Development of structural grade lightweight geopolymer concrete using eco-friendly materials

2.1 Ingredients

Lightweight geopolymer concrete was produced in this investigation using geopolymer concrete derived from ground granulated blast furnace slag (GGBFS), manufactured sand (M-sand), and crushed CS. Conventional geopolymer concrete used with crushed stone aggregate (CSA) was also produced for comparative study. The chemical composition of GGBFS, M-sand, CS, and CSA was determined using X-ray photoelectron spectroscopy, and the corresponding results are provided in Table 1. The sum of the chemical composition CaO, SiO₂, and MgO present in GGBFS was found to be around 81%, which is responsible for strength development. The remaining percentage contained Al₂O₃, MgO, and minimal amounts that included extra oxides. Active constituents, including CaO, SiO₂, and Al₂O₃, are discernible in GGBFS, M-sand, CSA, and CGA. The characteristics of the materials utilised are outlined in Table 2. GGBFS has a specific gravity of 2.65, a specific surface area of 4,280 cm²/g, and a maximum particle size of 12.3 µm (Table 2). The SiO₂ and Al₂O₃ content in the binder (Table 1) is the main factor that contributes to the properties and strength development pattern of geopolymers. Using this binder with high levels of reactive oxides helps to produce high-strength geopolymer concrete.

M-sand is extracted from local quarries near Chennai, Tamil Nadu, India. India. Sieve analysis was performed in accordance with IS 383:2016 [41] and ASTM C136 standards [42]. M-sand exhibits a dimensional variation with the following percentage passing through various sieves: 99% for the 4.75 mm sieve, 87% for the 2.36 mm sieve, 58% for the 1.18 mm sieve, 38% for the 0.6 mm sieve, 21% for the 0.30 mm sieve, and 6% for the 0.15 mm sieve. Based on these measurements, M-sand is classified as Zone II, indicating a medium grading that is suitable for various concrete applications.

Coconut husks are collected in the coconut industry in Erode district, Tamil Nadu, India. Scrap the raw CS using a shredder designed for shredding CS equipment (Figure 1). After CS is crushed, pass through a 12.5 mm sieve and be used as coarse aggregate. For coarse aggregate, CS and crushed granite with dimensions smaller than 12.5 mm and shell thickness of 8–10 mm have been used. The particle size distribution of CS and crushed granite is illustrated in Figure 1. Shredded CS has higher impact strength values (8.15%) and lower bulk density (650 kg/m³).

Figure 1

(a) Coconut shell aggregate preparation process. (b) Particle size distribution of CS and crushed granite stone aggregate (b to be drawn and added).

2.2 Alkaline activators

By combining sodium hydroxide (NaOH) solution with 10M and sodium silicate (Na₂O, 13%; SiO₂, 30%; and water, 57% by mass: Na₂SiO₃), an alkaline activator was produced for use in geopolymer conventional concrete (GPCC) and geopolymer coconut shell concrete (GPCSC). The NaOH and Na₂SiO₂ have been employed at a weight ratio of 1:2.5. Figure 2 presents a flow chart of the components needed to create GPCC and GPCSC.

Figure 2

Methodology of – GPCC and GPCSC concrete.

2.3 Mix proportion

The design of geopolymer concretes is subject to limited code regulations. When novel ingredients are introduced, there are no precise methodologies available for the design of concrete mixes; instead, trial mixes are typically used to establish the design [36]. Due to their disparate qualities, the ACI and IS techniques could not be used to develop a concrete mix that included agro-waste components [36,38]. All mixtures were initially formulated with volume proportions and then converted to weight quantities due to the fact that the performance of the mixtures can differ based on factors such as particle size, density, specific gravity, and percentage of void content of the components. The CS-to-CGA ratio was maintained throughout the creation of the control mixture. For this inquiry, more than 50 trial mixes were made with the goal of improving the use of CS as coarse aggregate and achieving the strength of concrete grade M₂₀–M₂₅. Table 3 lists a few trial mixes along with the mix proportions and other information. It was discovered that using CS% above the ratio of 1:2.97:1.50 caused increased segregation and incorrect bonding and was therefore not taken into consideration. From the other mixes provided in Table 3, Mix M5 for GPCC and M11 for GPCSC mix were selected for further studies since these mixes were seems to be homogeneous and no segregations were found.

Figure 3 shows the mixing of GPCC in a pan mixer, transfer of mix on a tray, and final prepared GPCC mix (M5), which was ready for cast. Similarly, Figure 4 shows the mixing of GPCSC in a pan mixer, transfer of mix on a tray, and final prepared GPCSC mix (M11), which was ready for cast. Scanning electron microscopy (SEM) images of mixes M7 and M11 of GPCSC were acquired to examine the segregation and production of C–A–S–H gel as a result of the generation of a pozzolanic/alkali-activated reaction [37,38,39]. These images are shown in Figures 5 and 6, respectively. Figure 6 depicts the homogeneous and dense packing of mix M11, while Figure 5 clearly depicts the segregated and inappropriate bonding with air gaps present in mix M7. It is obvious that the compatibility of the mixes created for GPCSC is mainly due to CS and binder ratio impact. Figure 7 gives the proportion of ingredients used in GPCC and GPCSC.

Figure 3

Mix 5 –Mix with crushed granite as coarse aggregate.

Figure 4

Mix 11 – Mix with CS as coarse aggregate.

Figure 5

Mix 7 – SEM.

Figure 6

Mix 11 – SEM.

Figure 7

(a) Ingredients used for GPCC mix M5. (b) Ingredients used for GPCSC mix M11.

2.4 Mixing, casting, and curing

The GGBS binder was added after aggregates had been thoroughly mixed in the machine, which took around 5 min. Alkaline activators were then added, and mixing lasted for an additional 4 min after that. For the GPCC and GPCSC mixes, the alkaline solution/binder ratio of 0.65 was maintained. The freshly mixed concrete was transferred into the respective moulds and kept on a table vibrator to remove air voids and to achieve a dense pack. Following the guidelines of ASTM C171-16 [43], the specimens were protected with plastic sheets immediately after being formed. After a period of 24 h, the specimens were removed from the moulds and subsequently covered with polythene until the testing date.

2.5 Microscopic analysis

For microscopic investigation, all samples were collected at 28 days of age to assess and forecast the weight variation, chemical composition, phase compositions, and morphology of both GPCC and GPCSC concretes. The GPCC and GPCSC samples were subjected to X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), SEM, and thermogravimetry in conjunction with energy-dispersive spectroscopy.

2.6 Slump test

In this study, the slump test was conducted following ASTM C143 [44] and IS1199:1959 standards [45]. Figure 8 demonstrates the procedure for measuring slump value, which serves as an indicator of the workability of fresh concrete. It is important to note that the slump test was carried out on two types of concrete: one containing crushed granite as aggregate and another using CS as coarse aggregate.

Figure 8

Slump test.

2.7 Experimental programme

In this study, several concrete properties were tested: workability, compressive strength, splitting tensile strength, flexural strength, impact resistance, elastic modulus, stress–strain behaviour, bond strength, and ultrasonic pulse velocity. The standards followed, and the respective calculations for each test are discussed in this section.

The workability of the concrete was measured using a conventional empirical slump test as per IS 1199:1959 and ASTM C14. For compressive strength of concrete, cube specimens 100 × 100 × 100 mm in size were used and tested as per IS 516:1959 [46]. As per ASTM C496-90 [47], 100 mm diameter by 200 mm long cylindrical specimens were used for the splitting tensile strength test. The splitting tensile strength fst of specimens was calculated as follows:

where P is the maximum applied load in Newtons, D is the diameter of the cylinder in mm, and L is the length of the cylinder in mm.

The two-point load method was adopted to measure the flexural strength of the specimen as per ASTM C78-84 [48]. Prisms 100 × 100 × 500 mm in size were used to find the flexural strength and tested as per IS 516:1959 [46]. The modulus of rupture ft of concrete specimens was calculated using the following equation:

where P is the maximum applied load in Newton, L is the supported length of the prism in mm, B is the breadth of the prism in mm, and D is the depth of the prism in mm.

As per ACI 544.1R-96 [49], specimens 152.40 mm thick and 63.5 mm in diameter were used for impact resistance tests. During the test, the number of blows until the first crack appeared on each specimen was counted, and the counting continued until the specimen was completely broken into pieces.

As per ASTM C469-02 [50], cylindrical specimens 100 mm in diameter and 200 mm in height were used for the determination of elastic modulus. This method involves applying compressive load longitudinally to the specimen to calculate the elastic modulus.

Bond strength was determined using the pull-out test, and the specimens were prepared as per ASTM C 234-91 [51]. Cylindrical specimens 100 mm in diameter and 200 mm in length embedded with deformed bars 10, 12, and 16 mm in diameter were used to determine the bond strength. To prepare the specimens for the bond test, each reinforcing bar was embedded in the center of the specimen. The pull-out test was carried out using the universal testing machine with 4,000 kN capacity. One end of the rod was fitted with grips provided in the machine that are vertically movable, and the load was applied by pulling the rod upward from the specimen until the specimen failed. The experimental bond strength fb of the concrete specimens was calculated using the following equation:

where P is the maximum applied load in Newton, D is the diameter of the reinforced bar in mm, and L is the embedded length of the reinforced bar in mm.

Ultrasonic pulse velocity test was used to predict the quality of concrete specimens for density, homogeneity, cracks, and voids. The quality of concrete was analysed as recommended by IS 13311-Part I:1992 [52].

3.1 Hydrated products

The chemical makeup of the precursor in the mixes determines how hydrated products are formed. Calcium-silicate-hydrate (C–S–H) and calcium-aluminium-silicate-hydrate (C–A–S–H) form as a result of the hydration of a precursor such as GGBFS [40], and these forms are aided by the use of an alkaline activator in the mixes, leading to excellent polymerization (Figure 6) [38].

3.2 XRD analysis

The change in crystallinity of the geopolymer samples at 28 days was examined using XRD. The major components of the initial raw material, XRD patterns, reveal the presence of minerals including calcite, quartz, and mullite, along with the products of hydration. Figure 9 displays the XRD pattern of the GGBS utilised in this study, which reveals an amorphous phase characterised by a broad and diffuse reflection spanning from 25 to 35°. A number of secondary crystalline phases, including Ca₂Al₂SiO₇ (JCPDS 00-034-1236), were also present in the GGBS spectra [38].

Figure 9

XRD – GGBFS.

Figures 10 and 11 show, correspondingly, the XRD patterns of GPCC and GPCSC mixes. In the GPCC and GPCSC, mixtures of alkaline activated elements exhibit the creation of products of reaction. Figures 10 and 11 provide patterns of XRD that illustrate the presence of various reaction products like aluminosilicate and C–S–H gel. It was observed that these products were identical for both GPCC and GPCSC. However, no fresh stages were found. The formation of C–S–H was plainly seen in both mixtures [38].

Figure 10

XRD – GPCC.

Figure 11

XRD – GPCSC.

3.3 FTIR spectra analysis

Both mixtures clearly contained C–S–H [28]. The FTIR spectra of GGBS are shown in Figure 12 with two distinct, highly strong bands of internal vibrations in TO₄ tetrahedra (T = Al, Si). Water deformation waves were thought to be the cause of the intensity at 1,635 cm⁻¹ (peak 1) (H–O–H). Peak 2 was located at 966 cm⁻¹, and it was connected to waves of the T–O bond bending and the glass structures of the base material (Si–O). The low wave number indicates that the high calcium concentration in the structure of the binding materials caused less cross-linking to occur in the amorphous phase of those materials. Bending waves were created by the Al–O bonds in the AlO₄ groups, resulting in the third peak of the anhydrous GGBS, which was visible as a band at 688 cm⁻¹ (peak 3). The internal deformation vibrations of the T–O bond (O–Si–O) caused the fourth one, which corresponded to peak 4 at 488 cm⁻¹ for the slag [38].

Figure 12

FTIR GGBFS – Binder.

The FTIR spectra of the GPCC and GPCSC mixtures are shown in Figures 13 and 14, respectively. The primary absorption lines in the alkali-activated slag’s FTIR spectrum were 1,600, 1,413, 897, and 460 cm⁻¹. At 1,600 cm⁻¹, distinct water molecule O–H group modes were found in (peak 1) of both mixtures. The development at (peaks 2 and 4) bands is 1,413 and 460 cm⁻¹, which shows the presence of calcium carbonate. In the activated slag (897 cm⁻¹), the initial base materials’ T–O band at 966 cm⁻¹ sharpened and descended to lower frequencies (peak 3). These peaks show how the GGBS glassy component and the activator of alkali interacted to form a range of new products, principally C–S–H gel and C–A–S–H gel [38].

Figure 13

GPCC – FTIR.

Figure 14

GPCSC – FTIR.

3.4 SEM analysis

The GGBFS, M-sand, CS, and CSA SEM with EDX analysis are displayed in Figures 15–18. The microstructure of GGBFS, M-sand, CS, and CSA is shown in Figures 15–18 and demonstrates the content of silicate and aluminate. These sources of aluminosilicate are activated by silicate solution and NaOH; the quantities of alumina and silica are essential elements of the process of polymerization that results in the formation of strength.

Figure 15

GGBFS – SEM with EDX.

Figure 16

M.Sand – SEM with EDX.

Figure 17

CS – SEM with EDX.

Figure 18

CSA – SEM with EDX.

Figures 19 and 20 illustrate, respectively, the SEM images of the GPCC and GPCSC that incorporate EDX data. They unequivocally show the content of aluminate and silicate, that is desired for the geopolymerization-induced synthesis of C–S–H and C–A–S–H gel [38].

Figure 19

SEM with EDX (GPCC).

Figure 20

SEM with EDX (GPCSC).

The reactivity of GGBS in alkali-activated solution resulted in the formation of amorphous aluminosilicate gels. The GPCC mix has a more uniform and homogeneous matrix as a consequence of these gels, as can be seen in the SEM image of GPCC in Figure 21.

Figure 21

GPCC – SEM.

More pores and reaction gels in the form of whitish granules can be found in the interfacial transmission zone (Figure 22). This is implied by the fact that a large proportion of CS aggregate might cause more water and alkali solution to collect around the aggregate. GPCC has considerably higher strength than equivalent GPCSC because it contains less porosity with more gel content than GPCSC. GPCC has a dense matrix with a remarkably consistent structure and almost no pores. This uniformity might lead to mechanical reliability. However, owing to the texture of the CS, results in a porous structure with void in matrix’s continuity, as shown in Figure 22.

Figure 22

GPCSC – SEM.

Overall, GGBFS as a binder with alkaline activator significantly changes the total properties of concrete in both GPCC and GPCSC. The density and homogeneity of the microstructure have typically improved while the void volume has decreased.

3.5 TGA of GPCC and GPCSC

Figure 23 displays TGA of the GPCC and GPCSC mixes for the materials collected after cornuate 550°C; only 2% of weight loss was seen, according to the GPCC thermogram. In contrast, more weight loss was seen with the GPCSC blend than with the mixes that included crushed granite. At 550°C, an 8% weight drop in the GPCSC mix was noted.

Figure 23

TGA of GPCC and GPCSC.

3.6 Properties of concrete mixes

Table 4 shows the properties of concrete tested after 3, 7, and 28 days of GPCC and GPCSC. Table 5 shows the comparison of mix parameters, workability, and strength with previous studies.

3.6.4 Failure mode

The cube’s shape has changed once it reaches its point of failure in the compression testing equipment. Whether it is satisfactory or not is shown by the failure shape. Figure 24a depicts multiple cube failure types that are acceptable according to BS EN 12390-3:2009 [55]. Figure 24b depicts the tested cubes. According to code [56], the test results for the GPCC and GPCSC demonstrate a satisfactory failure mode.

Figure 24

Failure mode of samples tested (GPCSC and GPCC): (a) failure mode as per code [55,56] and (b) failure of GPCSC and GPCC specimens before and after testing.

Table 5 compares the current study to previous research. Despite variations in the ingredients and mix quantities, the slump test is the most often used workability test. The current study’s findings are consistent with those reported in the literature. The proportion liquid binder ratio used to make the mix is the key difference between this investigation and the prior study. As shown in Table 5, the binder concentration is substantial; however, the activator employed in this study was lower than in previous investigations. The larger binder concentration was primarily employed to generate early strength, which would be advantageous since it would facilitate the construction process for structure, rapid construction, and formwork reuse.

3.7 Mechanical properties of GPCC and GPCSC

Tables 6–10 gives the mechanical properties of GPCC and GPCSC tested for 28 days. Studying the specimens’ mechanical characteristics included measuring their impact resistance, flexural strength, elastic modulus STS, and ultrasonic pulse velocity.

3.7.1 STS of GPCC and GPCSC

The tensile strengths at which GPCC and GPCSC separate are presented in Table 6. The experimental 28-day splitting tensile strength of GPCC and GPCSC was 3.13 and 3.03 MPa, respectively. The tensile strength of the GPCC mixes was approximately 3% higher than that of the GPCSC, which could be because of texture of CS aggregate in the interfacial transition zone, which has a substantial impact on the samples. ASTM C330 requires structural-grade lightweight aggregate concrete to have a minimum splitting tensile strength of 2.0 MPa. Figure 25 shows a comparison of the test findings with those of other researchers. The splitting tensile strength obtained by a standard [42] is found to be closer to the experimental results of the previous investigation. However, Jenima et al. [12] demonstrate a larger STS range, owing mostly to the % substitution of CS for a lower compressive strength of 26.70 MPa. Figure 26 depicts the test setup and failure mode of the sample tested for split strength.

Figure 25

STS and compressive strength relationship with earlier studies.

Figure 26

STS – sample specimen – GPCSC mix (M11 – Table 3).

3.7.2 Flexural strength of GPCC and GPCSC

The experimental flexural strength at 28 days for GPCC and GPCSC was calculated to be between 3.67 and 4.12 MPa, as shown in Table 7. Tensile strength obtained by the GPCSC mixtures was 11% higher than that of GPCC. It might be because of the interaction between the fibre CS, alkaline activator, and binder. Figure 27 illustrates the experimental configuration and a failure pattern sample utilised in the flexure strength test. The failure pattern during the flexural strength test clearly demonstrates bond failure between the binder and CS. Similar results for concrete made from oil palm shells were reported in [42]. But for Gunasekaran et al. [24], the ratio of flexural strength to compressive strength was 17%, and this is mainly due to the weaker interfacial zone in the mix. The difference in flexural strength between the current study and earlier literature for GPCC and NWGC (Figure 26) is generally attributable to the quantity of binder and alkaline activator utilised in the mix.

Figure 27

Flexural testing – sample specimen – GPCSC mix (M11 – Table 3).

3.8 Bond strength of GPCC and GPCSC

Table 10 lists the bond strength of deformed bars with diameters of 10, 12, and 16 mm. For GPCC and GPCSC, the bond strength ranged from 5.31 to 7.10 MPa and 6.09 to 8.25 MPa, respectively. Comparing all bars’ values to the guidelines bond strength advised by BS 8110 (2.42 MPa) [57] and IS456 (2.24 MPa) [58], the GPCSC and GPCC values were all greater. All of the examples were destroyed by longitudinal cracks and cracking of the concrete covers. Furthermore, as bar size increases, there is a decrease in bond strength, owing to the volume of the concrete surrounded and the lesser confining pressure created by the larger bar. Similar findings were reported in [26].