and it is observed that for 1,7,14 and 28 days with an l/b ratio of 0.54, there is a significant improvement in compressive strength for all the mixes. For 28 days of compressive strength the mix G 45 M 50 A 5, there is an increase in 2.68%, for the mix G 40 M 50 A 10 there is an increase in 9.60%, for the mix G 35 M 50 A 15 there is an increase in 13.94%, for the mix G 50 M 45 A 5, there is an increase in 3.38%, for the mix G 50 M 40 A 10 there is a negligible increase of 2.28%, for the mix G 50 M 35 A 15, there is a decrease in 0.70% than that of the compressive strength of the mix G 50 M 50 A 0 . It is observed and evidenced that Alccofine-1203 has improved the compressive strength significantly up to the first five mixes. When the amount of Metakaolin in the last mix is lowered, there is also a significant reduction in compressive strength. It is also evident from this graph that the mix G 35 M 50 A 15 has obtained the maximum compressive strength, and it is the optimum value.
It is observed that the mix proportion of G 35 M 50 A 15 with the l/b ratio of 0.5 has obtained the maximum compressive strength, and it has been chosen as a mix proportion for geopolymer concrete.

Geopolymer Concrete (GPC) Specimens
G 35 M 50 A 15 grade geopolymer concrete was made with coarse aggregate and then used to manufacture more geopolymer concrete. The mixes were weighed and blended for 3-4 min in a dry environment before use. The dry mix was combined with sodium hydroxide, sodium silicate solution, and a superplasticizer to generate an alkaline solution. Then, water (about 15% of the binder's weight) was added to improve the binder's workability. It took about 6-8 min to mix everything. After mixing, the concrete was poured into a steel mould and crushed to the desired consistency. Precautions were taken to ensure that the materials were mixed evenly. Two forms of curing were used on the specimens that will be examined for compressive strength. The cubes were dried in the oven on one side and outside on the other. They were cured for 8 h at 60 °C and 8 h at 90 °C in the oven. The cubes were reintroduced to the oven for another 8 h at the same temperatures after de-molding. They were then removed from the oven and allowed to cure at room temperature until the test day. After casting, the cubes were demolded and allowed to dry for one day at room temperature before being tested. The cubes were evaluated three, seven, and twenty-eight days after casting. For all tests, the specimens were then ovencured at 60 °C. A superplasticizer based on Naphthalene Sulphonate was utilized to improve the workability of a recently prepared geopolymer mix. The alkaline solution is suggested to be ready by at least 24 h ahead of time [17]. The reaction of several geopolymer paste formulations with alkaline solutions is being studied (Metakaolin, GGBFS, and Alccofine 1203).

Compressive Strength of GMA GPC (GGBS, Metakaolin, Alcoffine-1203 Geopolymer concrete) and Control Concrete
Compressive strength is assessed to deteremine the mechanical property and strength quality of geopolymer concrete and mortar mixtures. Ternary geopolymer concrete's compressive strength evolution is depicted in Table 6. Geopolymer concrete's compressive strength changed dramatically after 1, 3, 7, 28 and 90 days when alccofine1203 was used as a ternary binder. Figure 11 illustrates the compressive strength of GMA GPC graphically.

Split Tensile Strength
The split tensile strength of GMA-Geopolymer concrete and conventional concrete was determined using split tensile strength testing, and the results are presented in Table 7.
The split tensile strength of GMA-Geopolymer concrete was higher than that of conventional concrete. GPC exhibited split tensile strength in the range of 5.71 to 6.34 MPa at various ages when tested. Control concrete on the other hand, displayed strength in the range of 3.10 to 3.25 MPa. The split tensile strength of GPA GPC is represented graphically in the Fig. 12 Table 8 shows the flexural strength of GMA-Geopolymer concrete. GMA-Geopolymer concrete had higher flexural strength than CC. GPC demonstrated split tensile strength in the ranges 5.34 to 6.54 MPa at various ages. However, control concrete had a strength of 4.67 to 6.54 MPa. Figure 13 illustrates the flexural strength of GPA GPC. By using a finer replacement material as binder the voids will reduce and increase the better packing of the material and also the polymerization process of the geo concrete helps in increasing the mechanical behaviour. Figure 14 illustrates the SEM images of the samples at various ages. The pattern of the microstructure, particularly its uniformity, dense packing, and particle shape undergoes a   substantial alteration. The SEM picture of the source materials reveals that metakaolin has irregularly formed glassy particles, whereas GGBS contains flake-shaped particles. Alccofine-1203 contains silica and alumina in significant proportions, resulting in regular formed spherical particles. The microstructure of samples from various ages varied significantly. They appeared to be quite dense and uniform in composition. An increase in microstructure homogeneity, a significant indicator of greater strength occurs as the days are extended. As the geopolymer reactions occur, the fracture area of the concrete begins to fill with geopolymer products, thereby increasing its strength.

EDAX Analysis
All samples on GMA-GPC had significant amounts of Fe, Al, Ca, and Si [18], as shown in Figs. 15. But the samples have varying percentages. Tables 9, 10, 11 exhibit EDAX spectrograms at various ages. It is clearly found that in the geopolymer combinations the percenetages of silica (Si) and Alumina (Al) shows a noticeable variation because of it gel formation at different ages.

Effect of Alccofine-1203 in GPC
Alccofine-1203 improved the microstructure of the GPC, resulting in fewer pores. In contrast, GPC made with alccofine and activated with sodium silicate gel has a denser microstructure, less breaking, and higher compressive strength. An improved density of the matrix and fewer micro-fractures, flaws, and holes in the alccofine based GPC structure were verified by SEM examination. None of the GPC samples with alccofine had a cracking or alkali-silica reaction at the aggregate cement matrix interface.

Effect of Metakaolin (MK) in GPC
Based on maximum compressive strength, utilizing 35% MK as an additional material as one of the constituents of GGBS enhances the strength qualities of the blended GPC mix. It is observed from the mortar mix that by reducing the amount of Metakaolin, there is a significant reduction in strength.

Effect of GGBS in GPC
The mechanical properties of GGBS-based GPC are improved by addition of alkaline activator solution. This increase in strength may be due to pH. Increasing the molarity of the solution allows more sodium and hydroxide ions to react with the silicon (Si) and aluminium (Al) minerals in GGBS, increasing the rate of geopolymerization

Conclusion
A systematic procedure were followed and experimentation were carried out. The geopolymer mortar has been tested for its compressive strength with the addition of alccofine-1203 with different l/b ratios; the strength has been improved till the optimum value, and a significant decrease and increase in strength observed after the optimum level.
• By using a finer replacement material as binder the voids will reduce and increase the better packing of the material and also the polymerization process of the geo concrete helps in increasing the mechanical behaviour. The optimum value of the compressive strength of GMA GPC for 0.49 l/b ratio is 33.86 N/mm 2 , 0.50 l/b ratio is 37.10 N/mm 2 , 0.52 l/b ratio is 35.10 N/mm 2 , 0.54 l/b ratio is 34.41 N/mm 2 . • GMA GPC with an l/b ratio of 0.50 the compressive strength of mortar 37.10 N/mm 2 is the highest and maximum when compared to the other mix, and the corresponding mix proportion G 50 M 35 A 15 will be considered as an optimum mix. • The Alccofine-1203 may have plugged microspores, increasing the compressive strength of geopolymer mortar and Geopolymer Concrete.
• With the mix ratios of G 50 M 35 A 15 and the coarse aggregate, the Geopolymer concrete has over 40% better compressive strength, 84% better splitting tensile strength, and 14% better flexural strength than conventional concrete. • Due to the high molar alkali activator, the GPC had stronger strength and early strength growth than control concrete. • Micrographs of geopolymer samples reveal that additional geopolymeric gel was generated over time, resulting in an increase in microstructure density and a remarkable increase in the strength of geopolymer concrete. • Principal elements such as silica, sodium, and iron were used to validate strength growth as observed in EDAX quantitative analysis. • To summarise, the GPC ternary blend comprising GGBS, Metakaolin and alccofine-1203, may be used efficiently as a sustainable alternative as a binder.

Declarations
Ethics Approval Authors like to declare that no conflict of research results arises from the present work on "ENHANCEMENT OF THE MECHANICAL PROPERTIES OF A GEOPOLYMER CONCRETE DUE TO CHEMICAL AND MICROSTRU CTU RAL INTERACTION OF THE BINDER MATERIAL" if it is published. Authors also assure that this paper was not sent for any other organization in this connection.