New insights into the production of sustainable synthetic aggregates and their microstructural evaluation

2.1. Materials

Ordinary Portland cement (OPC) of grade 53 was purchased from Zuari Cement located in Vellore, India and used as a binder following ASTM C150 (2525. ASTM C150. (2001) Standard specification for Portland cement. Annual Book of ASTM Standards.
). According to Table 1, the specific gravity and specific surface area of cement are 3.15 g/cc and 3194 cm²/g, respectively. Ground granulated blast furnace slag (GGBFS) was obtained from the steel production industry (Salem, India) as per ASTM C989 (2626. ASTM C989. (2005) Standard specification for ground granulated blast-furnace slag for use in concrete and mortars. ASTM Int. 2-6.
). The quantity of oxides and some other chemical components present in the binders are shown in Table 2. Metakaolin (MK) was purchased from Jeetmull Jaichandlall Madras Pvt. Ltd., Chennai. India., which class N was as per ASTM C618 (2727. ASTM C 618. (2014) Standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete, ASTM Int. 1-5. https://doi.org/10.1520/C0618.
). Hydrated lime powder (LP) is a dry powder manufactured from limestone. This lime stone calcined under high temperature. Then the calcined lime stone is slaked with water. To convert oxides into hydroxides, quicklime (CaO) is hydrated with water. This hydrated lime is chemically known as calcium hydroxide (Ca (OH)₂). In this study, hydrated lime powder with a particle size of 3.39 µm (as depicted in Table 1) was used as filler material to fill the pores present in the synthetic aggregates. The incorporation of lime into mortars enhances workability and reduces porosity.

A quaternary blended binder system such as cement, GGBFS, metakaolin, and lime powder is utilized to manufacture synthetic aggregates. Figure 1 depicts the distribution of particles size of the binders used in this study. Riverbed sand collected from Vellore with a specific gravity of 2.66 and fineness modulus of 2.72 was utilized as fine aggregates. These fine aggregates had a loose bulk density of 1468.86 kg/m³. The water absorption was determined to be very less as 1.04%. A high-range water reducer (Polycarboxylic ether based) named Master Glenium SKY 8108 was used in this study to minimize the water required and to improve the workability significantly. As indicated in Figure 2, this investigation uses polypropylene fibers as fiber reinforcement. The fibers have an L/D aspect ratio of 300 and a tensile strength of 450 MPa in the form of continuous of 12 mm length monofilaments with a 40μm diameter of circular cross-section. Large peaks of calcium carbonate (calcite) are visible in the diffractograms for powdered limestone. Quartz peaks are observed in metakaolin diffractograms. Additionally, several peaks of kaolinite are identified. Alite and Belite were identified as the predominant phases of ordinary Portland cement, as shown in Figure 3.

2.2. Experimental design - Taguchi’s approach

The experimental investigation conducted by Xavier et al. (2828. Belarmin Xavier, C.S.; Abdul Rahim, A. (2023) Optimization and characterization of the ternary blended iron rich natural binder concrete system. Constr. Build. Mater. 363, 129838. https://doi.org/10.1016/j.conbuildmat.2022.129838.
) on the ternary blended concrete system, which is rich in iron, was developed using the Taguchi method. Using the Taguchi approach, Guo et al. (2929. Guo, L.; Wang, W.; Zhong, L.; Guo, L.; Wang, L.; Wang, M.; Guo, Y.; Chen, P. (2022) Influence of composite admixtures on the freezing resistance and pore structure characteristics of cemented sand and gravel. Mater. Construcc. 72 [347], e290 https://doi.org/10.3989/MC.2022.16021.
) developed experiments to examine cemented sand and gravel’s freezing resistance and pore structural properties. Rahim et al. (3030. Tan, Ö.; Zaimoglu, A.S. (2007) Optimization of compressive strength in admixture-reinforced cement-based grouts. Mater. Construcc. 57 [288], 91-98. https://doi.org/10.3989/mc.2007.v57.i288.67.
) employed the Taguchi technique to determine the best mix proportion of high strength concrete subjected to elevated temperatures to obtain the highest residual compressive strength. Tan et al. (3131. ASTM C305. (2011) Standard practice for mechanical mixing of hydraulic cement pastes and mortars of plastic consistency. ASTM Int. 1-3.
) used the Taguchi approach to optimize the unconfined compressive strength of admixture-reinforced grouts using fly ash, bentonite, and silica fume as binders in their study.

Design of experiments was employed to progress an experimental method for determining the optimum mix proportion of binders to produce synthetic aggregates. Specifically, the Taguchi method was preferred to optimize the binder content and water to binder ratio to produce durable synthetic aggregates. As shown in Table 3, the selection of parameters and their appropriate levels was decided from prior studies. In this research, the four design factors: Ordinary Portland cement content (%), GGBFS content (%), metakaolin content (%), and w/b ratio, were considered with three levels, which makes a maximum of nine trials. By Taguchi’s experimental design, the L9 (3⁴) orthogonal array (as depicted in Table 4) was employed for this experimental investigation.

2.3. Synthetic aggregates production

Synthetic aggregates were produced using quaternary binders with optimized proportions, as shown in Figure 4. According to ASTM C305-06 (3232. ASTM C330.(2009) Standard specification for lightweight aggregates for structural concrete. Annual book of ASTM Standards.
) a mortar mixer consisting of a planetary motion mixing paddle with a microprocessor-based speed and program controller was used in the manufacturing of synthetic aggregates. The quaternary blended mixture of 50% ordinary Portland cement, 40% ground granulated blast furnace slag, 5% Metakaolin, and 5% lime powder was mixed for two minutes at a lower rotational speed of 140 rpm in a mortar mixer. The fine aggregate is combined to the binder mixture and blended well approximately 2 Min. Then, the dry blend was wetted with about 80 % of the required water combined with a high-range water reducer and allowed to mix for 2-3 min. A linear type of synthetic polypropylene fiber (PPF) was dispersed in the mixture at 0.3% of the total binder content. Then the remaining 20% of superplasticizer mixed water was poured to the mixture, which was then agitated for an additional 1 Min. The sides of the mixing bowl were scraped and retained for 90 seconds. After a small resting, the mixture was mixed for 30 seconds at a higher rotational speed of 285 rpm. When a homogeneous mix has been formed, the prepared mortar mixture was poured into various shaped silicone mould and left to dry for 24 hours. As illustrated in Figure 6, four distinct shapes and sizes of synthetic aggregates were manufactured, including 10 mm cubical-shaped aggregates, 12 mm pyramid-shaped aggregates, 14 mm elliptical-shaped aggregates, and 16 mm frustum-shaped aggregates. Synthetic aggregates were taken from their moulds and cured at room temperature (28 °C) until their various properties were evaluated. Finally, hardened aggregates was sieved into appropriate sieve fractions in accordance with ASTM C330 grading specifications (3333. ASTM C127-15. (2015) Standard test method for relative density (specific gravity) and absorption of coarse aggregate. Annual Book of ASTM Standards.
). Figure 5 depicts the production method of synthetic aggregates. As indicated in Table 5, except for the control mix, all the mixtures were prepared with the incorporation of polypropylene fibers. The control mix is the gauge mix to compare with the optimized mix. We have produced two compositional aggregates. One is Synthetic aggregates (SA) made with an optimized mortar mix (OM). Another one is Fiber Intruded Synthetic aggregates (FISA), which are also made up of optimized mortar mix along with the incorporation of polypropylene fibers. We have compared the results of produced SA and FISA with Natural coarse aggregates (NA) obtained naturally from rocks in section 3 and 4.

2.4. Testing

Physical properties such as bulk density, water absorption and specific gravity, mechanical properties such as crushing value, impact value, abrasion value and soundness value, and durability properties such as soundness value of produced synthetic aggregates were evaluated as described in the subsequent sub-sections.

2.5. Physical properties of synthetic aggregates

The loose and bulk density of the aggregates was estimated by a cylindrical mould of a distinct volume using the below given Equations [1] and [2], respectively. As per codal provision ASTM C127 (3434. BS 812-110. (1990) Testing aggregates. BS 812 Part 110.
), the results were taken for a water absorption test after 24 hours of soaking is inappropriate for porous nature aggregates. Using Equation [3], the amount of water absorbed by aggregates has been computed. For the mass measurements, dry oven samples, surface saturated dry samples, and fully saturated water samples were employed to obtain the oven-dry specific gravity of synthetic aggregates using Equation [4] as per ASTM C 127 (3434. BS 812-110. (1990) Testing aggregates. BS 812 Part 110.
).

Where: D_l = loose bulk density (kg/m³); D_c= compacted bulk density (kg/m³); w₁ = weight of loose aggregates (kg); w₂ = weight of compacted aggregates (kg); v= cylinder mould volume (m³).

Where: W = water absorption (%); w_s = dry weight of the surface saturated aggregate (g); w_o = oven dry weight of the aggregate (g).

Where: Gsb = Bulk specific gravity; w_o = the oven dry weight of the aggregate (g); w_s = dry weight of the surface saturated aggregate (g): w _w = weight of aggregate in water (g).

2.6. Mechanical properties of synthetic aggregates

Generally, aggregate crushing value test (ACV) was conducted to determine the capability of coarse aggregates to withstand the force under applied loads. According to BS 812-110 (3535. IS 5640 (1970) Method of test for determining aggregate impact value of soft coarse aggregates.
). The crushing value was represented in a numerical index by using Equation [5]:

Where: M₂ = mass of test sample passed through 2.36 mm sieve after crushing testing (g); M₁ = mass of test sample taken initially (g).

The aggregate impact value test (AIV) measured the synthetic aggregate resistance to rapid impact loads. This impact value test was done per the IS 5640-1970 (3636. BS 812-112. (1990) Testing aggregates. BS 812 Part 112.
) and BS 812-112 (3737. IS 2386. (2016) Methods of test for aggregates for concrete, Part IV : mechanical properties. Bur. Indian Stand.
) test procedures. The Equation [6] is used to determine the aggregate impact value as given below:

Where: M₂ = mass of test sample passed through 2.36 mm sieve after impact testing (g); M₁= mass of test sample taken initially (g).

According to IS: 2386 -1963 (Part IV) (3838. IS:2386. (1963) Methods of test for aggregates for concrete. Bur. Indian Stand. 5, 1-14.
), the aggregate abrasion test value (AAV) test has been conducted to determine the abrasion resistance of synthetic aggregates. The aggregate abrasion value was determined by following Equation [7]:

Where: M₂ = mass of test sample passed through 1.7 mm sieve after abrasion testing (g); M₁ = mass of test sample taken initially (g).

2.7. Durability properties of synthetic aggregates

The aggregate soundness value (ASV) test was performed to determine aggregates’ durability and evaluate the resistance to disintegration under extreme weather conditions. For this test, synthetic aggregates shown in Figure 7 were submerged in a 15 mm depth of sodium sulphate solution for 18 hours. Then the samples were taken from the sulphate solution and allowed to fully drain for 15 minutes before being placed in the drying oven. Afterwards, the oven was switched to initiate a drying process to attain uniform mass. After removing the aggregates from the oven, they were allowed to cool at room temperature. The dried coarser material was sieved with an 8 mm sieve to determine the mass loss. This percentage of mass loss is termed as soundness value, and it was computed by using Equation [8] This procedure was conducted as per IS 2386 -1963 (Part V) (3939. ASTM C109/C109M-02. (2020) Standard test method for compressive strength of hydraulic cement mortars. ASTM Int. 04, 9.
).

Where: M₂ = mass of sample passing through 8 mm test sieve after the soundness test (g); M₁ = mass of test sample taken initially (g).

2.8. Compressive strength testing

Compressive strength testing has been performed to evaluate the hydration effect on quaternary binders. In accordance with ASTM C109 (3636. BS 812-112. (1990) Testing aggregates. BS 812 Part 112.
), a 50 mm cube mould was used to cast mortar cubes. Then the set of designed experimental trial mixtures of the produced mortar samples was tested for compressive strength, as presented in Table 5.

2.9. Micro hardness testing

In this work, the micro-hardness testing was conducted to examine the surface properties of the produced aggregates. The samples of natural aggregates (NA), manufactured synthetic aggregates (SA), and fiber-intruded synthetic aggregates (FISA) were preferred for this test. The test sample was prepared with dimensions of 10 mm x10 mm x 2 mm slices. Initially, these samples were ultrasonically cleaned in acetone. After the sample was fully dried, it was polished to produce a smooth, glossy surface to identify indentation locations easily. The micro-hardness value was obtained by indenting the sample at distance intervals of 10μm over the aggregate surface by applying a force of 10 g for 10 seconds using a Vickers micro-hardness tester as shown in Figure 8.

2.10. Microstructural analysis

Utilizing X-ray fluorescence spectroscopy (XRF), the chemical composition of the raw binders was analyzed as shown in Table 2. Bruker model S8 Tiger X-ray spectrometer was used to assess binder mineral phases and oxide percentages. This spectrometer has SPECTRA plus software for qualitative and quantitative element identification. The powder X-ray diffraction technique was executed using a Bruker D8 advanced model powder X-ray diffractometer to detect the phase change of raw materials and produced synthetic aggregates.

Scanning Electron Microscopy (SEM) along with Energy Dispersive X-ray spectroscopy (EDX) has been performed to establish the morphological properties of produced synthetic aggregates. ZEISS EVO 18 model scanning electron microscope from Carl Zeiss Microscopy is employed to examine the microstructure of produced synthetic aggregates. FTIR spectroscopy was used to detect the incremental stages generated through the hydration effect of cement and mineral admixtures. Using a Shimadzu make IRAffinity-1 model FTIR spectrophotometer, the peaks of hydrated aggregate samples are observed. It has a wavelength limit of 4000-400 cm^-1. After 28 days of hydration, a spectral comparison was conducted between the control and optimized mixture. This comparison enables the determination of the absorption spectrum and the identification of unknown compounds in various mixtures

TA instruments USA’s SDT Q600 thermo-gravimetric analyzer was employed to determine the quantitative mass loss of produced synthetic aggregates. This technique establishes the relationship between bound water and the degree of hydration at an earlier stage. A powdered aggregate sample was soaked for 48 hours in ethanol to prevent further hydration. The samples were placed in an oven with hot air to prevent the produced aggregate specimens from being hydrated. TGA was conducted using the temperature ramp of 20ºC till 800ºC at an amount of 20ºC/min in an inert N₂ gas flow chamber.

Atomic Force Microscopy (AFM) is a faster scan probe microscopic technique used to visualize the topography of a sample surface. Topographic images are acquired with the help of a probe with a sharp point at the end of a cantilever. The cantilever tip moves across a specimen’s surface. As the tip contacts the specimen’s surface, its deflection is measured, and the surface’s topography is recorded. 2D and 3D scans of AFM images are recorded.

3.1. Compressive strength

Trial experiments were performed to identify the control factors and their levels in the Taguchi technique. According to ASTM C109 (4040. Vignesh, R.; Abdul Rahim, A. (2022) Mechanical and microstructural properties of quaternary binder system containing OPC-GGBS-Metakaolin-Lime. Mater. Today Proc. 64 [2], 970-975. https://doi.org/10.1016/j.matpr.2022.05.074.
), mortar cube compressive strength was measured from nine trail mixes and an optimal mix designed using Taguchi’s experimental design. From the control factors listed in Table 3, the main effects of S/N ratios were determined based on the “Larger is Better” condition (as depicted in Figure 4) bestowed by Taguchi, where the compressive strength is higher for the optimized mix. The test results of compressive strength of the control mixture gained after 7 and 28 days of curing are 30 and 34 MPa, respectively. The results suggested that the compressive strength of the mortar made for aggregates improved with the length of its curing age. The increased GGBFS content enhanced the strength of mortar at 28 days rather than 7 days. This strength represented that the GGBFS, at an early age, showed the lowest strength and enhanced the strength of later-aged mortar. The used binders including GGBFS, Metakaolin, and lime powder chemically reacted with one another and produced C-S-H gel, which enhances the compressive strength after 28 days of curing (4141. BS EN 13055-1(2002) Lightweight aggregates - Part 1: Lightweight aggregates for concrete, mortar and grout. european committee for standardization.
). According to Figure 4, the optimal values for the parameters were Ordinary Portland cement (50%), Ground granulated blast furnace slag (40%), Metakaolin (5%), and w/b ratio (0.31), resulting in the optimized mixture having a highest compressive strength of 36 MPa at a curing period of 28 days. The control mix comprised mortar made only with OPC as a binder and, according to the Taguchi orthogonal array experimental design (L₉ (3⁴)), comprised nine initial experimental mixes. This is revealed in Table 5, and they are labeled as TM1 to TM9. According to the mix proportions calculated in Table 5, the mortar cube samples (50mm x 50 mm x 50 mm) were cast. Six mortar cubes were cast for each trail mix (TM1 to TM9) including the control mix and kept in the curing for 28 days. Totally, 60 mortar cubes were cast for nine trail mixtures and control mix. According to ASTM C109 (4040. Vignesh, R.; Abdul Rahim, A. (2022) Mechanical and microstructural properties of quaternary binder system containing OPC-GGBS-Metakaolin-Lime. Mater. Today Proc. 64 [2], 970-975. https://doi.org/10.1016/j.matpr.2022.05.074.
), three mortar cube samples were tested for compressive strength testing at 7 days, and the remaining three mortar cube samples were kept and then tested at the age of 28 days. The average measured values of tested mortar cube samples were noted. This has been plotted in Figure 9. Taguchi optimization analysis was carried out on the compressive strength of 9-nine trail mixes. After that, the “Larger is better” condition was used to obtain maximum compressive strength, yielding one ideal mixture distinct from the above nine trail mixtures. This mix is termed an optimized mix (OM), as shown in Figure 4. After that, six mortar cubes were cast again to confirm the optimized mix and tested compressive strength for 7 and 28 days. The obtained the maximum compressive strength of 36 MPa at 28 days, higher than the control and trail mixtures, as shown in Figure 9.

The mortar mixture optimized from cubes was used to manufacture synthetic aggregates in four different shapes and sizes, as shown in Figure 6, and cured for 28 days. As depicted in Figure 7 (a), the physical, mechanical, and durability properties of manufactured synthetic aggregates of well-graded (equivalent combination of four various shapes and sizes) were tested. The mean values of test results were recorded at the age of 28 days.

3.2. Bulk density

Generally, the bulk density is used to estimate the unit weight of concrete as well as to access the dead weight of structural concrete. After 7 days of curing the aggregate samples, the densities of both natural and synthetic aggregates were determined as shown in Figure 10a. The loose bulk density of synthetic aggregates (SA) and fiber intruded synthetic aggregates (FISA) fall between 1100 and 1250 kg/m³. While the density of FISA was measured at 1146 kg/m³, it met the requirement that such density of lightweight aggregate be below 1200 kg/m³ according to BS EN 13055-1 (4242. Cioffi, R.; Colangelo, F.; Montagnaro, F.; Santoro, L. (2011) Manufacture of artificial aggregate using MSWI bottom ash. Waste Manag. 31 [2], 281-288. https://doi.org/10.1016/j.wasman.2010.05.020.
). Table 6 shows that FISA is lighter than synthetic aggregates (SA). This lightweight is due to the inclusion of fibers in FISA, and its aggregate density was lower than that of SA. The degree of compaction and the presence of voids are determined by aggregates different size and shapes, which considerably influence the rodded bulk density of aggregates. Artificial aggregates produced prior to this investigation have densities between 1170 and 1330 kg/m³ (43-4543. Gesoğlu, M.; Güneyisi, E.; Öz, H.Ö. (2012) Properties of lightweight aggregates produced with cold-bonding pelletization of fly ash and ground granulated blast furnace slag. Mater. Struct. Constr. 45, 1535-1546. https://doi.org/10.1617/s11527-012-9855-9.
44. Colangelo, F.; Messina, F.; Cioffi, R. (2015) Recycling of MSWI fly ash by means of cementitious double step cold bonding pelletization: Technological assessment for the production of lightweight artificial aggregates. J. Hazard. Mater. 299, 181-191. https://doi.org/10.1016/j.jhazmat.2015.06.018.
45. Bate, S.C.C. (1979) Guide for structural lightweight aggregate concrete: report of ACI committee 213. Int. J. Cem. Compos. Light. Concr. 1 [1], 5-6. https://doi.org/10.1016/0262-5075(79)90004-6.
). The addition of cement, which has higher specific gravity than GGBFS, is responsible for the increased density of produced synthetic aggregates (4444. Colangelo, F.; Messina, F.; Cioffi, R. (2015) Recycling of MSWI fly ash by means of cementitious double step cold bonding pelletization: Technological assessment for the production of lightweight artificial aggregates. J. Hazard. Mater. 299, 181-191. https://doi.org/10.1016/j.jhazmat.2015.06.018.
). Also, a comparison between synthetic aggregates (SA) and natural aggregates (NA) revealed that SA (1217 kg/m³) had much lower densities than NA (1478 kg/m³). The aggregates with the highest and lowest densities were produced by synthetic aggregates (SA), and fiber intruded synthetic aggregates (FISA), which were 17.67% and 22.46% lighter than natural aggregates (NA), respectively.

3.3. Water absorption

After three days of immersion in water, natural and synthetic aggregates were subjected to a water absorption test. Similarly, a water absorption test after the completion of 7 days and 28 days of curing period is reported in Table 6. The water absorption capabilities of produced SA were determined to be within the usual limit of less than 25% after 7 days of curing specified by ACI 213R (4646. Gunning, P.J.; Hills, C.D.; Carey, P.J.(2009) Production of lightweight aggregate from industrial waste and carbon dioxide. Waste Manag. 29 [10], 2722-2728. https://doi.org/10.1016/j.wasman.2009.05.021.
). The absorption test results of 28 days revealed that the decrease in water absorption of SA and FISA with the continuous hydration of specimens resulted in the formation of a sufficient amount of hydration products. The XRD analysis further confirmed these hydration products. Hydrated cement products made a further pozzolanic interaction between GGBFS and metakaolin, developing more amount of calcium-silicate-hydrate (C-S-H) products, which leads to a denser microstructure (4444. Colangelo, F.; Messina, F.; Cioffi, R. (2015) Recycling of MSWI fly ash by means of cementitious double step cold bonding pelletization: Technological assessment for the production of lightweight artificial aggregates. J. Hazard. Mater. 299, 181-191. https://doi.org/10.1016/j.jhazmat.2015.06.018.
). The produced synthetic aggregates can also be compared to the naturally occurring aggregates, which have a very lower water absorption rate of 0.75%. However, the water absorption of manufactured SA and FISA was higher than that of commercially available natural aggregates after 7 days of curing, according to the current investigation. As depicted in Figure 8b, water absorption values decreased significantly as curing period was extended from 7 to 28 days. Numerically, water absorption at 28 days of curing was decreased by 25.88% and 42.69% compared to 7 days of curing for SA and FISA samples, respectively. SA aggregates are more porous than FISA aggregates. In prior investigations, paper ash (4747. Shi, M.; Ling, T.C.; Gan, B.; Guo, M.Z. (2019) Turning concrete waste powder into carbonated artificial aggregates. Constr. Build. Mater. 199, 178-184. https://doi.org/10.1016/j.conbuildmat.2018.12.021.
), waste concrete powder (4848. Venkata Suresh, G.; Karthikeyan, J. (2016) Influence of chemical curing technique on the properties of fly ash aggregates prepared without conventional binders. J. Struct. Eng. 43, 381-389.
), fly ash (4949. Gesoĝlu, M.; Güneyisi, E.; Mahmood, S.F.; öz, H. öznur, Mermerdaş, K. (2012) Recycling ground granulated blast furnace slag as cold bonded artificial aggregate partially used in self-compacting concrete. J. Hazard. Mater. 235-236, 352-358. https://doi.org/10.1016/j.jhazmat.2012.08.013.
), slag (5050. Tang, P.; Xuan, D.; Cheng, H.W.; Poon, C.S.; Tsang, D.C.W. (2020) Use of CO₂ curing to enhance the properties of cold bonded lightweight aggregates (CBLAs) produced with concrete slurry waste (CSW) and fine incineration bottom ash (IBA). J. Hazard. Mater. 381, 120951. https://doi.org/10.1016/j.jhazmat.2019.120951.
), concrete slurry and fine incineration bottom ash (5151. Güneyisi, E.; Gesoglu, M.; Ghanim, H.; Ipek, S.; Taha, I. (2016) Influence of the artificial lightweight aggregate on fresh properties and compressive strength of the self-compacting mortars. Constr. Build. Mater. 116, 151-158. https://doi.org/10.1016/j.conbuildmat.2016.04.140.
), and other raw materials were employed to produce artificial aggregates. However, it was noted that several aggregates were deficient in showing the required strength and had a higher water absorption rate. While the aggregates developed in this research, namely SA and FISA, exhibited comparatively high strength and low absorption of water (6-9%) that are useful for real-time applications. However, it has been observed that the performance of fiber intruded synthetic aggregate can be greatly enhanced by utilizing polypropylene fibers during production, achieving the lowest water absorption of 3.88 %.

3.4. Specific gravity

The aggregate specific gravity is the proportion of the aggregate weight to the weight of water for the same volume. The specific gravity of the manufactured synthetic aggregate (SA) and the fiber-intruded synthetic aggregate (FISA) is 2.56 and 2.52, as shown in Table 6. It was found to be nearer to the specific gravity of natural aggregates. Hence, the produced synthetic aggregates belong to weight of natural aggregates. Due to the increase in GGBFS content, the specific gravity of produced synthetic aggregates increased. Spherical fly ash particles adhere to one another during the pelletization process and retain a certain number of voids, leading to the lower specific gravity of the pellets. The low specific gravity and increased water absorption form porous aggregates (5151. Güneyisi, E.; Gesoglu, M.; Ghanim, H.; Ipek, S.; Taha, I. (2016) Influence of the artificial lightweight aggregate on fresh properties and compressive strength of the self-compacting mortars. Constr. Build. Mater. 116, 151-158. https://doi.org/10.1016/j.conbuildmat.2016.04.140.
).

3.5. Aggregate crushing value

The aggregate crushing value (ACV) test was performed to evaluate the strength of aggregate. The test of aggregate samples of results obtained for 7 days and 28 days of curing as illustrated in Figure 8c. It was noted that the crushing value results of FISA are similar to the findings of impact value test results. It reveals that adding fibers improved the crushing value of the aggregate. ACV development after 28 days of curing was 13.84% for SA aggregates and 20.37% for FISA aggregates, respectively. It is important to observe that the difference in ACV between aggregates cured for 7 and 28 days was more than the improvement in impact resistance, indicating that aggregates were more resistant to compressive loads than impact loads. FISA aggregates performed better than SA aggregates in terms of strength gain over time. In addition, ACV test results were aligned with density and impact value test results. The strength of manufactured synthetic and natural aggregates was also compared. As illustrated in Figure 11, visual observations were also taken for natural and produced synthetic aggregates after crushing.

3.6. Aggregate impact value

The aggregate impact value (AIV) is a measurement of an aggregate resistance to a sudden impact or shock. It is different from its resistance to a progressively applied compressive force. AIV of both natural and produced SAs was performed, and the setup was shown in Figure 12. The aggregate impact value of natural and synthetic aggregates gained 8.08 % and 18.22 %, respectively. Obviously, the inclusion of lime content increases the impact resistance of SA and FISA aggregates and densifies the microstructure of aggregates. This impact strength was owing to the enhanced production of C-S-H, calcium hydroxide (Ca(OH)₂) and portlandite (CH) as a result of an increased hydration process (1313. Rehman, M.U.; Rashid, K.; Haq, E. U.; Hussain, M.; Shehzad, N. (2020) Physico-mechanical performance and durability of artificial lightweight aggregates synthesized by cementing and geopolymerization. Constr. Build. Mater. 232, 117290. https://doi.org/10.1016/j.conbuildmat.2019.117290.
). Additionally, cement reacts with CaO, present in GGBFS, and activates it for strengthened aggregate at micro level (5252. Gesoǧlu, M.; Özturan, T.; Güneyisi, E. (2007) Effects of fly ash properties on characteristics of cold-bonded fly ash lightweight aggregates. Constr. Build. Mater. 21 [9], 1869-1878. https://doi.org/10.1016/j.conbuildmat.2006.05.038.
). On the other hand, GGBFS and MK react with CH present in cement and stimulate strength development. A comparative graph of water absorption with aggregate impact value is shown in Figure 10b. It can be noted from Figure 10b, that aggregate impact value and water absorption have an inverse relation. As shown in Figure 10b, SA and FISA exhibited a good correlation between aggregate impact value and water absorption values. Thus, aggregates with less amount of water absorption possess greater impact resistance. After 28 days, the AIV of produced SA and FISA aggregates was within the permissible limit (less than 30%) as specified by BS-812-112 (3737. IS 2386. (2016) Methods of test for aggregates for concrete, Part IV : mechanical properties. Bur. Indian Stand.
). Similarly, SA aggregates containing greater GGBFS offered high impact resistance, reduced porosity, and significantly improved strength (5353. Alqahtani, F.K.; Ghataora, G.; Dirar, S.; Khan, M.I., Zafar, I. (2018) Experimental study to investigate the engineering and durability performance of concrete using synthetic aggregates. Constr. Build. Mater. 173, 350-358. https://doi.org/10.1016/j.conbuildmat.2018.04.018.
). The natural aggregates had an aggregate impact value of only 7.89%, much lower than all developed synthetic aggregates.

3.7. Aggregate abrasion value

The percentages of abrasion values at 28 days of curing for natural (NA) and synthetic (SA) aggregates are 22.34% and 26.51%, respectively. As shown in Figure 10d, fiber intruded synthetic aggregates (FISA) have the lowest percentage of abrasion value (21.42%) conducted at 28 days. This indicated that these aggregates have a good abrasion resistance due to the inclusion of polypropylene fibers. The maximum material loss for aggregates intended for the pavement layer is 40%. As a result of these findings, FISA and SA can be used as structural materials for applications of lightweight structural concrete requiring a high level of wear resistance and mechanical performance (5353. Alqahtani, F.K.; Ghataora, G.; Dirar, S.; Khan, M.I., Zafar, I. (2018) Experimental study to investigate the engineering and durability performance of concrete using synthetic aggregates. Constr. Build. Mater. 173, 350-358. https://doi.org/10.1016/j.conbuildmat.2018.04.018.
).

3.8. Vickers micro hardness test

Figure 13 compares the micro hardness of natural aggregate (NA), synthetic aggregate (SA), and fiber-introduced synthetic aggregate (FISA). For all three samples, the micro hardness was observed to be constant up to a surface distance of 40 μm. After that, the micro-hardness value was observed to be increased around the distance of 40-60 μm on the aggregate surface and remained constant. While comparing the micro-hardness value of SA and FISA samples, it was observed that the SA sample had a lower micro-hardness value up to 40 μm. This lowest micro hardness is due to micro-level cracks and pores presented in synthetic aggregates (5454. Mo, K.H.; Ling, T.C.; Cheng, Q. (2021) Examining the influence of recycled concrete aggregate on the hardened properties of self-compacting concrete. Waste Biomass Valoriz. 12, 1133-1141. https://doi.org/10.1007/s12649-020-01045-x.
).

3.9. Aggregate soundness value

The soundness test was conducted to measure an aggregate’s resistance to weathering and freeze-thaw cycles-induced disintegration. The soundness values of natural and synthetic aggregates were 0.996 % and 1.843 %, respectively. Thus the results suggest that, despite their relatively high water absorption, the manufactured synthetic aggregates met ASTM C 330 (3333. ASTM C127-15. (2015) Standard test method for relative density (specific gravity) and absorption of coarse aggregate. Annual Book of ASTM Standards.
) standards for a concrete aggregate without causing any soundness defects.

3.10. Significance of shape parameters for produced aggregates

The size and shape of aggregates immediately impact the binder requirement, workability, concrete strength, durability, and cost of concrete. Usually, natural aggregates with angular shapes were used for concrete production. Generally, rounded, elongated, and flaky aggregates provide superior workability but lower in bond strength and mechanical properties. Hence, we have preferred four distinct shapes cubical (10 mm), pyramid (12 mm), elliptical (14 mm), and frustum (16 mm). Cubical aggregate has flat surfaces in all directions. An aggregate with a pyramid shape comprises clearly defined edges formed at the junction of a nearly flat surface. Elliptical-shaped aggregates have the smallest surface area and require less cement paste for bonding than other shapes. Frustum-shaped aggregates have more surface area and require more cement paste for bonding. Three tests, Aggregate Impact value (AIV), Aggregate Crushing value (ACV), and Aggregate Abrasion value (AAV) were conducted to measure the toughness of natural and synthetic aggregates with an equal combination of four geometric shapes, as explained in 3.4 to 3.7.

4.1. Scanning electron microscopy

Figure 14 illustrates the SEM pictures of the raw binders utilized in this examination. Magnification of 500X was used in SEM analysis. The EDX spectra of aggregate samples were acquired from the entire region of images taken from SEM, as shown in Figure 14. SEM images of natural aggregates are shown in Figure 14 (a). The SEM/EDX examination revealed the distinctive morphological characteristics of the produced synthetic aggregates. The EDX measurements reveal that silica and calcium are more prevalent minerals in the natural aggregate, indicative of sedimentary rock. C-S-H is the primary product of cement hydration and a crucial role in the strength of cement and other products formulated from cement. Due to the usage of mineral admixtures, EDX obtained results demonstrated clearly that the change of components occurred in the SA and FISA. Figure 14 (b) depicts increased C-S-H formation. The Synthetic aggregates had greater pores at the age of 7 days of curing, and the volume of pores was reduced after 28 days of curing. Due to the pozzolanic interaction between slag and metakaolin, cement mortar densifies at later ages. Figure 14 (c) indicates that adding polypropylene fibers decreased the porosity and micro-cracks of fiber intruded synthetic aggregates.

4.2. X-ray diffraction

Using X-ray diffraction (XRD), the compositional changes (increase) of NA, SA, and FISA samples were studied. Figure 15 illustrates the XRD patterns for NA, SA, and FISA. The montmorillonite (Al₂H₂O₁₂Si₄), Anorthite (CaAl₂Si₂O₈), Calcium alumino silicate (Ca₂Al₂SiO₇), Lawsonite (CaAl₂Si₂O₇ (OH)₂·H₂O), Quartz (SiO₂) and Calcite (CaCO₃) peaks were identified for NA, SA, and FISA are shown in Figure 15. Based on the XRD results of SA and FISA samples, it was determined that the enhanced strength of the both samples is due to the development of calcium alumino silicate (5555. Shahane, H.A.; Patel, S. (2022) Influence of design parameters on engineering properties of angular shaped fly ash aggregates. Constr. Build. Mater. 327, 126914. https://doi.org/10.1016/j.conbuildmat.2022.126914.
). In addition, Anorthite peaks indicated that GGBFS reacted with metakaolin and lime powder. Adding lime powder in SA and FISA reduced the intensity of the quartz peak, as shown in Figure 15. The peaks support this result at 26.58º (2θ), where the quartz peak is prominently visible. Thus the SEM/EDX study results validated the strength enhancement in produced aggregates.

4.3. Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared spectroscopy (FTIR) was employed to discover an absorption of infrared spectrum, emission, and photoconductivity of produced synthetic aggregate with diverse functional groups. The Infrared spectra are typically measured between the range of 4000 and 400 cm⁻¹. The FTIR spectra of aggregate samples NA, SA, and FISA were shown in Figure 16. NAs were identified with prominent bands around 2343.51 cm^-1, 1431.18 cm^-1 and 997.20 cm^-1, respectively. In SA, vibrations of 2335.80 cm^-1 specified the occurrence of the C-H stretching group. Bands at 1431.18 cm^-1 (C-O stretching group) indicate the presence of calcite at the site of formation of the principal crystalline phase. The signal at 997.20 cm^-1 shows the presence of vibrations in the C-C skeleton. The bending vibration of -OH between 3250 and 3750 cm^-1 is caused by the combination of structural and free water. It was observed that raw binders exhibit absorption maxima in this region, showing that these binders contain trace amounts of water. In contrast, free water presented in aggregates was absorbed on the surface and confined within the pores of reaction products. Aluminum (Al) availability in metakaolin enhanced the hydration level. Similarly, Al ions replaced Si ions presented in GGBFS to produce Si-O-Al, that restructured to develop efficiently organized alumino silicates (5656. Karthik, A.,; Sudalaimani, K.; Vijayakumar, C.T.; Saravanakumar, S.S. (2019) Effect of bio-additives on physico-chemical properties of fly ash-ground granulated blast furnace slag based self cured geopolymer mortars. J. Hazard. Mater. 361, 56-63. https://doi.org/10.1016/j.jhazmat.2018.08.078.
). The spectrum exhibits a significant broadband signal at 972.12 cm^-1, which results from the asymmetrical Si-O-Al stretching (5757. Samad, S.; Shah, A. (2017) Role of binary cement including Supplementary Cementitious Material (SCM), in production of environmentally sustainable concrete: A critical review. Int. J. Sustain. Built Environ. 6, 663-674. https://doi.org/10.1016/j.ijsbe.2017.07.003.
). Therefore, FTIR test results showed the structural reorganization occurred in aggregate samples (NA, SA and FISA).

4.4. Thermo-gravimetric analysis (TGA)

Thermo-gravimetric (TG) curves provide a very important indication of the temperature stability of the produced aggregate matrix. Figure 17 depicts images of the TGA of NA, SA, and FISA samples, respectively. Initially, an endothermic between 50 and 150ºC was identified, resulting from the evaporation of water present in the synthetic aggregates produced with cementing binders. Consequently, the C-S-H presented in SAs could be estimated using the predicted mass losses derived from the TGA curves among 25ºC as starting temperature and 125ºC as ending temperature. From this observation, the corresponding mass losses of NA, SA, and FISA aggregate samples in this range were determined to be 4.983%, 4.723%, and 4.427%, respectively. This result showed that SA and FISA had a greater C-S-H concentration.

In addition, this occurrence shows that the pozzolanic reaction between GGBFS and MK enhances the strength properties of the manufactured SA and FISA. The results exposed that the strength properties of SA were less than those of FISA. The identical C-S-H contents of these samples are an interesting finding. In addition, TGA data showed that between 400 and 800ºC, carbonates formed as in the temperature phase. Hence, the carbonate contents of SAs was determined using the mass losses obtained from the TGA plotted curves roughly in between 550ºC as starting temperature and 625ºC as ending temperature. The observed mass losses of NA, SA, and FISA samples between 550ºC and 575ºC were 1.846%, 1.548 %, and 0.775%, respectively. According to these findings, the carbonate concentrations of SAs were near one another. In addition, it is established that the complete proportion of mass retained after heating to 700ºC remains consistent in all cases, indicating its thermal stability and the absence of any further thermal degradation.

4.5. Atomic force microscopy

Using atomic force microscopy (AFM), aggregate surface roughness and multiple roughness parameters were determined. Figure 18 depicted 2D and 3D topographical images of produced synthetic aggregates. Darker parts suggested a relatively low topographical surface and in contrast lighter spots showed a higher topographical surface as observed in 3D topographical images as shown in Figure 18. Different roughness indices such as arithmetical mean deviation (S_a), root mean square deviation (S_q), maximum height deviation (S_y), maximum peak height deviation (S_p), maximum valley depth deviation (S_v) of NA, SA and FISA were presented in Table 7. It was observed that several roughness parameters of fiber intruded synthetic aggregate samples (FISA) were significantly improved compared to synthetic aggregates (SA). The improved average roughness (R_a) of FISA may have been caused by the coarser surface of the binder reaction products with fibers increased the surface area. Other FISA characteristic factors, including S_q, S_y, and S_p, were greater than SA. Also it favorably altered the topographical surface of produced aggregates including more prominent hills and valleys, resulting in the binder reaction products with a rough surface (5858. Karthik, A.; Sudalaimani, K.; Vijaya Kumar, C.T.; (2017) Investigation on mechanical properties of fly ash-ground granulated blast furnace slag based self curing bio-geopolymer concrete. Constr. Build. Mater. 149, 338-349. https://doi.org/10.1016/j.conbuildmat.2017.05.139.
). Developing a harder, stronger, and more organized binder arrangement improved the actual contact area and enhanced binding, strengthening the mechanical and durability properties of the manufactured synthetic aggregates and fiber intruded synthetic aggregates.

4.6. Optical microscope

An optical microscope was used to analyse fractured synthetic aggregate surface morphology and microstructure. There was a noticeable change in morphology between the aggregate edges and interiors. The inside part was densely packed with pores ranging from 10 to 50 μm. Pores within the aggregate were frequently self-contained and detached from the outer surface. Moreover, SEM images reveal that the aggregate surface was not as smooth and resulting in a larger contact area between the binder particles, facilitating densely packed microstructure. The pores of synthetic aggregates were smaller than capillary voids, showing that lime powder leads to a reduction in pore dimensions as self-healing potential and an increased its C-S-H development, as shown in Figure 19.

5.1. Economical analysis

Table 8 provides an overview of the total costs incurred in India during the manufacturing and transport of essential materials used in the manufacturing of synthetic aggregates. In addition, water expenses are based on the cost of industrial purpose water, which is marginally more expensive than the domestic water. The costs of various mortar mixtures can be estimated using the proportions indicated in Table 5.

Figure 20 depicts the correlation between concrete strength and its production cost. It is abundantly clear that the entire manufacturing cost of aggregate and the compresive strength of the mortar do not have a linear relationship. The cost usually varied depending on the quantity of cement that is required; if the proportion is high, then the cost is also increased. The control mixture, which contains no mineral admixtures as binders, has the highest cost of 94.82 dollars per cubic metre. While both target strength and cost are considered, the optimal mix is the best possible option for reaching a higher target strength at a minimal cost. Moreover, the GGBFS dosage of 40% has a significant impact on the cost of producing synthetic aggregates due to its low cost. Ultimately, it was clear that implementing the GGBFS would have positive effects on the economy, both directly and indirectly, in the form of benefits such as reduced disposal costs and improved dependability of waste management.

5.2. Environmental factors analysis

To evaluate the environmental factor analysis of incorporating GGBFS and other binders into the production of synthetic aggregates, the emissions of CO₂, consumption of energy, and raw material ratio (RMR) were preferred. Table 9 provides a summary of the emissions of CO₂and consumption of energy of all binders used in this study, as indicated in (59-6259. Meddah, M.S.; Ismail, M.A.; El-Gamal, S.; Fitriani, H. (2018) Performances evaluation of binary concrete designed with silica fume and metakaolin. Constr. Build. Mater. 166, 400-412. https://doi.org/10.1016/j.conbuildmat.2018.01.138.
60. Mukherjee, A.; Sumit; Deepmala; Dhiman, V.K.; Srivastava, P.; Kumar, A. (2021) Intellectual tool to compute embodied energy and carbon dioxide emission for building construction materials. J. Phys. Conf. Ser. 1950, 012025. https://doi.org/10.1088/1742-6596/1950/1/012025.
61. Yu, J.; Chen, Y.; Leung, C.K.Y. (2019) Mechanical performance of Strain-Hardening Cementitious Composites (SHCC) with hybrid polyvinyl alcohol and steel fibers. Compos. Struct. 226, 111198. https://doi.org/10.1016/j.compstruct.2019.111198.
62. Medjigbodo, G.; Rozière, E.; Charrier, K.; Izoret, L.; Loukili, A.(2018) Hydration, shrinkage, and durability of ternary binders containing Portland cement, limestone filler and metakaolin. Constr. Build. Mater. 183, 114-126. https://doi.org/10.1016/j.conbuildmat.2018.06.138
).

The ratio of raw materials is computed using Equation [9]:

Where m_t is the total weight of raw materials in the proportion of the mortar mixture and m_g is the weight of GGBFS.

Figure 21 depicts the correlation between carbon dioxide emissions and environmental sustainability factors such as energy usage and ratio of raw materials. The unit of energy consumption was adopted as MJ/0.1 m³ to display the three parameters in a logical manner on the same graph. Typically, as the dosage of cement lowers and the GGBFS dosage rises, environmental sustainability parameters were declined. As expected, RMR is directly related to GGBFS dose, with the lowest raw material ratio identified in OM at 90.16 %. Even though TM1, TM2, TM3, and TM6 have the lowest carbon dioxide emissions and energy consumption among the nine mixtures, their compressive strength is the less preferred. The maximum reductions in carbon dioxide (CO₂) emissions, consumption of energy, and RMR were 44.50 %, 30.74 %, and 9.8 %, respectively, for optimized mixture.