*Review* **A Review on the Optimization of the Mechanical Properties of Sugarcane-Bagasse-Ash-Integrated Concretes**

**Nisala Prabhath 1, Buddhika Sampath Kumara 1,\*, Vimukkthi Vithanage 1, Amalka Indupama Samarathunga 1, Natasha Sewwandi 2, Kaveendra Maduwantha 1, Madawa Madusanka <sup>3</sup> and Kaveenga Koswattage <sup>1</sup>**


**Abstract:** Leading sugar-producing nations have been generating high volumes of sugarcane bagasse ash (SCBA) as a by-product. SCBA has the potential to be used as a partial replacement for ordinary Portland cement (OPC) in concrete, from thereby, mitigating several adverse environmental effects of cement while keeping the cost of concrete low. The majority of the microstructure of SCBA is composed of SiO2, Al2O3, and Fe2O3 compounds, which can provide pozzolanic properties to SCBA. In this paper, literature on the enhancement of the mechanical properties of SCBA-incorporating concrete is analyzed. Corresponding process parameters of the SCBA production process and properties of SCBA are compared in order to identify relationships between the entities. Furthermore, methods, including sieving, post-heating, and grinding, can be used to improve pozzolanic properties of SCBA, through which the ideal SCBA material parameters for concrete can be identified. Evidence in the literature on the carbon footprint of the cement industry is utilized to discuss the possibility of reducing CO2 emissions by using SCBA, which could pave the way to a more sustainable approach in the construction industry. A review of the available research conducted on concrete with several partial replacement percentages of SCBA for OPC is discussed.

**Keywords:** carbon footprint of cement; bagasse ash composites; green concrete; pozzolanic; sustainable construction

#### **1. Introduction**

Construction is always proceeding and reciprocal waste is generated in high volumes, as the demand for further construction has been rising throughout the past several decades. A 2.5% increase in cement production highlights the high consumption of concrete in recent times [1]. In 2005, global cement production was 2300 million tons and in 2020, it rose to 3500 million tons. The cement demand is expected to be around 4400 million tons in 2050 [2]. The global construction industry has been excessively dependent on ordinary Portland cement material (OPC), raising environmental, economic, and health concerns. An ideal solution for cement is needed to ensure greater sustainability for the construction sector as well [3].

OPC production is considered as one of the major global CO2 emitting processes, largely thanks to being the third-most energy intensive operation for the production of one ton of produce. Therefore, the current process of OPC production cannot be considered as sustainable [4]. Health-related issues that emerge around dumpsites due to SCBA are notable [5]. Furthermore, the contribution of concrete to the total cost of a typical construction project is high, which can be reduced with suitable low-cost cement replacement materials [3]. In order for a material to be categorized as a suitable replacement for OPC, an initial requirement is that it has to possess pozzolanic characteristics.

**Citation:** Prabhath, N.; Kumara, B.S.; Vithanage, V.; Samarathunga, A.I.; Sewwandi, N.; Maduwantha, K.; Madusanka, M.; Koswattage, K. A Review on the Optimization of the Mechanical Properties of Sugarcane-Bagasse-Ash-Integrated Concretes. *J. Compos. Sci.* **2022**, *6*, 283. https://doi.org/10.3390/jcs6100283

Academic Editor: Krishanu Roy

Received: 13 July 2022 Accepted: 31 August 2022 Published: 27 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Several replacement options have been mentioned in the literature [6–9], including fly ash, blast furnace slag, silica fume, wood ash, and ceramic waste. Sugarcane bagasse ash (SCBA) from the sugar industry is also considered as a potential replacement material for cement.

Fly ash contains aluminum and silica within its microstructure, which help to improve key aspects in concrete, such as workability, cohesiveness, ultimate strength, and durability. Blast furnace slag, which is a byproduct of the iron extraction process, provides concretes with enhanced workability properties and better resistance to adverse effects from chemicals, while reducing the early temperature rise after mixing the concrete. Silica fumes are generated during silicon production, which can be introduced into concrete mixtures and help to improve compressive strength, bond strength, and abrasion resistance and to reduce permeability. The workability and compressive strength of concrete structures are enhanced after wood ash from combustion boilers is introduced. Ceramic waste dust is generated from dressing and polishing of ceramic products. The strength and durability of concrete benefit when ceramic waste dust is present in the mix [6–9].

In this review (see Figure 1), the effects of SCBA on important mechanical properties (compressive strength, workability, split tensile strength) in concrete and evidence of a reduction in the cost of concrete by utilizing SCBA are discussed. Moreover, carbon footprint analysis of the cement industry is reviewed so that the importance of utilizing SCBA and other potential cement replacement materials can be highlighted as well. Furthermore, potential new research areas are identified so that future research can be systematically planned.

**Figure 1.** Scope of the review article (Sugarcane Bagasse Ash—SCBA).

#### **2. Sugar Manufacturing Industries**

Around 115 countries produce over 1850 million tons of sugarcane annually to supply sugar, alcohol, and paper to the global markets [7]. Sugarcane was also considered the world's second-largest crop-production industry during 2017–2018 [10]. The sugar industry in the global market is majorly dominated by nations, including Brazil, India, China, Thailand, etc. Brazil, the world's leading producer of sugarcane bagasse, produced more than 700 million tons in 2019, accounting for roughly 40% of global production. India and China, respectively, are the nations with the next-highest contributions to global sugarcane production [11–14]. In Thailand, it was recorded that 98 million tons of sugarcane were produced, placing Thailand in fourth place in global sugarcane production rankings in 2013 [14,15]. Nigerian sugarcane production recorded a high volume of over 15 million tons in 2013 [16].

Significant byproducts of the sugar industry are bagasse, molasses, SCBA, and filter press mud, which can be processed to the status of economically valuable byproducts in later processes of sugar production [17]. During 2017–2018, Indian sugarcane bagasse production reached a maximum of 30 million tons. China has an annual production of 1.2–2 million tons of SCBA from sugarcane bagasse [17]. It is commonly mentioned that high volumes of sugarcane bagasse and SCBA are produced in sugarcane manufacturing facilities around the world [11–14].

Over 25% of the initial sugarcane weight is converted to sugarcane bagasse during the sugar-making process [18,19]. SCBA can be generated up to an amount of 3–5% of the total weight of sugarcane bagasse used in the combustion chamber [7,8,13]. The burning process/technology and the content of materials that consist of crushed sugarcane bagasse contribute significantly to defining the final quality of SCBA. At the same time, quantum impurities are reduced from a complete and effective combustion while the level of crystallinity is altered simultaneously. In addition to that, the other most-critical factors that determine the properties of SCBA are the parameters of the post-processes, including grinding, post-burning, and sieving [17,20].

The initial stage of the SCBA production process (see Figure 2) is the harvesting of sugarcane crops. If paper mills are available for production, approximately 50% of the leftover bagasse is transferred to the paper mills to produce paper after the extraction of sugar juice from the sugarcanes [21]. Later, in the cogeneration area, sugarcane bagasse is used as a fuel to produce steam, which powers the generation of electricity driven by turbines. Burning of bagasse is performed in a controlled environment inside the boiler where the temperature is altered with care to achieve efficient complete combustion. The average temperature inside the boiler is set at or above 500 ◦C. A three-hour burning process at 600 ◦C calcination temperature produced the highest pozzolanic activity [11]. During this process, minimum silica, alumina, and iron oxide for natural pozzolans reach above 70% by weight, which is the requirement according to the American Society for Testing and Materials ASTM C618 [17]. Finally, the produced SCBA is collected from the bottom of the boiler and ash-contaminated air from the boiler can be filtered to collect another sample of SCBA [14,17]. SCBA is commonly used as a fertilizer in Brazil and India, where SCBA is commonly dumped in landfills [7,17]. In Sri Lanka, the majority of the bagasse is used as a biofuel to generate electricity needed to power the sugar production operation [21].

**Figure 2.** SCBA manufacturing process.

#### **3. Sugarcane Bagasse Ash**

Use of sugarcane as a biofuel for cogeneration of electricity is in practice at the moment [21,22] while it is mentioned that over 7% of Indian national electricity demand can be supplied using sugarcane bagasse as a fuel for steam turbines [23].

SCBA is generated as a byproduct during the sugarcane bagasse burning process. The main components that consist of sugarcane bagasse are cellulose (50%), hemicellulose (25%), and lignin (25%). Because sugarcane bagasse contains up to 50% moisture, it is dried before being introduced into boilers [24], although some sugar manufacturing plants do not contain a drying stage within their process [25].

Potential applications of SCBA byproducts of bagasse burning can be identified, including applications in glass-ceramic, Phillip site zeolite synthesis, geo polymers, Fe2O3- SiO2 nanocomposites to remove chromium ions, sodium water glass, silica aerogels, and mesoporous silica as a catalyst silica and as an absorbent to clarify sugarcane juice [14,26,27]. The main requirement for SCBA to be used as a replacement material for OPC in concrete is its pozzolanic action. This depends on chemical properties and physical characteristics of SCBA produced from combustion (see Table 1).

**Table 1.** Physical properties of SCBA.



**Table 1.** *Cont.*

DNR—Data Not Recorded.

#### *3.1. Physical Properties*

The physical properties (see Table 1) of SCBA are defined (see Table 1 for available data on density, surface area, particle size, and color of SCBA), starting from the soil of the sugarcane plantation all the way to the SCBA collection method. The composition of the soil on which the crops are grown supplies nutrients to these sugar plants and the heavy metals present in the soil also rest within the plant bodies. Additional nutrients that are used as fertilizer by the farmers contribute to the composition of SCBA as well. Furthermore, the sugarcane variant and growth of plantation decide the internal composition of the

sugarcane bagasse [14,37]. If the collected bagasse contains other impurities while it is inserted inside the boiler, the properties of such impurities will affect the properties of SCBA. It is crucial to be conscious about the location of plantations and the bagasse collection method. Combustion period and temperature inside the boiler affect the SCBA's physical properties significantly. SCBA is collected from leftover ash at the bottom of the boiler or from the air-filtration system in the plant. Finer SCBA particles are present in the filtration system with less carbon content as opposed to coarser SCBA particles collected from the bottom of the boiler. Samples from the boiler are likely to have more carbon from unburnt bagasse volumes. If the collected SCBA is milled, the physical properties can be determined based on the milling time period [28,37].

As per the findings of Qing Xu et al. [14], different morphologies were identified in three SCBA samples, which were processed at different calcination temperatures (600 ◦C, 700 ◦C, and 800 ◦C). Within each individual sample, the processing time period inside the boiler altered its morphology. In general, all three samples illustrated different textures after 1 h, 2 h, and 3 h calcination periods. Olubajo Olumide Olu et al. carried out similar research [37] for samples calcinated at 600 ◦C, 650 ◦C, and 700 ◦C for 60, 90, and 120 min and observed dissimilar compositions and morphologies in each sample.

Uncontrolled burning temperatures above 800 ◦C for longer periods convert the amorphous silica into crystalline silica phase. It was identified that temperatures below 800 ◦C are the most cost-effective process parameters in SCBA production, while performing grinding processes in the following step is performed to enhance the pozzolanic properties of SCBA even further [38].

#### *3.2. Micromorphology*

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 [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.

#### *3.3. 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 [7,14]. According to information listed in Table 2, the majority of SCBA found in the literature fulfills this requirement. Apart from the most-common chemical compounds that are listed in Table 2, trace amounts of Ag, As, Ba, Cd, Cr, Hg, Pb, and Mn heavy metals can also be found in SCBA [17,41].

**Table 2.** Chemical composition of SCBA.


LOI—Loss on Ignition.

#### *3.4. Pozzolanic Activity*

It is mentioned that application of pozzolans as supplementary cementitious material can improve the mechanical and durability properties in concrete [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 [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 [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 [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 [39].

The fineness in SCBA is reportedly directly related to the pozzolanic activity. Muhammad Izhar Shah et al. [59] carried out an investigation to identify the effect of milling time period over an SCBA surface area by using samples that were passed through a 200-micron standard sieve. The samples were grinded in a ball mill using ceramic balls as the grinding media for grinding periods of 15, 30, 45, and 60 min. Later, investigations to find surface area were carried out under the guidelines of ASTM C204, which revealed that SCBA samples increased their specific surface areas to the sequence, with the 60 min processed sample having the highest surface area and the 15 min milled sample having the lowest surface area. T. Murugesan et al. [60] observed that the strength activity index (percentage) value of processed SCBA is higher than 75%, which is the margin separating pozzolans and non-pozzolans, while raw SCBA did not satisfy the requirement.

Bahurudeen et al. [38] investigated pozzolanic activity by varying the temperature in the boiler and discovered that 700 ◦C produces the highest pozzolanic activity in SCBA samples ground up for 120 minutes.

According to the experimentation carried out by Marcela M.N.S. de Soares et al. [48], on pozzolanic activity comparison between SCBA, amorphous silica and crystalline silica led to the conclusion that SCBA has pozzolanic properties at low levels that are more similar to the pozzolanic properties in crystalline silica than amorphous silica.

#### *3.5. 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% [61]). Therefore, high amounts of quartz are present in SCBA [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. [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 [62].

#### *3.6. 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 [4].

In Figure 3, Image (A)—[4] depicts pores in elongated oval-shaped particles, which absorb water and oxygen. Image (B)—[31] depicts unburnt, carbon-rich fibrous particles. Image (C)—[45] depicts filter bagasse ash prismatic particles from combustion fumes. Image (D)—[47] depicts well-defined burnt flakes of SCBA. Image (E)—[55] depicts well-

burnt flakes of SCBA. Image (F)—[63] depicts SCBA particles with lamellar aspect of superimposed layers.

**Figure 3.** Microstructure of SCBA (source: (**A**)—[4], (**B**)—[31], (**C**)—[45], (**D**)—[47], (**E**)—[55], (**F**)—[63]).

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 [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.

Gritsada Sua-iam et al. [15] investigated microstructural properties of SCBA samples that were collected from an open dump site in Thailand. After drying, homogeneous samples were prepared and, later, XRD analysis on SCBA samples indicated the presence of quartz phases in SCBA. SEM images with ×1000 magnification confirmed the availability of crystalline silica from particles with distinguishable sharp edges. A Malvern Instruments Mastersizer 2000 particle size analyzer was used to analyze the particle size distribution of SCBA, OPC, and lime stone (LS) samples. SCBA particles (107.9 μm) were substantially larger compared to OPC particles (23.32 μm), while LS particles (15.73 μm) were slightly smaller than OPC.

Chidanand Patil et al. [20] carried out SEM and EDS investigations to analyze the microstructure and chemical composition of SCBA in comparison to OPC. SEM images of OPC revealed angular, irregular particles while EDS analysis indicated a high composition of calcium, oxygen, and silicon, whereas analysis of four SCBA samples that were collected from four different factories indicated unique characteristics compared to one another and OPC. Particle density, crystallinity, porosity, and particle shape varied depending on the source of the sample. Silicon, oxygen, and calcium were the predominant materials for all four samples of SCB.

Mao-Chieh Chi [30] conducted microscopic investigations on SCBA samples processed through a 900–1100 ◦C boiler and a no. 325 sieve. It was noted that SEM analysis presented particles of irregular shapes, rough surface texture, and high porous characteristics. XRD analysis provided evidence to confirm the presence of Silica (quartz), which was three-times higher than the Silica concentration in the reference OPC sample. Exothermal peaks of calcium silicate hydrate (C-S h) range between 115 and 225 ◦C, ettringite at 120–130 ◦C and calcium hydroxide (CH) 430–550 ◦C. After conducting TG tests in a range of 115 ◦C to 550 ◦C, it was observed that 10% SCBA substitution to cement provided mortar with accelerated hydration after 56 days of curing.

An investigation carried out by Daniel Véras Ribeiro et al. [32] compared the effect of calcination temperature on the pozzolanic activity of SCBA. Calcination temperatures (500 ◦C, 600 ◦C, and 700 ◦C) were chosen based on the SCBA TG curve. The XRD peaks of three samples indicated the presence of the amorphous silica phase and, from the samples collected at 600 ◦C and 700 ◦C, the presence of calcium silicate and calcium aluminate was observed, corresponding to the increased reactivity in SCBA. Density and grain size differed for each calcination temperature, while surface area remained unchanged.

Jijo James et al. [44] studied the microstructure of SCBA and found that SEM images of the samples consisted of well-defined burned flakes of bagasse. High temperatures inside boilers preserved the structure of bagasse. Crystalline bulky grains and pyrolyzed organic fractions were present in the microstructure. XRD analysis of SCBA indicated the presence of quartz, cristobalite, and calcite.

Moisés Frías et al. [45] conducted an analysis on three different types of SCBA samples (LBA—Laboratory Bagasse Ash, FBA—Filter Bagasse Ash, and BBA—Bottom Bagasse Ash) that were calcined at different temperatures and exhibited more than 75% SiO2, Al2O3, and Fe2O3 combined percentages. Similar characteristics were identified in the XRD spectrum of three SCBA samples as well. Crystalline quartz is the major common component in the samples. TG/DTA indicated that 0 to 1000 ◦C heating resulted in a maximum weight loss in FBA while the minimum was in LBA. FTIR analysis aligned closely with the results from other techniques, indicating the presence of amorphous or not very crystalline substances. BBA and LBA analysis were very similar, while FBA had appreciable statistical differences in substance content. Different particle morphologies were identified in three samples from their SEM images as a result of the unique calcination process, from which each sample was collected. Particles were coarse in nature while particle sizes varied in the order of FBA < LBA < BBA. EDX analysis indicated that coarse particles were a result of quartz that were mentioned in XRD curves.

A solid waste study by Jijo James et al. [47] explained the presence of both crystalline and amorphous phases in the SCBA microstructure by using XRD patterns where peaks had high intensities but an increase in 2-theta angle corresponded to lowering of intensities. In SEM images of SCBA samples, charred remains of bagasse fibers were present, which could have affected the intensity peaks of the XRD curve.

Frequently mentioned information in the literature about the microscopic structure of SCBA is its pozzolanic property, unburnt particles, crystallinity, particle size, and shape, which contribute to the characteristic mechanical properties of SCBA-incorporated concrete. Relationships between the location of the SCBA samples and their microscopic properties

can also be identified. SCBA samples collected from exhaust gas flirtation systems indicate better pozzolanic and microscopic properties to be used as binding materials.

#### *3.7. Optimization of Mechanical Properties*

Plenty of evidence for the productive use of SCBA in structural applications, such as concrete, bricks, soil, and steel, is found in the literature [12,42,46,50,51,64]. The chemical and microstructural properties of SCBA were utilized to explain the enhancement in mechanical properties of samples that were prepared with SCBA.

#### 3.7.1. Workability of Concrete

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) [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 [15].

Priyesh Mulye et al. [7] carried out research work by replacing OPC of grade 53 with SCBA. Out of the concrete samples that were prepared with different SCBA percentages by weight (0%, 5%, 10%, 15%, and 20%), test results indicated the workability of concrete was increased significantly by 28% (slump increased from 70 mm to 90 mm) by replacing cement up to 15% and 20% with SCBA, compared to that of concrete with 0% SCBA in the mix.

As per the findings of R. Srinivasan et al. [43], it was observed that M20 grade concrete samples with SCBA percentages of 5%, 10%, 15%, 20%, and 25% by weight displayed higher slump values compared to the sample prepared with 100% OPC. In comparison to the slump of a 0% SCBA sample (60 mm), the 25% SCBA sample had the highest slump of 230 mm, while all the other samples also displayed slump values above 60 mm. This was noted as a clear indication of the positive effect SCBA has on the workability of concrete. It was suggested that the reason for this effect could be the high surface area of SCBA, which produced cement particles moistened with less water [66].

Sajjad Ali Mangi [67] conducted research work involving SCBA in M15 and M20 grade concrete. Six samples were prepared by replacing OPC with SCBA in amounts of 5%, 10%, and two control samples with 0% SCBA. It was observed that slump values of M15 grade 5%, 10% samples increased, respectively, by 15% and 28%, while the slump values of M20 grade concrete increased by 34% and 45%. It is recommended that use of super plasticizer is not essential because the recorded slum values can be categorized into low and medium degrees of workability.

#### 3.7.2. Compressive Strength

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 [67]. ASTM C109, BS EN 196-1, or AASHTO 106-02 are considered globally accepted standard methods of testing.

According to the work performed by Priyesh Mulye et al. [7], compressive strength enhancement can be achieved only up to a 15% partial replacement of SCBA. From the five-sample set prepared with 0%, 5%, 10%, 15%, and 20% SCBA, it was discovered that the sample with 15% SCBA has the highest average compressive strength compared to reference samples. Compressive strength values of samples were enhanced with longer curing periods, such that after 3 days of curing, compressive strength was increased by 6%, after 7 days of curing, a minor decrease was noticed and, again, after 28 days, compressive strength was enhanced by 2.1%. Samples with 20% SCBA in their composition indicated that the amount of SCBA negatively affects compressive strength properties.

Experimentation on SCBA by Pamela Camargo Macedo et al. [28] revealed that the compressive strength of concrete increases from the incorporation of SCBA and data from the research suggest that an enhancement in compressive stress occurs over longer periods. From the five sets of samples, including the reference, 3%, 5%, 8%, and 10% SCBA replacements, the highest values of compressive strength were recorded in the 10% specimen. A 23.23% increase in compressive strength was recorded after 56 days of curing for a 10% sample.

Prashant O Modani et al. [18] carried out research work with concrete samples prepared with 0%, 10%, 20%, 30%, and 40% SCBA and they observed that samples with 10% replacement had the highest compressive strength after 28 days of curing, Figure 4.

**Figure 4.** Compressive strength characteristic [18].

Mao-Chieh Chi [30] conducted research involving SCBA in mortars and his work indicated that the compressive strength of samples prepared from OPC replaced with 10% SCBA was increased after a 56-day curing period. Other samples with 20% and 30% replacements reduced their compressive strength compared to the control sample. It is also mentioned that the particle size of SCBA has the potential to fill the voids in concrete structures, which is associated with compressive strength enhancement.

T. Murugesan [31] et al. conducted research on the effects of SCBA and marble waste on concrete. From the five samples that were prepared with different percentages of SCBA, it was noticed that optimum compressive strength was achieved by the sample with 10% SCBA in its microstructure. The 20% replacement specimen also exhibited higher compressive strength than the control specimen, whereas the 30% SCBA sample had reduced compressive strength after 28-day and 56-day curing periods. A comparison of raw bagasse ash and sieved bagasse ash indicated that samples prepared with sieved bagasse ash had a higher strength activity index after 7 days and 28 days of curing.

According to the findings of S.Sanchana sri et al. [33], compressive strength of M20 grade concrete can be optimized by replacing OPC with 10% SCBA. Data indicated that compressive strength was gradually decreased below the compressive strength of the control specimen after 7 days and 28 days for the samples with 15% and 20% SCBA replacements.

Olubajo Olumide Olu et al. [37] investigated mortar with SCBA and observed that replacing 5% of OPC with SCBA resulted in a sample with the highest compressive strength, while samples with SCBA contents above 7.5% had lower compressive strength throughout the 60-day curing period.

In other research carried out by Jijo James et al. [46], soil block samples with SCBA and OPC were compared for their structural performance. It was noticed that 8% SCBA addition to the mix increased the compressive strength of soil blocks to 2.95 Mpa compared to 0% SCBA blocks, which had a compressive strength just above 2.5 Mpa.

Findings by K. Ganesan [52] et al. indicated that the compressive strength of concrete can be increased by replacing OPC with SCBA in a range of 5% to 20%. Samples with percentage replacements of 25% and 30% did not illustrate any positive influence on compressive strength. Maximum strength was attained when SCBA was added by replacing 10% OPC. Data were collected for a 90-day curing period and it was noticed that while compressive strength is increased with curing time, the percentage that increased relative to the control specimen was decreased for a 20% SCBA sample (see Figure 5).

**Figure 5.** Relative increase in compressive strength of 20% SCBA blended concretes [52].

Research by Sajjad Ali Mangi et al. [67] provides evidence of an increase in compressive strength when SCBA is introduced by replacing 5% and 10% of OPC. The average compressive strength of M15-grade concrete was increased by 10.1% for a 5% SCBA sample and 4.8% for a 10% SCBA sample after 7 days of curing. The 5% SCBA sample of M20-grade concrete enhanced its compressive strength by 21.9%, while the 10% replacement sample had an increase of 12% after 7 days. A similar trend was observed with samples cured for 14 days and 28 days.

Noor-ul Amin [68] conducted research on concrete with SCBA and it was identified that the compressive strength of concrete can be increased by adding SCBA as a replacement to OPC in percentage amounts of 5%, 10%, 15%, and 20%. The optimum strength corresponded to the sample prepared by adding 10% SCBA. This work also revealed that the addition of 25% and 30% SCBA reduces the compressive strength below the values of the reference sample.

In an experiment by Chandan Kumar Gupta et al. [69], cement mortar samples were prepared with 0% to 25% SCBA replacements and the compressive strength of the 5% SCBA sample was noticeably higher than the sample prepared with 100% OPC. It is mentioned that samples with high percentages of SCBA indicated signs of very poor bonding between the materials due to insufficient water supplied to the mortar. This agrees with Noorwirdawati Ali et al. [70] who identified that introducing 20% of SCBA to be the optimum cement replacement value in earth bricks provided the maximum compressive strength and additional SCBA in 25% and 30% samples weakened the bond strength in the bricks.

Furthermore, it is reported that lightweight concrete has a more significant enhancement in its compressive strength when cement is partially replaced by SCBA in comparison to other concrete grades [1].

#### 3.7.3. Split Tensile Strength

Tensile strength investigations of mortars from diametral compression were carried out by Pamela Camargo Macedo et al. [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 (see Figure 6).

**Figure 6.** Tensile strength by diametral compression [28]. (Translation: 7 dias is 7 days, 28 dias is 28 days, and 56 dias is 56 days).

Work by S.Sanchana sri et al. [33] revealed that the tensile strength of concrete can be optimized by replacing 5% of OPC with SCBA. Compared to the control specimen, the compressive strength of a 5% SCBA sample was enhanced by 4.5%.

R. Srinivasan et al. [43] investigated concrete and SCBA, which indicated that SCBA can be effectively used in concrete up to 15% as a replacement for cement and the split tensile strength of such samples was been improved. A 39.9% increase in tensile strength was observed compared to that in the control sample for a sample with 5% SCBA in the mix.

Research and analysis by K. Ganesan [52] indicated that the slit tensile strength of concrete can be increased by incorporating SCBA into the mix design. It was noticed that replacing 15% of OPC with SCBA would optimize the tensile strength of concrete to a maximum, while it is possible to add SCBA up to 20% and have the sample's tensile strength enhanced. The addition of SCBA further reduced the tensile strength of the concrete samples (see Figure 7).

**Figure 7.** Splitting tensile strength of SCBA blended concretes at 28 days curing [52].

Findings by Noor-ul Amin [68] indicated that the tensile strength of concrete can be optimized by introducing 10% of SCBA to the mix. Compared with the control specimen, tensile strength was increased above 11%. It can be identified that with additional SCBA beyond 20%, tensile strength is considerably reduced.

The tensile strength of concrete with 20% SCBA in its composition increased to 4.81 MPa, whereas the value decreased to 3 MPa when the SCBA quantity was increased above 25% [71].

An experiment conducted by D. Patel [72] produced M20-grade concrete samples with enhanced flexural and split tensile strength properties after 28 days of curing with 10% cement replaced by SCBA. It is observed that tensile strength is increased by 4.1% after 28 days of curing. Tensile strength decreased when SCBA content reached 15% or higher.

Selvadurai Sebastin [73] conducted extensive research on the split tensile strength characteristics of mortars with SCBA. Eleven cylindrical and cubic sample sets with different SCBA contents (0–25%) were tested and it was noticed that both cylindrical and cubic samples have similar tensile strength characteristics. It is possible to identify data indicating enhancements in tensile strength of the samples, but a clear relationship with percentage SCBA and tensile strength cannot be identified.

Samples of M40-grade concrete prepared with reference to BIS: 10262-2009 (mix design ratio 1:1.56:2.42) showed enhanced flexural and split tensile properties after the introduction of SCBA to the mix (15% replacement) [13].

With the evidence available in the literature, it is emphasized that SCBA has physical and chemical properties to positively influence the enhancement in mechanical properties of concrete, mortar, soil, and brick materials. The pozzolanic characteristics of SCBA provided a meaningful explanation for this behavior. Manipulation of SCBA particle size and calcination temperature are mentioned as major parameters to be monitored in order to improve pozzolanic activity.

#### *3.8. Cost Optimization*

The economical aspect of concrete with SCBA was analyzed by Priyesh Mulye et al. [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 m<sup>3</sup> of concrete compared to the cost of concrete with 15% OPC replacement by SCBA, with the same mix design proportions.

Similar results were reported by Mangesh V. Madurwar et al. [74], where self-compacting concrete with SCBA cost 35.63% less for ingredients compared to the control concrete, while both had 34 Mpa in similar compressive strengths.

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.

#### *3.9. 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 8) [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 [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 [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 [78].

**Figure 8.** Atmospheric CO2 amounts and annual emissions [75].

As far as the ordinary Portland Cement industry is concerned, cement is considered one of the most widely used essential materials in construction, which simultaneously contributes to 5–8% of annual global CO2 emissions [7,13,73]. The reports indicate that approximately 1 kg of CO2 is released into the atmosphere in the process of manufacturing 1 kg of Portland clinker [79]. For 1 kg of cement clinker, around 0.55 kg of CO2 is generated inside the cement kiln while the calcination process of cement is occurring [78].

According to reports, OPC's 1 km long concrete pavement in China produced 8215.31 CO2e (carbon dioxide equivalent), with the concrete processing stage accounting for 7.2% of the total emission [79]. The demand for OPC has been increasing and, as a result, the environmental concerns have intensified. The Kyoto Protocol commitments also urged the industry to move towards the implementation of clean development mechanisms [80].

Lightweight concrete blocks manufactured in Europe with 8–12% of cement additives are responsible for the emission of 239.7 kg of CO2e for 1 kg of the product, while 1 kg of precast concrete emits 120.5 kg of CO2e to the environment [81].

In an experiment carried out by Woubishet Zewdu Taffese et al. [82], it was identified that concrete, hollow concrete bars, and reinforcement bars were the major energy consumers and CO2 emitters in a sample of five multi-storied buildings. They consumed 94% of the embodied energy while contributing to 98% of the CO2 emissions.

The high CO2 emissions in the cement and concrete industries are highlighted in the literature. The cost of recovering from the adverse effects of CO2 is higher. Therefore, research towards low-cost, sustainable alternatives to cement is a better way to solve this problem.

#### *3.10. Other Problems Affiliated with Sugar Industry*

The world has identified the safe dumping of agricultural waste as another emerging issue in the field of agriculture. With respect to the sugar industry, both sugarcane bagasse and SCBA have the potential to cause adverse effects on the environment if they are discarded without a proper method. Unburned matter and oxides, such as silicon, aluminum, and calcium, have the potential to pollute soil, air, and water, posing environmental and social concerns. Harmful medical conditions can be observed in the lungs of factory workers and the public around dump sites (chronic lung condition pulmonary fibrosis) if processing and disposal of sugarcane bagasse ash are not conducted in a secure way [7].

Utilization of SCBA in concrete as a partial replacement to OPC will potentially reduce the amount of SCBA disposed to the environment. As a result, the aforementioned negative effects will be mitigated to some extent.

#### **4. Conclusions**


**Author Contributions:** The contribution of each author is mentioned here. Conceptualization, B.S.K., N.S., V.V. and K.K.; methodology, N.P., B.S.K., K.K. and V.V.; software, N.P.; validation, N.P., B.S.K., N.S., A.I.S. and M.M.; formal analysis, K.K., A.I.S., K.M. and N.S.; investigation, N.P., B.S.K., N.S., M.M. and A.I.S.; resources, B.S.K. and K.K.; data curation, V.V., N.S. and K.M.; writing—original draft preparation, N.P., B.S.K. and V.V.; writing—review and editing, B.S.K., V.V., N.S. and K.M.; visualization, N.P.; supervision, K.K.; project administration, B.S.K. and K.K.; funding acquisition, K.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by SCIENCE AND TECHNOLOGY HUMAN RESOURCE DE-VELOPMENT PROJECT (STHRDP)-ASIAN DEVELOPMENT BANK (ADB), grant number STHRD/ CRG/R2/SB3 and the APC was funded by STHRD/CRG/R2/SB3.

**Acknowledgments:** The authors gratefully acknowledge the support provided by the Research Grant: Competitive Research Grant Round 2, Science and Technology Human Resource Development Project (STHRDP)-Project Output 03 (Grant No: STHRD/CRG/R2/SB3).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

