Optimizing rice husk ash for ultra-high-performance concrete: a comprehensive review of mechanical properties, durability, and environmental benefits

2.1 Physical characteristics

The physical properties of RHA, including particle size, surface area, and morphology, are crucial in determining its effectiveness as an SCM in UHPC. The particle size of RHA varies depending on the grinding process used after combustion. Generally, RHA particles are finer than those of ordinary Portland cement but coarser than silica fume. The typical particle size ranges from 10 to 75 μm [19], 20]. Finer RHA particles offer a greater surface area for pozzolanic reactions, thereby improving the strength and durability of UHPC. However, their high surface area also increases water demand, which may negatively impact workability if not properly managed [20].

RHA has a high specific surface area due to its porous structure, which in turn influences its reactivity [21]. A higher surface area allows for greater interaction between the silica in RHA and the calcium hydroxide in the cement paste, promoting the formation of additional C–S–H. This increased reactivity is beneficial for improving the mechanical properties of UHPC [22], 23].

The morphology of RHA particles typically consists of irregular, porous structures, remnants of the original cellular structure of the rice husk [16]. This porous structure contributes to the high surface area and also influences the packing density of RHA in the concrete mix. The irregular shape can lead to a more interlocked structure within the cement matrix, contributing to the overall strength of the concrete [20], 24]. Ultra-fine particles, however, may agglomerate, hence lowering dispersibility and sometimes requiring further mixing energy or dispersing agents.

As shown in Figure 1, the Scanning Electron Microscope (SEM) image of RHA highlights its porous and irregular surface morphology. The visible pores and rough texture demonstrate the unique physical characteristics of RHA, which contribute to its high surface area and reactivity as SCM in concrete. These features enhance the pozzolanic activity of RHA, thereby improving the mechanical properties and durability of UHPC.

Figure 1:

SEM image of RHA revealing porous and irregular morphology [25].

The physical properties of RHA, including particle size, surface area, and morphology, significantly influence its performance as SCM in UHPC. Table 1 concisely encapsulates the essential physical characteristics of RHA as an SCM.

2.2 Chemical composition

RHA is primarily composed of silica (SiO₂), which typically constitutes about 85–95 % of its total chemical composition, depending on the combustion conditions during its production [30], 31]. This high silica content is vital for its role as SCM in concrete, especially in UHPC [7], 9]. The silica in RHA is predominantly in an amorphous form, which is highly reactive and essential for pozzolanic activity [19], 32]. The RHA must adhere to ASTM C 618 [33] in order to be utilised as a cementitious supplementary material. For instance, the LOI must not exceed 12 %, and the overall percentage of Si, Al, and Fe oxides must be at least 70 %. LOI shows RHA’s unburning carbon content. By adsorbing water and admixtures, high LOI RHAs can prevent cement hydration by lowering the effective water-to-cement ratio and thereby affecting the dispersion of superplasticizers.

The temperature and duration of the heat treatment must be carefully regulated to preserve the requisite chemical composition of RHA, including the SiO₂ phase [34]. RHA’s pozzolanic efficiency is primarily influenced by its amorphous silica concentration. Under regulated burning circumstances (500–700 °C), a large concentration of amorphous, non-crystalline silica is preserved and easily interacts with CH to generate C–S–H. This enhances strength, durability, and matrix densification. On the other hand, RHAs containing crystalline silica phases, such as those resulting from overburning, exhibit reduced pozzolanic reactivity, which in turn reduces their mechanical performance.

Pozzolanic materials, when combined with calcium hydroxide (Ca(OH)₂) in the presence of water, react to form additional C–S–H, the compound responsible for the strength and durability of concrete [9], 35]. The high amorphous silica content in RHA enhances this pozzolanic reaction, contributing to the densification of the concrete matrix, reducing porosity, and improving overall strength and durability [7], 22], 23]. Other minor components in RHA include alumina (Al₂O₃), ferric oxide (Fe₂O₃), and trace elements like potassium oxide (K₂O), which together typically make up less than 10 % of RHA [16], 21]. These components can slightly influence the color and reactivity of RHA but do not significantly affect its primary role as a pozzolanic material [30], 31].

Table 2 summarizes the typical chemical composition of RHA, highlighting its high silica content in comparison to other components. SiO₂ constitutes the majority of RHA, making up 85–95 % of its composition, which is crucial for its role as SCM in concrete. Other components, such as alumina (Al₂O₃), ferric oxide (Fe₂O₃), potassium oxide (K₂O), and trace elements, are present in lower percentages, typically accounting for less than 5 %, 3 %, 2 %, and 1 %, respectively. This composition underlines the importance of silica in enhancing the pozzolanic activity and overall performance of RHA in applications such as UHPC.

3.1 Hydration reactions

RHA plays a crucial role in the hydration process of UHPC by influencing the formation of hydration products, particularly calcium silicate hydrate (C–S–H), which is responsible for the concrete’s strength and durability [36], 37]. The primary hydration reaction in traditional concrete involves the hydration of cement to form calcium hydroxide (Ca (OH)₂) and C–S–H. In UHPC with RHA, the pozzolanic reaction occurs when the amorphous silica in RHA reacts with the calcium hydroxide produced during the hydration of cement. This reaction consumes calcium hydroxide, which is relatively weak and prone to leaching, and converts it into additional C–S–H [38], 39]. The formation of more C–S–H enhances the density and strength of the concrete matrix [40].

The inclusion of RHA in UHPC leads to an increased volume of C–S–H due to the higher silica content available for the pozzolanic reaction [38], 40]. This results in a more compact microstructure, reducing the amount of free lime (calcium hydroxide) and enhancing the overall durability of the concrete. The secondary C–S–H formed is typically more refined, contributing to a denser and less permeable matrix [37], 41].

Beyond C–S–H, the hydration process in RHA-enhanced UHPC may also produce small amounts of calcium aluminate hydrate (C-A-H) and calcium aluminosilicate hydrate (C-A-S-H), depending on the presence of alumina in the RHA [36], 38]. These products further contribute to the matrix’s refinement and strength, albeit to a lesser extent compared to C–S–H [37], 40], 42].

Figure 2 illustrates the impact of RHA on the hydration process of cement. On the left, in the absence of RHA, the hydration process shows larger capillary pores and porous calcium hydroxide. In contrast, the diagram on the right illustrates hydration with RHA, where the pozzolanic reaction yields a denser calcium silicate hydrate (C–S–H) structure, thereby reducing capillary pores and enhancing the overall matrix of the concrete. This transformation highlights the role of RHA in enhancing the durability and strength of concrete by densifying its microstructure.

Figure 2:

Schematic comparison of hydration in the presence and absence of RHA [43], 44].

3.2 Microstructure improvement

RHA enhances the microstructure of UHPC, resulting in improved performance characteristics, including reduced porosity, increased density, and enhanced mechanical properties. The pozzolanic reaction facilitated by RHA contributes to the reduction of capillary pores within the concrete matrix. As more C–S–H is formed, it fills the available voids, leading to a denser structure [45]. This reduction in porosity not only increases the compressive strength of UHPC but also enhances its resistance to harmful substances, such as chlorides and sulfates, which can lead to corrosion of the reinforcement in conventional concrete [40], 42], 46].

The high surface area and fine particle size of RHA allow it to effectively fill the voids between cement particles, further enhancing the packing density of the UHPC matrix. This densification results in more impermeable concrete, which is less susceptible to water ingress and freeze-thaw damage [45], 47].

Including different RHA particle sizes and replacement levels, Figure 3 displays the SEM micrographs of UHPC samples after 28 days of curing [48]. Along with clearly visible holes, fractures, and unreacted RHA particles, the pictures show major hydration products including needle-like ettringite, fibrous C–S–H gel, and flaky or layered calcium hydroxide (CH). While larger particles are associated with insufficient hydration and potential microstructural flaws, finer RHA particles contribute to creating a denser matrix and enhanced interfacial transition zones (ITZ). The secondary hydration products, primarily C–S–H, formed due to the pozzolanic reaction, are typically more refined and contribute to a more homogenous microstructure [45]. The fine and continuous C–S–H gel fills in micro-cracks and voids, which can otherwise serve as pathways for the ingress of deleterious agents [38]. The overall improvement in the microstructure results in concrete that exhibits superior long-term durability and mechanical properties [42], 46].

Figure 3:

SEM images of UHPC with RHA: (a) Control; (b) R5–5; (c) R5–40; (d) R25–15; (e) R50–15; (f) R100–15 [48]. Note: In the notation Rx–y, x represents the RHA particle size in microns (5, 25, 50, or 100 µm), and y indicates the replacement percentage of cement with RHA (e.g., 5 %, 15 %, or 40 %).

Table 3 compares the microstructural properties of conventional UHPC with RHA-enhanced UHPC. RHA-enhanced UHPC demonstrates improvements, including reduced porosity, fewer capillary pores, increased packing density, and more refined C–S–H gel formation. These enhancements lead to a denser, more durable concrete matrix with greater resistance to chlorides, sulfates, water ingress, and freeze-thaw cycles, thereby offering superior long-term durability and mechanical performance.

3.3 Role of the pozzolanic property of RHA in UHPC

Although this feature is still underappreciated in some studies, the pozzolanic character of RHA is crucial in improving the performance of UHPC. Amorphous silica, high in RHA, combines with CH generated during cement hydration to create extra C–S–H gel [49]. The long-term strength, microstructural refinement, and resilience of UHPC depend much on this secondary reaction [50]. Over time, the pozzolanic reaction occurs, and it is especially significant in thick matrices, such as UHPC, where internal hydration proceeds far beyond early ages. RHA’s finely divided amorphous silica reacts with portlandite (CH) according to the reaction:

By increasing the quantity of C–S–H, the concrete matrix becomes stronger and more impermeable while decreasing the amount of CH, which is comparatively susceptible to leaching [51]. RHA refines the pore structure and reduces capillary porosity through pozzolanic action, which facilitates microstructural densification [52]. The extra C–S–H gel creates a denser and more cohesive matrix by filling up interstitial gaps and microvoids. In UHPC, where high packing density and low porosity are essential performance criteria, this effect is very advantageous.

The pozzolanic reaction of RHA becomes increasingly crucial at later ages (28 days and beyond), whereas cement hydration and particle packing have the most effect on the early-age strength of UHPC [51]. Numerous studies have demonstrated that adding RHA results in a continuous increase in strength due to the pozzolanic activity that continues to occur, particularly when RHA is treated to produce a high amorphous silica content and a small particle size. The synergistic enhancement of mechanical and durability qualities can result from the pozzolanic features of RHA when combined with other SCMs such as silica fume or metakaolin [53]. Superplasticizers enhance the reactivity and uniform distribution of RHA particles in the UHPC matrix by facilitating their dispersion.

Several variables, including burning conditions, particle fineness, and crystallinity, influence the effectiveness of RHA as a pozzolan [54]. A high concentration of amorphous silica is typically obtained through controlled combustion at temperatures ranging from 500 °C to 700 °C. Reactivity is increased for finer particles with a larger specific surface area. When opposed to amorphous forms, crystalline silica forms (like quartz) show noticeably less pozzolanic activity [55]. In order to guarantee constant pozzolanic efficacy, RHA needs to be meticulously treated and characterized. The pozzolanic behavior of RHA in UHPC has not yet been well investigated or measured despite its significance. To evaluate the kinetics and byproducts of pozzolanic processes, future studies should make use of methods such as SEM, thermogravimetric analysis (TGA), and isothermal calorimetry. Optimizing RHA’s performance contribution will need an understanding of how it interacts with other UHPC components under different curing regimens.

3.4 Case studies and examples

Several real-world applications and case studies demonstrate the practical benefits and challenges of using RHA in UHPC. In Southeast Asia, RHA has been successfully integrated into UHPC for various infrastructure projects, including high-performance bridges and urban infrastructure [21], 38]. The use of RHA in these applications has improved sustainability and durability while also addressing environmental concerns related to the disposal of rice husks [42], 46].

In Malaysia, RHA-enhanced UHPC has been used in coastal structures, where resistance to chloride penetration and sulfate attack is crucial [21]. The dense microstructure provided by the RHA reduces the ingress of harmful ions, thereby extending the service life of these structures in aggressive marine environments [56]. In the Middle East, high-performance structures in hot climates are required, and RHA-based UHPC can enhance thermal resistance and reduce the heat of hydration [12]. The RHA-based UHPC application is also observed in Southeast Asia, the USA, and the northern part, as presented in Table 4.

Table 4 also outlines the regional applications of RHA-enhanced UHPC in Southeast Asia and Malaysia. It highlights the specific benefits, such as improved sustainability, durability, and resistance to harsh environmental conditions, as well as challenges like scaling integration and maintaining performance consistency. The widespread use of RHA in UHPC addresses local sustainability needs by repurposing rice husk waste while enhancing concrete performance in demanding infrastructure projects.

4.1 Mechanical properties

4.1.1 Compressive strength

The incorporation of RHA into UHPC influences its compressive strength. Generally, RHA enhances compressive strength, mainly when used at optimal replacement levels, typically between 10 % and 15 % [59], 60]. This increase in strength is attributed to the pozzolanic activity of RHA, which leads to the formation of additional calcium silicate hydrate (C–S–H), thereby densifying the concrete matrix [38], 46]. However, variations in compressive strength have been observed depending on the quality of RHA and the specific replacement level. At lower replacement levels (below 10 %), the benefits of RHA may not be fully realized, as the quantity of RHA may be insufficient to impact the pozzolanic reaction significantly [61]. Conversely, at higher replacement levels (above 20 %), there may be a reduction in compressive strength due to the dilution effect, where the decrease in the amount of cementitious material outweighs the benefits provided by RHA [21], 59].

Pu, et al. [60], as shown in Figure 4(a), finds a clear increase in compressive strength with RHA up to 15 %, with the highest strength achieved at a 15 % replacement level. Beyond 15 %, further increases in RHA content led to a plateau or slight reduction in strength, particularly at longer curing times. According to Huang, et al. [62], the fineness of RHA is probably between that of SF and cement. Therefore, a combination of SF, RHA, cement, and quartz sand, with a replacement ratio of 2/3, most likely forms the densest packing condition. Apart from the filling effect, both the pozzolanic reactivity and internal curing action of RHA contribute to the growth of compressive strength, as previously reported [63].

Figure 4a:

Compressive strength of concrete with varying RHA replacement percentages at different curing times [60].

4.1.2 Tensile strength

The tensile strength of UHPC is also positively influenced by the inclusion of RHA, although the effect is generally less pronounced than that observed in compressive strength [64]. The increase in tensile strength can be attributed to the refinement of the interfacial transition zone (ITZ) between the cement paste and the aggregates, which is enhanced by the presence of fine RHA particles [65]. This refinement reduces microcracking and improves the bond strength within the matrix [66]. However, similar to compressive strength, excessive RHA can lead to a reduction in tensile strength, particularly if the material’s pozzolanic activity is not fully realized due to poor quality or improper processing of RHA [67], 68].

Studies show that RHA replacement at 5 % increases both split and flexural tensile strengths by up to 25.09 % and 3.9 %, respectively. Pandey and Kumar [69], Singh, et al. [70]. Optimal improvements are observed at 10 %, with tensile strength gains up to 7.03 % Hasan, et al. [71] and an average increase of 6.2 % in high-strength concrete Priyanka [72]. However, beyond 10 %, the tensile strength decreases gradually, with a reduction in strength noted at 20 % and 30 % RHA replacement [19], 73]. This suggests that RHA replacement is most beneficial within the 5–10 % range for enhancing tensile strength.

Hasan, et al. [71], as shown in Figure 4(b), finds a slight increase in tensile strength with RHA content, particularly at 5 %–10 % replacement, with minimal improvement at higher levels. Strength increases over time, with the most significant gains observed at 28 and 56 days. These findings suggest that up to 10 % RHA is optimal for enhancing tensile strength, while higher percentages offer limited additional benefits.

Figure 4b:

Tensile strength of concrete with varying RHA replacement percentages at different curing times [71].

4.1.3 Flexural strength

Flexural strength, crucial for applications involving bending stresses, is enhanced by the use of RHA in UHPC. The pozzolanic reaction and the subsequent formation of C–S–H contribute to a denser and more cohesive matrix, improving load distribution and crack resistance in flexural applications [38], 59]. Studies have shown that RHA can increase the flexural strength of UHPC, particularly at replacement levels of 10–15 % [64], 67]. However, at higher replacement levels, the flexural strength may plateau or even decrease, as the reduced cement content offsets the benefits of additional C–S–H formation [74], 75].

Hasan, et al. [71], as shown in Figure 5, indicate that RHA enhances flexural strength, with the highest strength observed at 20 % RHA after 56 days. However, the increase in strength becomes less pronounced beyond 10 % RHA, suggesting that moderate RHA replacement (up to 10 %) provides the best balance between performance and material efficiency. Flexural strength improves with curing time, highlighting the progressive effects of RHA.

Figure 5:

Flexural strength of concrete with varying RHA replacement percentages at different curing times [71].

The use of RHA as a cement substitute in concrete mixtures shows varied effects on compressive strength, depending on the percentage used. Studies indicate that a 5 %–10 % replacement generally leads to an increase in strength, with gains up to 9.78 % in specific mixes. For high-performance applications, even 20 % RHA blends perform comparably to alternatives like silica fume. However, beyond 12.5 %, the compressive strength tends to decline, suggesting a threshold for RHA use in structural applications.

4.2 Rheology and workability

The inclusion of RHA in UHPC impacts the rheology and workability of the concrete mix, influencing factors such as flowability, setting time, and overall ease of handling. Studies have shown that the addition of RHA improves the workability of self-compacting concrete (SCC), which suggests similar benefits in UHPC [82]. However, the fine particle size of RHA can increase the water demand, necessitating adjustments in the water-binder ratio or the use of superplasticizers to maintain optimal flowability and workability [83]. Additionally, the use of RHA in 3D printing concrete has demonstrated its suitability in construction, particularly due to its influence on the rheological properties of the mix [29].

One of the primary challenges associated with the use of RHA in UHPC is its effect on workability. Due to its fine particle size and high surface area, RHA increases the water demand of the concrete mix, leading to a reduction in workability [82]. This can result in a stiffer mix that is more difficult to handle and place, particularly in applications requiring high flowability, such as self-compacting concrete [83], 84]. To mitigate this issue, the use of superplasticizers is often necessary to maintain the desired level of workability without compromising the mix’s strength and durability [29], 85].

The increased fineness and surface area of RHA also influence the flow characteristics of UHPC. While RHA can improve the cohesiveness of the mix, reducing segregation and bleeding, it can also lead to a decrease in flowability if not properly managed [86]. The setting time of UHPC with RHA may be delayed due to the slower pozzolanic reaction compared to the hydration of Portland cement [87]. This delay can be advantageous in hot climates, where rapid setting might lead to issues with workability and curing. However, in colder climates or when rapid strength gain is required, this delay may necessitate the use of accelerators to ensure timely setting and hardening [68], 88], 89].

Table 6 summarizes the effect of RHA content on workability, flowability, and water demand in UHPC. Studies indicate that at RHA contents above 15 %, workability is drastically reduced due to the ash’s low density and high porosity, which increases water demand [92], 93]. For instance, mixes with 20 % RHA require higher superplasticizer dosages to achieve self-compatibility, accompanied by a corresponding increase in viscosity [90]. However, the use of superplasticizers improves flowability and enables the retention of fresh properties, even in high-volume RHA mixes. It was also found that incorporating materials like alumina nanoparticles in combination with RHA helps offset workability loss, particularly at replacement levels of up to 30 % [91]. Thus, while RHA contributes to sustainability in UHPC, careful management of water demand and superplasticizer use is crucial to maintaining its workability and flow characteristics.

4.3 Durability

The durability of UHPC incorporating RHA is one of its most noteworthy advantages, particularly in terms of resistance to environmental degradation and chemical attacks [94], 95]. The use of RHA as a partial cement replacement enhances UHPC’s resistance to chloride ion penetration and sulfate attack, particularly in marine environments, thereby extending the lifespan of concrete structures exposed to harsh conditions [94]. Moreover, incorporating RHA in UHPC contributes to sustainability by reducing CO2 emissions and minimizing landfilling areas, further enhancing the environmental durability of the concrete [95], 96].

5.1 Optimal replacement levels

Determining the optimal replacement level of RHA in UHPC is crucial for striking a balance between enhanced mechanical performance and environmental sustainability [107]. Research indicates that RHA can effectively replace a portion of cement in UHPC, contributing to reduced carbon emissions and utilizing agricultural waste while maintaining or even improving the properties of concrete [108]. The majority of studies suggest that replacing 10 %–20 % of the cement with RHA provides the best balance between performance and sustainability [109]. At these levels, RHA enhances the compressive and flexural strengths due to its high silica content, which promotes the pozzolanic reaction and the formation of additional calcium silicate hydrate (C–S–H) [73]. This reaction not only strengthens the matrix but also reduces porosity, leading to improved durability against environmental factors such as chloride penetration and sulfate attack [110].

While higher replacement levels (above 20 %) can further reduce cement usage, they often result in diminished returns in terms of mechanical properties. At these levels, the dilution effect becomes more pronounced, where the reduction in cement content outweighs the pozzolanic benefits of RHA, leading to decreased strength and potential issues with workability [93], 109], 111].

Figure 6 illustrates the compressive strength of concrete mixes with different percentages of RHA replacement at 3, 7, 28, and 91 days. The data shows that increasing the RHA content up to 10 % generally improves the compressive strength over time, with the highest strength achieved at 91 days for the RHA 10 mix. After 10 % RHA replacement, a slight decline in compressive strength is observed.

Figure 6:

Compressive strength of concrete with varying RHA replacement at different ages [79].

Table 8 provides an overview of the effects of different RHA replacement levels in concrete on compressive strength, environmental footprint, workability, and durability. As the RHA content increases, compressive strength generally peaks at 15 % replacement while the CO₂ reduction improves significantly. Workability decreases slightly with higher RHA percentages, especially above 20 %, but durability remains superior at moderate replacement levels (10–15 %). Beyond 20 %, there is a noticeable reduction in both strength and workability, with the highest CO₂ reduction achieved.

5.2 Water-to-binder ratio

The water-to-binder (W/B) ratio significantly influences the workability, strength, and durability of UHPC, and its optimization is crucial for achieving optimal performance with RHA [84]. UHPC typically requires a very low W/B ratio, often below 0.25, to achieve its high strength and dense microstructure. When RHA is added to the mix, it tends to increase the water demand due to its fine particle size and high surface area. Therefore, maintaining a low W/B ratio is crucial to prevent the mix from becoming too stiff and unworkable [112]. Superplasticizers are often used to enhance flowability without increasing the water content, ensuring that the concrete remains workable while achieving the desired strength [113], 114].

Research suggests that the optimal W/B ratio for UHPC with RHA ranges between 0.18 and 0.22 [84], 115]. Within this range, the concrete achieves a good balance between workability and mechanical performance, with the enhanced pozzolanic activity of RHA contributing to both strength and durability while maintaining sufficient flowability for practical applications [84], 115], 116].

The effects of adding RHA to UHPC at 28 days on the compressive strength, workability (as shown by slump flow), and water-binder (W/B) ratio are shown in Table 9. Both the RHA concentration and the W/B ratio clearly affect compressive strength and workability, according to the data. According to [79], compressive strengths of up to 164 MPa may be obtained without RHA at a low W/B ratio of 0.18 and considerable flowability (230 mm). A lower compressive strength of 100 MPa was found in another investigation [60] with a greater W/B ratio of 0.32 and better workability (275 mm), suggesting that strength might be compromised even in the absence of RHA.

According to Pu, et al. [60], adding 10 % RHA at a W/B ratio of 0.32 produced a notable strength increase (142 MPa) when compared to the control mix at the same W/B ratio (100 MPa). This demonstrates how RHA contributes pozzolanicly, improving matrix densification and strength development. Nevertheless, further research demonstrates that despite large slump flows (510–520 mm), the compressive strength significantly dropped to 51.48 MPa and 48.22 MPa, respectively, even at comparable RHA levels (10–15 %) and somewhat lower W/B ratios (0.3). Variations in mix design might cause these disparities, the use of high-range water reducers, or RHA quality (such as silica concentration, particle size, and burning state).

According to the workability data, RHA can contribute to high slump flow values when mixed appropriately, even though it is a very porous and fine material. This could be because of better particle packing and the usage of admixtures. Higher strength does not necessarily follow from such great flowability, either, especially if the mixture gets too diluted or the binder reactivity is insufficient. All things considered, this table emphasises how crucial it is to optimise the RHA content.

5.3 Synergistic effects with admixtures and fibers

The performance of UHPC incorporating RHA can be further enhanced through the synergistic use of admixtures and fibers, such as superplasticizers and steel or polypropylene fibers. These combinations optimize the mechanical properties, workability, and durability of the concrete [119], 120]. Superplasticizers help to disperse the fine RHA particles more evenly throughout the mix, reducing the risk of agglomeration and ensuring a homogenous paste [84]. This results in a mix that is both flowable and capable of achieving high strength. The inclusion of steel fibers in UHPC with RHA enhances tensile and flexural strength, as well as toughness and crack resistance [121], 122].

Polypropylene fibers are often added to UHPC to improve resistance to shrinkage and thermal cracking. When used in combination with RHA, these fibers contribute to a more ductile concrete that can withstand temperature fluctuations and prevent the formation of shrinkage cracks [84]. Additionally, the fibers help to reduce spalling during fire exposure, making the concrete more resilient in fire-prone environments [119].

Figure 7 illustrates the combined effects of RHA with various admixtures and fibers on the performance of UHPC. Van Tuan, et al. [79] highlights that incorporating a ternary blend of 10 % RHA with 10 % silica fume into UHPC results in superior compressive strength compared to control samples. This confirms the synergistic benefit of combining RHA with silica fume to enhance concrete performance, achieving strengths exceeding 150 MPa. The combination of RHA and steel fibers further improves the mechanical properties of concrete. A study showed that this blend increases tensile strength by 28.15 %, flexural strength by 9.3 %, and compressive strength by 8.45 %, enhancing the overall performance of UHPC [123]. These findings confirm that RHA, in conjunction with fibers like steel and polypropylene, plays a crucial role in enhancing the workability, strength, and durability of UHPC.

Figure 7:

Synergistic effects of RHA, admixtures, and fibers in UHPC.

6.1 Carbon footprint and sustainability

The incorporation of RHA into UHPC offers substantial environmental benefits, particularly in reducing the carbon footprint associated with cement production [124]. Cement manufacturing is responsible for approximately 8 % of global CO₂ emissions due to the calcination of limestone and the high energy demand of the production process [124]. By partially replacing cement with RHA – typically at levels of 10 %–20% – the CO₂ emissions associated with concrete production can be substantially reduced [125]. This reduction is particularly beneficial in regions with high rice production, where RHA is readily available as an agricultural by-product, turning waste into a valuable resource for sustainable construction [13], 126].

6.1.2 Sustainability

The use of RHA in UHPC aligns with global sustainability goals, particularly those aimed at reducing the environmental impact of the construction industry [126]. By replacing a portion of the cement with RHA, the embodied energy and carbon footprint of UHPC are reduced, making it a more sustainable option for high-performance concrete applications [130]. Moreover, the local sourcing of RHA in rice-producing regions can further reduce transportation-related emissions, enhancing the overall sustainability of the concrete [131], 132].

Table 10 presents a comparative analysis of the emissions associated with conventional UHPC and RHA-enhanced UHPC, which incorporates RHA as a partial replacement for cementitious materials. The emissions data reflect the environmental advantages of utilizing agricultural waste in construction materials. The table underscores the environmental benefits of RHA-enhanced UHPC. Specifically, CO₂ emissions from RHA-enhanced UHPC are reduced to 85–90 % of those from conventional UHPC, illustrating the effectiveness of integrating agricultural waste in reducing the carbon footprint. Similarly, energy consumption is lowered to 80–85 %, contributing to sustainability in concrete production.

Table 10:

Environmental impact comparison: RHA-enhanced UHPC versus conventional UHPC.

Property	Conventional UHPC (%)	RHA-enhanced UHPC (%)	Ref.
CO2 emissions	100 %	85–90 %	[79]
Energy consumption	100 %	80–85 %	[123]
Material extraction emissions	100 %	70–75 %	[79]
Waste disposal emissions	100 %	60–65 %	[123]

Furthermore, the use of RHA results in a 70–75 % reduction in material extraction emissions, highlighting the potential for resource conservation and reduced environmental degradation. Additionally, waste disposal emissions see a considerable decrease to 60–65 %, emphasizing the importance of recycling agricultural waste in reducing landfill contributions. Overall, these findings advocate for the broader adoption of RHA in UHPC formulations as a strategy to enhance environmental sustainability in the construction industry.

Figure 8 compares the emissions associated with conventional UHPC and RHA-enhanced UHPC. The comparison encompasses key emission categories, including CO₂ emissions, energy consumption, material extraction emissions, and waste disposal. RHA-enhanced UHPC shows a reduction in all emission categories due to the use of RHA, a sustainable byproduct. The incorporation of RHA as a partial replacement for cement in UHPC contributes to emission reductions. The CO₂ emissions from RHA-enhanced UHPC can be reduced by 10–15 %, while energy consumption decreases by 15–20 %. RHA also minimizes the need for raw material extraction by 25–30 %, and waste disposal emissions decrease by up to 40 %. These environmental benefits make RHA-enhanced UHPC a more sustainable alternative to conventional UHPC without compromising on structural integrity.

Figure 8:

Emission reduction potential of RHA-enhanced UHPC compared to conventional UHPC [79], 123].

6.2 Economic viability

6.2.3 Long-term savings

The use of RHA in UHPC can lead to long-term economic benefits through enhanced durability and reduced maintenance costs. The improved durability provided by RHA, including better resistance to chloride penetration, sulfate attack, and freeze-thaw cycles, can extend the service life of concrete structures [126]. This reduction in maintenance and repair needs translates into significant cost savings over the lifespan of a structure, making RHA-enhanced UHPC an economically viable option for long-term infrastructure investments [62], 129].

Figure 9 illustrates the cost-benefit analysis associated with incorporating RHA into UHPC. It categorizes the analysis into two main components: Initial Costs and Long-Term Savings. Initial costs encompass expenses related to equipment, labor, and energy needed for the production of RHA. On the other hand, long-term savings highlight the benefits achieved through reduced maintenance costs, extended service life, and material savings. This visual representation helps stakeholders understand the financial implications and overall economic viability of utilizing RHA in UHPC.

Figure 9:

Cost-benefit analysis of using RHA in UHPC.

6.3 Global and regional perspectives

7.1 Quality and consistency

The quality of RHA can vary due to differences in combustion methods, which directly impacts its effectiveness as SCM in UHPC. Controlled combustion methods, typically carried out at temperatures between 500 °C and 700 °C, produce RHA with a high proportion of amorphous silica, essential for the pozzolanic reaction that enhances the strength and durability of UHPC [25], 79]. In contrast, uncontrolled combustion, often done in open-air conditions, can result in RHA with high carbon content and crystalline silica. High carbon content reduces pozzolanic reactivity, leading to a decrease in the overall effectiveness of RHA in UHPC [132]. Additionally, the presence of crystalline silica, which forms at higher temperatures, can negatively affect the durability of the concrete by increasing brittleness [140], 141].

The variability in RHA quality can lead to inconsistencies in the performance of UHPC. For instance, batches of concrete made with high-quality, amorphous silica-rich RHA may exhibit superior compressive strength and durability, while those made with low-quality, high-carbon-content RHA may show reduced performance. This inconsistency poses a challenge in standardizing the use of RHA in UHPC, as the material’s effectiveness is highly dependent on its physical and chemical properties [141]. To address these issues, it is crucial to implement stringent quality control measures during the production of RHA, including monitoring the combustion process to ensure optimal conditions for producing high-quality ash and conducting regular testing for key parameters such as silica content, carbon content, and particle size distribution [25], 132].

Table 12 illustrates the differences between controlled and uncontrolled combustion methods in the production of RHA. Controlled combustion produces high-quality RHA with amorphous silica, which enhances the strength and durability of UHPC. In contrast, uncontrolled combustion results in RHA with a high carbon content and crystalline silica, which reduces the concrete’s performance and makes it more brittle.

7.2 Processing and availability issues

The availability of high-quality RHA is closely tied to the rice production cycle, which varies by region. In rice-producing countries like India, Vietnam, and Thailand, RHA is abundantly available, making it a cost-effective SCM [132]. However, in regions where rice is not widely grown, sourcing RHA can be challenging and costly, as it often requires importation from distant locations [138]. This can lead to increased costs and logistical challenges, potentially reducing the economic viability of RHA-enhanced UHPC in those areas [69].

Processing RHA to meet the stringent requirements of UHPC involves several steps, each of which can introduce variability in the final product. These steps include controlled combustion to produce ash with high amorphous silica content, followed by grinding to achieve the desired fineness [25], 141]. The availability of appropriate processing facilities and technology can be limited, particularly in developing regions [133]. This can result in RHA of inconsistent quality, which may not meet the standards required for use in UHPC. Inadequate grinding can result in a product with inconsistent fineness, which negatively affects the packing density, workability, and strength of UHPC. Additionally, the supply chain for RHA can be affected by factors such as seasonal production cycles and transportation challenges [79], 142].

7.3 Performance limitations

One of the most common performance limitations observed with the use of RHA in UHPC is reduced workability. The fine particle size and high surface area of RHA increase the water demand of the mix, leading to stiffer and less workable concrete [141], 143]. This can make the concrete more challenging to handle, place, and compact, especially in applications that require high flowability, such as self-compacting concrete. To mitigate this issue, higher doses of superplasticizers are often needed, which can increase the overall cost and complexity of the mix design [132]. Additionally, the inconsistencies in RHA quality, particularly variations in silica content and carbon content, can lead to fluctuations in compressive, tensile, and flexural strengths, making uniformity and predictability in performance more challenging [124].

Figure 10 illustrates the interrelationships between RHA Quality, Workability, and Strength in UHPC. Each circle highlights key factors: RHA Quality depends on silica and carbon content, Workability relates to water demand and flowability, and Strength includes compressive, tensile, and flexural strengths. The overlaps demonstrate how poor RHA quality affects both workability and strength, resulting in reduced performance in UHPC. The diagram emphasizes the importance of maintaining a consistent, high-quality RHA to strike a balance between these attributes, ensuring optimal concrete performance.

Figure 10:

Interrelationship of RHA quality, workability, and strength in UHPC.

8.1 Innovative processing techniques

To enhance the pozzolanic activity and overall performance of RHA in UHPC, future research should focus on developing and refining processing techniques that optimize the quality and reactivity of RHA. Controlled combustion techniques, particularly those that maintain optimal temperatures between 500 °C and 700 °C, are essential for producing RHA with a high proportion of amorphous silica, which is crucial for its pozzolanic activity. Additionally, exploring oxygen-enriched combustion environments could further improve the quality of RHA by reducing residual carbon content and enhancing its reactivity [16], 18].

Future research should also investigate ultrafine grinding techniques to reduce the particle size of RHA to the nanoscale, increasing its surface area and reactivity. Combining ultrafine grinding with mechanical activation methods, such as high-energy milling, could further enhance the pozzolanic activity of RHA. Additionally, chemical and thermal treatments, such as acid or alkaline activation and post-combustion calcination, can be explored to enhance the structural and chemical properties of RHA [144].

Figure 11 illustrates potential advancements in the processing techniques of RHA to enhance its application in UHPC. It outlines various methods, such as controlled combustion techniques and oxygen-enriched combustion, aimed at optimizing combustion conditions and reducing residual carbon content. Additionally, the chart emphasizes the importance of ultrafine grinding and mechanical activation to achieve nanoscale particle sizes, thereby increasing the surface area and reactivity of RHA. Other advancements, including chemical activation and thermal treatments, focus on modifying the material’s chemical properties and improving its physical structure. Collectively, these advancements aim to enhance the pozzolanic activity and overall performance of RHA, promoting its sustainability and effectiveness in construction applications.

Figure 11:

Potential advancements in RHA processing techniques.

8.2 Long-term durability studies

The long-term durability of UHPC incorporating RHA is necessary for its broader adoption in construction, particularly in harsh environmental conditions. Future research should focus on conducting extended-duration tests on UHPC with RHA in chloride-rich and sulfate-rich environments, as well as in cold climates subjected to repeated freeze-thaw cycles. These studies should evaluate the material’s resistance to chemical attack, cracking, spalling, and carbonation over decades, providing data on the long-term stability and performance of RHA-enhanced UHPC. Field studies on actual structures using RHA-enhanced UHPC in these environments would also provide valuable insights into its real-world performance [145], 146].

Figure 12 highlights areas for long-term durability research focused on RHA-enhanced UHPC. Each area addresses specific environmental challenges, such as chloride-rich and sulfate-rich conditions, where the concrete’s resistance to chemical attacks and durability can be evaluated. The effects of freeze-thaw cycles are examined to assess resistance to cracking and spalling in cold climates. Additionally, the chart highlights the importance of analyzing chemical attack resistance over time, monitoring cracking and spalling for structural integrity, and investigating the effects of carbonation on potential durability loss. Ultimately, field studies on actual structures are essential for gathering data on real-world performance, providing valuable insights that contribute to the long-term stability and applicability of RHA-enhanced UHPC in diverse construction contexts.

Figure 12:

Key areas for long-term durability research on RHA-enhanced UHPC.

8.3 Standardization and broader adoption

For RHA to be widely adopted in UHPC, the construction industry needs standardized guidelines and certifications that ensure the consistent quality and performance of RHA-enhanced concrete. Collaborating with industry stakeholders to develop comprehensive guidelines for the production, processing, and use of RHA in UHPC is essential. These guidelines should specify the optimal combustion conditions, particle size requirements, and quality control measures for RHA. Additionally, establishing certification programs for RHA producers would ensure that their products meet industry standards, providing construction professionals with confidence in the material’s quality and performance [15], 147].

Efforts should also be made to include RHA-enhanced UHPC in national and international building codes, as well as in green building certification systems such as LEED and BREEAM. This would standardize its use and encourage its application in a broader range of construction projects, particularly those focused on sustainability. Promoting education and training programs for engineers, architects, and construction professionals on the benefits and best practices of using RHA in UHPC would further support its broader adoption [148].

The mechanical properties and durability of ternary or quaternary blended pozzolans, including RHA, should be examined to determine the optimal mix proportions for pozzolans in UHPC. Further research may determine the optimal water-to-binder ratio for UHPC, including RHA with OPC, based on its endurance. The mechanical and durability characteristics of UHPC, including RHA, may be examined for variations in curing temperatures.