1 Introduction

Extraction of coal on a massive scale as a primary fuel is vital to India for meeting its energy demands, much like the United States, China, and Australia. India's future energy demand is expected to surge due to the economic and demographic growth [1]. Moreover, as per provisional coal statistics of 2022–23 by the Ministry of Coal, power sector continued to be the largest consumer of coal, with 89.60% of overall coal mined [2]. Coal seams can be located either deep below or close to the surface of sedimentary rock formations. Coal from shallower seams is extracted by opencast mining techniques, which entail blasting and digging, while deep coal seams are dug out by underground mining techniques. Both of these activities leave large voids post coal extraction. It is imperative to backfill these voids in order to prevent surface subsidence and other related problems in the future. As such, the techniques to be employed for backfilling applications can broadly be categorized as mechanical, pneumatic, paste, and hydraulic [3, 4]. Mechanical backfilling uses conveyors in underground mining to efficiently backfill waste rock and build pack walls, reducing labor and speeding up material replacement. Whereas, pneumatic backfilling entails dumping dry, freely flowing materials into mine voids via a pipeline under air pressure. Paste backfilling contains a high percentage of solids, typically 75 to 88 by weight. In hydraulic backfill, also known as hydraulic stowing, mine voids are filled with slurry or fluid with a solids-to-weight ratio of 55 to 75%.

In general, river sand is employed as a stowing material in hydraulic backfilling applications of underground coal mines to alleviate the surface subsidence of ground [5]. However, perpetual dredging and excessive exploitation makes the river sand availability scarcer in watercourses and watersheds, simultaneously threatening the riverine ecology. Consequently, many underground coal mines had to be abandoned after coal extraction without backfilling. In this context, the mining industry has been facing an acute challenge in finding an apt alternative material to river sand for backfilling purposes. The review of literature reveals that there are studies on the use of fly ash, mine tailings, overburden dump waste, and pond ash as an alternate stowing material [3, 5,6,7,8]. It is important to note that gradation characteristics, one of the fundamental physical properties of a backfill material, have a direct impact on its performance as a stowing material [9]. In this context, the above listed materials failed to comply with the gradational requirements, resembling that of river sand, as prescribed by the standards. Furthermore, these materials also do not meet the requirements of drainage, filtration, stability, and durability criterion as desired for a backfilling material [6]. The tendency to settle down and clogging of pipes during hydraulic conveyance are also a few other negative features that render these materials unsuitable for the backfilling application [7]. Moreover, leaching of heavy metals that could contaminate the groundwater reserves is also a cause of concern when employing these materials without appropriate treatment [10]. The above all circumstances compel the research community to develop an alternate backfilling material, complying with the above functional requirements while replicating that of river sand.

Fly ash, a residue after coal combustion, is abundant and is less harmful to the environment than other industrial wastes/by-products [11]. Thus, the prime idea of this study is aimed to synthesize and develop fine aggregates or artificial sand and assess its potential as an alternate backfilling material in mine void applications. The key objective of this study is to develop fly ash based sand (FAB Sand) using fly ash alone as a resource material. Geopolymerization with the cold bonding pelletization technique is relied upon to develop the desired FAB sand gradation (4.75 mm to 0.075 mm), mimicking exactly that of river sand. Extensive experimental investigations are conducted to endorse the suitability of developed FAB sand as an alternate backfilling material to mine voids and substitute for river sand. The outcome of the study is a significant contribution towards exploring an alternate backfill material to be used in underground and open mine void stowing applications. The proposed novel sustainable solution of synthesis of FAB sand not only alleviates mining of sand from rivers but also resolves mining industry problems through valorization of industrial wastes/by-products.

2 Materials and methodology

2.1 Materials

Fly ash (FA) employed in the present study was brought from the nearest Aluminum refinery, M/s. Vedanta Limited at Lanjigarh, Odisha, India. The particle size analysis of FA was performed in accordance with ASTM D6913/D6913M-17 [12]. Natural river sand was procured from a local supplier at Bhubaneswar, Odisha, India.

Sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) were chosen as alkali activator solutions (AAS) for geopolymerization purpose. Na2SiO3 was procured from Panchi Chemicals, Hyderabad, and NaOH pellets (96% pure), were procured from Central Drug House. The ratio of Na2SiO3 to NaOH is fixed at 1.5, as per the literature [13] and NaOH concentration was fixed at 6 M [14]. For making a 6 M NaOH solution, 250 g (considering 96% purity of pellets) of NaOH pellets were dissolved in water to prepare one liter of solution. The reaction between water and NaOH is an exothermic nature, which results in a lot of heat liberation. Therefore, the prepared mixture is kept in undisturbed condition for 24 h to ensure that the solution cools down to room temperature. The specific gravity of 6 M NaOH and Na2SiO3 solution is measured as 1.24 and 1.45 respectively.

2.2 Development of fly ash based sand (FAB sand)

As regards to equipment, bespoke disc pelletizer equipment (make, MECHMIN, Maharashtra, India) was used to create sand size fraction with the FA. Pelletization fundamentally involves the agglomeration of moistened particles in a rotating drum. For agglomeration or bonding, geopolymer technology is relied on. Geopolymerization is an inorganic polymeric technique that converts pulverized particles of aluminosilicate rich precursor material into hardened substance upon addition of alkali solutions by forming a three-dimensional network of linkages [15]. Cold bonding, wherein the temperature is generally set below 100 °C [16], is adopted to cure the agglomerated sand particles. Based on intergranular water, pelletization includes three stages: pendular, funicular, and capillary state [17].

The sand of desirable gradation was manufactured by varying AAS to binder solids (BS) ratio of 0.32, 0.34, and 0.36, and by fixing the pelletization duration as 20 and 30 min. Based on the numerous trials, pelletizer tilt angle and speed were set at 50 degrees and 10 rpm, respectively. The process of sand synthesis would start by feeding fly ash (i.e. binder solids) of 1 kg to the disc pelletizer. Preceding to it, AAS is sprayed on the dry ash particles while the drum is in rotation. The spraying action triggers the formation of thin film on the surface of powder particles. The moistened particles under rotation would start converting into spherical particles. Under continuous rotation, bonding forces within the particles grow with time at the contacting points, resulting in the formation of sand, termed as FAB Sand. After the desired stipulation period, sand particles were collected and cured at 60 °C for 24 h. A detailed procedure employed in the current study to synthesize FAB Sand is depicted in Fig. 1.

Fig. 1
figure 1

Process of developing FAB sand using fly ash

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2.3 Characterization of FAB Sand for stowing applications

An optimal stowing requires chemically inert materials that do not contain carbonaceous elements, as they could induce spontaneous heating. On top of it, hydraulic stowing is heavily dependent on the stowing material’s density, particle size and shape, permeability, and so on. Hence, in order to evaluate FAB Sand efficacy as a stowing material and to endorse its suitability as an alternative backfilling material, its physical, chemical, mechanical, morphological, and mineralogical characteristics are established. Since FAB sand is made of industrial solid waste such as fly ash, its toxicity characteristics, environmental and human health risks are further assessed. To authenticate the suitability of FAB Sand, its properties were compared vis-à-vis that of river sand. To have a clear understanding, the synthesized FAB sand is divided into fractions of coarse sand (4.75–2 mm), medium sand (2–0.425 mm), fine sand (0.425–0.075 mm), and whole sand (4.75–0.075 mm). Corresponding to each parameter, results in triplicate were considered representative.

2.3.1 Physical properties

Particle size distribution (gradation analysis), specific gravity, water absorption, density (loose, rodded, compacted), permeability, and angle of internal friction are some of the physical features that are of special relevance when using FAB Sand as stowing in mining activity.

Gradation analysis was performed in accordance with ASTM C136-06 using a set of sieves from 4.75 mm to 75 µm [18]. Specific gravity (G), water absorption, and density were measured as per ASTM C128-22 and C29/29M-17a standards respectively [19, 20]. The permeability of FAB and river sand was measured by the constant head method in relevance with ASTM D2434-68 [21]. The angle of internal friction was measured following the ASTM D3080-4 standard [22]. It can be noted that all physical properties were determined on FAB sand gradation encompassing whole sand, coarse sand, medium sand and fine sand categories.

2.3.2 Chemical properties

Under chemical properties, pH and acid solubility of fly ash, FAB and river sand were measured in accordance with the ASTM D4972-19 and IS 8419 Part 1 standards [23, 24].

2.3.3 Mechanical properties

Under mechanical properties, aggregate crushing strength tests of FAB Sand and river sand were measured in accordance with the IS 2386 Part 4 standard [25].

2.3.4 Leaching analysis

Toxicity characteristic leaching procedure (TCLP) analysis was performed on FAB Sand resorting to the procedure laid down by USEPA leachability test method 1311 [26]. For testing purpose, FAB Sand was ground and sieved using 0.3 mm sieve size. The aqueous solution was prepared by adding 5.7 ml of concentrated glacial acetic acid and 64.3 ml of 1N NaOH. The contents are mixed thoroughly, and then the resultant solution is scaled up to one liter by adding distilled water. The prepared solution and ground samples were mixed at L/S ratio of 50. The solution was then agitated for 16 h at 30 rpm using a rotary tumbler. After completing the desired agitation period, the mixture was filtered with the help of 0.2 µm glass membrane filter. Before testing, the obtained analyte was acidified with 1N HNO3 to stabilize the pH. The preserved filtrate samples were analyzed for heavy metal elements such as Arsenic (As), Cadmium (Cd), Chromium (Cr), Cobalt (Co), Mercury (Hg), Lead (Pb), Zinc (Zn), Copper (Cu), Manganese (Mn), and Nickel (Ni) with the help of ICP flame photo meter. Other elements such as Iron (Fe), Sodium (Na), Magnesium (Mg), Potassium (K), and Calcium (Ca) were also measured. Leachable elemental concentrations were measured in mg/l or ppm. The measured heavy metals were also compared with various limits prescribed by international standards such as Hazardous and Other Wastes Rules 2016, MoEF&CC [27], ABNT NBR 1004 [28], USEPA [29], VROM [30], CONAMA [31].

2.3.5 Environmental and human health risk assessment

To comprehend the possible pollution effects of heavy metals present in FAB Sand on soil and water, the single factor index method and the potential ecological environment risk method were employed. The evaluation was done considering eight elements: Cu, Zn, Pb, Cr, Cd, Hg, Ni, and As. In addition, toxicity response coefficients and standard reference values of several elements were considered as per the literature [32, 33].

Heavy metals in FAB Sand not only pose a safety risk to soils, but they may also be carcinogenic to humans. Therefore, The American health risk assessment approach was employed to assess the non-carcinogenic human health risks of Cu, Zn, Pb, Cr, Cd, Hg, Ni, and As, and the carcinogenic human health risks of Cr, Cd, Ni, and As [34,35,36]. For health risk assessment of heavy metals in FAB Sand to humans (i.e. adults and children), the present study primarily relies on two different ingestion modes: inhalation and skin contact. The present study considered the reference values as per the literature [32, 33, 37].

2.3.6 Morphological and mineralogical analysis

Scanning electron microscope (SEM) analysis was performed on FA, FAB and river sand to observe the changes in the morphology of particles. Prior to SEM analysis, samples were sputtered with gold using a Q150R ES sputter coater to capture the particle's distinct morphological features (Quorum, Lewes, UK). The morphological characteristics of samples were subsequently recorded with MERLIN compact field emission scanning electron microscope (ZEISS FEI Quanta 25, Berlin). Energy dispersive X-ray spectroscopy (EDS) analysis was additionally performed to comprehend the elemental compositions of FA, FAB and river sand.

A D8 Advance X-ray powder diffraction apparatus was used for the X-ray diffraction (XRD) analysis to examine the mineralogical compositions of FA and FAB Sand. The samples were ground to powder form and oven dried at 95 °C–105 °C for 24 h prior to XRD analysis. Samples were scanned using a copper X-ray tube (Cu-Kα) radiation at 40 mA current and 30 kV voltage for 10°–70° of 2θ value (Bragg angle) with a step size of 0.2. X’Pert High Score Plus, which uses the peak search approach to identify mineral phases by comparing the generated peaks with common patterns of mineral interplanar distances, was employed to detect the dominant minerals.

3 Results and discussion

3.1 Physical characteristics

3.1.1 Particle size distribution

Figure 2 presents gradation curves of FA, FAB and river sands while Table 1 presents percent conversion of FA into FAB sand. From Fig. 2, a clear distinction in gradation curves between FA and FAB sand can be seen, indicating successful conversion of FA into sand size fractions. For a better validation, the gradation of river sand is superimposed on Fig. 2. Evidently, the gradation curves of all six trials excellently match vis-à-vis with river sand. These results highlight that sand can be manufactured with FA alone a resource material and by resorting to geopolymerization with pelletization technique.

Fig. 2
figure 2

Gradation curves of fly ash, FAB sand, and river sand

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Table 1 Percent conversion of FA into FAB sand
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It can be witnessed from Table 1 results that 100% of ash is converted into sand (~ 90%) and gravel (~ 10%). For a superior understanding, the overall gradation of sand is fractioned into fine, medium, and coarse size and their percentages calculated are presented in Table 1.

The particle size analysis (Fig. 2) reveals that FA comprises 0, 15, 75, and 10% of gravel, sand, silt, and clay size fractions. Clearly, dominance of silt size fractions in fly ash alone can be seen. The coefficient of uniformity (Cu) and coefficient of curvature (Cc) of FA is calculated as 16 and 3.06. River sand contains 3, 75, and 22% of coarse, medium, and finer fractions respectively. Whereas in FAB Sand, fractions of coarse, medium, and finer sand are calculated in the range of 12–15%, 14–35%, and 32–61% for six samples respectively. The variation in percent fraction of FAB Sand is attributed to the change in AAS to BS ratio from 0.32 to 0.36 and duration of pelletization from 20 to 30 min. The value of Cu and Cc for river sand and FAB Sand is calculated as 2.72 and 2.2–4.2, and 1.2 and 0.5–0.7 respectively. Based on the measured Cu and Cc values, both river and FAB Sand can be classified as poorly graded sands (SP).

Gradation analysis is one of the critically governing parameters of natural or waste materials in the realm of mine void and backfilling applications. Gradation analysis is an indirect indication of pore size and pore connectivity for effective water drainage. The size of individual pore in a sample is determined by the size of fill material particle size distribution. Backfill material gradation differs depending upon the technique used, with paste backfill requiring at least 15% particles in 20 µm range while hydraulic fill requiring 15% particles in the range from 40 to 150 µm [3]. Further, guidelines limit the percent particles of 75 µm to 10 in the case of hydraulic filling applications [3]. Similarly for pneumatic stowing, percent particles under 3 mm shall be limited to 20 [6]. Understandably, the developed FAB Sand excellently meets these gradational requirements, underscoring its suitability for mine void and backfilling applications as that of river sand. Based on the overall gradational analysis, it can be affirmed that FAB sand is suitable for stowing and could be a potential alternative to river sand in mine void backfilling applications.

3.1.2 Specific gravity and water absorption

The specific gravity of FA, FAB sand (whole sand of FABS6) and river sand is measured as 2.13, 1.9, and 2.63 respectively. The water absorption of river sand and FABS6 is measured as 2.8% and 16%. The higher water absorption of FAB sand can be attributed to the presence of voids in it formed during the agglomeration process in the disc pelletizer. Since FAB Sand possesses a higher water holding capacity than FA and river sand, it will absorb more water after stowing, allowing less to drain out for pumping and so assists to diminish pumping costs. Typically, finer fractions have higher specific gravity than coarser fractions. Conversely, the coarser fraction absorbs more water than the finer fraction.

3.1.3 Density

The density of fill material including loose, rodded, and compacted states is determined in their dry condition. The loose density of FA, FAB sand (whole sand of FABS6) and river sand is measured as 893, 878, and 1545 kg/m3. The rodded density of FA, FABS6 and river sand is determined as 1076, 940, and 1621 kg/m3, respectively. The compacted density of FA, FABS6 and river sand is determined as 1204, 1033, and 1728 kg/m3, respectively. The detailed densities of fine, medium, coarse, and whole sand fractions of both river sand and FABS6 are presented in Table 2. According to ASTM C330, the developed FAB Sand can be categorized as lightweight aggregate because the density value is < 1120 kg/m3 [38]. The percent voids in FA are calculated as 58%, whereas the same for river sand and FABS6 is measured as 41 and 54%. Typically, the void percentage of finer fractions is higher than that of coarser fractions.

Table 2 Comparison of physical properties of river sand and FAB sand
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It is to be noted that the density of a material to be used in backfilling application plays a crucial role in determining both mass ratio and backfill body’s compression ratio. Moreover, the volumetric weight of a backfill material is fundamental to calculate the backfill volume, which in turn is essential to estimate the volume of mining and backfilling by volume mass relationships [39]. In this context, the lower density recorded for FAB sand is advantageous, as it imposes lesser weight in a given volume and exerts lesser lateral pressure as compared with other conventional materials.

3.1.4 Permeability

Permeability is one of the crucial engineering property requirements for hydraulic backfilling, as it establishes the ground support rule. Easy and quick drainage is one of the essential requiring features of hydraulic stowing technique. The permeability of FA, FAB sand (whole sand of FABS6) and river sand is measured as 3.5 × 10–4, 5.22 × 10–3, and 3.88 × 10–3 cm/s. It can clearly be comprehended that the permeability values of both river and FABS6 match very well, suggesting that FAB Sand is a viable substitute for natural river sand. The similarity in permeability emphasizes that there is no extra cost associated with dewatering when considering FAB sand as a backfilling material. Further, as the water absorption capacity of FAB sand is higher than river sand, it can even reduce the dewatering cost. The gradation has a very sensitive influence on the permeability concerning the drainage requirements. To promote good drainage, it is recommended to have an effective grain size (D10) greater than 10 µm and hydraulic conductivity exceeding 100 mm/h [40]. Evidently, FAB Sand precisely complies with the drainage criterion based on permeability and effective grain size.

3.1.5 Angle of internal friction

The angle of internal friction of river and FAB sand (whole sand of FABS6) is measured as 39° and 40° respectively. Cohesion for both of these materials is measured as zero. It is to be noted that these values are determined after densifying the sand to maximum compacted state, as presented in Table 2. Similar shear strength results evidentially suggest an identically in the particle size distributions of river and FAB sands. Gradation further advocates proper binding and bonding of FA particles in the presence of AAS that has been pelletized. The very high value of frictional angle indicates that FAB sand can have good stability and bearing capacity.

Based on the overall physical properties, it can be inferred that FAB Sand is an ideal substitute for river sand and can be rendered as an alternate mine backfilling material.

3.2 Chemical characteristics

3.2.1 pH

The pH of 6 M NaOH and Na2SiO3 solution is measured as 10.1 and 11.8 respectively. The pH of FA, FAB Sand, and river sand is measured as 8.5, 11.5, and 7.6 respectively. The measured pH value of FAB sand shows that, it is alkaline in nature. The inherent alkalinity could potentially aid in the mine void filling applications where acidic water or acid mine drainage issues arise [41].

3.2.2 Acid solubility test

The acid solubility of FAB sand and river sand is measured as 3.6 and 2.2% respectively. According to the filtration requirements, the sand should contain no more than 5 percent acid soluble matter [24]. Notably, the acid solubility of FAB sand is well within the prescribed limit, emphasizing that FAB Sand meets the filtration requirements.

3.3 Mechanical characteristics

The aggregate crushing strength is measured as 8.9% for FAB Sand and 5.2% for river sand. The acceptable crushing strength value of sand is 30%, as per the IS 2386 Part 4 standard [25]. The low crushing value of FAB Sand, within the permissible limit of the standard, accentuates its remarkable strength, making it an ideal substitute for river sand. This exceptional strength indicates that the backfilled area can be effectively repurposed for other applications.

3.4 Leaching analysis

Table 3 presents the heavy metal analysis results of FAB sand along with the acceptable limits specified by various international standards such as MOEF&CC [27], ABNT NBR 1004 [28], USEPA [29], VROM [30], CONAMA [31]. Understandably, the measured concentrations are well below the prescribed limits, indicating that FAB Sand is not hazardous in nature and is safe to the environment. From these results, it can be inferred that FAB Sand can serve as a resource material for mine void stowing applications. Other elementals such as Iron, Sodium, Magnesium, Potassium, and Calcium compositions (mg/l) present in FAB Sand are measured as 7.6, 628, 3.7, 14.5, and 16 respectively.

Table 3 Measured concentration of heavy metals in FAB sand
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3.5 Environmental risk assessment

3.5.1 Single Factor Index Method

In general, Single Factor Index (Pi) is used as an indicator for the risk assessment in soil environment. This method is also often chosen to assess the ecological hazards and risks of heavy metals. Pi ≥ 0.5 indicates that heavy metals pose potential ecological hazards, Pi > 1 indicates that the soil is polluted. As such, larger the value of Pi, the more serious is the pollution.

3.5.2 Potential ecological environment risk method

The potential ecological risk of heavy metals can better reflect the ecological benefits and environmental behavior of heavy metals. According to Wang et al. [33], a risk index (RI = ∑Ei) of less than 40 indicates a low ecological risk grade, 40–80 implies moderate risk, 80–160 represents considerable risk, 160–320 shows high risk, and RI > 320 signifies extreme risk.

Detailed calculations of the single factor index and potential ecological risk index are done and the results obtained are listed in Table 4. The interpretation of results reveal that the Pi value of FAB Sand is 0.04, which is far less than 0.5, indicating that FAB Sand does not pollute the soil. From the results in Table 4, RI is summed up as 0.747, which again is far lower than the above prescribed values. It can thus be acknowledged that FAB Sand has no risk and its potential ecological hazard can be ignored.

Table 4 Environmental risk assessment results of FAB sand
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3.6 Human health risk assessment

The results of non-carcinogenic risk in terms of hazard index (HI) are presented in Table 5. Whereas, carcinogenic risk assessment results are given in Tables 6 and 7 respectively.

Table 5 Hazard index (HI) of FAB sand
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Table 6 Carcinogenic risk (CR) index through different pathways
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Table 7 The value of CR and TCR calculated for FAB Sand
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Based on the results presented in Table 5, it can evidently be seen that the HI of FAB sand is < 1. From this inference, it can be concluded that FAB Sand has no non-carcinogenic risk to human health, including both adults and children, when stockpiled on the soil. When CR or TCR > 10–6, it is considered that, there is no carcinogenic risk. When 10–4 < CR or TCR < 10–6, there is a certain risk of harm, and CR/TCR > 10–4 represents a very high cancer risk. From Table 6, it can be understood that FAB Sand has no carcinogenic effects on humans, including adults and children, via inhalation or dermal contact. Furthermore, based on TCR values as presented in Table 7, it can be established that children and adults have no carcinogenic risk with FAB Sand.

3.7 Morphological and mineralogical analysis

Figure 3 presents micrographs of FA, FAB and river sands. As depicted in Fig. 3a, FA particles are dominantly spherical in shape, with bright surfaces that can be identified as cenospheres. The river sand particles are angular with diameter as 258.1 µm (Fig. 3b). Whereas the morphology of FAB Sand is entirely distinct with the presence of angular and rounded grains as well. A close comparison of Fig. 3a (FA alone) with the rest of images (Fig. 3c–f) reveals agglomeration of FA particles and thereby, formation of FAB Sand of variable particle sizes because of geopolymerization in conjunction with pelletization technique, as discussed above. As such, the variability in grain sizes very well corroborates the gradation results presented in Fig. 2 for FAB sand. Images of Fig. 3c–f clearly substantiate aggregation of FA particles by bonding and binding to yield the FAB Sand size fractions. This can be attributed to pelletization in the presence of alkali activator solutions (NaOH and Na2SiO3) and dense packing of Si–O–Al–O– units formed by the dissolution and polycondensation of the reaction products [42, 43].

Fig. 3
figure 3

SEM images of a FA, b river sand, and c–f FAB Sand

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The rounded and angular particle morphology of FAB Sand allows it to flow and be stable under loading conditions in FAB Sand-water combinations. As a result, it can significantly improve slurry rheology and provide lubricating action via the ball bearing effect, resulting in frictionless flow in the stowing range and decreasing pipeline wear and tear [5]. It can be understood from Fig. 3c, f that void ratio of FAB sand shall be higher because individual sand particle is formed as a result of agglomeration of several ash particles. The process of agglomeration can have repercussion in terms of increased specific surface area of FAB sand. This may be a reason why higher water absorption and lower density is measured (Table 2) for FAB sand vis-à-vis that of river sand. As illustrated in Fig. 3f, the existence of voids can contribute to higher water absorption and the formation of lightweight FAB Sand. The resemblance in terms of size and shape between the micrographs of river and FAB sand shows that FAB sand is a better substitute for river sand as a backfilling material in underground mine activity.

Further, colour mapping is used to analyze the distribution of Si, Al, Na, Ca, and Fe elements in micrographs, as depicted in Fig. 4. The colour mapping confirms that FAB Sand encompasses Si and Al as major elements. EDS analysis also reveals that FA and FAB Sand contain mostly Si, followed by Al, Fe, and Na, as given in Table 8. Similar to the FAB sand, river sand also contains Si followed by Al as major elements.

Fig. 4
figure 4

Colour mapping of FAB sand

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Table 8 Elemental composition of FA, FAB Sand, and River sand
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The mineralogical compositions established of FA and FAB sand are presented in Fig. 5. In FA, quartz (SiO2) and mullite (3Al2O3·2SiO2) are identified as the primary crystalline minerals. Peaks corresponding to quartz (Q) are detected at 2θ (degrees) of 20.85 and 26.75 while mullite (M) peaks are identified at 2θ of 16.46, 26.02, 26.27, 33.26, 35.36, 40.87, and 60.88. Alike FA, crystalline Q and M peaks are also detected in FAB sand, ruling out phase conversion of crystalline minerals such as Q and M during the geopolymerization. Despite employing extremely potent AAS, Q and M remain stable and did not undergo dissolution easily. Evidently, reflections belonging to calcium-silicate-hydrate (CSH) gels that had formed as a result of geopolymerization process are identified in FAB sand. These peaks, notably at 2θ of 42.91 and 51.02, can be evidenced from Fig. 5.

Fig. 5
figure 5

X-ray diffractograms of FA and FAB sand

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The formation of C–S–H phases in fly ash based geopolymer fine aggregates are also reported by Parvathy et al. [13] and Sekhar and Rao [11]. Additionally, Reddy et al. [44] have identified peaks belonging to N–A–S–H gels between 2θ of 15–35°, and peaks between 2θ of 30–49° indicating a possible formation of poorly crystalline C–A–S–H and crystalline C–S–H gel phases in alkali activated fly ash and slag based concrete. It is obvious in this study that salg contains high calcium content. Research by Lodeiro et al. [45, 46] observed C–S–H gels exhibiting broad peaks spanning from 29–51° in high calcium environments, along with an amorphous halo containing calcite between 20–35° in low calcium environments. Moreover, the formation of these calcium compounds within geopolymers is greatly dependent on Si/Al ratio and pH levels [47]. Given that, the developed FAB Sand originates by geopolymerization of FA with low calcium oxide content, neither N–A–S–H nor C–A–S–H gels are identified. Instead, only peaks corresponding to C–S–H gels are detected, as depicted in Fig. 5. Furthermore, the colour mapping images (Fig. 4) depict the distribution of Ca, Si, Al, and Na within the FAB Sand. The binding and bonding of agglomerated particles of FAB sand, as presented in Fig. 2 and depicted in Fig. 3, can be linked to CSH gels. The presence of gels very well corroborate with the formation of FAB sand, as depicted in Fig. 3. An increase in intensity of CSH peaks with rise in AAS quantity, from 0.32 to 0.36, can also be noticed in Fig. 5. This suggests a direct correlation with the agglomeration of fly ash particles in the presence of AAS, ultimately leading to the formation of FAB Sand. The glassy phase of FA dissolves in an alkaline environment, releasing Si and Al ions. These ions then polymerize to form aluminium–silicate networks, which play a vital role in solidifying the binder and enhancing the strength of the geopolymer material.

4 Conclusions

The present study demonstrates an innovative idea of developing fly ash based sand, as an alternate backfilling material for stowing applications in mine voids as a substitute for naturally available river sand. It is successfully demonstrated that FAB sand can be developed using industrial by-product (i.e. fly ash alone) by resorting to geopolymer technology with a cold bonding process. The extensive characterization results authenticate that FAB Sand complies excellently with the functional requirements for mine void backfilling applications, much like the river sand. The results of leaching substantiate that FAB sand is non-hazardous and is safe to the environment when employed. More importantly, the risk analysis concludes that FAB Sand poses no risk to the environment and humans when stacked over the soil. The low crushing strength results advocate that FAB sand filled area can be repurposed for any general use. The finding of the study, by introducing a doable and sustainable alternate backfilling material for the mine void applications, avoids over exploitation of natural sand while concurrently promoting bulk utilization of abundantly available fly ash by converting it to green sustainable material such as sand. Since FAB sand (4.75–0.075 mm) is manufactured by adopting geopolymerization with pelletization process, the technology is simple to adopt in all practical scenarios and can be appreciated as a significant contribution for the mining activity.

Furthermore, the present study directly supports the Sustainable Development Goals (SDGs), specifically SDG 9 (Industry, Innovation, and Infrastructure) by introducing an innovative, sustainable alternative (FAB Sand) to natural sand, and SDG 12 (Responsible Consumption and Production) by promoting the use of industrial by-products such as fly ash. By aligning with these global goals, our research not only offers an environmentally sound solution but also contributes to broader efforts toward sustainable development and resource conservation.