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Article

An Assessment of the Impact of Locally Recycled Cementitious Replacement Materials on the Strength of the Ultra-High-Performance Concrete

by
Thuc V. Ngo
1,
Viet Ba Tran
2,
Bao Hoai Le
1,
Huyen T. Dang
3,
José Matos
3,
Minh Q. Tran
3 and
Son N. Dang
3,*
1
Urban Infrastructure Faculty, Mien Tay Construction University, Vinh Long 85100, Vietnam
2
Vietnam Concrete Association, Ha Noi 11100, Vietnam
3
Department of Civil Engineering, ISISE, ARISE, University of Minho, 4800-058 Guimarães, Portugal
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(17), 7484; https://doi.org/10.3390/app14177484
Submission received: 19 July 2024 / Revised: 13 August 2024 / Accepted: 22 August 2024 / Published: 24 August 2024
(This article belongs to the Special Issue Mechanical and Structural Behavior of Fiber-Reinforced Concrete)
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Versions Notes

Abstract

:
Withstanding extreme events is increasingly a significant challenge for the construction industry. Where civil infrastructures remain using traditional concrete, which has low tensile strength, poor durability, and weak crack resistance, in this regard, ultra-high-performance concrete (UHPC), with its outstanding mechanical properties and high strength, offers the prospect of wide application. This advanced technology allows for the fabrication of thin and light-dimensional structures to accelerate construction while increasing corrosion resistance to minimize maintenance intervention and extend the service life of the infrastructures. Despite this, UHPC is less eco-friendly due to consuming more cement than the usual material, which requires replacement materials, such as silica fume (SF) and rice husk ash (RHA), which are readily available from other local material production. This study proposes an experimental approach to assess the influence of SF and RHA content on the properties of UHPC. Different SF and RHA compositions will be adjusted to analyze their effects on slump flow, compressive strength, flexural strength, tensile strength, and the stress–strain relationship in UHPC tension testing. Based on the results, the most effective ratio is RHA replacing 50% of the SF in the UHPC mixture. Specialized tensile experiments reveal enhanced tensile strength with judicious RHA incorporation at 5-day and 28-day stages, particularly in initial crack and damage conditions. Stress–strain curves for 5% to 15% RHA samples show increased ductility, indicating that optimal RHA-SF ratios enhance UHPC cracking characteristics. Based on the results, a discussion on the appropriate proportions for utilizing most local materials will be derived, especially for regions of Vietnam. It is evaluated as a feasible and promising solution to reduce greenhouse gas emissions threatening global climate change.

1. Introduction

With high and still-climbing greenhouse gas levels, 2023 is assessed to be the warmest year ever recorded. Since atmospheric carbon dioxide levels are now 50% more than before industrialization, global warming will persist for the foreseeable future due to heat-trapping [1]. Current research pointed out that the negative impacts of climate change have been more evident in recent years and have significantly influenced numerous aspects of human life. The human race is subject to natural disasters and extreme events, which can break infrastructure and severely affect people, property, and the economy [2]. Any disruption or destruction of this infrastructure will hurt society’s ability to continue operating normally in the long term. Therefore, studying advanced materials is extremely necessary to ensure the sustainability and resilience of critical infrastructure [3].
Regarding concrete material, ultra-high-performance concrete (UHPC) is an advanced technology that has received significant worldwide attention from the construction industry. Several mechanical properties, such as high strength, durability, and crack resistance, characterize it. UHPC is created by combining cement, fine aggregates, steel fibers, superplasticizers, and other advanced materials, resulting in a concrete that possesses significantly greater resistance capacity as well as durability in comparison to traditional concrete [4,5,6,7,8]. The utilization of UHPC has demonstrated its ability to enhance structural performance and decrease maintenance demands in contrast to conventional concrete structures. One of the key factors contributing to UHPC’s superiority is its exceptional compressive strength. With compressive strength reaching 150–200 MPa or even higher, UHPC allows for the design of thinner and lighter structures while still ensuring high load-bearing capacity. This not only reduces the amount of material used but also decreases the load on the foundation, thereby saving construction costs and increasing economic efficiency. UHPC’s crack resistance is significantly improved thanks to the use of steel fibers and superplasticizers. The steel fibers are evenly distributed within the concrete mix, creating a micro-reinforcement network that prevents the development of cracks from the early stages. Superplasticizers not only improve the workability of the concrete mix, making it easier to pour and mold, but also enhance the bond between the components in the concrete, increasing durability and impermeability. Furthermore, UHPC exhibits excellent resistance to harsh environmental impacts, such as corrosion, abrasion, and temperature changes. Advanced components in UHPC, like silica fume and other mineral additives, create a dense microstructure that reduces water permeability and enhances resistance to environmental aggressors. This is particularly important in bridge construction, port facilities, and offshore structures, where materials with high durability and environmental resistance are required. A typical application of UHPC is in bridge construction. Using UHPC in bridge components, such as beams, columns, and bridge decks, not only enhances load-bearing capacity but also extends the bridge’s lifespan, reducing maintenance and repair costs. Additionally, UHPC is applied in high-rise buildings, retaining walls, and other structures requiring high durability, opening up new possibilities for the design and construction of modern structures. UHPC represents a breakthrough in concrete technology, offering numerous economic and technical benefits for the construction industry. Researching and widely applying UHPC not only improves the performance and durability of structures but also contributes to promoting sustainable development in construction [7,8].
One of the most commonly used mineral admixtures for manufacturing UHPC is silica fume (SF). Numerous authors have studied the use of SF and various mineral additives to replace cement in order to enhance the properties of different types of concrete. The Pozzolan materials, such as silica fume and fly ash, are used to improve certain properties of concrete in lightweight concrete mixtures by replacing a portion of the cement weight [9]. Another research study also pointed out that with the use of 30% SF, the fiber can better resist brittleness, thus improving 45% of the bending strength and 55% of the tensile strength, which significantly increases the performance of the UHPC. In practice, the use of 30% SF exhibits an equivalent performance not only under unaged conditions but also under aged conditions, where the bending strength remains around 14.5–15.5 MPa while fracture toughness value fluctuates between 8.0–8.7 kJ/m2 [10].
The replacement for cement contributes to optimizing the UHPC mixture in compactness and mineral densification. When incorporated into the concrete mixture, SF not only fills the microvoids, increasing the density of the concrete, but also participates in chemical reactions with the hydration products of cement, creating compounds with superior mechanical properties. This results in UHPC having high compressive strength, excellent crack resistance, and long-term durability. Moreover, SF significantly enhances UHPC’s strength through pozzolanic reactions. Nevertheless, the present elevated expense of silica fume has significantly increased the material expenses for UHPC in comparison to conventional concrete. The research and development of alternative materials with similar properties but lower costs are becoming an important direction. Several studies have shown the potential of industrial waste materials, such as fly ash, blast furnace slag, and rice husk ash. These materials are not only available in large quantities but can also reduce CO2 emissions, contributing to environmental protection.
In order to promote the widespread application of UHPC, it is necessary to find an alternative material with similar properties to silica fume but at a lower cost. Therefore, utilizing local and waste materials in a suitable and standardized manner as a replacement for cement is regarded. The use of local and recycled materials not only helps reduce the production costs of UHPC but also lessens the negative environmental impact. The construction industry can create more sustainable concrete types that meet modern technical and environmental standards. Additionally, the development and application of alternative materials will promote a circular economy, minimizing waste and conserving natural resources. It aims to be an essential issue for the progress of the economy by resulting in concrete that is more eco-friendly to the environment [11,12,13,14,15].
Various waste materials have been initially investigated to understand their effects on the structural resistance of materials. For example, using rice husk ash (RHA) in soil improvement is one of the findings that discovered that the substance could be regarded as an excellent stabilizing agent for enhancing the various soils’ geotechnical properties [16]. Furthermore, it can be partially utilized as a cement replacement. To obtain the highest possible compressive and split tensile strengths, the ideal ratio is 7.5% of cement replacement for RHA. This ratio produces the best results, and the cost is really reasonable. RHA and paper ash are two materials that might be substituted for the paste typically used to produce self-compacting concrete (SCC) [17,18].
Regarding the interfacial transition zone (ITZ), the porosity or total content of Ca(OH)2 can be reduced by incorporating RHA or SF into concrete. In addition, compared to the control samples, the ITZ is also narrower [19,20]. Furthermore, the utilization of RHA with 84% SiO2 content can result in a remarkable 50% improvement in the concrete’s compressive strength [21]. Moreover, impermeability can be improved with more durability; on the other hand, resistance against chemical attacks and frost can also be advantageous. Eventually, using those cement replacement materials, besides improving the concrete properties, is expected to prevent environmental pollution or save costs significantly.
Vietnam, a leading rice producer, is harnessing its agricultural abundance for sustainable construction practices. The primary objective is to employ rice husk ash, an aftereffect of the production of rice, for the purpose of producing concrete. This innovative approach not only minimizes agricultural waste but also enhances the qualities of concrete, including its strength and durability. Rice husk ash, when incorporated into the concrete mix, serves as an additive that enhances the properties of cement, resulting in the creation of construction materials that are both high-performing and eco-friendly. This initiative reflects Vietnam’s dedication to resourcefulness and eco-conscious advancements in concrete technology, paving the way for greener and more resilient infrastructure. The integration of rice husk ash into concrete mixtures offers several key benefits. First, it reduces the reliance on traditional cement, which is known for its high carbon footprint due to the energy-intensive production process. By partially replacing cement with rice husk ash, Vietnam can significantly lower the environmental impact of its construction activities. Second, rice husk ash improves the microstructure of concrete, leading to increased compressive strength and enhanced durability. This enhances the durability of the concrete, making it more resistant to environmental deterioration. As a result, the lifespan of structures is prolonged, and maintenance expenses are reduced. Moreover, this approach supports the principles of a circular economy by turning agricultural waste into valuable construction materials. It demonstrates how industries can collaborate to address environmental challenges while creating economic opportunities. Utilizing rice husk ash as a substance in concrete also highlights Vietnam’s commitment to innovative solutions that promote sustainability in the construction sector.
This study not only builds upon previous research recognizing the potential of RHA as a substitute for SF but also addresses a critical gap: a comprehensive analysis of how workability, entrapped air content, and the mechanical properties of UHPC interrelate. By conducting technically demanding direct tensile tests to assess tensile strength and stress–strain relationships, the research aims to optimize the RHA-SF ratios for UHPC. The study not only highlights the increased ductility and fracture energy in UHPC with 5% to 15% RHA but also emphasizes the practical application of locally sourced materials in Vietnam, contributing significantly to UHPC research and promoting sustainable construction practices. The assessment of various mechanical properties strengthens the case for RHA’s integration into UHPC, particularly in regions where this material is abundant.

2. Materials and Methods

2.1. Materials

We performed research on using Pordland PC50 cement to produce UHPC. The cement properties were tested and complied with the Vietnamese standard regarding Portland cement [22]. The aggregate was natural quartz sand, ensuring cleanliness and no salt content. The coarse quartz sand (largest particle size 1 mm) accounted for 31.8–35.8%, and the fine sand (particle size smaller than 0.314 mm) accounted for 7.9–8.9%. The sand’s specific gravity was 2.64 g/cm3, moisture content 3%, and modulus of fineness 2.1. Quartz powder filled the voids in the structure, enhancing the bond with the cementitious phase and promoting the formation of crystal nuclei in the Calcium Hydrosilicate structure, thereby increasing strength, especially early-age strength. The quartz powder with particle size smaller than 63 μm accounted for 1–1.2%. The superplasticizer additive used was PCE based, with a water-reducing rate of 40%, pH 6.33, specific gravity of 1.06 kg/L, and dry matter content of 37.8%. The steel fiber used had a fiber dosage selected at 2% and fiber aspect ratio of 62.5, with a 2800 MPa tensile strength and 13 mm in length. The study used SF that complied with ASTM C1240 standard [23], with a specific gravity of 2.22 g/cm3, SiO2 content of 91.6%, bulk density of 310 kg/m3, and moisture content of 2.6%, along with chemical composition as determined in Table 1. The rice husk is burned at 700 °C, and after 3 h, the RHA is collected. After burning, the rice husk ash was finely ground using a vibrating ball mill for 30 min. The RHA had a SiO2 content of 87.4%, as shown in Table 1, and had a mean particle size of approximately 2.67 µm. The SEM analysis results of RHA showed a porous structure [24], while the XRD analysis indicated that RHA mainly consisted of amorphous SiO2 [25]. Table 2 shows the physical property results of SF and RHA.

2.2. Mixture Proportion

Optimizing particle components is a critical step in designing UHPC mixtures [26,27]. The cement content and other supplementary mineral additives can be determined based on the required structural strength. The content of mineral additives should not be less than 10% of the cement, and the total content of binders should not exceed 50% of the total mass of UHPC components [28].
The dosage of dispersed steel fibers is also determined at this stage depending on the specific requirements in terms of tensile and flexural strength, which can be adjusted at different levels. The even dispersion of steel fibers in the concrete mix is crucial for creating an effective reinforcement network. Steel fibers help prevent and limit the development of microcracks, thereby enhancing the crack resistance and load-bearing capacity of the concrete. This is particularly significant in applications requiring high durability, such as bridges, high-rise buildings, and heavy-load structures. Table 3 below presents the mixing ratios of the UHPC samples.
The composition of UHPC mixtures is designed based on an absolute volume balance approach, assuming a certain amount of air voids, from which the maximum amount of chemical additives is determined. The next step is to adjust the volume balance and ensure volume density. The UHPC mixture must simultaneously meet three requirements regarding workability and various mechanical properties, especially in compressive, tension, and flexural strength [28]. UHPC is implemented at 140 MPa compressive strength, using different percentages of steel fibers (SF) as follows: 10% (SF10), 15% (SF15), and 20% (SF20) of the binding material. In this study, RHA was added and used to replace SF with corresponding ratios of 25% (SF15RHA5), 50% (SF10RHA10), 75% (SF5RHA15), and 100% (RHA20).

2.3. Experimental Methods

2.3.1. Mixing and Handling UHPC Mix

The technology or mixing process significantly impacts the quality of UHPC and its solidified form [25]. Mixing UHPC is a complex process that can be divided into two parts. First is the distribution mixing, where low shear forces and velocities alter the state of the mixture particles. The second is dispersion mixing, where high shear velocities disperse the accumulation of mixture particles [28,29]. Theoretically, these effects manifest in UHPC being highly flowable (self-compacting). Therefore, mixing parameters, such as particle shape and size, distribution, density variation, and surface smoothness, are paramount. For the powder particles in the mixture to change their state, sufficient kinetic energy for mixing must be provided, which is significantly higher than that required for regular concrete. Depending on the corresponding consistency and viscosity, power and electricity consumption increase significantly. Dispersion mixing needs to ensure that the voids between coarse particles and fibers are filled with finer particles and that the gaps between solid particles are filled with water. This process helps release water, break up aggregates, and achieve the dispersion and homogenization of fibers [30,31,32]. To achieve uniformity in the production of UHPC specimens, all mixtures were mixed according to a standardized process, as shown in Figure 1.
After casting and leveling the surface, UHPC samples were cured under the same conditions. Initially, the samples were naturally cured in a moist environment. After an additional 24 h, they were further cured in a controlled environment with a temperature of 80 °C for 72 h. Then, the samples were cured in laboratory conditions until they reached the age of 5 days and, finally, until they reached the age of 28 days for testing [33]. The elevated temperature significantly speeds up the chemical reactions in the cement hydration process. At 80 °C, hydration occurs much faster, leading to rapid development of strength and other mechanical properties. High-temperature curing promotes the formation of a denser microstructure. The increased temperature facilitates the more complete and more efficient hydration of cement particles, reducing porosity and increasing the density of UHPC. A 5-day curing period is sufficient to ensure that the hydration process of the cement is thorough, providing the necessary strength and durability for UHPC. The combination of 5 days and high temperature helps UHPC to develop early strength, saving time compared to traditional curing methods. It also aids in the reaction between hydrated cement lime and active mineral additives to produce hydro-silicate minerals, enhancing bonding and density. Additionally, it boosts the reaction between the surface of silica powder particles and lime produced during cement hydration, increasing the bond between the CKD phase and fine silica particles. Furthermore, strengthening the structure reduces the shrinkage of UHPC, preventing structural cracks after curing. These effects collectively increase the density, bonding phases, strength, impermeability, and corrosion resistance of UHPC.

2.3.2. Test Methods

The flow of UHPC mixtures over time was measured using the Suttard viscometer according to the ASTM standard specification regarding the flow for use in tests of hydraulic cement [34]. The measurement was conducted for all UHPC mixtures at different intervals, including 0 min (after mixing), 15 min, 30 min, and 45 min.
To determine the entrapped air content of the UHPC mixture, an air content meter based on ASTM C185 was used [35]. The fresh UHPC mixture was carefully placed into the meter, ensuring minimal disturbance. The meter was then sealed, and pressure was applied according to the device’s guidelines. The air content was read directly from the gauge, representing the percentage of air within the mixture.
The UHPC’s compressive strength was tested in accordance with ASTM C39 using 10 × 20 cm cylinder samples [36]. The test specimens were measured after a heat curing period of 72 h (5 days) and 28 days. Six samples represented each concrete mix at each age level, and the experimental results were averaged over these six samples.
The tensile strength in flexure was measured on 5 × 10 × 50 cm specimens according to ASTM C78 standards at two age levels (5 days and 28 days) [37]. The number of test specimens corresponded to the compressive strength determination experiment.
A direct tensile experiment was conducted on a “dog-bone” shaped specimen, as shown in Figure 2. The test sample, with a length of 50 cm, featured two elongated anchor heads of 10 cm each, securely fastened to a tensile testing machine. The experimental procedure followed standards applicable to other materials, such as ASTM B557M [38], ASTM E8/E8M [39], and ASTM D638 [40]. In addition, the experimental apparatus for this study was established by consulting research studies regarding the approach to measure the tensile strength of concrete by [41,42].
The stress–strain relationship curve for UHPC was determined for all mixing proportions by employing LVDT displacement sensors regarding both sides of the sample during the direct tensile experiment.
To ensure the reliability of the experimental results, each characteristic or stress–strain relationship was tested on three replicate samples.

3. Results

3.1. UHPC’s Workability and Entrapped Air Content

The results of the slump flow experiments for various UHPC mixtures are presented in Figure 3. The slump flow of UHPC mixtures gradually decreases when using SF as a replacement for cement, and the reduction corresponds to the increased content of SF. This outcome also aligns with findings from other studies [43,44,45]. The ultrafine particle size of SF necessitates a higher water content for surface lubrication.
Conversely, as observed in Figure 3, the slump flow of UHPC mixtures tends to increase when using RHA as a replacement for SF. It is attributed to RHA’s smaller characteristic surface area compared to SF, which reduces the need to lubricate with water.
The experimental results indicate that the utilization of SF can lead to a reduction in the entrapped air content of UHPC mixtures. When using 20% SF, the entrapped air content decreased by more than 40% compared to the control sample SF10. Similarly, when incorporating 5–10% rice husk ash (RHA), in comparison to the SF20, the entrapped air content tends to decrease. It can be attributed to the capability of SF and RHA to fill voids. The diverse combination of particle sizes among SF, RHA, cement, and quartz sand contributes to the overall compactness of the mixture. Figure 4 shows the result of the trapped air content of the UHPC mixture.
However, using RHA at 15–20% increases the entrapped air content. It is primarily due to the insufficient presence of SF to facilitate the void-filling effect, especially within micro-scale voids. This finding aligns with the research results [46]. Moreover, the elevated presence of RHA particles in the mix, particularly at higher percentages, contributes to a less effective void-filling mechanism than SF. The larger particle size of RHA, in conjunction with its limited ability to fill micro-scale voids, leads to increased entrapped air. This observation underscores the importance of carefully optimizing the proportions of RHA in UHPC mixtures to strike a balance between the benefits of void-filling and the potential drawbacks associated with entrapped air. It also further supports the notion that a nuanced approach is required when incorporating RHA into UHPC formulations to achieve optimal performance [46].

3.2. Compressive Strength

For samples using only SF without RHA, the strength of compression is proportionally increased with the SF content increase. The results show that samples with 15% and 20% SF exhibit a significant increase in compressive strength compared to the sample with 10% SF, with respective increments of 6.9% and 12.7% at the 28-day stage. It aligns with previous studies by [13,14], where it was explained that SF can enhance hydrolysis reactions. This results in a reduction in unreacted cement, and less stable components such as Ca(OH)2 are neutralized through ettringite reactions, leading to the formation of new C-S-H gels with improved quality.
Regarding samples incorporating RHA at levels of 5–10%, it has been observed that this enhances the compressive strength of UHPC at both the 5-day and 28-day stages compared to the sample with 20% SF. The rationale behind this improvement may be attributed to RHA’s ability to fill microscale voids, creating a denser state within the UHPC matrix. According to Huang and colleagues [46], RHA particles fall between the size range of cement particles and SF particles, facilitating the formation of a more compact and robust UHPC mixture. In this particular situation, the substitution of cement with SF accounted for a 20% replacement rate. However, this proportion might fall short of effectively occupying all the system’s empty spaces and voids [47]. The insufficiency of this replacement percentage to adequately fill these gaps could potentially impact the overall performance and characteristics of UPHC. Figure 5 shows the compressive strength of UHPC at 5 days and 28 days of age when using different ratios of SF and RHA.
The increase in the compressive strength of UHPC with the incorporation of RHA is primarily due to its impact on workability and entrapped air content. RHA enhances the densification of the concrete matrix through its pozzolanic reaction with Ca(OH)2, resulting in the formation of additional C-S-H gel. This reaction not only contributes to a denser microstructure but also improves the packing density of the concrete mix. Consequently, voids within the concrete are more effectively filled, leading to reduced porosity and enhanced compressive strength.
The finer particle size of RHA further contributes to this effect by improving workability. Better packing density due to finer RHA particles allows for a more uniform distribution and consolidation of the mix, which reduces the likelihood of entrapped air. Since entrapped air can create internal voids that weaken the concrete, reducing its content is crucial for achieving higher compressive strength.
Additionally, RHA’s ability to enhance pozzolanic reactions, fill microscale voids, and mitigate autogenous shrinkage also plays a significant role in increasing compressive strength. However, when RHA is used at higher proportions, such as 15–20%, a decrease in compressive strength compared to a 10% RHA sample may occur. This reduction could be due to an increase in entrapped air, insufficient silica fume (SF) to fill microscale voids, and the limited compensatory effect of RHA particles in addressing these issues.

3.3. Flexural Strength

UHPC’s flexural strength is directly proportional to the silica fume (SF) content employed in this investigation. At the 5-day age of UHPC, using 15% and 20% SF resulted in increases of 28.4% and 42.3% compared to 10% SF. At 28 days, these increases were 17.5% and 43.9%. When replacing SF with RHA at 5% and 10% of the binder’s mass, the flexural performance of UHPC improved both at 5 days and 28 days. The increase in samples using 10% RHA compared to those using only SF (SF20) was 8% at 28 days. It is in line with research findings from [46], who concluded that replacing SF with RHA can enhance UHPC’s flexural strength by 10.5% at 28 days of age. The flexural strength of UHPC is enhanced as a result of the improved bonding of high-strength steel fibers regarding the matrix of concrete, which is facilitated by the increase in compressive strength. Figure 6 shows the flexural strength of UHPC at 5 days and 28 days of age when using different ratios of SF and RHA.
The outcome can be elucidated primarily through improvements in the packing of granular mixtures. Insights gleaned from the study outlined highlight that in mixtures of a 0.165 water-to-cement ratio, a significant portion of up to 50% of the cement fails the approach of substituting cement with alternative materials. Furthermore, the incorporation of finer materials emerges as a strategic avenue capable of yielding a more favorable packing density. It contributes to an amelioration in the fundamental properties of the concrete, signifying a notable enhancement in its overall structural performance.

3.4. Tensile Strength

The tensile strength of UHPC specimens is directly determined using a specialized measuring device. The system consists of components installed on the machine, including a load cell, LVDT, and a datalogger system, aiding in the experiment to ascertain the stress–strain response during tensile deformation of the samples (Figure 2).
The results in Figure 7 and Figure 8 demonstrate that the tensile strength of UHPC increases when using RHA as a substitute for SF, with quantities ranging from 5 to 15% in both the 5-day and 28-day stages. The elimination of Ca(OH)2 components is a critical factor to improve the concrete tensile strength. The ultrafine particles of SF and RHA not only assist in the pozzolanic reactions of cement but also initiate secondary reactions that consume Ca(OH)2. These effects contribute to the improvement of the C-S-H gel and transition zones between the matrix and high-strength steel fibers, particularly evident in the early-age stage.
The size of RHA falls between cement particles and SF, which also plays a significant role in creating a mixture with higher compactness. This aspect has also been addressed in studies by [46]. Additionally, the tensile strength of UHPC is enhanced by the stronger bond between the concrete matrix and high-strength steel fibers.
The stress–strain relationship was obtained from the UHPC samples at the 5-day age stage (Figure 9) and 28-day age stage (Figure 10).
From Figure 10, the samples with a combination of SF and RHA show significant improvement in ductility and deformation capacity, which is in line with current similar research [48]. Notably, the SF10RHA10 sample maintains a good balance between tensile strength and deformation, suggesting an optimal mix of SF and RHA. The SF15RHA5 sample has a tensile strength close to SF15 but with better deformation capacity, indicating that RHA improves the ductility of UHPC. Using too much RHA (RHA20) may reduce tensile strength but significantly improves the ductility of UHPC, which can be beneficial in applications requiring high ductility. This graph provides crucial information on how the ratios of SF and RHA affect the mechanical properties of UHPC, guiding the design and application of these high-performance concrete types in practical scenarios.
The curves effectively illustrate the behavior of UHPC samples corresponding to varying levels of SF and RHA usage. Based on the chart, we can determine the strength at which the first crack appears for different UHPC mixtures when normalized to the ratio of initial crack strength to ultimate strength. The post-peak curves of samples using RHA (5–15%) exhibit different slopes compared to those using only SF; they have higher ductility and a lower slope, resulting in a larger area under the curve, indicating higher fracture energy. Therefore, it can be concluded that the fracture characteristics of UHPC will be enhanced by combining RHA and SF at appropriate levels. The utilization of fine particles can enhance the fracture properties of both concrete as well as UHPC, as evidenced by prior studies [49,50].
Along with the above results, it is noteworthy that an emphasis on the analysis of the microstructure is necessary. The incorporation of RHA into UHPC enhances tensile strength by refining its microstructure. RHA, rich in amorphous silica, actively participates in the pozzolanic reaction, consuming calcium hydroxide (Ca(OH)2) and producing additional calcium silicate hydrate (C-S-H) gel. This process not only densifies the matrix but also reduces porosity, particularly in the interfacial transition zone (ITZ), which is typically a weak point in concrete. Moreover, the harmonious combination of SF and RHA significantly enhances the microstructure of UHPC. This synergy not only reduces the porosity of the concrete but also strengthens the bond between the aggregates and the cement matrix, resulting in a denser and more uniform microstructure.
Several studies have supported this mechanism. For instance, Tuan et al. found that RHA significantly enhanced the hydration process and the distribution of hydration products, thereby strengthening the bond between aggregates and the cementitious matrix. This microstructural improvement is critical in resisting the initiation and propagation of cracks, thus improving the tensile strength of UHPC [20]. Another study by Wang et al. demonstrated that the introduction of RHA led to a more refined pore structure, which directly contributed to improved mechanical properties, including tensile strength [21].

4. Conclusions

The study effectively combined silica fume and rice husk ash in UHPC production, revealing several key findings:
  • RHA improves workability but can increase entrapped air at higher percentages (15–20%).
  • Replacing 50% of SF with RHA (SF10RHA10) enhances compressive strength beyond that of 100% SF. The compressive strength increases by 15.7% compared to the sample with only 10% SF and by 4% compared to the sample with 20% SF.
  • A mixture of 10% RHA and 10% SF enhances both the workability and compressive strength of UHPC. The optimal RHA–SF ratios lead to improved flexural and tensile strengths, with tensile strength increasing by approximately 20% compared to UHPC samples containing 10% SF and about 5% compared to those with 20% SF.
  • The stress–strain analysis shows increased ductility and fracture energy with 5% to 15% RHA. This suggests that RHA can effectively substitute a portion of SF.
This study emphasizes the potential of RHA as a partial substitute for SF, optimizing mechanical properties and promoting the sustainable development of UHPC using locally sourced materials in Vietnam. To advance the research, we propose investigating the effects of RHA and SF on UHPC’s microstructure, analyzing their carbon footprint and energy consumption, and assessing long-term durability through environmental exposure tests (salt, sulfate, acid). Further studies should focus on optimizing RHA-SF ratios, comparing other materials, and exploring the scalability of RHA-based UHPC in construction projects. A life cycle assessment (LCA), alongside numerical simulations and experimental testing, will provide a comprehensive evaluation of the environmental and economic benefits, ensuring the long-term effectiveness and sustainability of UHPC.
It is important to note that this study evaluates sustainability only for UHPC mixtures with RHA in specific regions of Vietnam. Consequently, the weighting factors and evaluation criteria may need to be adjusted to reflect local conditions and requirements. To verify the generalizability and broader applicability of the conclusions and findings, further case studies should be conducted using the proposed sustainability assessment model.

Author Contributions

Conceptualization, T.V.N., V.B.T., B.H.L., M.Q.T., J.M., and S.N.D.; methodology, T.V.N., V.B.T., B.H.L., M.Q.T., J.M., and S.N.D.; software, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., and S.N.D.; validation, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., J.M., and S.N.D.; formal analysis, J.M. and S.N.D.; investigation, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., J.M., and S.N.D.; resources, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., and S.N.D.; data curation, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., and S.N.D.; writing—original draft preparation, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., J.M., and S.N.D.; writing—review and editing, M.Q.T., J.M., and S.N.D.; visualization, T.V.N., V.B.T., B.H.L., M.Q.T., H.T.D., and S.N.D.; supervision, J.M. and S.N.D.; project administration, T.V.N., J.M., and S.N.D.; funding acquisition, J.M. and S.N.D. All authors have read and agreed to the published version of the manuscript.

Funding

This study received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available at the request of the corresponding author due to the host institution’s rules.

Acknowledgments

Minh Q. Tran acknowledges the support by the doctoral grant reference PRT/BD/154268/2022, financed by the Portuguese Foundation for Science and Technology (FCT) under the MIT Portugal Program (2022 MPP2030-FCT).

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 14 07484 g001
Figure 1. The mixing process of UHPC at the laboratory.
Figure 1. The mixing process of UHPC at the laboratory.
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Figure 2. Test specimens and setup (ASTM standard): (a). compressed sample; (b). tensile sample; (c). bending sample; (d). compression test; (e). tensile test.
Figure 2. Test specimens and setup (ASTM standard): (a). compressed sample; (b). tensile sample; (c). bending sample; (d). compression test; (e). tensile test.
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Figure 3. Slump flow of UHPC mixture.
Figure 3. Slump flow of UHPC mixture.
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Figure 4. Entrapped air content of UHPC mixture.
Figure 4. Entrapped air content of UHPC mixture.
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Figure 5. Effect of SF and RHA on compressive strength of UHPC.
Figure 5. Effect of SF and RHA on compressive strength of UHPC.
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Figure 6. Effect of SF and RHA on flexural strength of UHPC.
Figure 6. Effect of SF and RHA on flexural strength of UHPC.
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Figure 7. Effect of SF and RHA on tensile of UHPC at 5 days.
Figure 7. Effect of SF and RHA on tensile of UHPC at 5 days.
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Figure 8. Effect of SF and RHA on tensile of UHPC at 28 days.
Figure 8. Effect of SF and RHA on tensile of UHPC at 28 days.
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Figure 9. Tensile stress–strain relationship of UHPC at 5 days.
Figure 9. Tensile stress–strain relationship of UHPC at 5 days.
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Figure 10. Tensile stress–strain relationship of UHPC at 28 days.
Figure 10. Tensile stress–strain relationship of UHPC at 28 days.
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Table 1. Chemical composition of cement, SF, and RHA (%).
Table 1. Chemical composition of cement, SF, and RHA (%).
MaterialsSiO2Fe2O3CaOMgOAl2O3SO3Na2OK2O
Cement17.233.4564.562.113.984.330.150.89
SF91.60.20.30.40.580.10.160.65
RHA87.40.631.150.580.820.410.382.58
Table 2. The results of testing the physical of silica fume and rice husk ash (%).
Table 2. The results of testing the physical of silica fume and rice husk ash (%).
MaterialsSpecific Weight (g/cm3)Porous
Volume (kg/m3)		Humidity (%)Loss on Heating (%)
SF2.223102.60.6
RHA2.264102.31.92
Table 3. Mix proportions of UHPC.
Table 3. Mix proportions of UHPC.
MixCement (kg/m3)SF (kg/m3)RHA (kg/m3)Fine Sand (kg/m3)Coarse Sand (kg/m3)Steel Fiber (kg/m3)SP 2
(%)		w/b 1
SF101035.0115.0-2058181552.40.165
SF15997.5172.5-2058181552.40.165
SF20920.0230.0-2058181552.40.165
SF15RHA5920.0172.557.52058181552.40.165
SF10RHA10920.0115.0115.02058181552.40.165
SF5RHA15920.057.5172.52058181552.40.165
RHA20920.0-230.02058181552.40.165
1 w/b: water to binder (by weight); 2 SP: Superplasticizer (% by weight of binder).
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MDPI and ACS Style

Ngo, T.V.; Tran, V.B.; Le, B.H.; Dang, H.T.; Matos, J.; Tran, M.Q.; Dang, S.N. An Assessment of the Impact of Locally Recycled Cementitious Replacement Materials on the Strength of the Ultra-High-Performance Concrete. Appl. Sci. 2024, 14, 7484. https://doi.org/10.3390/app14177484

AMA Style

Ngo TV, Tran VB, Le BH, Dang HT, Matos J, Tran MQ, Dang SN. An Assessment of the Impact of Locally Recycled Cementitious Replacement Materials on the Strength of the Ultra-High-Performance Concrete. Applied Sciences. 2024; 14(17):7484. https://doi.org/10.3390/app14177484

Chicago/Turabian Style

Ngo, Thuc V., Viet Ba Tran, Bao Hoai Le, Huyen T. Dang, José Matos, Minh Q. Tran, and Son N. Dang. 2024. "An Assessment of the Impact of Locally Recycled Cementitious Replacement Materials on the Strength of the Ultra-High-Performance Concrete" Applied Sciences 14, no. 17: 7484. https://doi.org/10.3390/app14177484

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