Agro-industrial-based wastes as supplementary cementitious or alkali-activated binder material: a comprehensive review

An attempt to shed light on the use of agro-industrial-based wastes as supplementary cementitious material (SCM) or as alkali-activated binder (AAB) material is mainly focused in this review paper. The SCM or AAB acts as a suitable replacement for cement, as the cement manufacturing tends to contribute to 8–10% of global CO2 emissions. To gain acceptability in the construction industry, diverse agro-industrial-based wastes must demonstrate their potential as precursors in AAB or as SCM. The binder materials discussed among the agricultural wastes are rice husk ash, olive waste ash, and coconut-based waste ash, and the industrial wastes are ground granulated blast furnace slag, fly ash, and ultra-fine steel slag. The paper also analyses the effects of calcined and uncalcined agro-industrial-based wastes on pozzolanic properties of binder system. It is clear from the studies that the calcined wastes tend to provide greater reactivity compared to the uncalcined wastes, that helps in early strength development in the binder system. The review findings indicate that the agro-industrial-based wastes provide pozzolanic and hydraulic properties, thereby making them suitable as an AAB or as a SCM.


Introduction
Cement as a binder is being used in concrete and other concrete-related applications for about two centuries. With time, its applications have become more popular, thereby leading to its increased production which automatically led to environment-related problems [1]. The reason for the environmental issues is the fact that 8-10% of global CO 2 emissions are contributed by cement production. An increase in cement production in the future will lead to an increase in CO 2 in the atmosphere [2]. Owing to environmental problems posed by the cement production, it becomes a necessity to decrease the amount of CO 2 emitted into the atmosphere by utilizing various industrial and agricultural by-products. The utilization of these by-products would reduce the dumping of the industrial and agricultural wastes produced, and will also help in increasing the production of sustainable agricultural and industrial products in the global market [3].
Researchers are conducting studies to understand various characteristics of the binders prepared using various agricultural and industrial wastes. As the agro-industrialbased waste materials, such as silica fume (SF), ground granulated blast furnace slag (GGBS), rice husk ash (RHA), bagasse ash (BA), fly ash (FA), and coconut-based waste ash (CBWA) shown in Fig. 1, are rich in silica, they are largely incorporated in concrete since a few decades. From the studies on the use of a few of above-mentioned by-products, it is observed that they provide various merits such as enhanced durability and strength properties, depletion in cost of construction by reducing cement utilization, benefits the environment by reducing the emission of carbon dioxide, and also contributes to easy disposal of the waste materials that otherwise pollute the environment [4].
Similar to the conventional form of the cement-dependent binder system, nowadays, alkali-activated binders (AAB) are used, which possess various properties similar to the conventional one. Different raw materials can be turned into useful cementitious-like compounds with little environmental effect via alkali activation [8]. AAB properties influence the workability, temperature requirement, and setting time of the concrete. It has been found that fresh properties of AAB can be subjected to specific applications depending on the processing condition and mix design [9]. The alkali-activating materials (AAM) provide an alkaline environment helping the AAB materials to react and form low CO 2 cementitious binders. Alkalis are widely available, and researchers have discovered abundant sources of silica and alumina in a variety of industrial wastes [5]. The commonly used AAM are sodium hydroxide (NaOH), sodium silicate (Na 2 SiO 3 ), potassium hydroxide (KOH), potassium silicate (K 2 O 3 Si), calcium hydroxide (Ca(OH) 2 ), sodium carbonate (Na 2 CO 3 ), etc. There are mainly two issues related to the preparation of AAM, the first one being the problem of handling a high concentration of viscous and corrosive alkaline liquid, while the second is the necessity for heat curing in order to boost the polymerization and activation. AABs are generally subjected to 50 °C and above temperatures for several hours as part of heat treatment [10]. The cost of global production, which is dependent on several factors such as the location of the construction site, the location of the precursors, and the mode of transportation used to move both the raw materials and the finished product, is one of the drawbacks of using alkali-activated cement as a replacement for traditional cements in some applications [11]. These problems need to be addressed in order to make these materials popular and accepted widely as a recognized building material.
The total wastes generated and cement manufactured globally per year are 2.01 billion and 4.1 billion tonnes, respectively, as shown in Fig. 2a, which would keep on increasing in the coming years [12][13][14]. Out of these wastes material, 44% of the contribution is from agricultural and food wastes, while remaining 56% contributes from industrial and other wastes, which is shown in Fig. 2b [12,14]. According to "CemWeek research's 2020 update of the Global Ground Granulated Blast Furnace Slag Market Report and Forecast", total amount of GGBS production is about to reach 269 million tonnes between 2020 and 2025, while fly ash production which was 225 million tonnes in year 2020 would also drastically increase [15]. The silica fume produced globally every year is around 1 million tonnes [16], while by-products such as metakaolin, ferrochrome slag, and ultra-fine slag contribute to the remaining portion of industrial wastes produced, as shown in Fig. 3a. As for the agricultural wastes, 70 million tonnes of rice husk ash is produced globally every year, 40 million tonnes/year of olive mill waste, approximately 45 million tonnes/year of sugarcane bagasse ash, and 30.5 million tonnes/year of corn stalk ash, while remaining amount is contributed by other Fig. 1 a Fly ash; b silica fume; c GGBS; d rice husk ash; e bagasse ash; and f coconutbased waste ash [5][6][7] agricultural and food wastes as shown in Fig. 3b [17][18][19][20][21]. Various studies have been and are being conducted on the utilization of industrial wastes in concrete production. The application of agro-industrial-based wastes in the construction industry would be a game-changing opportunity to reduce or reuse the amounts of waste which are produced by the industrial and agricultural sectors. This would also provide a sustainable solution to the problems caused due to agro-industrial-based wastes dumped in landfills [22].
As these by-products from industries and agricultural sector are abundantly available, they can be used in construction industry along with cement for small building works such as constructing footpaths, plastering, interlocks, and grouting, when procured in large quantity, which is less expensive. Its application varies depending on its usage, whether used in alkali-activated slag mortars as a replacement to cement or used as a replacement of natural coarse aggregate providing a better strength and durability properties [23][24][25]. The major drawback in the use of these materials is higher water absorption rate due to the large surface area of the waste materials, difficulty in handling alkali-activating material, difficulty in large-scale practical applications, and burning of wastes for better reactivity. For long, agro-waste burning has been a major environmental concern in agricultural sector, as it degrades air quality and causes serious health issues [26,27]. Calcined wastes provide a better reactive phase, which not only tends to improve pozzolanic properties, but also helps in optimizing the amorphous silica present in it [28][29][30][31]. While, on the other hand, the uncalcined wastes ashes tend to be in the amorphous phase and have higher carbon content compared to that of calcined ash [32]. Therefore, the present review not only focuses on the study of agro-industrial-based wastes as SCM or AAB materials, but also the pozzolanic effects on the properties of binder   [15][16][17][18][19][20][21] 125 Page 4 of 23 system when it is calcined and uncalcined. The graphical abstract representing the present study carried out for agroindustrial-based wastes used as SCM or AAB materials, and its effect after calcination is shown in Fig. 4.

Agro-industrial-based wastes
The agricultural and industrial sectors are major contributors of by-product wastes in countries such as India, China, and the US which are generally dumped as landfills [33]. A controlled combustion procedure (i.e. 700-900 °C) known as calcination transforms amorphous silica in agro-industrial wastes into highly reactive silica while retaining the ashes. Non-reactive crystalline silica is produced by heating it above 900 °C [34]. Generally, two types of ashes develop when the agricultural wastes are burnt for producing biomass fuel, which are biomass bottom ash (BBA) and biomass fly ash (BFA) [35]. Due to its course granulometric composition, BBA is not widely used in the manufacturing of alkaline cement and concrete. The surface area and reactivity of this substance should be increased by grinding it more. Forming blends with various active aluminosilicate materials is another possible technique to boost the reactivity of BBA precursor [36]. The main ashes generated form industries are GGBS and fly ash, which are further divided into various categories depending on its oxide compound and other properties [37]. RHA, olive waste ash (OWA), coconut-based waste ash (CBWA), etc., are a few of the agro-based wastes used in concrete.

Fly ash
Fly ash (FA) is the fine residual particles generated from the coal-fired power plants, which can be used as a SCM as well as to prepare AAB concrete [38]. These are the aluminosilicate materials which may or may not have high calcium oxide concentration, based on which they are classified into Class C and Class F, respectively, as per the American Society for Testing and Materials (ASTM C618-12a) [39]. The majority of "Class F" FA is obtained by burning bituminous coal with a mixture of SiO 2 , Al 2 O 3 , and Fe 2 O 3 concentrations of more than 70%, while the "Class C" FA is obtained by burning sub-bituminous coal in the range of 50-70% of above-mentioned components. Based on the field emission scanning electron microscope (FESEM) study of FA, it is found to have relatively smooth and spherical shaped articles as shown in Fig. 5.
The major chemical composition of oxide components in "Class F" FA and "Class C" FA are shown in Fig. 6, which tend to vary from maximum to minimum values depending on the source [41]. FA has specific gravity of around 2.15, is considered to be pozzolanic in nature and in the absence of chemical activators such as lime or cement display very less self-hardening properties [42]. As the existence of crystalline or amorphous phases of FA determines the reactivity of fly ash, its composition alone is insufficient to understand its behaviour [43,44].
The maximum proportion of fly ash used in the making of Portland Pozzolana cement is limited to 35%, according to an Indian standard (IS 1489-1991) [45]. When utilized at more than 35 per cent of replacement, FA is unable to improve the strength qualities in its natural state. In such circumstances, FA-cement binder mix must be modified to produce additional hydration products, which would improve the strength [41]. The modification includes use of nano modified fly ash and alkali activation process in  Chemical composition of Class F and Class C fly ash [41] FA-cement binder mix to accelerate its early-age hydration. The nanoparticles operate as nuclei for cement, speeding up the hydration process and densifying the microstructure as well as interfacial transition zone (ITZ), thus lowering permeability [46]. While in alkali activation process, the FA is mixed with various binder mixes such as GGBS, palm oil fuel ash, rice husk ash, and silica fume, and activated using AAM such as NaOH [24, 47,48]. Best temperature endurance was witnessed in the binder mix containing 75% of GGBS and 25% of FA, and with complete replacement of natural coarse aggregate by ferro chrome slag (FCS) [25]. On studying its durability properties, it was concluded that the suitable AAB concrete mix would be obtained using 50% of GGBS, 50% of FA, and complete replacement of natural coarse aggregate by FCS, giving a better performance [24]. The application of FA in a quaternary binder system also proved to provide an eco-friendly zero-cement concrete made with 30% by wt. GGBS, 65% by wt. FA, 5% by wt. nano-silica, and 15% by wt. of binder as hydrated lime giving a compressive strength of 42.5 at 28 days, which satisfies the minimum strength requirement of ASTM standard, i.e. 41.4 MPa [49]. The influence of the SiO 2 /Al 2 O 3 molar ratio (3.25, 3.37, 3.50, and 3.75) on the characteristics of FA-based geopolymer with FA (78%, 85%, 92%, and 96%) and GGBS (22%, 15%, 8%, and 4%) was studied, and the maximum compressive strength was obtained for the SiO 2 / Al 2 O 3 ratio of 3.37 which was mainly due to the Al 2 O 3 -SiO 2 bonding in the amorphous region comprising a larger portion of the microstructure of the geopolymer. It was also found that increasing FA with GGBS increased the compressive strength of geopolymer mixes and improved the setting time [50].
To understand the reactivity of fly ash, it is crucial to determine the quantity of reactive crystalline and amorphous phases. The reactive crystalline phase of the fly ash is developed only after it is calcined. For a concrete prepared by partially replacing cement with calcined FA, the slump value decreases largely as compared to the uncalcined FA [51]. The decrease in slump is due to the finer particle sizes in calcined FA, which fills the pores in concrete, thereby increasing the density as shown in Fig. 3. On the other hand, the water absorption ability of the calcined FA is more as compared to that of the uncalcined FA as shown in Fig. 7.
Irrespective of whether calcined or uncalcined fly ash is used as partial replacement in a binder, significant improvement in strength was observed as shown in Fig. 8, as the small pore structure of fly ash concrete adds to greater compressive resistance. Calcined fly ash has been proven to reduce the pore volume of fly ash-cement paste when compared to uncalcined fly ash. The calcination of fly ash reduces or eliminates all organic components, which may have a negative or non-existent impact on the concrete's strength.
The major drawback of using FA as AAB is its longer setting time, which would otherwise require high-temperature curing for early setting. One of the main issues about using Class C fly ash in Portland cement concrete is that its durability is inconsistent and inferior to that of Class F fly ash. According to several studies, Class F fly ash can be utilized to attenuate the alkali-silica reaction; however, its performance is better at lower replacement rates due to the reduced alkali concentration [52][53][54]. In case of longterm durability study of fly ash-based geopolymer concrete, pores refinement can be seen which would reduce the permeability, and also provides a higher resistance to chloride attack [55].

Ground granulated blast furnace slag
GGBS is a greyish-white-coloured powdered material that is formed from the waste by-product of the iron and steel industries. In the construction field, the cement industry has been using GGBS as partial replacement of cement on a large scale to produce low-to-moderate strength cement [56]. The AAB pastes produced with several GGBS from various industries had vastly varied setting behaviours, which are influenced by their particle size, oxide components, and specific surface area.
It has a specific gravity ranging between 2.4 and 2.9, and the specific surface area of GGBS may range from 0.3 m 2 /g to 0.5 m 2 /g [58]. The texture of particles of GGBS indicates irregular and angular shapes as shown in Fig. 9. It has been found through studies that the reaction rate increases after including alkalis such as Ca(OH) 2 , NaOH, and Na 2 SO 3 , thereby improving the setting time and strength gain of the GGBS concrete [40]. The major chemical composition of oxide components in GGBS which tend to vary depending on the source are shown in Fig. 10. GGBS also tends to show a crystalline phase due to the components called merwinite (Ca 3 Mg (SiO 4 ) 2 ) and melilite [59].
GGBS hardens slowly on its own and must be activated with OPC before being used in concrete. The most frequent mix is 50% OPC and 50% GGBS; however, GGBS ratios ranging from 20 to 80% are also prevalent. The influence on concrete characteristics will be larger as the percentage of GGBS increases [60,61]. The properties of the high strength and stable ternary binder system using GGBS, FA, and metakaolin (MK) were studied, and the FA-based ternary sample consisting of 30% FA, 50% GGBS, and 20% MK showed higher compressive strengths of 102.04 MPa after 56 days. With the generation of different gels such as NASH, CASH, CSH, and (N, C)-ASH, the ternary mixes showed a homogenous and heavier microstructure [40]. Mortars prepared using AABs such as metakaolin (MK) and GGBS had the highest 28 days flexural strength of 9.28 MPa and compressive strength of 82.5 MPa, respectively, which were around 41 per cent and 29 per cent greater than the corresponding OPC mortar [62]. Maximum compressive strength of 63.8 MPa was obtained with smaller particle sizes of GGBS having a mean particle size of 3.1 µm which   [40,57] was activated with higher dosage of 6% Na 2 CO 3 [63]. GGBS is frequently combined with fly ash (Class F) to increase the alkali-activated concrete response mechanism [64]. GGBS can be used alone in alkali-activated concrete, although the high calcium concentration of GGBS accelerates the reactivity of alkaline binders, resulting in an early setting time [65,66]. The GGBS-based AAB proved to provide better strength improvement compared to that of FA-based AAB subjected to ambient curing technique, while the water curing technique is not suitable in the AAB system consisting of GGBS owing to the problem of leaching alkali [67]. In a one-part AAB utilizing GGBS as the precursor to provide high strength mortar, 10-20% increase in the compressive strength of mortar were obtained on blending GGBS, phyllite dust, along with silica fume (SF) ternary mixes of AABs, reporting to give 145 MPa at 28 days. This was the highest strength reported in a one-part AAB system, which also provided the potential of an economic benefit in terms of its availability [68].
GGBS is obtained as the calcined by-products from the iron and steel industries, thereby indicating the absence of uncalcined GGBS for research. But there were studies on GGBS stating various types (slag types) based on their particle size. Figure 11 shows particle size distribution of slags I, II, and III of various sizes along with the compressive strength of FA (30%)-slag (70%) mortar prepared using sodium carbonate [63]. Form the figure, it is clear that at 28 days, the highest compressive strength of slag III activated 30% FA and sodium carbonate was 55.5 MPa, and at 90 days, it was 63.8 MPa.
As GGBS is a by-product of the iron manufacturing sector, it is estimated that the energy used during 1 tonne GGBS production is around 1300 MJ, compared to 5000 MJ for the production of 1 tonne OPC. The mineral extraction done for the production of OPC is around 1.5 tonnes and produce 0.95 tonnes of carbon dioxide equivalent. GGBS, on the other hand, would produce only 0.07 tonne of CO 2 equivalent of 0.07 tonne. The major limitation during the GGBS replacement is the percentage of strength increment is up to certain limits past which the decrease in percentage of strength can be witnessed due to increase in GGBS percentage in concrete. While the later strength increment can be witnessed due to slower rection between GGBS and Ca(OH) 2 [61].

Ultra-fine slag
Ultra-fine slag (UFS) or ultra-fine blast furnace slag is a fine material obtained from the controlled granulation of the by-product of steel mills. Since it has a large content of calcium oxide (CaO) and silicate (SiO 2 ), this material can be used as a binder in producing geopolymers. The range of specific gravity and specific surface area of the UFS is found to be 2.19 to 2.7 and 1.2 m 2 /gm and above, respectively. Calcium oxide and alumina content are in higher proportion in UFS, which would help in the formation of calcium-based supplemental hydration products in the binder matrix [69]. Figure 12 shows the SEM image of the ultra-fine slag giving smooth spherical particle shape and indicated the presence of crystalline phase, calcite from the XRD.  [58,59] UFS has a similar situation to that of GGBS where no studies conducted on calcined and uncalcined UFS are found, since UFS is obtained as the by-products from the iron and steel industries wherein the formation of UFS itself is taking place in calcined condition. The UFS can be divided into two categories, readymade ultra-fine slag (RUFS) which can be directly obtained from steel industries and grounded ultra-fine slag (GUFS) which are generally synthesized by grinding the precursor slag in dry ball mill [71]. The major chemical composition of oxide components in UFS tend to vary depending on the source as shown in Fig. 13.
The percentage replacement of UFS in a concrete is similar to that of GGBS. While replacing the cement with 0, 5, 10, 15, and 20% of UFS, the strength development at 3, 7, and 28 days is shown in Table 1 and Fig. 14. When compared to other replacements of UFS, the 20% replacement had a greater strength, due to the high CaO level in UFS,   [75] providing both pozzolanic and hydraulic reactivity, resulting in a denser pore structure and higher strength. As compared to control concrete mix, all UFS mixes demonstrated early-age strength and the ultimate strength of all the UFS concrete mixes were higher than that of PPC concrete. At 3 days, the 20 per cent replacement mix had a compressive strength that is 29% higher than PPC concrete. As a result, UFS is considered to be a good microfine material for precast industries and may be utilized in concrete for high early strength [75]. UFS is also used in an AAB system consisting of different waste materials used as binders having various particle sizes. The particle size distribution of the ultra-fine slag along with corncob ash and rice husk ashes (RHA) is depicted in Fig. 15, which indicates that UFS has more finer particles with mean particle size of 4.4 µm as compared to RHA and corncob ash [70]. Ultra-fine slag helps in the early as well as final strength development of the concrete. The overall surface area for reaction grows as the fineness of the UFS increases, thereby increasing the apparent rate of hydration and pozzolanic reactions at an early age [76]. With the inclusion of UFS, the structure becomes stronger and more resistant to deformations induced by applied forces [77].
The strength parameters vary depending on the percentage of the UFS as an addition or as a replacement to the binder system. At the age of 28 days, the characteristic compressive strength of concrete replaced with 10% RUFS had practically reached the required compressive strength of 51.8 MPa. Beyond 10% replacement of cement with RUFS, there was no substantial increase in performance. Cement replacement with GUFS having lower particle sizes performed better than cement replaced with RUFS in concrete [71]. At all ages, sustainable geopolymer concrete with UFS outperformed RHA-based geopolymer concrete in terms of mechanical and durability. The results demonstrated that adding UFS to a geopolymer can improve its characteristics [70]. With the addition of UFS and greater RHA, the compressive strength of RHA geopolymer concrete is enhanced. RHA geopolymer concrete may also achieve a much greater compressive strength of 50 MPa at 28 days when RHA content of 400 kg/m 3 is combined with 10% UFS [69]. While at 28 days and 90 days, the compressive strength of concrete prepared using UFS containing 20% FA activated with 4% sodium oxide equivalent (Na 2 O-E) of sodium sulphate (NS) reached up to 41.5 MPa and 60.8 MPa, respectively, as shown in Fig. 16. In comparison with the typical alkaliactivated system, CO 2 emissions from sodium silicateactivated slag/FA were reduced to 53-72 kg/t, a reduction of 45-64 per cent [73]. Since UFS-FA mortar has consistently enhanced compressive strength while emitting very little CO 2 , it is projected to be developed as a replacement material for strong bases-activated slag.

Rice husk ash
RHA is an excellent pozzolana material, and its colour ranges from black to white-grey depending on the raw material source, type of incineration, temperature of burning, and period of burning [4]. The fineness of the RHA is determined by the grinding process, and the mean particle diameter of RHA decreases as grinding energy and duration increases. In a ball mill, the mean diameter of RHA particle drops from 86.2 to 5.7 µm at grinding time of 0-540 min [78]. Because of its honeycomb structure, its specific surface area ranges typically from ten to hundred times more than that of cement, and five to ten times greater than that of silica fume. Brunauer-Emmett-Teller (BET) values of adequately burned RHA are in the range of 100-150 m 2 /g [79]. A maximum specific surface of 274 m 2 /g can be achieved in RHA if it is treated [80]. Many studies show that RHA is not only utilized as a binder material but also can be utilized in Fig. 15 Particle size distribution of UFS along with RHA and corncob ash [70] Fig. 16 Compressive strength of sodium sulphate (NS)-activated UFS-FA mortar [73] the preparation of sodium silicate (Na 2 SiO 3 ) solution. Most commonly, this sodium silicate (Na 2 SiO 3 ) solution is used as an alkali activator for blending fly ash and GGBS, providing better compressive strength and effective setting time. The specific gravity of RHA varies between 2.1 and 2.6, at the same time, the particle size varies from 5 to 96 µm [81]. A minimum of 402 °C is necessary for the release of silica for completely destroying the organic materials from rice husk by isothermal heating [82]. At 500 °C combustion, RHA produced the most reactive silica [82,83]. The reaction mechanism of RHA particle with aqueous solution is shown in Fig. 17 and discussed below [78].
• The amorphous silica present on the exterior surface of RHA particles dissolves in aqueous solution when attacked by OH ions present in the solution. When these amorphous silica dissolves, pores are formed in the RHA particles, which leads to water absorption in the RHA particles. • C-(Na, K)-S-H gels are formed when Ca and OH in the solution reacts with silica, which subsequently precipitate on RHA particle surfaces and can further absorb Na, K, and Al ions. As a result, pozzolanic reaction rim is formed, and RHA particles are partially or totally packed inside the rim. • Over time, thickening of the reaction rim takes place. It functions as a semi-permeable barrier that enables K, Na, Ca, and OH ions to pass through but prevents alkalisilica reaction gels from leaking out when the pozzolanic reaction rim is formed.
• Finally, alkalis combine with silica to form alkali-silica reaction gels, which then react with Ca ions, adsorb water in RHA particle pores, and expand. • The cracking of RHA particles and the surrounding cement matrix is caused by the resulting expanding alkali-silica reaction (ASR) hydrates accumulating inside RHA particles.
There are many studies conducted on utilization of RHA produced at different temperature in concrete. Based on the temperature variation used for preparing RHA, it can be termed as calcined and uncalcined. Figure 18 represents SEM image, particle size distribution, and XRD of RHA burnt at 500 °C which is indeed uncalcined and subjected to quick cooling (RHA500-12Q) and slow cooling (RHA500-12S). The samples RHA500-12Q and RHA500-12S demonstrate that particle size rises considerably in the slow cooling regime as compared to the quick cooling regime. The silica structure of uncalcined RHA is amorphous, as observed with an XRD diffused peak of 80 counts at 22° [83,84]. Figure 19 represents the SEM image, particle size distribution, and XRD of calcined RHA burnt at considerably high temperature of 780 °C, where it provides crystalline structure with two peeks of cristobalite and a trace of tridymite crystalline phases. The particles of calcined RHA are angular and still have porous structure after grinding. The particle size distribution of metakaolin, calcined RHA, and silica fume determined using LS 13,320 laser diffraction showed that RHA is finer than metakaolin, although the silica fume is Fig. 17 Reaction mechanism of RHA particles with aqueous solution [78] equivalent in size to metakaolin. It is found that the calcined RHA has a D50 of 12.5 mm [78,83,85].
The maximum to minimum values of major oxide components of RHA for both calcined rice husk ash (CRHA) and uncalcined rice husk ash (URHA) vary depending on the source of availability as shown in Fig. 20.
RHA as a pozzolanic reactive substance can be utilized in order to enhance the surface area of the interfacial transition zone in the preparation of high-performance concrete. Replacing 25%, RHA would result in making the blended concrete very much impermeable compared to ordinary concrete [86]. As rice husk ash is an agricultural by-product whose disposal is a major problem, the use of RHA in geopolymer concrete with ultra-fine slag at room temperature resulted in a considerable improvement in strength and durability [69]. Also, RHA can be used to make low-cost construction bricks instead of cement. When RHA is used as a partial replacement for energy-intensive Portland cement, it saves both energy and money [79].
A study conducted on the 28th and 91st day compressive strength for concrete in which 10, 20, and 30% of the cement have been substituted with RHA, as shown in Table 2 [96]. The increase in the 28th day compressive strength for concrete containing 10% RHA compared to control concrete prepared with cement was between 15 and 27 per cent for w/b ratios of 0.35, 0.50, and 0.65, while this increase for the 91st day compressive strength was 10-21 per cent. The studies on the 28th and 91st day compressive strengths of 20% RHA concrete were 11-34% and 19-26%, respectively, while for 30% RHA concrete, the results were between 6 to 26% and 7 to 27%, respectively [89].
The compressive, split tensile, and flexural strengths of geopolymer concrete incorporating UFS and corncob ash were somewhat lower than those of UFS and RHA-based geopolymer concrete, indicating it as a better replacement [70]. The maximum compressive strength of 81 MPa and 75 MPa was obtained for 28 days when unprocessed RHA replaced fine aggregate by 10% and OPC by 15%, respectively [93]. All other waste ashes tend to show SiO 2 content similar to FA except for RHA, which is much higher, contributing to early setting and exceptional stability at high temperatures of binders made from RHA [67]. In terms of wider application, silica fume as well as RHA both play a vital role in geopolymer systems based on metakaolin in order to improve its freezing resistance and mechanical properties by improving the pore structure [85]. Even though calcined RHA consumes more energy during its preparation, on comparing the strength parameters of fresh and hardened properties of calcined RHA with the untreated (uncalcined    [28,69,70,80,81,[84][85][86][87][88][89][90][91][92][93][94][95] and ungrounded) RHA, it was found that calcined RHA provides better properties [93,97].

Olive waste ash
Olive crops generate a large quantity of leftover biomass each year (about 2-3 tonnes of pruning wastes from one hectare of olive trees), the majority of which are discarded carelessly [98]. Olive mill wastes such as husk, pulp, and residual oil are burned and pulverized to produce olive waste ash (OWA), which can be further categorized into olive biomass fly ash (OBFA) and olive-stone biomass ash (OBA) or olive biomass bottom ash (OBBA), which possesses pozzolanic characteristics [99,100]. OBFA is formed after combustion of olive mill wastes and can be captured with electrostatic precipitators or fabric filters or cyclone in combustion plants, while OBBA is formed at bottom of the boilers as solid waste [100]. The specific surface area and the specific gravity of OWA range between 0.41-0.42 m 2 /g and 2.13 and above, respectively [101]. Figure 21 depicts the field emission scanning electron microscopy of OBA and SEM of OWA which shows more of porous and irregular shape, and some particles presented a smoother surface which might be due to the unburned olive waste particles. The XRD patterns of olive biomass fly ash and olive biomass bottom ash are given in Fig. 22, where the K (potassium magnesium silica) formation can be seen larger in number giving peek 2θ value, indicating the presence of large concentration of potassium.
The maximum values of major oxide components of calcined OWA and major oxide components of uncalcined OWA varying depending on the source of availability are shown in Fig. 23, indicating major presence of calcium oxide (CaO) ranging from 3.22 to 54.82% and K 2 O ranging from 0.33 to 52.93%, with lower amounts of other oxide components. Several studies have been done in the past on the use of OWA as a partial cement substitute in concrete with 7-22% replacement at controlled temperature providing increased compressive strength after exposing to elevated temperature of 400 and 600 °C [99]. On comparing calcined and uncalcined OWA used in manufacture of bricks, the maximum compressive strength of 58.98 MPa was obtained by specimen consisting of water to chamotte (crushed ceramic clay) ratio of 0.2 and 30% wt of chamotte replacement by calcined OWA [103]. When using OWA as a filler material in self-compacting concrete, it tends to show low workability reducing the performance of sample, on the other hand, higher concentration of alkali like K 2 O along with high concentration of CaO makes it more suitable in the geopolymer matrix [98,104]. Figure 24 depicts the strength variation of concrete on replacing OPC by 0, 5, 10, and 15% of OWA. The compressive strength of the OWA0 and OWA5 mixes exceeded the objective of 25 MPa after 28 days, but not the OWA10 and OWA15 combinations. In general, an increase in the proportion of OWA as a cement replacement results in a decrease in compressive strength of concrete. This decrease in strength owes to the reduction in the amount of cement in the binder paste which would lead to a decrease in the amount of hydration outputs such belite (C 2 S) and alite (C 3 S) responsible for the blended paste's increased strength and absorbency.  [99,102] Alternatively, the interlocking between OWA, cement, and sand particles may be reduced, resulting in an increase in the amount of voids in the concrete [106]. When OWA was mixed with blast furnace slag as an alternative material in AAB system using NaOH and KOH solutions ranging from 4 to 12 mol/kg as AAM, the compressive strength increased to about 31.25 MPa on 30% replacement of blast furnace slag by OWA and 38.38 MPa on 25% addition of OWA to blast furnace slag [105]. When OWA was introduced as a partial replacement of fine aggregate up to 15%, the compressive and flexural strengths increased by up to 21% and 40%, respectively, which was attributable to the OWA's filler effect [107].

Coconut-based waste ash
The coconut-based waste ash (CBWA) includes ashes of waste materials such as coconut shell, coconut fibre, and coconut husk, out of which major alkali activation can be witnessed with coconut shell and husk ash. These waste materials can be utilized as a filler or as a partial replacement of binder in conventional as well as AAB systems. An uncalcined coconut shell ash burnt below 550 °C gave

Fig. 23
Chemical composition of calcined and uncalcined OWA [33, 35, 98-100, 103, 105] specific gravity 1.33, water absorption value of 25%, and fineness modulus 8% [108]. While calcined coconut shell ash is burnt at 650 °C and above for 3 h gave specific gravity 2.3, moisture content of 1.46%, and bulk density 505 kg/ m 3 [109]. The SEM image of both coconut shell ash and coconut husk ash is shown in Figs. 25 and 26, respectively. It is clear from the images that the coconut shell ash shows spherical shape with an average size of 42 µm and a density of 2.04 g/cm 3 [110], while the coconut husk ash shows irregular structure due to thermal change taking place at 800 °C causing variation in particle surface, shrinking, and splitting of particles [111]. The calcined CBWA provides finer and reactive product consisting lesser organic components. The XRD of calcined CBWA, calcined in an electric arc furnace at 900 °C for 3 h, revealed the presence of crystalline phase components with varying peak intensities in coconut shell ash as shown in Fig. 27 [110]. The major oxide components of calcined and uncalcined CBWA are shown in Fig. 28 which mainly include calcium oxide (CaO), silica (SiO 2 ), alumina (Al 2 O 3 ), and potassium oxide (K 2 O), while the other oxides components are less than 10%. Both workability and strength show a considerable improvement when coconut husk ash partially replaces cement in concrete [113]. CBWA as a partial replacement of cement (SCM), and also as AAB, has proved to give better performance. The initial setting time and final setting time of OPC replaced with 0%, 2%, 3%, 5%, 10%, and 15% of CBWA increased with increase in percentage replacement, which was caused by the adsorption of water on CBWA surface [114]. The initial and final setting times of concrete containing coconut husk ash were found to be somewhat longer than those of 100% OPC mix, although all were within the required standard [115].
With higher quantities of coconut husk ash, the compacting factor increases, and it was discovered that concrete made with CBWA mixtures is more workable and compactable than concrete made entirely of OPC [115]. Concrete with 10% CBWA and 10% silica fume (SF) replacement provided   [110,112] Fig. 26 FESEM of coconut husk ash [111] greater compressive strength than that of concrete with only CBWA replacement [120]. With addition of CBWA content of 5, 10, and 15% as replacements of cement, the 28-day strengths of concrete were 103.1, 106.2, and 109.8%, respectively, showing an increasing trend of about 3% [121]. The 90-day compressive strength of OPC at 5-10% CBWA replacements was higher than the control values [122]. The variation in compressive strength of the binder system on partially replacing it with CBWA from different studies is given in Table 3.
When compared to the control mix, the mechanical qualities of the concrete/mortar containing CBWA appeared to be adequate. The early or later compressive strength of CBWA concrete/mortar is also affected by the particle size of the CBWA [116].

Conclusion
The present study gives a comprehensive assessment of previous research investigations and current developments in concrete incorporating agro-industrial wastes, ashes from industrial plants, and agricultural farming wastes. It is a challenge to find an inexpensive, long-lasting, and highquality building material for the scientific community, for which many research investigations have been conducted. These studies look at the qualities of concrete made with agro-industrial waste ashes as a pozzolanic additive. Based on the study of research work that has been carried out on agro-industrial-based wastes, the conclusions that are drawn are presented below.
1. The industrial-based waste such as fly ash and GGBS provides great results. The lesser the particle size of industrial-based wastes larger the surface area, which would lead to a stronger bond formation in the AAB system. 2. Calcined industrial-based wastes have increased CaO content, forming high calcium precursors, thus helping in early setting time. While low calcium precursors such as FA would require heat curing due to their lengthier setting time.  [108-110, 113, 115-119] 3. Due to the high silica concentration in agro-based waste, the materials display pozzolanic reactivity, which, in turn, are favourable to concrete's later age strength development. It is observed that improved treatment, regulated burning, and increased fineness allow for higher percentage of agro-based waste replacement of cement in concrete. 4. The comprehensive study of literature shows that fly ash, GGBS, ultra-fine slag, rice husk ash, olive waste ash, and coconut-based waste ashes can be used as pozzolanic materials in cement binder system or as alternative activators in AAB system while maintaining the requisite building material qualities. 5. It is clear from the studies that calcined wastes tend to provide greater reactivity compared to the uncalcined wastes, which help in early strength development of the binder system.
Author contributions Each of the authors confirms that this manuscript has not been previously published and is not currently considered by any other journal. Additionally, the authors have approved the contents of this paper and have agreed to the Journal of Innovative Infrastructure Solutions submission policies. Each named author has substantially contributed to conducting the underlying review and drafting this manuscript. Additionally, to the best of our knowledge, the named authors have no conflict of interest, financial, or otherwise.
Funding Open access funding provided by Manipal Academy of Higher Education, Manipal. The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data availability
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Conflict of interest
The authors have no relevant financial or non-financial interests to disclose.
Ethical approval Human subjects are not involved in the study and hence it is not applicable.
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