Comparative characteristics assessment of calcined and uncalcined agro-based waste ash with GGBS and its application in an alkali-activated binder system

Abstract

A lot of energy is released during the cement manufacturing process, and a large amount of carbon dioxide (CO₂) is discharged into the environment. Presently, researchers are focusing on reducing CO₂ emissions by researching sustainable alternatives to traditional Portland cement-based materials. A comparative study on the material characterization of both calcined agro-based waste ash (calcined ash) and uncalcined agro-based waste ash (uncalcined ash) and its utilization as a binder along with granulated blast furnace slag (GGBS) in the alkali activation process is carried out in this paper. The study regarding calcined and uncalcined ash would help in better understanding the variation in the properties of the material and its behavior during the alkali activation process. The tests conducted on both calcined and uncalcined ash along with GGBS include specific gravity, X-ray fluorescence, X-ray diffraction, scanning electron microscopy, reaction degree, Brunauer-Emmett-Teller (BET) analysis, particle size distribution, and the Puntke test for understanding its microstructural characteristics. Later, alkali-activated mortars are prepared using 30% weight of agro-based waste ash (both calcined and uncalcined) and 70% weight of GGBS. A comparison of the compressive strength of alkali-activated mortars prepared using calcined and uncalcined ash was studied, which showed a promising increase in the strength by 11.02% of mortar prepared using calcined ash (23.46 MPa) for 28 days when compared to mortar prepared using uncalcined ash (21.13 MPa). The major finding from this study indicates that calcined agro-based ash with GGBS shows better results compared to uncalcined agro-based ash with GGBS.

Keywords:

• Agro-based waste ash
• material characterization
• calcined ash
• uncalcined ash
• GGBS
• alkali-activation

PUBLIC INTEREST STATEMENT

A lot of energy is released during the manufacture of cement, with a large amount of carbon dioxide being discharged into the environment. The current research focuses on reducing cement utilization as a binder by incorporating the use of waste ashes produced from agricultural and industrial sectors. These waste ashes, which are directly obtained from various industries, have binding properties that can be activated using alkaline solutions such as sodium hydroxides. Even the binding properties of these ashes vary depending on whether they are incinerated at a high temperature or not. A comparative study on the material properties of both calcined (incinerated at high heat) agro-based waste ash and uncalcined (burned at low heat) agro-based waste ash and its utilization as a binder along with granulated blast furnace slag (GGBS) in the alkali activation process is carried out in this paper.

1. Introduction

For the last two centuries, the use of cement as a binder material has been in practice with a large number of applications, even though there are environmental-related problems associated with it (Chakraborty et al., 2013; Indukuri et al., 2019; Gupta & Kashani, 2021). A lot of energy is released during the manufacture of cement, with a large amount of carbon dioxide (CO₂) being discharged into the environment. Thus, reduced cement usage will result in lower CO₂ emissions and savings in energy (Meyer, 2009; Turner & Collins, 2013). Cement manufacturing is an energy-intensive process that requires significant amounts of electricity and fossil fuels. As reported in a study published in the Journal of Environmental Development, the high energy consumption of cement production is a major challenge for the industry, and alternative fuel sources and energy-efficient technologies are needed to reduce energy consumption and associated costs (Sahoo et al., 2022; Singh et al., 2019). Presently, researchers are focusing on reducing CO₂ emissions by researching alternate binding materials that are naturally available or are formed as a by-product of agricultural and industrial products (Poudyal & Adhikari, 2021; Frittelli, 2019; Adesina et al., 2021).

Ettringite (Aft), calcium hydrate gel (C-H Gel), and calcium silicate hydrate gel (C-S-H Gel) are examples of cement composites frequently developed during the hydration reaction (Medina et al., 2018; Zhang et al., 2021). The alternate materials proposed for a partial or complete replacement of cement must also be able to undergo a similar reaction mechanism in order to form a strong bond. Ground granulated blast furnace slag (GGBS), fly ash (FA), silica fume (SF), rice husk ash (RHA), etc. are a few of the alternate materials that can undergo a similar type of reaction through alkali activation (Blesson & Rao, 2023). These alternate materials are generally the by-products of industrial or agricultural wastes. According to a study by Provis and van Deventer (2014), alkali-activated binders (AABs) can potentially reduce CO₂ emissions by up to 80% compared to traditional cement-based materials. Additionally, alkali-activating materials (AAMs) can be made from industrial waste materials, such as fly ash, blast furnace slag, and mining tailings, which can reduce waste generation and disposal costs (Kashani et al., 2014).

To achieve the best performance from AAB systems as compared to cement concrete systems, a large number of factors must be taken into account, such as the type of binder materials used, their properties, the type of alkali-activating material, its molarity, and the type of curing (Chilukuri et al., 2021; Oyebisi et al., 2020). An alkaline environment is created by the AAMs, which aids in the reaction of the AAB materials to create cementitious binders with reduced CO₂ emissions. Alkalis are abundantly available, and scientists have revealed numerous sources of alumina and silica in a range of industrial wastes (Usha et al., 2014). The alkali-activation of various agro-based waste ashes and their impact on the performance of agro-based waste ash-based alkali-activated binders are other issues that have not yet been resolved or are not entirely clear (Athira et al., 2021).

The properties of the binder system prepared using agricultural and industrial waste ashes vary depending on whether the waste ashes are calcined or uncalcined (Tantri et al., (2022). Calcination is an a thermal treatment process used to remove volatile matter from materials such as ceramics, metals, and minerals. During this process, the material is heated to a high temperature, causing the bond between the binder and the material to break down (Alla & Asadi, 2022). Under 700°C regulated burning (calcination) of agro-based waste products, the ash’s silica content is converted into amorphous silica, which can then be utilized as a pozzolanic material (Singh et al., 2000). Aluminosilicates fail to react with water or do so extremely slowly. However, if these materials have significant amorphous content, they will hydrolyze and condense in an alkaline environment, creating new inorganic polymers with a load-bearing capacity (Habert, 2014).

Several studies have examined the geopolymerization reaction of aluminosilicate precursors from the standpoint of amorphous and crystalline phases of raw materials, emphasizing the critical function of aluminosilicate precursors’ amorphous aluminosilicate glass in geopolymerization (Maldonado-Alameda et al., 2020; Rickard et al., 2011; Liu et al., 2022). X-ray diffraction (XRD) is frequently used to assess a material’s crystalline and amorphous compositions. By combining alkali leaching experiments with XRD and energy-dispersive spectroscopy (EDS) mapping, Chancey et al. (2010) tracked the alteration of FA’s amorphous and crystalline phases both before and after alkali dissolution. Amorphous content (alumina-silicate glass) is a higher reactive than crystalline mullite content, according to the results of the XRD and EDS maps. The fact that the crystalline mullite content did not dissolve at any point during the reaction suggests that it served as an inert filler (Liu et al., 2022).

Even though there are studies related to understanding the chemical (crystalline and amorphous phases) and physical properties of alkali-activated materials and their applications, there are very few studies related to comparative assessment of the physical and chemical properties of calcined and uncalcined agro-based waste materials that are used along with GGBS in alkali-activation process. The novelty of the present study mainly focuses on the material characterization of the agro-based waste ash (both calcined and uncalcined) along with GGBS using tests such as X-ray fluorescence, X-ray diffraction, scanning electron microscopy, reaction degree, and thermogravimetric analysis to understand the physical and chemical properties of calcined and uncalcined agro-based waste materials that are used in alkali-activation process. The materials are then taken in the proportion of 70% of GGBS and 30% of agro-based waste ash and activated using a 4 M sodium hydroxide (NaOH) solution. The binder mixes for both calcined and uncalcined ash are prepared separately and are checked for their performance by comparing the compressive strength, water absorption, ultrasonic pulse velocity, and loss on ignition. The main objective of the study is to perform a comparative characteristic assessment of the agro-based waste ash (both calcined and uncalcined) and GGBS to understand its applicability as a binder. The graphical abstract representing the present study is shown in Figure 1.

Figure 1. Graphical abstract.

2. Related works

One example of research in the field of the use of AAB as construction materials is a study by Provis et al. (2015), which investigated the use of alkali-activated materials in construction applications (Provis et al., 2015). The authors reported on the promising results of several case studies, demonstrating the potential of these materials for use in a range of structural and non-structural applications. Other researchers have explored the use of various precursor materials and activators, as well as the effect of curing conditions and additives on the properties of alkali-activated materials (Garcia-Lodeiro et al., 2014; Ke et al., 2015). These studies contribute to a growing body of knowledge on the potential of alkali-activated materials for sustainable construction.

A comprehensive review of biomass ashes obtained from agricultural waste as partial replacement of cement in the preparation of geopolymer concrete sheds light on the effect of various material properties of biomass ash on the durability and mechanical properties of hardened concrete. The partial replacement of Ordinary Portland cement (OPC) with biomass ashes, such as palm ash, sugarcane waste ash, Napier grass ash, wheat straw ash, plantain peel ash, bamboo-leaf ash, corncob ash, olive waste ash, and rice straw ash, helps in the reduction of CO₂ emissions, and it was confirmed that biomass ashes could feasibly be utilized as a pozzolanic material or as an alternate source of activator in the preparation of geopolymer concrete (Thomas et al., 2020).

The addition of hemp fiber and expanded perlite in a one-part AAB system improved the mechanical properties of the composites, yielding higher strength, and stiffness. The microstructure analysis revealed a more homogeneous and compact structure, leading to greater resistance to deformation. The researchers also found that the composites maintained their integrity and strength even when subjected to high temperatures. These findings suggested that hemp fiber along with expanded perlite could be used as sustainable effective additives to enhance the mechanical properties and high-temperature resistance of alkali-activated composites (Bayraktar et al., 2022).

A comprehensive overview of the use of pozzolanic materials in alkali-activated cement concrete to enhance its durability and mechanical properties reviews various types of pozzolanic materials, including fly ash, rice husk ash, metakaolin, and silica fume, and their impact on the properties of the concrete. Tayeh et al. highlight the advantages of using alkali-activated binder over conventional Portland cement and propose several factors that can influence the durability and mechanical properties of the concrete, such as the type and content of pozzolanic materials, curing conditions, pore structure, and chemical composition. The article concludes that the use of pozzolanic materials in alkali-activated cement concrete can significantly improve its mechanical and durability properties, making it suitable for various construction applications (Tayeh et al., 2022).

In a review conducted for AAB’s materials composition and fresh characteristics, the physical, chemical, and mineralogical characteristics of fly ash, GGBS, metakaolin, silica fumes, RHA, palm oil fuel ash, and other pozzolans used to create AAB were studied. Alkaline activator solutions based on sodium (Na) and potassium (K) were also emphasized. The effects of AAB characteristics on workability (slump, flow, consistency), setting time, reaction kinetics, and temperature have been synthesized from previous research. Based on mixed design and processing circumstances, the study demonstrates that the fresh characteristics of AAB may be adjusted for various uses (Elahi et al., 2020).

In a study performed on the preparation of new alkali activators prepared from coffee husk ash (CHA), the CHA with the greatest qualities for alkaline activators: high alkali concentration and low organic matter content, was calcined at several temperatures (600°C, 700°C, and 800ºC) for various periods (1–10 h). The optimum calcination temperature and time were found to be 700ºC for 6 h. The CHA that was produced had a high K₂O (Potassium oxide) concentration in the form of K₂CO₃ (Potassium carbonate). It was validated in one-part AAB combinations of CHA and blast furnace slag (BFS), in which hydrated products ((K, C)ASH) were discovered in pastes by microstructural testing, and mortar compressive strength was 16.4 MPa after 1 day at 60°C (Lima et al., 2021).

The use of agro-based waste ashes along with industrial-based waste ashes in partial replacement of OPC mixes or in the AAB system has proven to be more effective than its individual use. It is also understood that various factors such as mean particle size, calcination, and chemical oxide components of agro-based waste ashes (RHA, olive waste ash, bagasse ash, palm oil fuel ash, corncob ash, etc.) would affect the activation process of AAB systems. The agro-based waste ashes tend to show higher content of silica, alumina, sodium oxide, potassium oxide, and calcium oxide, which are major contributors to the formation of NASH, KASH, (C, N)ASH, and (C, K)ASH gels. Calcined agro-based waste ashes used in the AAB system or in partially replaced OPC mix show better mechanical properties compared to uncalcined agro-based waste ashes (Mo et al., 2014). Industrial-based waste materials are largely being incorporated into construction industries, causing a shortage of these materials. The combined use of agro-based waste ashes and industrial-based ashes reduces the use of industrial by-products to a large extent and provides better mechanical properties. The optimum replacement ratio of various agro-industrial-based waste ashes in the formulation of a zero-cement binder system not only promotes the properties and performance of concrete but also enhances sustainability thereby reducing the cost of manufacturing of concrete. The drawback being said is that, a higher water absorption rate is due to the large surface area of the waste materials and difficulty in handling AAM during a large-scale practical application.

There are many studies conducted on the use of agro-based waste ashes and GGBS in the AAB system, but the major significance of this study is that it is the first study that mainly focuses on checking the variation in properties of calcined and uncalcined agro-based waste ash along with GGBS in AAB system. The present study would help in understanding the properties of calcined and uncalcined agro-industrial-based waste ash along with GGBS, providing a better alternative to cement in the construction industry.

3. Materials and methods

3.1. Material

The materials utilized for the current experimental work are shown in Figure 2.

Figure 2. Materials used: (a) calcined ash, (b) uncalcined ash, and (c) GGBS.

• GGBS is a greyish-white-colored powdered material obtained as a by-product of iron and steel industries, such as JSW Steel Ltd. and Federated Steel Company (Parthiban & Vaithianathan, 2015; Oyebisi et al., 2022). According to CemWeek Research, the GGBS Market Report and Forecast for 2020, the total GGBS output would be 269 million tonnes between 2020 and 2025 (GGBFS production to grow over the next, 2023). In the present study, GGBS was procured from Astrra Chemicals, Tamil Nadu, India, having a particle size of less than 75 µm.

• When it comes to agro-based wastes, 2,50,000 tonnes of coffee waste and 70 million tonnes of rice husk ash are produced a year and landfilled globally (Blesson & Rao, 2023; Coffee Waste Statistics & Coffee Cups Recycling Facts, 2023). Uncalcined agro-based waste ash (uncalcined ash) used for the study is agro-based waste ash with a combination of a larger amount of coffee husk (about 80–90%) and lesser amount of rice husk (about 10–20%) burnt at 500°C and above in an uncontrolled manner, which was directly procured from smokehouses located in Shirady village, Karnataka. The waste ash was later sieved through a 75 µm sieve.

• The raw uncalcined ash was calcined in an electric furnace at a temperature of 800°C in a controlled manner, ground, and sieved through a 75 µm sieve to obtain calcined agro-based waste ash (calcined ash).

• Alkali-activating material such as sodium hydroxide (NaOH) is required for activating the agricultural and industrial waste materials used as binder mixes (Parthiban & Saravana Raja Mohan, 2014; Kathirvel et al., 2020). In the present study, 160 g of 97% pure NaOH flakes was mixed with water to prepare 1 L of 4 M NaOH solution (Rajamane & Jeyalakshmi, 2015).

In comparison with the other raw binder materials such as OPC, fly ash, metakaolin, and silica fumes, which have been used as AABs, the above-mentioned materials have a better advantage with respect to reduced energy consumption and CO₂ emissions associated with the preparation process. Materials such as OPC and fly ash are produced at a temperature of 1100–1600°C during the incineration of coals (Bicer, 2020; Sonebi et al., 2016), and silica fumes are generated at a temperature of about 2000°C (Claisse, 2014; Soomro et al., 2023), which is comparatively high when compared to the agro-based waste ash used in the present study.

3.2. Material characterization

An understanding of the physical and chemical properties of agricultural and industrial wastes is very much important in order to utilize them in an alkali-activated binder system (Qaidi et al., 2022; Tahwia et al., 2022). Various characterization techniques such as specific gravity using a pycnometer, particle size distribution, surface area using a BET analysis, X-ray fluorescence (XRF), scanning electron microscopy (SEM), and X-ray diffraction (XRD) can be used to understand the physical and chemical properties of the binder materials (calcined ash, uncalcined ash, and GGBS) (Wypych, 2016; Ríos-Parada et al., 2017; Provis, 2022; Cyriaque Kaze et al., 2022).

3.2.1. Specific gravity and specific surface area

The mass of a volume of a substance at a given temperature divided by the mass of the same volume of distilled water at the same temperature is known as specific gravity, which is most prominently used for determining air voids present in materials (Minhas, 2019). By dividing the solid particles’ surface area by their mass, one can calculate their specific surface area (Talib & Bheekhun, 2018). The specific gravity of the materials was tested using Le Chatelier’s flask method as per IS 4031 Part 11 (1988). The specific surface area of the materials was measured using Smart Sorb 92/93 which conducts BET analysis using gas adsorption techniques as per ASTM D 5604-96 (2001).

3.2.2. X-ray fluorescence

A quick, precise, and non-destructive method for determining and detecting material composition is XRF spectroscopy. It requires little to no sample preparation and works with liquid, solid, and powdered materials. In contrast to energy-dispersive spectroscopy systems, there is no requirement that the sample be placed in a vacuum chamber. The varieties of XRF spectrometers can range from small, hand-held gadgets to large, table-top instruments (Ardebili & Pecht, 2009). The Rigaku XRF instrument was used in this study to accurately determine oxide compounds and trace elements in powdered samples of the materials used.

3.2.3. Particle size distribution

The particle size distribution has a great influence on the binder properties (Alderete, 2016). The packing density of binder particles, the need for water, and the need for chemical admixtures are all significantly influenced by the particle size distribution and the binder phase composition (Mehdipour & Khayat, 2017). The particle size distribution of all three materials was carried out using Malvern Mastersizer 3000. The data obtained from the particle size distribution were plotted in the form graph using Origin Pro 2022.

3.2.4. Puntke test

For calculating the packing density of fine materials, there is no accepted automated technique. Electrochemical surface forces and van der Waals forces have an impact on how densely a granular material can be packed. The Puntke test was chosen in the current experiment to determine particle packing. The binder materials were measured in terms of volume and are taken into a bowl (V₁). Clean water was gradually added while stirring the mixture, and after many taps on the beaker, it developed a closed structure. The water was then added at the following stage, a drop at a time, with a pipette, gently combining each addition until saturation was achieved. After several taps on the beaker, the surface smoothens out and becomes shiny at this stage. Each experiment required roughly 10 min to complete. In order to find the volume of water needed to reach saturation (V₂), the experiment was repeated. The packing density Ø_p, is calculated using the formula: Ø_p = 1 − [V₂/(V₁ + V₂)] (Babu, 2016; Nanthagopalan et al., 2008).

3.2.5. Reaction degree

The reaction degree of the materials is determined using the selective dissolution method (Fernández-Jiménez et al., 2006; Puligilla & Mondal, 2015), with three trials for each material. For each trial, 1 mg of binder material (R1) was mixed thoroughly with 250 ml of 5% concentrated HCl, and after 3 h of stirring it was filtered using a 2 µm filter paper. The insoluble residues were cleaned with distilled water until a pH of neutral was achieved, which were then dried and stored for further analysis. The original structure of the materials is broken down by acid attack, leaving only the unreacted materials (R2) as the sole insoluble byproduct. The reaction degree (RD) for each of the binder materials was calculated as [RD = (R1-R2)/R1 × 100] (Gao et al., 2017; Kocaba et al., 2012).

3.2.6. Thermogravimetric analysis

Thermal characterization of the binder materials can be carried out using thermogravimetric analysis (TGA), which would help in understanding the percentage of mass loss or gain of material at various temperatures indicating the number of impurities in the material (Garcia et al., 2022). TGA can obtain some crucial data regarding the physical and chemical characteristics of materials (Aramesh et al., 2022). The TGA was carried out using PerkinElmer TGA4000 at a rate of 10°C/min ranging from 25°C to 850°C, and the study was held for 1 min at 850°C.

3.2.7. X-ray diffraction

The X-ray diffraction (XRD) technique is used to analyze the material for its amorphous or crystalline phase (Li et al., 2021). When a monochromatic X-ray beam strikes a crystal made up of unit cells with atoms arranged in a regular pattern, the X-rays that are dispersed by the various atoms interfere with one another, producing intense XRD in some specific directions (Rajagopal et al., 2023; Xiao et al., 2022). Rigaku Ultima IV with a wavelength of 1.540562 Å, filter as Ni, and a scan rate of 2 deg/min was used to produce the XRD pattern for all three materials.

3.2.8. Scanning electron microscopy

Scanning electron microscopy (SEM) is used to determine the microscopic image of any particular material by scanning the surface with a focused beam of electrons. One of the primary testing tools for an in-depth examination of micro areas is the SEM. It benefits from having a strong stereoscopic vision, high magnification, a deep depth of field, and high resolution (Wen et al., 2021). The SEM images are more lucid and have higher magnifications than those taken by optical microscopy. Excitation voltages of 5.00 kV were maintained throughout the experiment in order to eliminate electron cloud and distorted image conditions. Surface morphology for all three materials is analyzed using EVO MA18 (Syed et al., 2020).

3.3. Mortar cube preparation and testing

3.3.1. Compressive strength test

Two different sets of mortar cubes were cast using (a) GGBS and calcined ash (70G30C), and (b) GGBS and uncalcined ash (70G30U). The mortar mix proportions were 70% weight of GGBS and 30% weight of agro-based ash (calcined/uncalcined) with fine aggregate in the ratio 1:3 as per IS 4031–06 (1988). The mortar mixes were activated using 4 M NaOH solution, with a solution-to-binder ratio of 0.35. The mortar cubes of a size 70.6 mm × 70.6 mm × 70.6 mm were cast as per IS 4031:part 6 (1988), and the details of the mixed proportion for the mortar cube for a single trial are shown in Table 1. According to IS 383 (2016), the grading zone II fine aggregate utilized to make mortar was evaluated in accordance with IS 2386-01 (1963), and its specific gravity is 2.66.

Table 1. Mix proportion for mortar cube

Mix no.	Binder materials	Weight of binder (g)	Total weight of binder (g)	Total weight of fine aggregates (g)	4 M NaOH solution (0.35 times binder) (ml)	Mould size (mm)	Reference
70G30C	GGBS	140	200	600	70	70.6 × 70.6 × 70.6	IS 4031:part 6 (1988) For 0.35 alkali solution (Kamath et al., 2021)
	Calcined ash	60
70G30U	GGBS	140	200	600	70	70.6 × 70.6 × 70.6
	Uncalcined ash	60

3.3.2. Water absorption test

A water absorption test in mortar is conducted in order to understand the durability parameter of the mortar mix. The water absorption of the mortar cubes was tested as per IS 3459-02 (1991). For the water absorption test, cubes of 70G30C and 70G30U mortar mixes after 28 days of ambient curing were immersed in water for 24 h and weighed (W1). The cubes were then placed in an oven at 100°C for 24 h, cooled, and weighed (W2). The percentage of water absorption (WA) was calculated using the formula [WA = (W1 − W2)/W2 × 100] (Akthar et al., 2016).

3.3.3. Ultrasonic pulse velocity

A common nondestructive test to check for cracks, cavities, and other flaws in concrete is the ultrasonic pulse velocity (UPV) test (Kaliyavaradhan & Ling, 2019). A test for UPV measures the speed at which ultrasonic pulses with a frequency of 50–58 kHz, produced by an electroacoustic transducer, travel from one surface of an element to another. The density and elastic characteristics of the material being evaluated affect the ultrasonic pulses’ passage duration (Faraj et al., 2022). The UPV test was conducted using a Tico Ultrasonic Instrument, where the two transducers of the instrument are placed on opposite faces of the mortar surface and the transverse time (t) taken for the signal to pass from the input transducer to the output transducer is recorded. The distance between the transducers (L) is noted to be 70.6 mm, which is the mould size. The UPV is determined using the formula UPV = L/t in km/s as per IS 516 Part 5/Sec1 (2018).

3.3.4. Loss on ignition

The loss on ignition (LOI) test calculates the weight loss that occurs when a sample is ignited at a high temperature, simulating the impact of pouring molten metal into sand moulds. When the test is not conducted in an inert atmosphere, the loss is a sign of weight loss through gas formation, loss of chemically bonded water, and some weight increase from oxidation (Anwar et al., 2021). The LOI of the mortar binder mix was conducted using a gravimetric technique where the temperature was maintained at 850°C as per IS 8112 (2013). The percentage of the total mass reduction from each binder mix is considered to be the LOI of the corresponding mix.

4. Results and discussion

4.1. Material characterization

4.1.1. Physical and chemical properties

The physical and chemical properties of GGBS, calcined ash, and uncalcined ash using XRF, BET, and Le Chatelier’s flask method are shown in Table 2.

Table 2. Chemical and physical properties using XRF, BET, and Le Chatelier’s flask method

Materials		Calcined ash (% mass)	Uncalcined ash (% mass)	GGBS (% mass)
Chemical Properties	CaO	15.65	8.205	51.718
	SiO2	21.818	17.08	3.827
	Al2O3	1.683	0.944	2.075
	Fe2O3	2.350	8.727	1.243
	K2O	61.717	68.099	0.469
	Na2O	1.08	0.00	25.598
	MgO	1.878	0.494	7.964
	SO3	0.664	0.977	0.078
	MnO2	1.258	2.302	4.496
	TiO2	0.220	0.421	2.533
	pH	12.30	12.50	10.12
Physical Properties	Loss on ignition	-	-	0.26
	Specific gravity	2.44	2.52	2.85
	Specific surface area/fineness (m^2/g)	1.78	1.70	0.40

From Table 2, it is observed that even though the specific gravity of GGBS is slightly higher than agro-based waste ashes, indicating it to be a much more stable material to be used as a binder (Netinger Grubeša et al., 2016), the calcined and uncalcined ash has a specific gravity ranging closer to GGBS, indicating the materials to form a homogenous and relatively denser mix when used in the preparation of alkali-activated binder mix. The specific surface area for calcined ash is 1.78 m²/g, for uncalcined ash is 1.70 m²/g, and for GGBS is 0.40 m²/g, which indicates the materials to have a lesser specific surface area, indicating lesser water absorption of the materials (Hossain et al., 2019). The XRF indicates the presence of a high concentration of silica, calcium oxide, and potassium oxide in both the ashes that would act as a pozzolanic material to promote the alkali-activation process and help in the formation of CSH and KASH gels (Kamath et al., 2021; Lima et al., 2021, 2022), while GGBS has a higher concentration of calcium oxide acting as a latent hydrate than a pozzolanic material, due to the higher presence of CaO with very less silica (Cheah et al., 2021; Deboucha et al., 2017). The combination of these binder materials would provide a great pozzolanic and cementitious material that can be activated using alkali-activating material such as sodium hydroxide (Tantri et al., 2021).

4.1.2. Particle size distribution

It is well understood that the particle size distribution of the binder materials has a significant impact on mortar and concrete properties (Alderete, 2016). From the particle size distribution graph as shown in Figure 3, the mean particle sizes of GGBS, calcined ash, and uncalcined ash are 15.82 µm, 21.52 µm, and 26.42 µm, respectively. Even though all three materials have a closer range of particle size, GGBS, and calcined ash are comparatively finer than uncalcined ash, indicating that the mix of calcined ash and GGBS will form a more homogeneous and reactive mix as compared to the mix of uncalcined ash and GGBS (Tantri et al., (2022).

Figure 3. Particle size distribution of GGBS, calcined ash, and uncalcined ash.

4.1.3. Puntke test

The Punkte test provides a solution for optimizing cementitious materials in a wet state that strongly matches the actual scenario of cementitious materials in paste (Babu, 2016; Nanthagopalan et al., 2008). The packing density of the anhydrous binder materials is shown in Table 3, indicating the packing of GGBS and calcined ash to be 0.56 and 0.53, respectively, which is comparatively higher than that of uncalcined ash having 0.48. The reason for the lesser packing density of uncalcined ash is mainly due to its larger specific surface area asking for a larger volume of water requirement V₂ = 3.4, which is greater than the volume of uncalcined ash taken V₁ = 3.2 (Hossain et al., 2019). Indirectly increased packing density states boost in mortar strength (Babu, 2016). Hence, the combination of calcined waste ash and GGBS would provide comparatively higher strength than the combination of uncalcined waste ash and GGBS.

Table 3. Packing density of binder materials

Material/Mix	V2 (Volume of Water) (cm^3)	V1 (Volume of Solid) (cm^3)	Øp (Packing Density)
Calcined waste ash	3.20	3.67	0.53
Uncalcined waste ash	3.40	3.20	0.48
GGBS	6.60	8.42	0.56

4.1.4 Reaction degree

A discrete way of assessing the reactivity of any binder material is the reaction degree using the selective dissolution method (Fernández-Jiménez et al., 2006). It can also be used to determine how reactive hydrate and anhydrous materials are (Puligilla & Mondal, 2015). The reaction degrees of the anhydrous binder materials are shown in Figure 4, indicating the reactivity of GGBS and calcined ash to be 91.5% and 89.7%, respectively, which is comparatively higher than that of uncalcined ash having 78.9%. From comparing both reaction degree and the Puntke test, it can be seen that calcined ash along with GGBS as binder material has higher packing density and reaction degree compared to that of uncalcined ash, indicating it to provide more void-free binder mix with better reactivity and higher compressive strength during alkali-activation process (Babu, 2016; Gao et al., 2017; Kocaba et al., 2012).

Figure 4. Reaction degree of GGBS, calcined ash, and uncalcined ash.

4.1.5. Thermogravimetric analysis

The Thermogravimetric (TG) graph for all three materials plotted using Origin Pro 2022 is shown in Figure 5. The total percentage of mass loss of the GGBS and calcined ash is 3.274% and 17.881%, respectively, while the uncalcined ash shows a total percentage of 32.063%. The TG graph for all three binder materials can be separated into three separate phases to make it easier to understand the mass loss of those materials (Aramesh et al., 2022). During phase 1, dehydration (at a range of 50–400°C) takes place where the moisture content present in the binder materials gets evaporated, in this case, the larger amount of moisture content can be witnessed in uncalcined ash compared to that of GGBS and calcined ash (Deboucha et al., 2017). Phase 2 is dihydroxylation (at a range of 400–600ºC), where the hydroxyl group (OH) is released in the form of water molecules, and in the present study, all three binder materials exhibit carbon monoxide and carbon dioxide, which are produced when calcium carbonate is broken down, in roughly equal amounts (Zhou et al., 2019). The final phase is decarbonation (in a range of 600–850ºC) where unburnt carbons and impurities present in binder materials get disintegrated completely, in the present scenario the uncalcined ash tends to consist of a large number of impurities, indicating that it is unstable as compared to calcined ash and GGBS (Bernal et al., 2017). The TGA results showed a drastic mass loss for uncalcined ash when compared to that of calcined ash and GGBS, indicating a higher percentage of impurities and less reactivity of the material.

Figure 5. TG graph of GGBS, calcined ash, and uncalcined ash.

4.1.6. X-ray diffraction

The XRD graphs of GGBS, calcined ash, and uncalcined ash are plotted using Origin Pro 2022 as shown in Figure 6. The XRD of GGBS represents an amorphous phase with only noises and no peaks indicating its higher reactivity. The GGBS is mostly amorphous, as evidenced by the vast diffusional amorphous hump between 25° and 35° at 2Ɵ. Due to the amorphous phase of GGBS, the presence of a crystalline phase cannot be detected (Almutairi et al., 2021; Kamath et al., 2021). The XRD of calcined ash shows a semi-crystalline phase with larger peaks of potassium carbonate (K₂CO₃) (ICSD IDs: 662) and feldspar (KAlSi₃O₈) (ICSD IDs: 83534, 83535, 83536) along with noises, while XRD of uncalcined ash shows complete crystalline phase with only peaks of potassium carbonate (K₂CO₃) (ICSD IDs: 662) which indicates uncalcined ash as having less reactivity compared to calcined ash.

Figure 6. XRD graph of GGBS, calcined ash, and uncalcined ash with reference to International Crystal Structure Database (ICSD) standards.

4.1.7. Scanning electron microscopy

The SEM image analysis helps in understanding microstructural properties along with the reactivity of the binder materials (Kocaba et al., 2012). The structural morphology of the GGBS is completely different from that of the ashes (calcined and uncalcined). The SEM images of all three materials with 10 µm zoom and 4.00 KX magnification are represented in Figure 7. The SEM of calcined and uncalcined ash both show coalescence of the particles, forming agglomerate structures indicating the presence of potassium oxide (K₂O) in the form of potassium carbonate (K₂CO₃) in abundance, which is verified from the XRD graph in shown Figure 5. The presence of K₂O in the form of K₂CO₃ would help in the formation of KASH gels when activated along with GGBS (Lima et al., 2021). The presence of the SEM of GGBS shows irregular and angular structures, while the XRD indicates the presence of an amorphous phase required for high reactivity (Kamath et al., 2021).

Figure 7. SEM of (a) uncalcined ash, (b) calcined ash, and (c) GGBS.

4.2. Mortar cube preparation and testing

4.2.1. Compressive strength test

After the cubes were cast, they were kept steady for 24 h till they hardened, and later the cubes were demoulded and maintained for ambient curing. The tropical climate with a temperature of 30 ± 3°C was utilized for the ambient curing. The cubes were then later tested for their compressive strength after 3, 7, and 28 days using a compression testing machine as per IS 516 (1959). The results of the compressive strength test are shown in Figure 8, indicating that the cubes prepared using calcined ash (70G30C) tend to provide the highest compressive strength of 23.46 MPa as compared to cubes prepared using uncalcined ash (70G30U) having a compressive strength of 21.13 MPa at 28 days. The increment in the strength is very minimal due to the use of low molar NaOH solution (4 M), which may tend to increase with the increase in the molarity of NaOH (Parthiban & Saravana Raja Mohan, 2014). Due to the low molar NaOH solution (4 M), both the mortar mixes did not show much improvement in strength, owing to the lesser formation of NASH gels. The finer particle size, higher reaction degree, better packing density, lesser mass loss, and larger amorphous phase of calcined ash as discussed above have all contributed significantly to the higher strength development for the 70G30C mix when compared to the 70G30U mix (Babu, 2016; Bernal et al., 2017; Gao et al., 2017; Kamath et al., 2021; Kocaba et al., 2012; Tantri et al., (2022).

Figure 8. Results of compressive strength test for 70G30C and 70G30U cubes.

4.2.2. Water absorption, UPV, and LOI

The mortar blocks were also tested at 28 days for ultrasonic pulse velocity (UPV), loss on ignition (LOI), water absorption, and wet compressive strength, the results of which are shown in Figure 9. The water absorption of the mortar blocks (70G30C and 70G30U) is not mainly influenced by the NaOH concentration alone (Parthiban & Vaithianathan, 2015). The water absorption of the 70G30C mortar block (9.19%) is 22.77% lesser than 70G30U mortar block (11.9%), the reason for this is the better packing density, finer particle size, and higher reaction degree of the calcined ash and GGBS compared to that of uncalcined ash (Tantri et al., (2022). The less water absorption of 70G30C mortar block compared to 70G30U mortar block indicates fewer voids and better durability parameters of 70G30C mortar blocks.

Figure 9. Comparative study on mortar cubes (70G30C and 70G30U).

The low water absorption of 70G30C mortar blocks in turn influences the wet compressive strength, providing a higher wet compressive strength of 22.83 MPa due to the lesser voids, making it much more compact and dense. Based on IS 516 Part 5/Sec1 (2018), it is clear that 70G30C mortar blocks have better grading quality with UPV of 3.15 km/s compared to 70G30U with UPV of 2.56 km/s. Due to the presence of a larger number of uneven voids in 70G30U mortar blocks, the time taken for the pulse signal to travel from the input transducer to the output transducer is very high compared to 70G30C mortar blocks, which causes a decrease in UPV of 70G30U mortar blocks (Kamath et al., 2021). Even the LOI of 12.58% for 70G30C was comparatively better than 70G30U with an LOI of 16.34% shown in Figure 9. This indicated that the mix 70G30C consists of lesser impurities compared to that of 70G30U that gets ignited during LOI at 850°C (Anwar et al., 2021). The impurities present in the 70G30U mix is due to uncalcined ashes.

5. Conclusion

The following are the conclusions that can be interpreted from the study:

• The material characterization of the agro-based waste ash (both calcined and uncalcined) along with GGBS helped in understanding the micro-structural properties of these materials before using them in the AAB system.

• Calcined and uncalcined ashes are rich in calcium oxide, silica, and potassium oxide that act as latent hydrates and partial pozzolan, while GGBS having a large calcium oxide concentration has latent hydrate properties.

• Agro-based waste ash has a larger concentration of feldspar and potassium carbonate as determined by XRD and SEM, which are responsible for KASH gel formation in the AAB system.

• Calcined ash shows a semi-amorphous phase as determined by XRD, indicating higher reactivity than uncalcined ash which has a totally crystalline phase.

• When subjected to TGA, the calcined ash and GGBS have finer particles, better packing density, and lesser mass loss than uncalcined ash.

• The 28 days compressive strength of mortar mix 70G30C increases by 11.02% compared to 70G30U mix, while water absorption of mortar mix 70G30C decreases by 22.77% compared to 70G30U mix, indicating 70G30C as a better mix prepared using calcined ash.

• The mix 70G30C cubes have better grading quality with UPV of 3.15 km/s compared to 70G30U with UPV of 2.56 km/s indicating it to be of poor quality.

• The mix 70G30C consists of lesser impurities compared to that of 70G30U, indicated by the LOI results.

• While considering the AAB system using agro-based waste ashes (calcined and uncalcined), the calcined ash has finer particle sizes, lower mass loss, larger amorphous phase, and higher reaction degree that contributed to the higher compressive strength, lower water absorption, higher UPV, and lower LOI for 70G30C mix.

Disclosure statement

The authors have no relevant financial or non-financial interests to disclose.

Data availability statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request. We obtained our dataset for XRD analysis from ICSD database: https://icsd.fiz-karlsruhe.de