Applications of Sustainable Bio-Degradable Agro-Waste (Rice Husk Ash) in Improving the Flow and Mechanical Properties of Ultra-High-Strength Mortar ^†

Abstract

This study aims to develop a sustainable fibre-reinforced, high-strength mortar (UHSM) using Rice Husk Ash (RHA), cement, and steel fibres, with a view to developing a high-strength mortar that can be utilized for repair work in major industries where cracks occur due to vibrations and thermal conditions. RHA was used in 20%, 30%, and 40% replacement levels of cement. Steel fibres were used at a constant dosage of 1.5%, and a very low w/c of 0.25 was adopted. Five different types of curing conditions, namely 1-day hot water curing, 1-day oven curing and 7-day normal water curing, 1-day oven curing and 28-day normal water curing, and 7-day normal water curing and 28-day normal water curing, were adopted. The mechanical behaviour of the mortar was evaluated using a compressive strength test and a split tensile test, and a statistical analysis was done using two-way ANOVA. Results revealed that the replacement levels up to 30% yielded better strength results, and there was indeed a significant effect of the curing conditions.

1. Introduction

The construction industry in recent years has reached a significant boom, with innovative research such as ultra-high-performance concrete, 3D printing, and many other advancements. Even though engineers and researchers aim to develop very high- or ultra-high-strength concrete that is durable, these concrete structures are also being repaired worldwide, and there is a strong demand for new repair materials. So, it is essential not only to develop new concrete mixes but also to develop new mortar mixes. The main objective of this research is to develop a high-strength mortar that is sustainable—satisfying all greenhouse gas emission conditions—and possesses high strength, thereby restricting further crack development in structures. But when developing high-strength concrete or high-strength mortar, the use of large quantities of cement is inevitable. Many warnings have already been given regarding global warming and CO₂ emissions. It has been proven that cement industries contribute 7% of the global CO₂ emissions [1]. A significant amount of CO₂ is released due to the heating process involved in the production of cement [2]. Research states that it is not only CO₂ but also many other pollutants—such as sulfur dioxide and nitrogen oxides—which cause effects such as respiratory disorders and other health issues [3]. Therefore, it is necessary to use alternative methods to develop high-strength mixes, such as by reducing cement content in mixes with equivalent supplementary materials such as silica fume and fly ash and waste materials such as rice husk ash, sugarcane bagasse ash, and other materials possessing pozzolanic mixtures. The construction sector ranks among the top consumers of natural resources and is a major source of global carbon dioxide (CO₂) emissions, primarily due to the production of Ordinary Portland Cement (OPC), which is responsible for nearly 8% of the world’s CO₂ emissions [4].

The use of agro-waste in construction marks a significant advancement in sustainable construction materials, merging high-performance engineering benefits with significant environmental gains. Agro-waste ashes, including rice husk ash (RHA), sugarcane bagasse ash (SCBA), palm oil fuel ash (POFA), wheat straw ash, and corn cob ash, are abundant in amorphous silica and alumina, making them highly reactive in pozzolanic reactions [5]. One such alternative, which is abundant and creates disposal problems, is rice husk. Rice husk ash (RHA) is a highly pozzolanic by-product obtained by the controlled combustion of rice husks, which contributes to the strength enhancement of concrete by contributing to the formation of essential cementitious compounds [6,7]. These are waste materials from agriculture, and in countries like India, which are major contributors to agricultural products, disposing of the waste generated from agricultural products remains a major problem. Globally, rice cultivation generates millions of tons of husks annually, and their disposal often creates environmental challenges owing to open dumping or uncontrolled burning. Rice husks are mainly disposed of in empty lands, while in some countries, they are used as fuel [8,9]. Disposing of them on empty land requires more land for filling, and there are further chances that the dumped waste may contaminate groundwater and affect the environment.

India stands as the 8th largest exporter of agricultural goods worldwide, accounting for about 2.4% to 2.5% of the total global agricultural trade in recent years [10]. The leading agricultural products include rice, spices, and sugar. The global production volume of processed rice between 2008 and 2009 and between 2022 and 2023 indicates that the rice production worldwide was 514.8 million metric tons during 2021–2022 [11]. Reports state that 0.25 T of RHA can be obtained from 1 t of rice husk by the incineration process [12,13,14]. RHA has been effectively used as a supplementary cementitious material in concrete in various research works, and several researchers have discussed its mechanical and durability properties. Ramasamy et al. [15] showed that incorporating 10% RHA resulted in a significant improvement in compressive strength and concluded that 20% is the most effective dosage for enhancing durability. Nair et al. [16] mentioned in their experimental investigations that RHA in HSC has the potential to enhance compressive and flexural strength. Ganesan et al. [17] reported that the use of RHA up to 30% could reduce water permeability and chloride diffusion by 35% and 28% respectively. Silica, which is present in amorphous form and is responsible for the pozzolanic activity, is the primary chemical component in RHA [18]. So, RHA is classified under class N, as per ASTM C618 [19], as a pozzolanic material. So, it is evident that the use of RHA as a supplementary cementitious material has been justified by many researchers. When processed under controlled conditions, these ashes refine the pore structures, enhance particle packing, and densify the microstructure, thereby boosting both strength and durability.

Thus, from the earlier research works, it is evident that rice husk ash has emerged as a sustainable, cost-effective, and technically viable supplementary cementitious material. Its high pozzolanic reactivity, microstructural densification, and durability benefits align with global efforts to reduce CO₂ emissions and promote circular economy practices in construction industries. However, challenges such as variability in quality, reduced workability, and a lack of standardized guidelines hinder its widespread implementation. Optimizing the replacement level (5–20%), ensuring controlled processing, and developing standards are essential to maximize the benefits of RHA in UHPC. With continued research and industry adaptation, RHA can play a pivotal role in advancing eco-efficient high-performance concretes.

Research on rice husk ash (RHA) as a supplementary cementitious material has primarily focused on its use in conventional concrete and cement-based composites. However, there is a significant research gap in exploring their potential applications in ultra-high-strength mortars (UHSM). Previous findings suggest that RHA could potentially enhance concrete performance; however, its effects on this specific application remain largely unexplored. There is a clear need for comprehensive research on the use of RHA in UHSM applications. Future studies should investigate the optimal replacement levels, effects on various mechanical and durability properties, and potential synergistic effects of their combination. Additionally, future research should address the long-term performance and sustainability of UHSM incorporating agricultural waste materials.

2. Materials and Methods

The various materials adopted and the experimental methods involved are discussed in this section.

2.1. Cementitious and Agro-Waste Materials

OPC–53 grade cement from Dalmia cements, Tamilnadu, India, conforming to ASTM C150 [20] was used after a proper physical examination. Basic tests, such as consistency, setting time, and specific gravity tests, were conducted. Silica fume with an average particle size between 0.1 and 0.2 μm was used. Quartz powder was used as a filler to ensure that the particle size of the UHPC was suitable for its densely packed nature. Figure 1, Figure 2 and Figure 3 show the cementitious and agro-waste materials used in the current research. The silica fume and quartz powder were received from Astraa Chemicals, Chennai, Tamilnadu India.

The RHA received from AStraa chemicals, Chennai, India, was sieved through a 90-micron sieve, and the ash that passed through the sieve was used for the experiment. A scanning electron microscopic (SEM) image of RHA is shown in Figure 4a,b. The SEM image of rice husk ash (RHA) revealed a complex microstructure characterized by irregular, angular, and porous particles with layered, flaky structures. The surface exhibited a rough, uneven texture with visible cracks and crevices, creating a honeycomb-like network that contributed to its high specific surface area. This porous nature, resulting from the combustion of organic matter in rice husks, enhances the water demand and pozzolanic reactivity of the material. The presence of thin, sheet-like structures indicates an amorphous silica content, whereas agglomerated fine particles appear as lumps or clumps. These microscopic features, observed at 10.0 kx magnification, explain RHA’s effectiveness as a pozzolanic material in cement and concrete applications, with its reactivity influenced by factors such as burning temperature and particle size distribution.

The XRD pattern of the rice husk ash is shown in Figure 5. X-ray diffraction (XRD) analysis of rice husk ash revealed a complex structure, marked by a distinct, intense peak at low angles, suggesting the presence of crystalline phases, such as residual crystalline silica or other minerals. Furthermore, a broad, significant peak at approximately 20–25° was observed, which is typical of amorphous silica. This pattern, which is common in rice husk ash, indicates a high silica content and the thermal decomposition process involved in ash formation. The XRD pattern displays a broad hump between 15°and 35° (2θ), signifying amorphous silica, which contributes to pozzolanic reactivity. In contrast, a sharp peak near 26.6° indicates residual crystalline quartz content, confirming the presence of both amorphous and crystalline phases. The substantial amount of non-crystalline silica in the ash, as indicated by the broad peak, suggests that rice husk ash could be valuable in various applications, such as cement additives or as a source of reactive silica.

2.2. Fine Aggregate

River sand passing through a 600 µm sieve was used for the entire experiment. The particle size distribution of the river sand, determined using sieve analysis, is shown in Figure 6. Sieve analysis of the fine aggregates was performed in accordance with IS 2386 (Part I)-1963 [21]. The resulting gradation curve verified that the sand used in this study adhered to the Zone II grading criteria specified in IS 383:2016 [22]. With a fineness modulus of 2.65, the sand was deemed appropriate for ultra-high-performance concrete mixtures.

2.3. Steel Fibre

The use of steel fibres in concrete offers significant benefits in terms of improved mechanical properties, particularly the tensile and flexural strengths. Crimped steel fibres of 12.5 mm length and 0.3 mm diameter were used for this research. They help enhance the mechanical properties of concrete. Figure 7a,b show the steel fibres utilized and their sizes.

2.4. Chemical Admixture

To reach the required flow of concrete, a poly carboxylate ether (PCE)-based superplasticizer is used, which is helpful in providing a low water–binder ratio. Superplasticizers are widely used in concrete production to improve workability, reduce water content, and enhance durability. The admixture was thoroughly mixed with water before being poured into the concrete mixture.

2.5. Methodology

The detailed methodology is presented in Figure 8.

3. Experimental Investigations

The various experimental techniques that were adopted in this research are elaborated in this section.

3.1. Mix Proportions and Mixing Details

The preparation of 50 mm × 50 mm × 50 mm cubes for ultra-high-strength mortar (UHSM) involves meticulous steps to ensure accurate evaluation of compressive strength and material properties. Clean, non-absorbent moulds were lightly coated with oil to prevent adhesion, and the fresh UHSM mixture was added in thin layers. The mixture was then compacted with a tamping rod to eliminate air pockets and finished with a smooth surface. Testing was conducted at various intervals using a calibrated compression-testing machine in accordance with ASTM C109/C109M [23] standards. The use of 50 mm cubes, a common practice in concrete research, facilitates efficient laboratory handling, requires less material, and provides accurate data for comparative analysis, mix optimization, and the assessment of supplementary cementitious materials such as rice husk ash (RHA), ultimately advancing the development of sustainable concrete technology. In

Table 1. Composition (by fraction of cement) of UHSM mixtures.

Mixes	Cement	RHA	Silica Fume	QP	FA	HRWR	H2O	SF
(kg/m3)						(%)
Control	1213	-	283	192	605	2.5	17.5	1.5
RHA20	970.4	242.6	283	192	605	3	21
RHA30	849.1	363.9	283	192	605	5.5	23.5
RHA40	727.8	485.2	283	192	605	8	26
RHA50	606.5	606.5	283	192	605	11	29.5
RHA60	485.2	727.8	283	192	605	14	34

, RHA20, RHA30, RHA40, RHA50, and RHA60 represent the replacement percentages of cement by RHA with 20, 30, 40, 50, and 60%, respectively.

3.2. Casting and Curing of Cubes

Figure 9 depicts the concrete-mixing machine used throughout this study and the dry mixing of concrete. Figure 10 and Figure 11 illustrate the preparation process of UHSM and its flowability respectively. The cubes were cast according to the ASTM C109/C109M [23] standards. After an initial 24 h curing period, the specimens were removed from the moulds and subjected to different curing conditions, namely 24 h water curing at 90–100 °C, 24 h oven curing and 7-day normal water curing, 24 h oven curing and 28-day normal water curing, 7-day normal water curing, and 28-day normal water curing, and the curing conditions were designated as C1, C2, C3, C4, and C5 respectively.

3.3. Mechanical Properties

Compression tests on 50 mm × 50 mm × 50 mm cube specimens were performed following ASTM C109/IS 4031 (Part 6) [23,24] standards, utilizing a calibrated compression-testing machine (CTM) with a capacity of 2000 kN. The specimens were subjected to axial compression at a consistent load application rate of approximately 140 kg/cm²/min until they failed. The compressive strength was determined by dividing the load at failure by the cross-sectional area of the cube (2500 mm²).

3.4. Statistical Validation

The experimental results were validated by performing a two-factor analysis using Analysis of Variance (ANOVA) to study the effect of the replacement of RHA, the effect of different curing regimes adopted, and the interaction effects due to replacement levels and different curing conditions. The analysis was done at a 5% significance level and a 95% confidence level. The p-values obtained were analyzed and the results were interpreted accordingly. Generally, a p-value less than 0.05 indicates that the factors have a significant impact on the response, and if the p-value is less than 0.01, it indicates that the factors have a highly significant effect [25]. The statistical analysis was performed using PYTHON scripts in ANACONDA NAVIGATOR 2.6.3 with jupyter notebook 6.5.4 environment.

4. Results and Discussions

The experimental results and the statistical validation of the results are elaborated in this section.

4.1. Flowability

Figure 12 depicts the variations in the demand for the high-range water reducer (HRWR) and the water-to-binder (w/b) ratio across different mix designs. The x-axis represents the control and rice husk ash (RHA) replacement levels, which ranged from 20% to 60%. The left y-axis shows the HRWR content (%), and the right y-axis indicates the w/b ratio (%). Two lines are illustrated: the green line with square markers denotes the HRWR dosage, and the pink line with circular markers represents the w/b ratio. Both parameters steadily increased as the RHA replacement level increased, underscoring the influence of RHA on the fresh-state properties.

In the control mix, the HRWR requirement was minimal (~2%), with a w/b ratio of approximately 18%, indicating sufficient workability without the need for additional admixtures. However, as the RHA replacement level increased, both the HRWR and w/b ratio increased significantly, reaching approximately 14% and 34%, respectively, at a 60% replacement level. This trend is attributed to the high specific surface area and porous microstructure of the RHA particles, which increases the water demand in the mix. To maintain adequate workability under these conditions, higher HRWR doses were required. The nearly linear relationship between HRWR and the w/b ratio suggests that while the addition of HRWR aids in maintaining workability, the increasing RHA content exerts a dominant effect that cannot be entirely offset by chemical admixtures. These findings highlight the importance of identifying an optimal replacement level—likely between 10% and 20%—to balance sustainability benefits with acceptable workability and admixture demand.

The water-to-binder (w/b) ratio exhibited a significant increase from 17.5% in the control mix (0% RHA) to 34.0% in the R60 mix, nearly doubling as the rice husk ash (RHA) content increased to reach a flow of 180–210 mm, as shown in the flow table illustrated in Figure 13. This substantial rise in the w/b ratio highlights the considerable rheological challenges posed by the incorporation of RHA. The marked increase in water demand can be attributed to the distinct physical characteristics of RHA, particularly when used at high replacement levels in a low w/b matrix like ultra-high-strength mortars. High Specific Surface Area: RHA particles typically possess a much higher specific surface area than Portland cement or silica fume. This increased surface area necessitates a proportionally larger amount of water to fully wet the particles and create the necessary lubricating layer for flow, thereby increasing the frictional resistance of the mixture. This effect has been studied by various researchers [17,26,27,28], and it was mentioned that blending RHA with cement decreases workability; to balance this, the amount of superplasticizer should be increased accordingly to maintain the desired workability. Porous and Cellular Structure: RHA has a highly porous cellular structure. These pores absorb some of the mixing water, effectively reducing the amount of free water available to enhance the workability of the mix. Although this phenomenon is often referred to as internal curing at later ages, it results in a stiffer mix in the fresh state, requiring the addition of more water (or superplasticizer) to maintain the desired flowability (slump flow).

4.2. Compression Test

Compression tests on 50 mm × 50 mm × 50 mm cube specimens were performed following ASTM C109/IS 4031 (Part 6) [20,24] standards, utilizing a calibrated compression-testing machine (CTM) with a capacity of 2000 kN. The specimens were exposed to axial compression at a consistent load application rate of approximately 140 kg/cm²/min until they failed. The compressive strength was determined by dividing the load at failure by the cross-sectional area of the cube (2500 mm²).

Figure 14 depicts the changes in compressive strength (MPa) of ultra-high-performance concrete (UHPC) mixes that incorporated varying percentages of rice husk ash (RHA) as a partial substitute for cement. The mixes included a control (0% RHA) and RHA substitutions of 20%, 30%, and 40%. The control and 20% RHA mixes exhibited the highest early strength values, surpassing 100 MPa under hot water curing (HW-24 h). The 30% RHA mix also demonstrated relatively high strength, although slightly lower than the control, indicating some pozzolanic activity at early ages. These results are in line with the research by Jittan V et al. [18], in which it was mentioned that the use of RHA as a partial replacement for cement enhanced both compressive strength and tensile strength. The highest compressive strength for conventional concrete with RHA was reported to be between 10% and 15% [26].

Higher RHA contents (40–60%) led to a notable reduction in strength, suggesting that the dilution of cementitious content outweighs the benefits of pozzolanic reactivity at this stage. Under normal water curing (NW-7D), the control and 20% RHA mixes exhibited superior strength (≈85–90 MPa). The 30% RHA mix maintained good strength (≈78 MPa), confirming that up to 30% replacement can be advantageous without significant compromise on strength. Beyond 40% replacement, the strength decreased significantly, highlighting the reduced hydration products owing to excessive cement replacement.

A clear improvement was observed in the pozzolanic mixes after 28 d. The 30% RHA mix even surpassed the control mix (≈110 MPa), showing optimal replacement owing to the enhanced secondary C–S–H formation. The 20% RHA mix also exhibited comparable or slightly lower strength than the control. However, mixes with 40%, 50%, and 60% replacement continued to show substantially reduced strength, confirming that excessive RHA addition leads to underperformance, and so RHA50 and RHA60 were not included in the result analysis as they performed poorly.

Optimum RHA content is approximately 20–30% replacement of cement with RHA, which provides the best compressive strength performance, even surpassing the control at 28 days. The effect of the curing regime is that hot water curing accelerates early hydration, enabling high strength at 24 h, whereas normal water curing shows gradual but consistent strength gain. High replacement levels (≥40%) lead to the dilution of the binder phase and inadequate calcium hydroxide for the pozzolanic reaction, resulting in a significant drop in compressive strength.

4.3. Statistical Analysis

To validate the experimental results, a simple statistical analysis was performed using two-factor analysis, and it is explained in detail in this section.

4.3.1. Two-Factor Analysis

To check the effect statistically, a two-way ANOVA was performed considering the effects of replacement levels of RHA, curing conditions, and the interaction between them. For performing the analysis, both factors and the interaction between them were analyzed.

Table 2. Factors and levels.

Factors		Number of Levels
A	Material replacement	4 (a)
B	Curing conditions	5 (b)
The number of different treatments is (ab)		20
Number of observations per treatment (n)		3
Total number of observations (N) = abn		60

shows the factors considered, and the results are shown in

Table 3. The ANOVA results.

Source of Variation	SS	df	MS	F	p-Value	F Crit
Replacement	12,364.34	3	4121.448	310.4494	9.90817 × 10−28	2.838745
Curing temp	4396.577	4	1099.144	82.79339	8.41101 × 10−19	2.605975
Interaction	3366.785	12	280.5654	21.13368	1.28747 × 10−13	2.003459
Error	531.0299	40	13.27575
Total	20,658.73	59

.

The factor of interaction is significant if F > Fcrit and the p-value is <0.05. When considering Factor A and evaluating whether the results are significant, Table 2 shows that the results are indeed significant, as F > Fcrit (320.4494 > 2.83875) and p-value < 0.05 (9.90817 × 10⁻²⁸ < 0.05). Also, the effects of Factor B prove to be significant, as F < Fcrit (82.79339 > 2.605975) and p < 0.05 (8.41101 × 10⁻¹⁹ < 0.05). In addition, the interaction between replacement levels and curing conditions is also significant, as their ‘F’ values are much greater than ‘Fcrit’. This indicates that the replacement levels, due to curing conditions, affect the compressive strength both individually and combinedly. Normally, a p-value < 0.005 is considered a significant effect, and the values here are much less than 0.01 in all cases, from which it is understood that there is a highly significant effect due to changes in the curing and replacement levels.

4.3.2. Interaction Between RHA Replacement and Curing Temperature

Figure 15 shows the interaction effect between the material replacement levels and the curing conditions. From the interaction curve, it is understood that there is no consistent decrease or increase in compressive strength as the replacement levels change and the curing conditions are modified. The control (C) shows higher compressive strength under the curing condition (C1), but does not show much strength increase under the other curing conditions. RHA 30 shows a progressive strength increase up to C3 curing conditions, after which it shows a decrease in strength when exposed to other curing conditions. Mortars of all replacement levels show a considerable strength hike when exposed to the C3 curing condition. So, irrespective of the replacement levels, it is evident that among all curing conditions adopted, C3 contributes the most to improving strength.

4.4. Split Tensile Strength

The split tensile strength results shown in Figure 16 follow a similar trend to that of the compressive strength values. Like the compressive strength results, the optimum tensile strength has also been reported by earlier researchers to be between 10% and 20% [26,27,28], and the RHA20 subjected to hot water curing and oven curing showed higher tensile strength results with 10.29 MPa and 10.23 MPa respectively, which were the highest among all specimens. There was a reduction in the tensile strength of the RHA20 mix subjected to normal water curing for 28 days. The reason for higher strength at elevated temperatures is the triggering of the reactive components due to the controlled hot temperature. Compared with hot water curing for 24 h, the oven-cured specimens exhibited higher split tensile strength. The other mixes, namely RHA30 and RHA40, showed a slight decrease in strength. This may be due to the additional increase in the RHA content, which might have affected the pozzolanic reaction.

5. Conclusions

This study demonstrates the significant potential of rice husk ash (RHA) as a sustainable supplementary cementitious material for ultra-high-performance concrete (UHPC). Partial replacement of cement with RHA markedly influenced both fresh and hardened UHPC properties. Flowability decreased with increasing RHA content, which is attributed to its high surface area and porous microstructure, necessitating increased water and superplasticizer demand.

• Compressive strength development revealed an optimal replacement range of 20–30% RHA, maximizing pozzolanic reactivity and enhancing secondary C–S–H formation. This optimal range achieved strengths comparable to or exceeding those of the control mix at 28 days. However, higher replacement levels (≥40%) resulted in strength reductions due to dilution of cementitious phases and insufficient calcium hydroxide for effective pozzolanic reaction.
• Curing regimes significantly impacted strength development. Hot water curing accelerated early hydration and strength gain, while normal water curing facilitated steady long-term development, underscoring the importance of optimizing curing strategies to fully leverage RHA reactivity in UHPC systems.
• The incorporation of RHA at optimum levels not only enhances UHPC’s mechanical performance but also offers substantial environmental benefits by reducing cement consumption and associated CO₂ emissions, while addressing agricultural residue disposal challenges. However, large-scale adoption faces hurdles such as RHA quality variability, reduced fresh-state workability, and the lack of standardized guidelines for UHPC applications.
• Statistical validation using two-way ANOVA indicated that there is a significant effect due to the replacement levels of RHA and due to the curing condition. It is also evident from the interaction plots that the curing condition C3 has contributed to the increase in compressive strengths.

Future research directions should focus on developing controlled processing methods for consistent RHA quality, investigating hybrid replacement strategies with other supplementary cementitious materials and fibres, and establishing comprehensive design standards for practical applications. Addressing these challenges could position RHA-blended UHSM as a next-generation, eco-efficient construction material, combining exceptional mechanical performance with significant sustainability contributions.