Urbanization and industrialization have dramatically increased the manufacture of cement causing substantial pollution of the environment. The primary global concern related to cement manufacture has been the management of the large carbon footprints. The usages of environmentally friendly cementitious materials in the construction of structures have proved to be a viable option to deal with this environmental concern. Therefore, it is necessary to further explore the usage of cementitious materials which can replace cement albeit partially. In this direction of research, two such cementitious materials, namely, natural zeolite and metakaolin have been investigated in this study. High-strength concrete M60 with natural zeolite and metakaolin as the partial replacements for the cement has been prepared in this work. Polycarboxylic ether-based superplasticizer solution has been used to enhance workability. The test specimen cast and cured for 3, 7, 28, 60, and 90 days at ambient room temperature has been tested for compressive strength, split tensile strength, and flexural strength as per the Indian standards. The optimum mix of high-strength concrete thus manufactured has met the Indian standards, and the combination of cement +5% natural zeolite +10% metakaolin has exhibited the highest compressive, split tensile, and flexural strengths at 90 days of curing. Natural zeolite and metakaolin when used in smaller proportions have increased the concrete strength, and these materials are recommended for partial replacement of cement.

1. Introduction

Concrete is one of the most widely used construction materials in the world. Cement, water (active components), fine particles, and coarse aggregates (inactive members) are the fundamental ingredients of concrete [1, 2]. Cement manufacture produces a significant amount of carbon dioxide. Cement can be partially replaced by pozzolanic materials or industrial wastes to reduce CO2 emissions [3, 4]. By filling the pores and lowering the porosity and permeability of the concrete without sacrificing the required characteristics, natural zeolite and metakaolin materials improve the durability and strength of the concrete. By fulfilling current and future demands, the notion of sustainability enhances human well-being and quality without compromising it. Due to urbanization, population expansion, and economics, the construction sector directly influences the environment by utilizing cement [5, 6]. It is necessary to use sustainable building materials or processes to minimize carbon dioxide emission and moderate global warming [7]. Because cement is the primary building material in the construction sector, it is essential to convert to sustainable methods and materials owing to the dangerously increasing demand of cement and its significant environmental effect. Various admixtures have been utilized to produce concrete, and therefore the concrete may be made more tolerable by substituting cement components. Natural zeolite is a mineral composed of silicate and aluminate constituents. Metakaolin is a pozzolanic substance that can replace cement in concrete. Both metakaolin and kaolinite are clay minerals that have been dehydroxylated [8, 9]. The use of these mineral admixtures has attracted much attention in the last two decades especially in the building sector because they are ecologically benign and increase the strength and durability of concrete. The production and successful use of zeolite [10, 11] and metakaolin [12, 13] have been discussed. The major investigations for hardened concrete are the split tensile strength, compressive strength, water absorption, and modulus of elasticity. The laboratory experiment on high-strength concrete for durability and strength by adding zeolite was carried out by a descriptive-analytical approach [1416]. The endurance and mechanical characteristics of concrete containing zeolite, metakaolin, and tiny nanobubbles of water have been investigated [17, 18]. The ELM model was used to investigate the compressive strength of high-strength concrete utilizing 324 data records acquired from laboratory tests. The ELM model has been trained and verified [19].

A study was performed to assess the effects of magnetic water with different percentages of natural zeolite (NZ) on self-compacting concrete (SCC) mixes [20]. Physiochemical characterization of zeolite and appraisal of potential as a pozzolanic material in structural concrete were the subjects of two separate experiments wherein the compression strength of the hardened concrete was also evaluated [21].

A study on concrete using natural zeolite as a partial substitute for regular Portland cement of 53 grade with coarse aggregates of 10–12 mm, fine aggregates, zeolite, and water was carried out. Compressive strength tests were performed on 150∗150∗150 mm cubes for 7, 14, and 28 days [22]. In this study, the physical-mechanical characteristics of zeolite and cement and the chemical and mineral parameters of fine and coarse aggregates were estimated. Compressive strength, density, water absorption, porosity, ultrasonic pulse velocity, and freeze-thaw resistance were tested in the lab. The results revealed that replacing 10% of the cement with natural zeolite enhanced porosity and later improved freeze-thaw resistance. When 10% natural zeolite was substituted for 10% cement in the freeze-thaw resistance calculations, the concrete exhibited 3.3 times greater resistance [23]. Experiments were conducted on concrete incorporating natural zeolite as a cement substitute in blended Portland cement up to 60% by mass. The basic physical parameters, fracture mechanics properties, mechanical properties, durability qualities, and hydric thermal properties were investigated [24].

An examination of the impact of natural zeolite as an additive to concrete building from the perspective of freeze/thaw resistance was undertaken using computer simulations. The examination covered two types of concrete for hygrothermal behavior: reference concrete without any admixtures and zeolite concrete with 40% zeolite as cement replacement. The mathematical calculations were effectuated using the computer simulation program HEMOT, and the process input parameters were employed by the FEM [25]. A study was carried out on high-strength concrete using a trial combination designed to achieve goal strength of more than 90 MPa for a control mixture after 28 days of curing and a consistent water/binder ratio of 0.3 for all the mixes. Four distinct mixes, each with a partial replacement of cement with metakaolin of 0, 5, 10, and 15%, were created to test the effect of low water to binder ratio on the mechanical and durability qualities of concrete mixes containing metakaolin [26, 27]. The compressive strength and water absorption of cubes of size 100mm 100mm 100mm and split tensile test of cylinders of size 100mm 200mm, GWT water permeability test, and water penetration tests were conducted on 150mm150mm150mm cubes.[28]. An experimental examination of concrete durability and strength with cement substituted with zeolite [29, 30] and metakaolin [3133] was undertaken.

A study on concrete mixed with metakaolin and concrete mixed with zeolite subjected to compression strength tests for 3, 7, 28, and 90 days was conducted. The study used ordinary Portland cement (OPC), metakaolin (MK), silica fumes (SF), and fly ash as binders in concrete (FA). Polymetric naphthalene sulphonate was used as a chemical additive in the concrete [34, 35]. An examination of the crystallography of the binder components was conducted using XRD analysis. The W/B ratio was determined as 0.5 after 56 days of curing by SEM, EDS, and XRD techniques [36]. Durability studies with metakaolin as a substitute of cement were carried out [37, 38]. A study concluded that the concrete characteristics have been improved by metakaolin. The study proposed five percent, ten percent, and fifteen percent replacements by weight of cement [3941]. A study was conducted on the durability of concrete with metakaolin against sulphate attack [42].

1.1. Significance of Research

Concrete is globally consumed in large quantities in construction. Cement, the principal ingredient of concrete, needs substantial amounts of energy to manufacture and it releases large amounts of carbon dioxide, which has detrimental environmental effects. Therefore, research involving supplementary cementing materials has continuously proven the benefits of incorporating cement replacement materials. In furtherance to this research domain, in this study, naturally occurring zeolite and kaolin (used as metakaolin) have been investigated to arrive at recommended quantities of metakaolin and zeolite as partial replacements of cement.

2. Experimental Work

2.1. Raw Materials

Naturally occurring zeolite and kaolin were sourced from the Rajasthan mines in India. Zeolite was put through a filtration process for removal of impurities. Metakaolin was manufactured from kaolin by a burning process similar to cement at a temperature between 600 and 850°C in Astrra Chemicals, Chennai, India. Aluminum, silicon, and oxygen make up the majority of natural zeolites. They are aluminosilicates, which belong to the “molecular sieves” family of microporous solids. The framework comprises a tetrahedral assemblage of AlO4 and SiO4 linked together in different configurations with oxygen atoms, forming a simple crystal lattice with holes of molecular diameters through which molecules can pass.

Natural zeolite was utilized in a 10% aqueous slurry with a pH of 8.1, bulk density after 100 drops (g/cc) of 0.60, water absorption of 81.8 G/100 g, and specific gravity of 2.1. Metakaolin used had a pH (10 percent solids) of 4.5–5.5, bulk density (kg/lit) of 0.4–0.5, specific surface area (m2/g) of 19–20, and specific gravity of 2.6. The cement was of OPC 53 grade with a specific gravity of 3.15, normal consistency of 27%, initial setting of 78 minutes, and final setting of 528 minutes. The cement soundness test yielded 2 mm result.

The chemical composition of OPC 53 grade cement, natural zeolite, and metakaolin is represented in Table 1.

The superplasticizer was a polycarboxylic ether-based liquid with a pH of about 6.0, a volumetric mass of 1.09 kg/liter at 200C, and an alkali content of less than 1.5 g Na2O equivalent/liter of admixture. Concrete was made with fine aggregates (M sand) that passed through a 4.75 mm IS sieve and coarse aggregates that passed through a 20 mm sieve. Water with a pH of 7 was utilized.

2.1.1. Scanning Electron Microscopy (SEM) of Natural Zeolite and Metakaolin

Figure 1 shows the scanning electron microscopy (SEM) of natural zeolite. SEM was performed mainly to understand the morphology of the system. The heterogeneous nature of the surface was well evidenced by the micrograph. The surface exhibited a shapeless, rod-like, and spherical structure with a high degree of aggregation. The system showed the presence of voids in the natural zeolite.

Figure 2 shows the scanning electron microscopy (SEM) of metakaolin. The surface exhibited more voids having mostly elongated rod-like structures with fewer independent particles and grains. Some were shapeless, and others were in spherical form. The processing conditions might have driven the crystal growth mainly towards a rod-like morphology.

2.1.2. XRD Analysis of Natural Zeolite and Metakaolin

X-ray diffraction analysis is a material science technique for determining the crystallographic structure of a material.

Figure 3 shows the XRD graph for natural zeolite. The XRD pattern revealed the presence of crystalline compounds in the final system. Sharp and intense peaks with low full width at half maximum unambiguously confirmed the presence of crystallites with large sizes. Some compounds were not observed and some were observed with less intensity which might be due to their low concentration or due to their amorphous nature. The less exposure of miller planes due to the complexity of the system might be another reason for the absence of peaks corresponding to certain compounds in the pattern. The major compounds found were quartz alumina and calcium oxide with minor quantities of titania, ferrite, and magnesium oxide. The peaks at 2θ values around 23°, 27°, 30°, 37°, and 40° were indexed to the quartz structure.

Figure 4 shows the XRD graph for metakaolin. The sharp peak points refer to the presence of kaolinite, and in 2θ, the lower peak points refer to the presence of quartz. The sharp peaks at 2θ values around 18°, 25°, 28°, 32°, 34°, 36°, and 41° were indexed to the calcium oxide.

2.2. Test Methods

Table 2 shows the mix design details of mixes computed according to IS 10262-2019. The raw ingredients were weighed in a computerized balance and batched according to the optimal mix percentage. The raw materials such as fine aggregate, coarse aggregate, cement, natural zeolite, and metakaolin were measured and adequately mixed. A superplasticizer solution dissolved in water was then added to the mixture until it became homogeneous. Extreme caution was taken to avoid concrete bleeding and segregation. The sample preparation was completed by using a pan mixer.

2.3. Slump Test (IS 456-2000)

A slump test was performed according to IS 456–2000 to assess the workability of fresh concrete.

2.4. Preparation of Concrete Specimen

The pan mixer was used to mix concrete with a variety of different components. The inside surfaces of the chosen mould were lubricated. The produced homogeneous concrete mixture was put into the mould and compacted to prepare the necessary sized specimen. The concrete specimen was left in the mould for 24 hours until it hardened, after which it was cured at room temperature.

2.5. Compressive Strength

The concrete compressive strength test was carried out following IS 515–1959. Observations and results were recorded for 3, 7, 28, 60, and 90 days cured specimens.

Compressive strength = (maximum load/net area of concrete cube) MPa.

2.6. Split Tensile Strength

IS 515–1959 was used to conduct a split tensile strength test on concrete. Observations and results were recorded for 3, 7, 28, 60, and 90 days cured specimens.

Split tensile strength = (2P/πDL) MPa.

2.7. Flexure Strength

The concrete beam of size 100 × 100 × 500 mm was symmetrically supported by two parallel steel rollers at a distance of 40 cm from the centers of the two rollers. The load was applied through two rollers mounted at 1/3rd of the supporting span. The load on concrete was applied without shock, and the load was increased continuously at a rate of 0.06 + 0.04 N/mm2 per second. The flexural strength was expressed as the modulus of rupture as per the IS 515-1959. Observations and results were recorded for 3, 7, 28, 60, and 90 days cured specimens.

Flexure strength = (PL/bd2) MPa.

3. Result and Analysis

3.1. Slump Test

From Figure 5, it was observed that the slump values for fresh concrete mixed with natural zeolite and metakaolin were in the range of 43 mm to 48 mm. Mix 3 had attained the maximum slump value of 48 mm.

3.2. Compressive Strength Test

From Figure 6, it was been observed that the compressive strength values for hardened concrete for different mixes for 3, 7, 28, 60, and 90 days were in the range of 30 MPa to 72 MPa. At 90 days, mix 3 had attained the maximum compressive strength value of 72 MPa.

3.3. Split Tensile Strength Test

From Figure 7, it was observed that the split tensile strength values for hardened concrete for the different mixes for 3, 7, 28, 60, and 90 days were in the range of 3.1 MPa to 5.3 MPa. At 90 days, mix 3 had attained the maximum split tensile strength value of 5.3 MPa.

3.4. Flexural Strength Test

From Figure 8, it was observed that the flexural strength values for hardened concrete for different mixes for 3, 7, 28, 60, and 90 days were in the range of 4.03 MPa to 9.44 MPa. At 90 days, mix 3 had attained the maximum split tensile strength value of 9.44 MPa.

3.5. Mechanism Analysis
3.5.1. XRD Analysis

In this research, mix 2 and mix 4 were prepared considering metakaolin concentration extremities (5% and 15%) while keeping the natural zeolite concentration constant. Concrete cubes of size 150 mm∗150 mm∗150 mm for the two mixes were prepared and cured for 28 days, and then the cubes were finely ground. The study of XRD was performed on the ground material which had passed through a 90-micron sieve.

Figure 9 refers to the XRD graph of mix 2. The results were well in agreement with the XRF results. The peak at 2θ = 13° was attributed to kaolinite. Metakaolin was not observed in the pattern due to its amorphous nature. The peaks at 2θ values around 21°, 27°, 43°, 52°, and 60° were indexed to the quartz structure. Peaks around 23.4° and 29.4° were attributed to gypsum and calcite, respectively [4345]. The peak corresponding to gypsum was observed to be intense and sharp. A less intense peak at 24° was indexed to ferrite structure. Peaks at 2θ values around 28.5° and 34° were pointed towards mullite crystal planes.

Figure 10 refers to the XRD graph of mix 4. In mix 4, the majority of the peaks were found to be sharp and intense, which confirmed the presence of crystalline compounds. Some broad peaks with high full width at the half maximum supported the presence of amorphous compounds. A dominant peak of natural zeolite at 2θ value around 7° was observed in the pattern. The peak at 11° was indexed to the (100) miller planes of gypsum. The peaks at 17°, 28°, and 34° were attributed to the crystal planes corresponding to mullite. A sharp peak with a good signal-to-noise ratio was observed at 2θ = 21°, which was assigned to the (003) miller plane of muscovite. A peak with fairly good intensity at 29.5° was indexed to calcite. The peaks around 32°, 37.38°, and 64.30° were assigned to the miller planes of calcium oxide (CaO). Peaks at 27°, 43°, 46°, 55°, and 60° were attributed to quartz structures [4346].

3.5.2. FT-IR Analysis

Figures 11 and 12 refer to FT-IR patterns for mix 2 and mix 4, respectively. FT-IR patterns of both the mixes were more or less the same. The band at 464 cm-1 was attributed to the O-Si-O bending vibrations [47, 48]. A band with fairly good intensity at 587 cm-1 was indexed to the vibrations corresponding to the Si–O–Al bond [49]. Another band at 533 cm-1 was due to the vibrations of the Ca-O bond [50]. The sharp band at 777 cm-1 was assigned to the symmetric stretching vibrations of the Si-O-Si bond [51]. The strong band at 995 cm-1 supports the existence of aluminum in the octahedral position, which confirms the presence of mullite [52]. A well-resolved band around 1430 cm-1 was pointed towards the C–O stretching of the carbonate group. The harmonic vibrational mode of C-O was observed as a minor band at 2985 cm-1 [50]. Broadband around 3379 cm-1 was assigned to the asymmetric stretching mode of the hydroxyl group of natural zeolite. The results distinctly confirmed the existence of aluminosilicates and calcium compounds in the final system.

3.5.3. Thermogravimetric Analysis (TGA)

Figures 13 and 14 refer to thermogravimetric analysis (TGA) of mix 2 and mix 4, respectively. Thermogravimetry was used to study the thermal stability of the prepared systems. Both the mixes exhibited more or less identical decomposition patterns. A minor weight loss observed between 100°C to 200°C was due to removing adsorbed and lattice-held water. Another significant weight loss between 410°C to 420°C was attributed to the decomposition of impurities (mainly carbonaceous materials) present in the system. After 420°C, the compounds were found to be stable up to 800°C. There were no noticeable weight losses either due to the degradation of compounds or the phase changes in the analysis. The analysis proved the ability of the system to withstand high-temperature conditions which might be due to the presence of stable metal oxides to a large extent.

3.5.4. SEM: Scanning Electron Microscopy

Figure 15 refers to the SEM of mix 2. The system revealed a slightly porous nature, and the surface exhibited shapeless independent particles with a high degree of aggregation which might be due to the presence of natural zeolite. However, the pore distribution followed a nonuniform pattern. The processing conditions might have facilitated the crystal growth towards specific shapes and the aggregation process.

Figure 16 refers to the SEM of mix 4. Micrograph exhibited heterogeneous nature of the surface, which contains shapeless independent particles and rod-like structures with a high degree of aggregation. Due to the high concentration of metakaolin, bulk porosity may have become dominant in the system.

3.5.5. Evidence for the Regression Analysis

Results from the regression model are shown below.

As shown in equations (1) and (2), the single-factor regression model regresses concrete strength on mix combinations and curing days individually. Equation (3) is a double factor regression model that shows concrete strength as a function of concrete mix combination and curing days taken together.where Sm is the parameter referring to the strength of concrete, is the coefficient constant, is the coefficient for concrete mix, is the coefficient of curing duration,Ct is the combination of mineral admixtures in concrete (i.e., mixes), Dt is the curing days, and is the standard error.

Concrete mix combinations and curing days are the independent variables in the models. Concrete strength is the dependent variable. The intercepts are found larger in the single-factor model suggesting that the model does not adequately explain the strong link between the independent variables. This possibly might be a result of errors in parameters like the water-cement ratio.

According to the results of SPSS software for the differential equations describing concrete strength for single factor (curing days or concrete mix combination) and double factors (both curing days and concrete mix combination), equation (2) has the highest R square value. This shows a significantly stronger link among concrete strength, the number of curing days, and the concrete mix. The analysis obtained is shown in Table 3.

4. Discussion

The use of natural zeolite and metakaolin in concrete manufacturing can help to promote sustainable and environmentally friendly building practices without sacrificing strength. The use of a carboxylate ether-based superplasticizer as an admixture can contribute to improved workability. Mineral admixtures can reduce CO2 emissions when cement is partially replaced. The recent findings demonstrate that employing mineral admixtures like natural zeolite and metakaolin to make eco-friendly concrete is a step forward in the sustainable building strategy.

5. Conclusions

This study arrived at the following conclusions:(i)The optimum concrete mix is mix 3 which contains 85 percent cement, 5% natural zeolite, and 10% metakaolin. The mix achieved the highest compressive strength of 64.58 MPa after 28 days of curing and 72 MPa after 90 days of curing, thus indicating that it is a high-strength concrete.(ii)Mix 3 had the most significant split tensile strength of 4.95 MPa after 28 days and 5.3 MPa after 90 days of curing. The maximum flexural strength was 8.56 MPa after 28 days of curing and 9.44 MPa after 90 days of curing for the same mix.(iii)The existence of calcium oxide, quartz, gypsum, and small amounts of titania, ferrite, and magnesium oxide was verified by XRD analysis. These have assisted in the improvement of pozzolanic binding and strength. TGA analysis results are consistent with XRD analysis results in which the former has indicated that the presence of metal oxides in substantial amounts in concrete has aided in the improvement of mechanical characteristics.(iv)Aluminosilicates and calcium compounds were observed in concrete using FT-IR. They have assisted in the development of concrete strength and improved performance. SEM has exposed the presence of rod-like structures and shapeless independent particles of metakaolin and natural zeolite. This has made the paste structure more complex and has increased the voids and the bulk porosity. Therefore, higher percentages of metakaolin are likely to affect the strength of concrete adversely.(v)The R square value in the double factor model can be regarded as an excellent way to estimate concrete strength.

Data Availability

All the data, models, or analyzed during the study are included in this published article.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.