Several shapes can be found in fine SCBA particles, including prismatic, spherical, fibrous, and irregular. Spherical particles correspond to the melting of minor components, such as Mg, P, K, Si, Na, Fe, etc. Higher temperatures provide the thermal conditions required for spherical particle formation. Prismatic particles illustrate a crystallization effect within SCBA and it is disadvantageous to the pozzolanic properties present in SCBA. Large coarse fibrous particles indicate unburnt carbon bagasse components present in the SCBA profile ^[5][20][21][14,39,40]. The micromorphology of the final SCBA is influenced by the purity of the bagasse, the thermal conditions inside the boiler, and the biological profile of the sugarcane variant.

2.3. Chemical Properties

Individual samples’ chemical compositions are different from one another and SCBA has a characteristic chemical composition to be generally categorized under class F pozzolan material based on ASTM C618-08a specification. That is, the sum weights of SiO₂, Al₂O₃, and Fe₂O₃ compounds are more than 70% of the total mass of the SCBA sample ^[5][8][7,14].

2.4. Pozzolanic Activity

It is mentioned that application of pozzolans as supplementary cementitious material can improve the mechanical and durability properties in concrete ^{[14][22][23][24]}[24,29,43,56]. Pozzolanic strength in concrete is a result of the pozzolanic reaction between calcium hydroxide compounds present in cement materials from cement hydration, silicates, and/or aluminates in the chemical composition of SCBA and water in the concrete. Calcium hydroxide formation is executed during the cement hydration process where chemicals in OPC (calcium silicates and calcium aluminates) interact with water in the mix to form calcium hydroxides as one of the products. The need for calcium hydroxide is that silicates and aluminates are only soluble in highly basic media ^[25][26][57,58]. Calcium hydroxide molecules are then transported through water to combine with aluminum/silicates. As a result of this chemical reaction, calcium silicates and aluminum silicates are synthesized, which are responsible for the enhanced physical properties in concrete. This phenomenon occurs over longer periods, from months to years ^[25][57]. It is mentioned that curing samples at elevated temperatures within the first five hours of post-mixing enhances the reaction rates in concrete, which results in even stronger concrete ^[25][57]. SCBA samples with high LOI values have to be post-treated prior to using them in concrete as they do not possess acceptable pozzolanic activity. Some unburnt compounds in SCBA could be amorphous in nature and they might have the potential to enhance the reactivity of SCBA, which requires further experimentation to arrive at a conclusion ^[20][39].

2.5. Mineral Composition

Sugarcane crops, which are grown in silicic-acid-rich water-based soil, absorb compounds into plantations and polymerization into amorphous silica occurs inside the plant cells. The combustion process converts silica to reactive amorphous silica, which is identified in SCBA. Crystalline silica in SCBA is precent due to an uncontrolled incineration process and the sand in the soil being taken inside the boiler together with sugarcane bagasse (silica from sand is 4–10% ^[27][61]). Therefore, high amounts of quartz are present in SCBA ^[5][28][6,14]. Other miner minerals that were identified via X-ray diffraction (XRD) analysis using SCBA samples are mentioned as Calcite, Corundum, Hematite, Fluorite, Halite, Bornite, etc. ^[29][42]. Air flow conditions during the calcination process also affect the morphology of SCBA. It was identified that calcination without controlled air flow does not break down long bagasse fibers and, as a result, the LOI value of such SCBA is relatively higher ^[30][62].

2.6. SCBA Characterization

A wide range of characterization methods have been utilized throughout the literature to examine the microstructure and to identify the chemical compounds within SCBA, including Scanning Electron Microscopy (SEM), Energy Dispersive spectroscopy (EDS), X-ray Diffraction analysis (XRD), Thermogravimetric/Differential Thermal Analysis (TG/DTA), Energy Dispersive X-ray (EDX), and Fourier-transmission infrared (FTIR). These studies provided a better understanding of SCBA’s potential for improving concrete properties ^[31][4].

In Figure 23, Image (A)—^[31][4] depicts pores in elongated oval-shaped particles, which absorb water and oxygen. Image (B)—^[32][31] depicts unburnt, carbon-rich fibrous particles. Image (C)—^[33][45] depicts filter bagasse ash prismatic particles from combustion fumes. Image (D)—^[34][47] depicts well-defined burnt flakes of SCBA. Image (E)—^[35][55] depicts well-burnt flakes of SCBA. Image (F)—^[36][63] depicts SCBA particles with lamellar aspect of superimposed layers. From different characterizations, the presence of silica (SiO₂) has been highlighted in SCBA samples in both crystalline and amorphous phases. The roots of the sugarcane plants absorb soil, which then facilitates the formation of silica within the plant body. Pathogenic fungi that can potentially harm the plants are physically restricted from penetrating inside the plant by silica and water transportation within the plants is also facilitated by silica ^[36][63]. Depending on how the bagasse is collected, some sand from the fields may enter the boiler with the bagasse and the final SCBA material collected from the burner frequently contains this crystalline silica material.

2.7. Optimization of Mechanical Properties

The ability to transport, place, compact, fill, and resistance to segregation is generally defined as the workability of concrete. The common standard methods used to conduct testing on concrete to investigate workability are (ASTM C 143), American Association of State Highway and Transportation Officials (AASHTO T 119), or British Standards (BS EN 12350-2) ^[37][65]. It is important that concrete possesses low flow resistance as well, because this reduces safety issues, such as “white finger syndrome”, and minimizes adverse environmental effects, including sound pollution, while concrete placement is being performed ^[38][15].

One of the most important characteristics in concrete is its compressive strength properties, which contribute to the load-bearing capabilities without the occurrence of failure ^[39][67]. ASTM C109, BS EN 196-1, or AASHTO 106-02 are considered globally accepted standard methods of testing.

Tensile strength investigations of mortars from diametral compression were carried out by Pamela Camargo Macedo et al. ^[19][28]. It was identified that 3% replacement of OPC with SCBA was to be the optimal replacement content for enhanced tensile strength. Samples with SBCA content above 3% had lower tensile strengths than the control sample.

2.8. Cost Optimization

The economical aspect of concrete with SCBA was analyzed by Priyesh Mulye et al. ^[8][7] where they identified that normal concrete of grade M25 with mix design ratios 1:1.78:2.86 had a cost 12% more for 1 m³ of concrete compared to the cost of concrete with 15% OPC replacement by SCBA, with the same mix design proportions. SCBA is often produced as a byproduct of the sugar industry, which has a very low economic value. Therefore, in the above literature, the possibility of producing low-cost concrete using SCBA as a partial replacement for OPC is mentioned while the final product is capable of satisfying the standards of defined quality management systems.

2.9. Carbon Footprint Analysis

The contribution of CO₂ to total global greenhouse gases stands out at 77%. The Earth System Research Laboratory from the US National Oceanic and Atmospheric Administration measurements indicated that in 1980, the mean CO₂ concentration was approximately 335 ppm, which later increased to 394 ppm in 2012. CO₂ concentrations have risen to 414.72 ppm in 2021 (see Figure 38) ^[40][75]. To avoid a +3 °C temperature increase, the International Panel on Climate Change announced that the global CO₂ concentration has to be maintained below the level of 450 ppm ^[41][76]. It is also mentioned that the average cost for CO₂ capture is in an estimated range of EUR 20 to 50 per ton of CO₂, without transportation and storage costs ^[42][77]. Several energy consumption rates and CO₂ emission values are mentioned in the literature for the production of a unit mass of concrete and cement. Such data are dependent upon several factors, including the weather, production site conditions, transportation distances, types of energy sources used, and the conditions of the plant equipment. Fossil fuel energy generation, approximately, gives rise to 80 g of CO₂ per 1 MJ, while natural gas based on 1 MJ only generates 55 g of CO₂ ^[43][78].