Review and Evaluation of Agricultural Biomass Ashes as Supplementary Cementitious Materials for Sustainable Concrete

Abstract

Concrete is the second most consumed material after water, with cement as its primary binder. However, cement production accounts for nearly 7% of global CO₂ emissions, posing a major sustainability challenge. This review critically evaluates 35 agricultural biomass ashes (ABAs) as potential supplementary cementitious materials (SCMs) for partial cement replacement, focusing on their effects on concrete strength and durability and highlighting performance gaps. Using a systematic methodology, rice husk ash (RHA), sugarcane bagasse ash (SCBA), and wheat straw ash (WSA) were identified as the most promising ABAs, enhancing strength and durability—including resistance to chloride ingress, sulfate attack, acid exposure, alkali–silica reaction, and drying shrinkage—while maintaining acceptable workability. Optimal replacement levels are recommended at 30% for RHA and 20% for SCBA and WSA, balancing performance and sustainability. These findings indicate that ABAs are viable, scalable SCMs for low-carbon concrete, promoting greener construction and contributing to global climate mitigation.

1. Introduction

Concrete is the second most consumed material after water [1], valued for its strength, durability, fire resistance, and cost-effectiveness [2]. At its core is cement, a hydraulic binder responsible for approximately 7% of global CO₂ emissions, making it the second-largest industrial emitter of carbon dioxide [3]. Cement is produced from finely ground inorganic materials that react exothermically with water, undergoing a chemical process known as hydration, which forms a paste that binds aggregates such as sand and gravel to create concrete products [4].

The most commonly used cement in concrete production is ordinary Portland cement (OPC). The production of OPC involves several stages, beginning with the extraction and grinding of raw materials, primarily limestone (CaCO₃) and clay, which contain silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and iron oxide (Fe₂O₃). The materials are subsequently heated in a kiln to approximately 800–900 °C [5], where calcination takes place, producing calcium oxide (CaO) and releasing carbon dioxide (CO₂) as a byproduct [6]. The calcined feed is then heated to around 1500 °C, where a series of chemical reactions and mineral phase transformations occur, leading to the formation of clinker phases [5], as summarized in Table 1. The resulting clinker, consisting of dark gray spherical nodules, is cooled and finely ground with gypsum to produce cement [6].

Table 1. Key Raw Materials, Chemical Roles, and Reactions in Clinker and Cement Production, Adapted from [5,6].

Raw Material	Chemical Role	Reaction	Temperature	Product
Limestone (CaCO3)	Provides calcium oxide (CaO)	CaCO3 → CaO + CO2	800–900 °C	Calcium oxide (CaO)
Silicon Dioxide (SiO2)	Combines with CaO to form two important silicate phases	2CaO + SiO2	900–1250 °C	C2S (Belite)
		3CaO + SiO2	1250–1450 °C	C3S (Alite)
Aluminum Oxide (Al2O3)	Reacts with CaO to form aluminate phase	3CaO + Al2O3	950–1250 °C	C3A (Tricalcium Aluminate)
Iron Oxide (Fe2O3)	Reacts with CaO and Al2O3 to form ferrite phase	CaO + Al2O3 + Fe2O3	950–1250 °C	C4AF (Brownmillerite)

Note: Gypsum (CaSO₄·2H₂O) is inter-ground with the cooled clinker to form Portland cement. It plays a critical role in regulating the hydration rate of tricalcium aluminate (C₃A) and preventing flash setting of the final product.

Based on estimated global averages, cement production is highly energy-intensive, requiring 60–130 kg of fuel oil and 110 kWh of electricity per ton of cement produced. The production of one ton of cement results in the emission of approximately one ton of carbon dioxide (CO₂) [1]. Figure 1 illustrates the four countries with the highest CO₂ emissions from cement production in 2023, along with their respective emission levels. The data are based on territorial emissions, which include only those generated within a country’s borders and exclude emissions associated with imported goods.

Figure 1. CO₂ Emissions from Cement Production in 2023, adapted from [7].

As global cement demand is expected to rise by up to 20% by 2050, there is increasing urgency to reduce emissions from cement production [3]. The cement industry must reduce emissions by 4% annually from 2020 to 2030 to meet the International Energy Agency’s Net Zero by 2050 Scenario [3]. This reduction is essential to align with the Paris Climate Agreement’s goal of limiting global warming to well below 2 °C, with the aspiration of limiting it to 1.5 °C [1]. Approximately 50% of CO₂ emissions in cement production stem from clinker calcination, 40% from fuel combustion, and 10% from electricity consumption and transportation [1]. To reduce these emissions, various strategies are being explored, including improving energy efficiency, using alternative fuels, reducing clinker production, and implementing carbon sequestration. Among these, reducing the clinker factor through the use of supplementary cementitious materials (SCMs)—which include soluble siliceous, aluminosiliceous, or calcium-aluminosiliceous compounds—is a key approach. SCMs, which can be used as partial replacements for cement, not only lower clinker demand, energy consumption, and CO₂ emissions but also enhance the mechanical performance and durability of the resulting concrete, making it a cost-effective and environmentally friendly alternative to concrete made with 100% OPC [8].

While fly ash and slag are commonly used SCMs, their application faces certain challenges. Slag, which accounts for less than 10% of global cement production, is limited by the availability of steel recycling processes. Fly ash, although more widely available, suffers from inconsistent quality, and its long-term supply is threatened by the decline of coal-based electricity generation [9]. In this context, a promising, viable, and sustainable alternative SCM is biomass ash—particularly agricultural biomass ash (ABA)—produced from agricultural waste through a controlled incineration process.

This review analyzes existing research on the incorporation of ABAs into concrete mixtures as a strategy to reduce clinker production and, consequently, lower carbon emissions. In addition to these benefits, the use of ABAs helps mitigate harmful gas emissions from open-field burning—an issue particularly prevalent in countries such as China and India, which together account for approximately 33.4% of global open-field burning. Furthermore, controlled incineration of agricultural biomass can generate steam energy for electricity production, adding another layer of environmental and economic value [9].

Previous studies have shown that the partial addition of certain types of ABAs—such as rice husk [10,11], millet husk [12,13], wheat straw [14], highland barley straw [15], oat husk [16], palm leaf [17], rice straw [18,19], sugarcane bagasse [20,21,22], sorghum husk [23,24], and cashew nutshell [25]—into OPC can enhance the strength of concrete. In contrast, others—such as corn cob [26,27], sawdust [28,29,30,31], crushed coconut shell [32,33,34], bamboo leaf [35,36,37], and banana leaf [38,39,40,41]—have produced inconsistent results.

Although interest in ABA is growing, most studies—such as those referenced above—focus on individual ash types. When multiple ashes are examined within a single study, the rationale behind their selection is often not clearly explained. Additionally, few studies have conducted direct performance comparisons of different ABAs in cement, particularly in terms of their potential as SCMs. This absence of clear justification and comparative analysis limits the broader applicability of existing findings and hinders a comprehensive understanding of which ash types are most effective for cement applications.

In response to these gaps, this study systematically reviews the research performed on 35 types of biomass ash to assess their potential as SCMs in structural concrete, with a focus on strength, durability, and overall performance in concrete mixtures. By identifying the most promising types of biomass ash for use as SCMs, this research provides valuable insights into how these alternative materials can be integrated into concrete production to meet sustainability goals and enhance structural performance. Ultimately, the study aims to support the development of more sustainable and cost-effective alternatives to traditional cement, advancing the use of biomass ash in the construction industry.

2. Methodology

This study utilized three primary databases—Scopus, Web of Science, and Google Scholar—to identify various types of agricultural biomass ashes (ABAs) investigated in concrete production, including those not specifically reported as supplementary cementitious materials (SCMs). The potential of these ABAs as partial cement replacements was evaluated using ASTM C618-22 [42] as the primary benchmark for pozzolanic suitability, followed by further assessment of their performance in structural concrete applications.

As detailed in Table 2, 35 distinct types of ABAs applied in concrete production were identified and classified into nine categories based on their type. This classification scheme is grounded in the biological origin of the biomass and the stage of agricultural processing at which the residues are generated, both of which significantly influence ash composition and physicochemical properties. The nine ash categories include husk, straw, stalk, shell, wood-based, leaf, grass-derived, agricultural processing residues, and fruit/root processing residues.

Table 2. Types of Agricultural Biomass Ashes (ABAs) Used in the Cement Industry, Classified into Nine Categories.

No.	Category	Type of Biomass Ash	Abbreviated Name	References
1	Husk Ashes	Rice Husk Ash	RHA	[10,11,43,44]
		Millet Husk Ash	MHA	[12,13,45]
		Sorghum Husk Ash	SHA	[23,24]
		Oat Husk Ash	OHA	[16,46]
		Groundnut Husk Ash	GHA	[47,48]
		Coffee Husk Ash	CHA	[49,50]
		Cocoa Pod Husk Ash	CPHA	[51]
2	Straw Ashes	Wheat Straw Ash	WSA	[14,52]
		Rape Straw Ash	RpSA	[53]
		Highland Barley Straw Ash	HBSA	[15,54,55]
		Rice Straw Ash	RSA	[18,19]
		Soybean Straw Ash	SbSA	[56,57]
3	Stalks Ashes	Sesame Stalks Ash	SSA	[58]
		Sunflower Stalks Ash	SfSA	[59,60]
		Corn Stalks Ash	CSA	[61,62]
		Cotton Stalks Ash	CtSA	[63,64]
4	Shell Ashes	Crushed Coconut Shell Ash	CCSA	[32,65,66]
		Hazelnut Shell Ash	HSA	[67,68]
		Almond Shell Ash	ASA	[69,70]
		Cashew Nutshell Ash	CNSA	[25,71]
		Pistachio Shell Ash	PSA	[72,73]
		Walnut Shell Ash	WShA	[74,75]
		Groundnut Shell Ash	GSA	[76,77]
5	Wood-Based Ashes	Sawdust Ash	SDA	[28,31,78]
		Wood Ash	WA	[79,80]
6	Leaf Ashes	Palm Leaf Ash	PLA	[17,64,81]
		Banna Leaf Ash	BnLA	[38,39,40,41]
		Bamboo Leaf Ash	BmLA	[36,82,83,84]
7	Grass-Derived Ashes	Elephant Grass Ash	EGA	[85,86,87]
		Miscanthus Ash	MA	[88,89]
8	Agricultural Processing Residues	Sugarcane Bagasse Ash	SCBA	[20,21,22,90]
		Corn Cob Ash	CCA	[27,91,92]
9	Fruit/Root Processing Residues	Olive Waste Ash	OWA	[93,94]
		Cassava Peel Ash	CPA	[95,96,97]
		Tobacco Waste Ash	TWA	[98,99]

The studies published over the last two decades were included, prioritizing research that investigated the experimental use of ABAs in concrete production. This time frame was chosen to capture the growing emphasis on sustainable alternatives to traditional cement, particularly as global demand for concrete rises. Observed limitations of this study include the unavailability and consistency of experimental data across all ABAs, which may be influenced by variations in factors such as the origins of the ABAs, their treatment methods, and the water-to-binder ratio used in the experiments. Despite these limitations, the suitability of these 35 ABAs as SCMs was evaluated using a two-step methodology based on the experimental findings of the selected studies:

Step 1: Evaluation of ABA for Compliance with ASTM C618-22

The compliance of alternative ABAs with the ASTM C618-22 standard was evaluated to determine their suitability for use as pozzolans in cement for concrete production. This assessment focused on the chemical composition of the materials, as pozzolanic reactivity—a key factor in improving concrete performance—is strongly linked to the content of pozzolanic oxides. The percentages of pozzolanic oxides can be derived from the elemental composition data obtained using X-ray fluorescence (XRF), a widely used technique for determining the chemical composition of materials. According to ASTM C618-22, a material qualifies as a pozzolan if the combined content of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and iron oxide (Fe₂O₃)—collectively referred to as pozzolanic oxides—is equal to or greater than 50% by mass (i.e., SiO₂% + Al₂O₃% + Fe₂O₃% ≥ 50% [42]. This criterion represents a revision from earlier editions of the standard, which required a higher threshold of 70%. As shown in Figure 2, 20 out of the 35 ABAs analyzed met the minimum standard, qualifying them for further evaluation.

Figure 2. Comparison of the Combined Pozzolanic Oxide Contents of Agricultural Biomass Ashes (ABAs) with the ASTM C618-22 Minimum Requirement for Natural Pozzolans.

Step 2: Evaluation of ABAs for Structural Concrete Applications

ABAs that passed the initial screening were further evaluated for their suitability in structural concrete using a decision matrix. The original framework included four criteria—strength, durability, carbon emission reduction, and economic value. However, due to limited data for the latter two, they were replaced with the criterion of optimal replacement percentage. This serves as a proxy, based on the premise that higher replacement levels typically indicate both lower carbon emissions and greater cost-effectiveness. As a result, the evaluation focused on three key criteria: strength, durability, and optimal replacement level. Detailed subcategories were developed for both strength and durability, as illustrated in Figure 3. These criteria were chosen to address sustainability goals while ensuring essential performance characteristics for structural applications.

Figure 3. Evaluation Criteria for Agricultural Biomass Ash (ABA) as Partial Cement Replacement.

Table 3, which is a design matrix, presents the evaluation results for 20 ABAs evaluated using the finalized criteria.

Table 3. Design Matrix for Evaluating ABA-Blended Cement Concrete.

BMA Type	Strength						Durability														Optimal Replacement (%)
	Early-Strength		Standard Strength (28 D)		Long-Term Strength (Beyond 28 D)		Water Permeability		Sulfate Resistance		Acid Resistance		Chloride Resistance		Alkali–Silica Reaction (ASR)		Drying Shrinkage		Freeze–Thaw Resistance
Elephant Grass		[85,86]		[85,86]		[85]	N/A *		N/A		N/A		N/A		N/A		N/A		N/A		20% [85,86,87]
		[87]		[87]		[87]
Millet Husk		[13,45]		[12,13,45]		[12,13]		[12,100]		[101,102]		[102]		[12]	N/A			[100]	N/A		5–10% [12,13,45,100,103]
Wheat Straw		[14]		[14]		[14]		[104]		[105]	N/A			[106]		[106,107]		[108,109]		[110]	10–20% [14,52,104,108,111,112]
Rice Husk		[11]		[11]		[10,11]		[11,43]		[113,114]		[115,116]		[11,117]		[118,119]		[120,121]		[122,123,124]	10–30% [10,11,43,44,117,125,126]
Corn Cob		[27,91,92]		[91,92]		[91]		[127,128]		[91]		[127,129]		[27]	N/A			[26]		[130]	8%–10% [27,91,92]
				[27]		[27]				[129]
Saw Dust		[28,29,78]		[28,29,78]		[28,29]		[28,131]		[28]		[132]	N/A		N/A			[28]		[133]	5–15% [28,29,30,31,78]
						[30,31]
		[30,31]		[30,31]		[78]
Crushed Coconut Shell		[32,33,66]		[32,33,66]		[32,66]		[134,135]		[66] [136]		[137]		[138]	N/A			[139]	N/A		10–15% [32,34,65,66]
		[34]		[34,65]		[34]
Sugarcane Bagasse		[20,21,90]		[20,21,22,90]		[20,21,22,90]		[20,21,22]		[140,141,142]		[140,142,143]		[21,22,142]		[142,144]		[21] [145]	N/A		10–20% [20,21,22,90,142,146]
Highland Barly Straw	N/A			[15]		[15]	N/A		N/A		N/A			[147]	N/A		N/A			[148]	15% [15]
Oat Husk		[16]		[16]		[16]	N/A		N/A		N/A		N/A		N/A		N/A		N/A		15–20% [16,149]
Palm Leaf		[17]		[17]		[17]		[81]	N/A		N/A		N/A		N/A		N/A		N/A		5% [17,81]
Sorghum Husk		[23]		[23,24]		[23,24]		[24]	N/A		N/A		N/A		N/A		N/A		N/A		5% [23,24]
Miscanthus	N/A		N/A		N/A		N/A		N/A		N/A		N/A		N/A			[88]	N/A		20% [88]
Bambo Leaf		[35,36,150]		[36,82,83]		[35,37]		[151,152]		[35,82]		[36,151]		[37,82,151]		[153]		[152,154]	N/A		10–30% [35,36,37,82,83,84,150]
				[35,84,150]		[36]
Banana Leaf		[38,39,40]		[38,39,40]		[38,39,40]		[38,39]		[38]		[38]		[39,155]		[39,40]	N/A		N/A		10% [39,40]
		[41]		[41]
Cashew Nutshell		[25]		[25,71]		[25]	N/A			[25]	N/A		N/A		N/A		N/A		N/A		15% [25,71]
Cassava Peel		[95,96,97]		[95,96,97]		[95,96,97]		[97]		[156]		[97]		[97]		[157]		[97]	N/A		5–10% [96,97]
Rice Straw		[18]		[18,19]		[18,19]		[18,19]	N/A		N/A			[19]	N/A		N/A		N/A		8–10% [18,19]
Corn Stalk		[61,62]		[61,158]		[62]		[62]		[159]	N/A			[160]	N/A		N/A			[59]	7–10% [61,62,158]
				[62]
Wood		[80]		[79]		[79]		[80]	N/A			[161]		[79]		[79]		[79]		[80]	10% [79]
				[80]		[80]

Note: Arrow directions represent changes compared to control concrete (100% OPC): indicates an increase, a decrease, and same (no change). * Entries marked as N/A indicate that data for the corresponding criterion were not available in the literature.

A consistent and objective scoring system was applied across all criteria using the design matrix to facilitate a comprehensive evaluation, as shown in Table 4. For scoring purposes, a value of zero was assigned to criteria marked as N/A (data not available) to ensure consistency and comparability across all materials. These gaps reflect current limitations in the available literature and highlight opportunities for future research.

Table 4. Scoring System for the Design Matrix.

Criteria	Scoring Scale	Evaluation Basis	Score Description	Point Value
Strength	−1, 0, 1	Observed change compared to control concrete (100% OPC)	↑ = increase ↓ = decrease - = same ↑ or ↓ = positive effective change - = neutral ↑ or ↓ = negative ineffective change	+1 0 −1
Durability
Optimal Replacement Percentage	1, 2, 3	Percentage-based performance outcome	>20%	3
			10–20%	2
			<10%	1

3. Results and Discussion

The scoring system described in the methodology was applied to the data presented in Table 3 to identify the most promising agricultural biomass ashes (ABAs) for use as supplementary cementitious materials (SCMs) in concrete production. Based on this evaluation, the top three candidates for partial cement replacement are rice husk ash (RHA), sugarcane bagasse ash (SCBA), and wheat straw ash (WSA). These materials are depicted in Figure 4.

Figure 4. Top Three Agricultural Biomass Ash (ABA) Candidates for Partial Cement Replacement.

Concrete mixtures incorporating these ABAs demonstrated superior compressive strength and durability compared to those with other ABAs. Furthermore, these materials exhibited significant potential for reducing the carbon footprint of concrete production. The following sections provide a comprehensive analysis of the ABAs, encompassing their origins, performance characteristics, and sustainability advantages.

3.1. Global Agricultural (Rice, Wheat, Sugarcane) Production

Figure 5 illustrates the global production volumes of rice, sugarcane, and wheat. As previously discussed, the top four countries in CO₂ emissions from cement production—China, India, Vietnam, and the United States—are also among the leading producers of these key agricultural products. Specifically, China and India consistently rank within the top three for all three crops. Vietnam and the United States are among the top five producers of rice and wheat, respectively. Additionally, the United States ranks 9th in sugarcane production and 13th in rice production.

Figure 5. Global Agricultural (Rice, Wheat, and Sugarcane) Production by Country, adapted from [162,163,164].

3.2. Chemical and Physical Characteristics of the Selected ABAs

The effectiveness of ABAs as pozzolanic materials is primarily governed by their chemical and physical characteristics [104].

Figure 6 illustrates the combined content of the key pozzolanic oxides—SiO₂, Al₂O₃, and Fe₂O₃, collectively expressed as S% + A% + F%—for the selected ABAs in comparison with ordinary Portland cement (OPC). The ABAs exhibit significantly higher combined oxide contents than OPC, and all values comply with the requirements of ASTM C618-22 for pozzolanic materials.

Figure 6. Combined Content of SiO₂% (S%) + Al₂O₃% (A%) + Fe₂O₃% (F%) in Selected Agricultural Biomass Ashes (ABAs) and Ordinary Portland Cement (OPC).

A higher combined content, particularly of silica (SiO₂), is indicative of greater pozzolanic reactivity. During the hydration of OPC, tricalcium silicate (C₃S) and dicalcium silicate (C₂S) react with water to form calcium hydroxide (Ca(OH)₂) and calcium silicate hydrate (C-S-H), the latter being the primary contributor to concrete strength. Figure 7 shows that the ABAs, especially RHA, contain significantly higher silica content than OPC, facilitating additional secondary C-S-H formation through pozzolanic reactions. This leads to enhanced compressive strength, reduced permeability, and improved chemical durability over time [165].

Figure 7. SiO₂% (S%) Content of Selected Agricultural Biomass Ashes (ABAs) and Ordinary Portland Cement (OPC).

Table 5 presents the full chemical composition of the selected ABAs compared to OPC.

Table 5. Chemical Characteristics of the Selected Agricultural Biomass Ashes (ABAs).

Chemical Components	OPC	RHA	WSA	SCBA
SiO2 (%)	19.00–25.00 [52,166]	76.81–96.84 [2,119]	44.54–77.36 [14,112]	56.40–91.30 [140]
Al2O3 (%)	4.00–6.19 [125,166]	0.22–6.17 [11,119]	1.50–9.10 [52,104,111]	1.07–14.95 [22]
Fe2O3 (%)	0.60–5.44 [21,166]	0.20–4.19 [119,120]	0.50–4.36 [52,104,111]	0.42–12.25 [22]
CaO (%)	60.22–65.96 [104,111,125]	0.48–2.12 [2,11]	3.05–12.54 [105,109]	1.00–12.60 [22]
MgO (%)	0.83–4.56 [11,21]	0.28–1.41 [11,119]	1.10–2.7 [52,108]	0.30–6.42 [22,146]
Na2O (%)	0.08–0.46 [11,119]	0.03–1.83 [44,120]	0.10–2.70 [52,109]	0.12–3.4 [21,146]
K2O (%)	0.27–1.30 [21,166]	1.12–3.51 [2]	1.30–11.10 [52,121]	0.36–9.59 [21,144]
SO3 (%)	2.03–3.40 [10,167]	0.05–1.21 [10,120]	1.13–2.35 [168,169]	0.11–4.33 [20,22]
P2O5 (%)	0.05 [10]	0.80–0.98 [10,170]	1.24–3.80 [12,107]	0.59–2.13 [20,167]
Cl (%)	-	-	0.2–0.30 [52]	-
TiO2 (%)	0.28 [52]	-	0.04–0.97 [109,112]	0.30–1.30 [140]
LOI (%)	0.51–4.10 [10,167]	1.30–11.90 [121,170]	4.03–11.20 [107,168]	0.35–10.53 [171]

Note: Only the minimum and maximum percentage values were considered in the cited data.

While the pozzolanic oxides contribute positively to performance, other constituents—such as chloride (Cl⁻), sodium oxide (Na₂O), potassium oxide (K₂O), sulfur trioxide (SO₃), and loss on ignition (LOI)—can adversely affect concrete properties. Chloride ions accelerate steel reinforcement corrosion, while Na₂O and K₂O increase the total alkali content and pore solution pH, thereby heightening the risk of alkali–silica reaction (ASR) [144]. ASTM C150 [172] recommends that total alkalis—calculated as (Na₂O + 0.658 × K₂O)—should not exceed 0.6%. As this threshold is surpassed in the selected ABAs, mitigation strategies may be necessary, which are discussed in the following sections of this study.

In addition, ASTM C618-22 limits SO₃ content to a maximum of 5% to prevent the formation of deleterious ettringite. All selected ABAs fall within this permissible range. The standard also sets a preferred LOI value below 6%, although values up to 12% are acceptable if supported by adequate performance records or laboratory evidence [42]. When LOI exceeds these limits, pre-treatment or processing may be required, as elevated LOI can increase water demand and negatively impact concrete performance [9].

The physical characteristics that influence the performance of ABA-blended cement concrete include morphology, particle size, fineness, specific surface area, specific gravity, and color. Table 6 presents these properties for the selected ABAs in comparison with ordinary Portland cement (OPC). The influence of each parameter is discussed in the following sections.

Table 6. Physical Characteristics of the Selected Agricultural Biomass Ashes (ABAs).

Physical Characteristics	OPC	RHA	WSA	SCBA
Particle Size (µm)	4.0–86.56 [14]	1–50 [2]	3.56–134.3 [14]	4.7–107.90 [140]
Fineness Passing 45 µm	93% [22]	70–99% [22]	-	95–97% [22]
Blaine’s Fineness m2/kg	308–373 [52,170]	314–1620 [22,173]	367–552 [105,112]	453–1250 [22,144]
BET Specific Surface Area m2/g	1.14–11.69 [108,120]	18.89–65.60 [120,121]	2.75–68.01 [52,112]	6.1–60.50 [140]
Bulk Density kg/m3	3170 [170]	2290 [170]	1845 [104]	1200–2500 [20]
Specific Gravity g/cm3	2.9–3.15 [22,107]	2–2.7 [2,22]	1.97–2.41 [105,107]	1.36–2.88 [22]
Pozzolanic Activity Index (PAI) at 28 Days (PAI ≥ 75%)	-	80–110% up to 20% RHA replacement [174]	85.20–107.69% up to 20% WSA replacement [112]	81–103% up to 20% SCBA replacement [175]
Color	The color of ashes varies from black to whitish-gray, with lighter colors indicating a lower amount of unburnt carbon content in the ABAs [112]; however, SCBA also includes reddish tones [22].

Note: Only the minimum and maximum percentage values were considered in the cited data.

The morphology and microstructure of OPC and the selected ABAs exhibit notable differences that influence concrete performance. OPC consists of irregular, angular particles [176] with the main crystalline phases including C₃S, C₂S, C₃A, and C₄AF [177]. Upon hydration, it forms amorphous or poorly crystalline C–S–H gel—the main binding phase—alongside crystalline calcium hydroxide (CH) and ettringite [177,178], which exhibit sheet-like and needle-like morphologies, respectively [178]. These hydration products contribute to pore filling and microstructural densification [177].

In contrast, scanning electron microscopy (SEM) analysis shows that RHA exhibits either spherical [126] or irregular particle shapes with a porous, cellular [9,10], or honeycomb structure of varying pore sizes [126]. WSA consists of irregular and occasionally elongated particles [9], with rough surfaces [179], porous inner tubules, and a dense outer layer characterized by compact channels and the absence of pores [112]. SCBA consists of diverse particle morphologies—including spherical, prismatic, and fibrous forms—with a rough texture and porous structure [9,142].

Despite these differences, X-ray diffraction (XRD) analysis shows that all three ABAs are primarily composed of amorphous silica, with minor crystalline phases such as quartz and cristobalite [52,173,180]. SCBA additionally contains traces of calcite, corundum, and hematite [140]. The reactive amorphous silica in these ashes undergoes pozzolanic reactions with CH to form additional C–S–H gel. Simultaneously, their fine and porous particles act as microfillers, refining the pore structure and enhancing packing density, which contributes to overall microstructural densification [106,167,173].

3.3. Performance of the Selected ABA-Blended Cement Concrete

3.3.1. Fresh Properties of the Selected ABA-Blended Cement Concrete

Workability

The incorporation of ABAs as partial replacements for OPC generally reduces workability due to their physical and chemical characteristics. As shown in Table 6, the specific gravity of all ABAs is lower than that of OPC, primarily due to their inherently porous structure. This increased porosity raises the total binder volume, affecting the water demand of the mix [9]. Additionally, the hygroscopic nature and high surface area of ABA particles—especially finer particles—enhance water absorption, further decreasing workability [11,104]. These fine particles possess a significantly larger specific surface area, requiring more water for proper wetting and dispersion. Moreover, the irregular shapes [111,142] and rough textures of ABA particles, such as those in SCBA, reduce the flowability of the concrete, resulting in a stiffer mix [142]. Another influencing factor is the LOI content of ABAs. Higher LOI values are associated with lower specific gravity and increased water absorption, both of which negatively impact workability [9,11].

Improving Workability

Despite the challenges associated with reduced workability in concrete incorporating ABAs, some studies have reported that, under optimized conditions, the workability of ABA-blended concrete can be significantly improved—and in some cases, may even exceed that of control concrete (CC) [20]. These enhancements can reduce the overall water demand of the mix and eliminate the need for chemical admixtures such as superplasticizers. Several optimization strategies have been identified in the literature, including:

Modification of ABA’s physical properties: The pre-treatment of ABAs to remove fibrous materials prior to mixing enhances the homogeneity of the blend and contributes to a further reduction in flow resistance [20]. In addition to pre-treatment, the specific physical characteristics of ABAs—such as particle morphology, density, size, and distribution—can significantly influence the workability of concrete [20,126,173].

Sobuz et al. [20] demonstrated that incorporating up to 20% SCBA led to an overall improvement in workability by approximately 40–50%. This enhancement is attributed to the favorable physical properties of SCBA, including its spherical, uniform, and regular particle shape, which minimizes internal friction and particle interlocking, thereby promoting better flow. Furthermore, the glassy and smooth surface texture of SCBA particles facilitates easier movement within the concrete matrix. Similarly, Tian et al. [126] reported that grinding RHA for about 30 min resulted in particles with high surface area and optimal workability. This was largely due to the spherical morphology of RHA particles, which produces a ball-bearing effect that reduces internal friction and enhances flowability.

The lower density of ABAs, such as SCBA, compared to that of cement, also plays a crucial role in improving workability. This reduced density leads to a decrease in the fresh concrete’s overall density, an increase in entrained air content, and a reduction in internal resistance—all of which contribute to enhanced flow properties. These characteristics also highlight SCBA’s potential use in lightweight concrete applications [20].

Additional insights from Givi et al. [173] reveal that coarser RHA particles, with an average size of approximately 95 µm, can enhance workability at a water-to-binder ratio of 0.4. This improvement is primarily due to the lower water demand associated with coarser RHA. The ash particles adsorb onto the surfaces of cement grains, reducing flocculation and encouraging a more uniform dispersion. Moreover, RHA contributes to optimizing the overall particle size distribution, which increases the packing density of solids and decreases void content, ultimately reducing the amount of water required to achieve the desired level of workability.

Modification of mix design parameters for the ABA-blended cement concrete: Optimizing the replacement level of ABA and the water-to-binder (w/b) ratio can significantly enhance the workability of concrete. For instance, incorporating RHA at replacement levels up to 5% [44], or within the range of 5–10%, in combination with a w/b ratio of 0.53, has been shown to increase slump values by approximately 8–17% compared to CC [11]. Similarly, a study by Sudeep et al. [179] utilized response surface methodology (RSM) to determine optimal mix parameters. Their findings indicated that a w/b ratio of 0.45 combined with 10.12% WSA resulted in enhanced workability while maintaining adequate mechanical strength. In another study, Priya [146] reported that replacing cement with SCBA at levels up to 25%, in conjunction with a w/b ratio of 0.5, significantly improved workability. Notably, at the 25% replacement level, the concrete achieved a slump of 160 mm, compared to 70 mm for the control mix, highlighting a substantial improvement in flowability.

Initial and Final Setting Times

The initial and final setting times of ABA-blended cement concrete were longer than those of the control concrete (CC) at all levels of ABA replacement [21,52,119,142]. This delay is attributed to the reduced content of tricalcium aluminate (C₃A) in the mixture and the presence of unburned carbon, as well as sulfur trioxide (SO₃), which is specifically associated with the SCBA [142].

Studies report that replacing cement with up to 20% SCBA or RHA increases both initial and final setting times; however, all values remain within ASTM C191 limits (initial setting time > 45 min; final setting time < 375 min). Kazmi et al. [144] observed that the initial setting time increased from 97 min (control) to 114 min for 10% SCBA and to 187 min for 20% SCBA. Similarly, the final setting time increased from 189 min (control) to 234 min at 10% SCBA and to 287 min at 20% SCBA. Abbas et al. [119] reported that replacing cement with 10% RHA extended the initial setting time from 89 min (OPC) to 105 min, whereas 20% RHA increased it to 127 min. The final setting time increased from 159 min (OPC) to 179 min at 10% RHA and to 198 min at 20% RHA. Ahmad et al. [106] found that incorporating 21% WSA increased the initial and final setting times by 46.3% and 47.6%, respectively, compared to the control; however, these values also remained within acceptable operational thresholds.

3.3.2. Compressive Strength (CS) of the Selected ABA-Blended Cement Concrete

RHA replacement at 5–20% by cement weight has been shown to enhance the compressive strength (CS) of concrete, with improvements ranging from 2.4% to 18.7% compared to control concrete (CC) at 28 days [44]. Notably, finely ground RHA at a 10% replacement level produced a significant increase in CS of 30.8% relative to CC at the same curing age [125]. Furthermore, a higher replacement level of 30% RHA yielded CS values comparable to those of the control mix at 28 days [11].

Similarly, incorporating SCBA within the 5–15% range resulted in CS improvements of 3.72% to 20.81% compared to the CC at 28 days [167]. Replacement with SCBA up to 20% could further increase CS; however, exceeding this threshold negatively impacted strength relative to the control mix at 28 days [21,146]. Likewise, using WSA as a cement replacement between 5% and 15% enhanced CS, with peak performance observed at 10% replacement. Beyond 15%, WSA incorporation caused a decline in CS compared to CC at 28 days [104]. Notably, with appropriate treatment methods, WSA replacement could be increased to 20%, resulting in a 10.98% higher CS than CC at 28 days (from 60.1 MPa for CC to 66.7 MPa with 20% WSA) [169].

Treatment Methods to Improve CS of ABA-Blended Cement Concrete

The following treatment methods can enhance the CS of ABA-blended concrete:

• Sieving: Sieving removes oversized particles and separates particles by size (ranging from 75 to 2000 μm). This process improves the quality of ABAs by eliminating impurities and reducing carbon content and LOI. Additionally, sieving increases the concentration of pozzolanic oxides, thereby enhancing reactivity and potentially improving the compressive strength of concrete [142].
• Burning: Controlled burning conditions not only reduce carbon content and LOI but also increase the concentration of pozzolanic oxides [142], particularly amorphous silica [11]. Achieving a higher proportion of amorphous silica depends critically on the optimal temperature range and duration, as outlined below:

  ○

  RHA: Burning in the range of 570–670 °C for 5 h [104], with 650 °C for 60–80 min results in the highest amount of silica in amorphous form [11,126].

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  WSA: Burning in the range of 500–600 °C for 2 h, specifically at 600 °C for 2 h [112] or at 550 °C for 4–5 h, results in the highest amount of amorphous silica [14,108].

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  SCBA: Burning in the range of 450–650 °C for 1–3 h, with 550 °C for 1 h, results in the highest amount of silica in amorphous form [166].

However, exceeding a certain temperature threshold causes the transformation of amorphous silica into crystalline silica, which negatively impacts its pozzolanic activity [116].

• Water or Chemical Treatment: If necessary, water or chemical treatment is recommended. Water washing is particularly effective for the removal of soluble salts, including chlorides, sulfates, and alkalis (e.g., Na and K). For heavy metal removal, more advanced methods—such as chemical washing (using hydrochloric acid, sulfuric acid, or phosphoric acid), electro-dialytic remediation (with citric acid, ammonium citrate, or ammonia), and ammonium acetate extraction—have also been shown to be effective [181].
• Grinding: The reduction in particle size of ABAs through grinding significantly enhances their pozzolanic reactivity by increasing the surface area of the finer particles, thereby providing more active sites for chemical reactions [9]. In the case of RHA, although grinding reduces its average particle size, the surface area is largely governed by its intrinsic multilayered, angular, and microporous structure rather than particle size alone. This complex morphology contributes to its high reactivity [125].

In addition, the particle size distribution of ABAs, which affects their pozzolanic activity [170], along with the water-to-binder (w/b) ratio, can significantly influence the CS of ABA-blended cement concrete. Typically, the w/b ratio ranges from 0.35 to 0.55, with higher w/b ratios generally reducing the mechanical performance of the concrete. However, this adverse effect can be alleviated by incorporating finely ground ABAs, which enhance the microstructural properties of the material, thereby helping to maintain the desired strength and durability [104]. While ultrafine ABA is more effective at improving CS than coarser particles, it tends to reduce workability compared to coarser ABA-blended cement concrete at the same w/b ratio. This reduction in workability is mainly due to the increased specific surface area of the ultrafine particles, which raises the water demand of the concrete mixture [173].

Optimizing ABA Replacement Percentage Based on Concrete CS

Table 3 presents the optimal replacement level ranges of the selected ABAs in concrete, which are required to achieve CS equal to or greater than that of the CC. Based on these results, the highest reported replacement percentages are 30% for RHA and 20% for both WSA and SCBA. The rationale for selecting the highest feasible replacement level for each material is to maximize the reduction in carbon emissions while still meeting the strength requirements for structural applications. Figure 8 illustrates the recommended maximum replacement levels along with the corresponding treatment methods that enhance their effectiveness. It is important to note that these recommendations are based on the 28-day CS of concrete incorporating the selected ABAs. However, due to the continued pozzolanic activity of these materials, there is potential for further strength development at later curing ages compared to the CC [142].

Figure 8. Maximum Agricultural Biomass Ash (ABA) Replacement Percentages in Cement with Corresponding Treatment Methods for Compressive Strength (CS) Compliance [11,21,52,108,112,126,145,166,169].

3.3.3. Durability Properties of the Selected ABA-Blended Cement Concrete

The durability improvement observed from incorporating the selected ABAs into concrete is attributed not only to their pozzolanic activity—which enhances the formation of additional C-S-H gel—but also to their finer particle size compared to OPC. This finer particle size contributes to the filling effect by refining the pore structure and reducing porosity [142]. Table 7 presents the highest reported replacement percentages of the selected ABAs in cement that satisfy durability requirements.

Table 7. Maximum Agricultural Biomass Ash (ABA) Replacement Percentages in Cement for Durability Compliance.

Durability Properties	RHA	WSA	SCBA
Water Permeability	Up to 35% replacement resulted in a reduction in water permeability.	Up to 20% replacement resulted in a reduction in water permeability.	Up to 25% replacement resulted in a reduction in water permeability.
Sulfate Resistance	Up to 30% replacement demonstrated improved sulfate resistance in sodium sulfate (Na2SO4) and mixed sulfate environments.	Up to 24% replacement showed the highest CS at the lower sodium sulfate concentration (10,000 mg/L), while 16% replacement performed best at the higher concentration (40,000 mg/L).	Up to 20% replacement resulted in improved sulfate resistance.
	The use of ABA-blended cement concrete reduces the content of C3A and Ca(OH)2, limiting the availability of these reactive compounds and decreasing the formation of harmful products like gypsum and ettringite that contribute to sulfate-induced deterioration [109].
Chloride Resistance	Up to 35% replacement resulted in increased chloride resistance.	Up to 40% replacement resulted in improved chloride resistance.	Up to 25% replacement enhanced chloride resistance. At 28 days, 5% showed moderate corrosion risk, while 15% and 25% showed low risk, further reduced to very low by 56 days.
Acid Resistance	Up to 30% replacement resulted in increased resistance to nitric and acetic acids.	-	Up to 20% replacement resulted in increased resistance to sulfuric acid.
Freeze–Thaw Resistance	Up to 20%	Up to 15%	-
Alkali–Silica Reaction (ASR)	Up to 40% replacement resulted in a reduction in ASR expansion.	Up to 15% replacement enhanced ASR resistance.	Up to 40% mitigated the ASR.
	ASR, known as the cancer of concrete, causes expansion and cracking due to the reaction between reactive silica in aggregates, hydroxyl ions (OH−), alkali ions (Na+, K+), and moisture, forming an expansive gel. Alkalis in cement (Na2O and K2O) raise the pore solution pH, accelerating ASR and leading to structural damage [144]. To reduce risk, the total alkali content must meet ASTM C150. SCMs with higher alumina content help mitigate ASR by forming calcium aluminosilicate hydrate (C-A-S-H) gel, which binds alkalis and reduces their availability in the pore solution [119].
Drying Shrinkage	Up to 30% replacement resulted in a reduction in drying shrinkage.	Up to 20% replacement resulted in a reduction in drying shrinkage, due to WSA’s fineness and porous structure that absorbs water and acts as an internal curing agent.	Up to 25% replacement reduced drying shrinkage or showed similar values to CC.
Low Heat of Hydration	ABA-blended cement concrete has a lower heat of hydration than CC, reducing the risk of early-age thermal cracking [21]. This is due to fewer heat-releasing components (C3A, C3S, gypsum) [21,142] and a higher amount of pozzolanic oxides in ABAs.

3.4. Environmental Impact of the Selected ABA-Blended Cement Concrete

Ordinary Portland Cement (OPC) production is a major contributor to fossil-based carbon dioxide (CO₂) emissions and significantly depletes non-renewable raw materials, such as limestone and clay. In contrast, agricultural biomass ashes (ABAs) are derived from renewable agricultural residues. According to Khanna et al. [182], the production of ABAs involves the controlled combustion of these residues, generating ash and releasing CO₂ that is classified as biogenic—originating from carbon absorbed by plants during their growth cycle. As a result, the net increase in atmospheric CO₂ is negligible, since these emissions are part of the natural carbon cycle.

Importantly, the CO₂ emissions associated with ABAs are substantially lower than those from OPC; for instance, sugarcane bagasse ash (SCBA) emits approximately 7.5 times less CO₂ than OPC [142]. Moreover, the utilization of agricultural waste as an alternative energy source to replace conventional fossil fuels, such as coal or oil, offers additional environmental benefits by reducing reliance on non-renewable energy sources. The ash generated through this process can be used as a partial cement replacement in concrete mixtures, thereby supporting net-zero carbon emission targets and enhancing sustainability within the construction sector.

4. Conclusions

This review systematically evaluated 35 agricultural biomass ashes (ABAs) as supplementary cementitious materials (SCMs) for partial cement replacement, aiming to reduce CO₂ emissions while maintaining or enhancing concrete performance. Twenty ABAs were identified as chemically suitable through a pozzolanic oxide screening chart and further evaluated using a concrete performance matrix integrating strength, durability, and optimal replacement levels. Rice husk ash (RHA), sugarcane bagasse ash (SCBA), and wheat straw ash (WSA) emerged as the most promising SCMs.

Concrete incorporating these ABAs demonstrated improved compressive strength and enhanced durability (resistance to chloride ingress, sulfate attack, acid exposure, alkali–silica reaction, and drying shrinkage) compared with control concrete (100% OPC), while maintaining acceptable workability. Their pozzolanic activity, combined with chemical and physical properties and targeted retreatment methods, contributed to these performance improvements. Recommended replacement levels are 30% for RHA and 20% for SCBA and WSA, with higher levels feasible for most durability criteria. Global availability analyses confirmed their suitability, particularly in regions with high cement-related CO₂ emissions.

Remaining gaps include limited durability data for some of the 20 ABAs, particularly regarding freeze–thaw resistance; however, these ABAs are expected to enhance freeze–thaw performance through pore refinement, reduced air voids, and decreased permeability. Future research should use machine learning to optimize the blend of multiple ABAs to develop a novel SCM, with predictions validated through laboratory testing and economic assessment. By integrating chemical, physical, and performance data into a structured framework, this review positions ABAs as credible, scalable SCMs for sustainable concrete production.