1. Introduction
Concrete is one of the most widely used construction materials; it is usually associated with Portland cement as the main component for making concrete. Due to rapid urbanization and industrialization, there is a need for large infrastructural development, which is why utilization of concrete as a construction material is increasing day by day. It is estimated that the production of cement was increased to about 4.4 billion tons in 2021 [
1]. Concrete is used globally to build buildings, bridges, roads, runways, sidewalks, and dams. Cement is indispensable for construction activity, so it is tightly linked to the global economy. Cement production is growing by 2.5% annually and is expected to rise from 2.55 billion tons in 2006 to 3.7–4.4 billion tons by 2050 [
2]. Cement manufacturing requires extreme heat for its production. Producing a ton of cement requires 4.7 million British thermal units (BTUs) of energy, equivalent to about 400 pounds of coal, and generates nearly a ton of CO
2. Due to the high emissions and critical importance to society, cement is obviously responsible for increasing greenhouse gas emissions. The production of cement releases greenhouse gas emissions both directly and indirectly: the heating of limestone releases CO
2 directly, while the burning of fossil fuels to heat the kiln indirectly results in CO
2 emissions. The direct emissions of cement occur through a chemical process called calcination. Calcination occurs when limestone, which is made of calcium carbonate, is heated, resulting in the breaking down into calcium oxide and CO
2. This process accounts for almost 50% of all emissions from cement production.
The high ash content (30–50%) of the coal in the world makes this problem more complex. Safe disposal of the ash without adversely affecting the environment is also a major challenge. Hence, attempts are being made to utilize this fly ash rather than dump it. The coal ash can be utilized in bulk in geotechnical engineering applications, such as construction of embankments, as a backfill material, and as a sub-base material. Fly ash is a byproduct of electricity generating plants using coal as fuel. During combustion of powdered coal in modern power plants, as coal passes through the high temperature zone in the furnaces, the volatile matter and carbon are burned off, whereas most mineral impurities, such as clay, quartz, and feldspar, will melt at high temperatures. The fused matter is quickly transported to lower temperature zones, where it solidifies as spherical particles of glass. Some of the mineral matter agglomerates to form bottom ash, but most of it flies out with the flue gas stream and thus is called fly ash. The ash is subsequently removed from gas by electrostatic precipitators. The fly ash is a waste product of coal. Besides the use of fly ash in geotechnical applications, its use in concrete to produce greener concrete was also undertaken in the past [
3].
There is no harmful effect of geopolymer concrete used in structural members on load carrying capacity, and the same standard codes are used for the design of geopolymer concrete as used for conventional concrete [
4]. Geopolymer is a Portland-cement-free concrete and is a useful alternative to normal Portland cement concrete. It is useful for reducing the carbon footprint and has better performance in terms of structural behavior and durability [
5]. Geopolymer concrete has high compressive strength at a low elevated temperature, low to medium chloride ion penetration, and high resistance to acid attack and abrasive forces [
6]. The inclusion of fly ash and silica fume in geopolymer concrete is helpful for improving sustainability and reducing the cost of geopolymer concrete [
7]. It is concluded that geopolymer concrete with or without steel fibers has better durability properties as compared to conventional concrete [
8].
The inclusion of 1% to 2% glass fiber content in concrete is suitable for reducing shrinkage cracks, improvement in flexural toughness, and temperature resistance of light-weight concrete [
9]. Inclusion of glass fibers is helpful for enhancing the split tensile and flexural strength of normal and recycled aggregate concrete. There is no significant effect of glass fibers on the compressive strength of normal and recycled aggregate concrete [
10]. It is estimated that the use of FRP bars in concrete columns contributed 10% column capacity, which is very close to the capacity provided by steel bars, 12%; hence, they are suitable for compression members [
11]. There is significant improvement in the flexural strength of polymer composites having glass fiber and polyester contents [
12]. The strength and durability properties of concrete can be improved after the addition of glass fiber content up to 1%. Beyond 1%, a strength decrease takes place rapidly [
13].
The use of fibers in concrete has increased in building structures because the fibers in concrete may improve the toughness, flexural strength, tensile strength, and impact strength of concrete. Glass fibers have various applications in concrete, such as crack control, preventing coalescence of cracks, and changing the behavior of materials by bridging the fibers across the cracks. Glass fibers used in concrete are of several types, such as E-glass (polyethylene glass fibers), P-glass (polypropylene glass fibers), O-glass (woven roving glass fibers), and AR glass (alkali resistance glass fibers) [
14].
The basic purpose of using glass fibers in concrete is to provide reinforcement in concrete. The fibers are the main load carrying components, and their desired locations and orientations are maintained by a surrounding matrix, acting as a load transfer medium between them and protecting them from any damage caused by the external environment. Alkali-resistant glass fibers are mostly used in concrete as a reinforcement because of their resistance against alkali attacks caused by Portland cement [
15].
In this research, the comparison of the strength properties, such as compressive strength and split tensile strength of concrete with or without geopolymer and glass fibers, will be carried out. The main purpose of this research is to investigate the suitability of the use of geopolymer and glass fibers in concrete to check whether there is some positive effect on concrete strength or not as there is a need to produce greener concrete nowadays to save natural resources and to conserve energy.
2. Literature Review
A polymer concrete is an innovative construction material that is produced by the chemical action of inorganic molecules. Fly ash, which is a by-product of coal obtained from a thermal power plant, is also available around the globe. Fly ash is rich in silica and alumina and, when reacted with alkaline solution, resulted in the production of aluminosilicate gel that can be used as a binding material for concrete production in place of ordinary Portland cement (OPC) [
16]. The chemical composition of the geopolymer material is identical to the natural zeolitic materials, but the microstructure is amorphous. Any material that contains silicon (Si) and aluminum (Al) in amorphous form may be used as a possible source material for the manufacture of geopolymer suitable to produce concrete [
17]. A study on the properties of fly-ash-based geopolymer M20 grade concrete was conducted, and the results indicated that the geopolymer can be used as an alternate binding material in place of cement to produce economical concrete [
18]. The results of research work conducted on geopolymer concrete prepared by mixing sodium silicate and sodium hydroxide with processed fly ash subjected to curing at different conditions and different temperatures indicated that, with an increase in temperature, the strength of geopolymer concrete decreased, and geopolymer concrete made at 1200 °C had the least strength as compared to concrete made at a lesser temperature [
19]. Oven-cured geopolymer concrete specimen provides the higher compressive strength as compared to direct sunlight curing. It was also observed that geopolymer concrete is a more advantageous, economical, and eco-friendly method when compared with conventional concrete [
20]. With proper proportioning of total aggregate content and ratio of fine aggregate to total aggregate, along with the optimum values of other parameters, it has better engineering properties as compared to ordinary cement concrete [
21]. Due to its better application, it is suitable for various civil engineering infrastructures [
22]. Due to the higher concentration of sodium hydroxide solution, geopolymer concrete has high compressive strength, has easy handling to about 120 min, little drying shrinkage, and low creep. In addition, geopolymer concrete has excellent resistance to sulphate attack [
23].
The results showed that no significant change was observed in water permeability coefficient for the geopolymer with different parameters [
24]. It was also observed that there was a good correlation between the rheological parameters and slump for fly-ash-based geopolymer concrete incorporating plasticizer and super plasticizer [
25]. Geopolymer concrete production results in environmental protection due to recycling of waste by products obtained from industries into a high value construction material needed for infrastructure development [
26]. The geopolymer concrete has higher bond strength than ordinary concrete, as depicted by the pull out test conducted on both types of samples. The existing equation used for the calculation of bond strength of “reinforced” concrete can be equally applicable for geopolymer concrete [
27]. Geopolymer concrete can be used as an alternative for Portland cement in various construction applications for infrastructure development [
28]. Geopolymer concrete has better performance in acidic and basic environments due to its better durability, higher strength, and excellent volume stability as compared to ordinary concrete [
29]. For the structural elements subjected to external restraints, the geopolymer concrete has an excellent advantage over ordinary concrete due to its low heat of hydration and shrinkage and high tensile strength [
30].
The geopolymer concrete has excellent applications in precast concrete members in terms of cost and sustainable development [
31]. The flexural strength and elastic modulus of geopolymer concrete can be easily determined using the equation provided by ACI 318-08, the same as in the case of ordinary Portland cement concrete [
32]. Incorporation of geopolymer fibers improves the ductile nature of concrete suitable for structural performance for the infrastructure development [
33]. In order to develop significant strength of geopolymer concrete, high temperature curing and extended curing time are preferred [
34]. The type of application of geopolymer materials is determined by the chemical structure in terms of the atomic ratio of silicon and aluminum in the polymer material [
35]. The polymer materials were found suitable for use in concrete as a partial replacement of fine materials [
36,
37]. The results showed that the heat-cured fly-ash-based geopolymer concrete undergoes very low drying shrinkage. The drying shrinkage strain at one year as calculated using the Gilbert method was much higher, about five to seven times, compared to the measured drying shrinkage strain [
38]. The discussion on the chemical reaction, mechanism, role of materials, applications, and microstructure of fly ash geopolymer cement concluded that the atomic ratio of silicon and aluminum in the polysialate structure plays a major role in the selection of suitable material for application in different fields [
39]. The addition of admixture up to 2% by mass of fly ash improves the workability of fresh geopolymer concrete without segregation and bleeding [
40]. Due to its better fire resistance property, the geopolymer concrete is more sustainable and durable concrete suitable to be used as construction material in the future [
41]. Geopolymer concrete has better mechanical properties and a low carbon footprint, which makes it better construction material to be used in the near future in place of OPC concrete [
42]. The results indicated that there was a nominal increase in compressive strength, while the increase in flexural strength was significant for glass fiber concrete. The workability of glass fiber concrete decreased with an increase in the percentage of glass fibers [
43]. The research work concluded that the optimum fiber content was dependent on the strength properties of the concrete [
44]. The concrete having fiber glass is economical as compared to conventional concrete [
45]. An investigation on the effect of glass fibers on compressive strength, split tensile strength, and flexural strength of concrete demonstrated that the optimum percentage of glass fibers is 0.1% for an increase in compressive strength, split tensile strength, and flexural strength of concrete [
46,
47]. The use of glass fibers in concrete resulted in enhancement in the compressive strength of concrete; however, an excessive amount of glass fibers caused a reduction in compressive strength. There is no effect of the addition of glass fibers on the modulus of elasticity of concrete, but there is a positive effect on the stress strain curve and flexural strength of concrete, and, lastly, glass fiber concrete has more service life and less permeability than conventional concrete [
48]. The study concluded that 1% is the optimum dose of glass fibers to have positive effects of concrete properties [
49]. The glass fiber has a positive effect on the modulus of elasticity and damping properties of concrete. There was reduced water absorption and less wear upon addition of glass fibers in concrete [
50]. The addition of glass fibers in concrete resulted in thin sections of concrete of around ½ to ¾ inches in thickness. The glass-fiber-reinforced concrete is suitable for precast concrete applications [
51]. It is observed that the toughness, compressive strength, flexural strength, and modulus of elasticity of glass fiber concrete are more than conventional concrete [
52]. The durability properties of glass fiber concrete are also compensated due to the addition of steel fibers [
53]. The fatigue life of glass-fiber-reinforced concrete can be predicted at the desired level of survival probability [
54].
The length of glass fibers used in concrete has a significant effect on flexural strength of concrete as short fibers have lesser strength than long fibers. The same effect is also observed in the case of the compressive strength property of glass fiber concrete [
55]. The incorporation of wastepaper in glass-fiber-reinforced concrete results in the development of sustainable concrete production having better split tensile and flexural strength as compared to conventional concrete. However, the load carrying capacity of glass fiber concrete having wastepaper was found to be 42% lower than conventional concrete [
56]. Glass fibers used in concrete have negative effects on the fresh properties of concrete while having positive effects on the hardened and durability properties of concrete. There is adequate bonding between cement paste and glass fiber for strength development in the concrete mix [
57]. The use of glass fibers causes a slight increase in the compressive strength of concrete, while split tensile and flexural strength are increased considerably. The glass fiber concrete also helps in recovering strength loss of concrete [
58]. Inclusion of glass fibers in concrete helps in improving the fracture properties of concrete [
59]. Glass fibers also cause an increase in the strength properties of recycled aggregate concrete, such as split tensile strength and flexural strength [
60]. Addition of glass fibers causes an increase in the strength properties of moderate deep beams without stirrups [
61]. Replacing steel with reinforced glass fiber polymer tiles causes a reduction in the ultimate load capacity of reinforced concrete columns and has up to 80% of the ultimate load [
62]. Similarly, the mechanical properties of glass-fiber-reinforced Sulphur concrete are also higher than unstrengthened concrete [
63]. To improve the mechanical properties of glass-fiber-reinforced concrete, chemical binding is used to improve the interfacial bond strength [
64].
The main aim of this research work is to check the effect of fly ash and cement with proper alkaline solution, water, and super plasticizers on the workability, compressive strength, and split tensile strength of concrete. In addition to that, glass fibers in different ranges were also added in separate samples of concrete to check their effect on workability, compressive strength, and split tensile strength. The strength comparison of conventional concrete, geopolymer concrete, and glass fiber concrete will be completed in the form of bar charts and figures and conclusions will be drawn.
3. Materials and Methodology of Research
The materials used for making fly-ash-based geopolymer concrete specimens are low-calcium dry fly ash as the source material, aggregates, alkaline liquids, water, and super plasticizer SP-675. The cement to be used in a particular concrete or mortar is selected based on specific properties required, as mentioned in
Table 1. Chemical properties of cement were provided by the supplier received from cement manufacturer, while physical properties of cement were determined from different laboratory tests. For specific gravity test, set of pycnometers was used. Consistency and initial and final setting times were determined from Vicat apparatus. Le-Chatelier apparatus was used for soundness test of cement. Compression testing machine was used for finding compressive strength of 2 × 2 inches mortar cube, and, for fineness test, 90 Micron IS sieve was used.
Class F low calcium fly ash (class F fly ash has lower calcium content than class C fly ash; it mainly contains alumina and silica) obtained from thermal power plant was used for experimental work.
Table 2 shows physical properties of fly ash.
Natural river sand of size below 4.75 mm was used as fine aggregate.
Table 3 shows the physical properties of fine aggregates.
Natural crushed stone with 20 mm downsize was used as coarse aggregate.
Table 4 shows the physical properties of coarse aggregates.
The alkaline liquid used was a combination of sodium silicate solution and sodium hydroxide solution, as shown in
Figure 1. The sodium silicate solution (Na
2O = 13.7%, SiO
2 = 29.4%, and water = 55.9% by mass) was purchased from a local supplier in bulk. The sodium hydroxide (NaOH) in flakes or pellets with 97–98% purity was also purchased from a local supplier in bulk. The NaOH solids were dissolved in water to make the solution. The combination of NaOH and Na
2SiO
3 was used as alkaline solution. The ratio of both solutions, alkaline to fly ash, was taken as 0.35. Ordinary tap water used for drinking purposes was used for the preparation of concrete mixes.
E-glass fibers from industry were purchased and used in concrete. Different properties of glass fibers are shown in
Table 5 [
31].
The overall methodology required for the research is summarized below:
Collection of fly ash material from industrial (thermal power plant class F) used as filler material for concrete
Collection of glass fibers from industry
Casting of conventional concrete cubes and concrete cylinders
Casting of geopolymer concrete with combinations of cement and fly ash as 15% and 85%, respectively
Use of NaOH or KOH and sodium silicate or potassium silicate as alkaline solution for geopolymerization
Casting of glass-fiber-“reinforced” concrete cubes and cylinders
Curing of casted samples of concrete for 7, 14, 21, and, 28 days
Testing of concrete cubes and cylinders in compressing testing machines
Comparison of failure modes of all concrete samples with or without geopolymer and glass fibers