Environmental-friendly high-strength concrete using hydraulic cement and river gravel residue

2.1 Materials

This study utilized HC as the primary cementing agent, conforming to the Thai Industrial Standard (TIS 2594-2023) to manufacture HSC. A portion of HC was replaced with Class C FA from the Mae Moh power plant at replacement levels of 0%, 10%, 20%, and 30% to improve the chemical reaction of HC. The specific gravities of HC and FA were 3.15 and 2.30, respectively. Additionally, this study employed an MRWR, Types D and G according to ASTM C494, due to its compatibility with FA and its lower cost compared to high-range water reducing admixtures (HRWR). MRWR was found to provide adequate workability and compressive strength development when the mix design was properly optimized. Photos of the raw materials used in this study have been shown in Fig. 1.

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Fig. 1.

Binder mixture.

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Fig. 2 presents scanning electron microscope (SEM) images at 5,000x magnification of HC and FA. The HC particles exhibited irregular shapes with a rough and complex surface texture, while the FA particles were spherical with similarly rough surfaces. The particle size distribution and chemical composition of both materials have been shown in Fig. 1 and Table 1, respectively, with the average equivalent particle sizes (D₅₀) of HC and FA being 15.17 µm and 34.26 µm, respectively. HC primarily contains 12% calcium (Ca) and 6.06% silicon (Si) by weight. FA had a silicon content of 16.68% and an aluminum (Al) content of 9.14%. Both materials included oxygen (O) as a significant component, accounting for 45.38% and 42.03% by weight in HC and FA, respectively. Oxygen plays a crucial role in forming oxide compounds, which enhance the strength of the cement paste.

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Fig. 2.

Particle characteristics of Hydraulic Cement (HC) and Fly Ash (FA).

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The aggregates used in this study consisted of natural river sand and 0.95-mm (3/8-inch) river gravel residue. Both materials were sourced from the Nan River, located in Uttaradit Province, which is a major river in Northern Thailand. The gradation of the river sand complied with ASTM C33 specifications (ASTM, 2002), as illustrated in Fig. 2. The Nan River serves as a significant source of construction materials in the region, highlighting its importance in providing high-quality aggregates for this research.

The particle size distribution analysis revealed that the HC particles had a narrow and consistent distribution, which facilitated hydration and early strength development in concrete. In contrast, FA exhibited a broader, bimodal distribution, efficiently filling voids within the concrete (Fig. 3). The sand had a coarser distribution, enhancing concrete density, reducing voids, and improving overall durability (Assi et al., 2018; Cui et al., 2020; Kim et al., 2022). The aggregate properties used for the mix design have been shown in Table 2.

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Fig. 3.

Particle Distribution of HC, FA, and sand.

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2.2 Preparation of specimens

This study used a concrete mix design based on ACI 211.4R-93 (ASTM, 2000) and targeted a compressive strength of 60 MPa at 28 days. The water-to-binder ratios (w/b) was adjusted to 0.325, 0.320, 0.308, and 0.292, while varying the FA content at 0%, 10%, 20%, and 30% by weight of the binder (HC+FA) to maintain the concrete slump within the range of 75-125 mm (100±25 mm). The mix proportions of the specimens are shown in Table 3. Furthermore, the MRWR content was varied at 1%, 1.5%, 3%, 4.5%, and 6% by total binder weight. These variations were applied using the mix with the optimal FA replacement ratio, which provided the highest compressive strength, to evaluate the effect of MRWR content on compressive strength development and workability.

2.3 Experimental program

3.1 Compressive Strength

Fig. 4 indicates a relationship between compressive strength and curing age for concrete specimens in which HC was partially replaced with various proportions of FA. The compressive strength consistently increases over time for all mixtures. The 28-day compressive strength of HC concretes using limestone and river gravel residue met the strength target (> 60 MPa). However, the 28-day compressive strength of HC-FA concretes remains below the target value of HC concretes. This phenomenon is an expected behavior when incorporating FA. The 56-day results of the 10–20% FA mixes closely approached the target 28-day strength of HC concretes. Nevertheless, optimizing the w/b ratio to achieve appropriate workability has contributed to higher 28-day compressive strength values, reaching 45.9 MPa, 50.8 MPa, and 29.0 MPa for FA replacement levels of 10%, 20%, and 30%, respectively. In contrast, concrete specimens utilizing only HC with limestone (HCFA0) as coarse aggregate achieves 28-day compressive strength exceeding 60 MPa, aligning with the target design strength.

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Fig. 4.

Compressive strength at different curing times and various FA contents.

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The use of high volumes of FA results in a reduction of tricalcium silicate (C₃S), a key component in the hydration process, thereby limiting the formation of C-S-H, which is responsible for the early development of compressive strength. Consequently, the initial strength development is slower (Chindaprasirt et al., 2007; Rao et al., 2021). FA, as a pozzolanic material, requires calcium hydroxide (Ca(OH)₂) produced from C₃S hydration to react and form C-S-H over a longer period. Though, the reduction in C₃S content due to FA replacement leads to lower early-age compressive strength, this pozzolanic reaction contributes to improved concrete strength and durability in the long term.

The test results exhibit a consistent trend in compressive strength development across all curing periods. The optimum w/b providing the highest strength is found to be 0.320 for HCFA10 and 0.308 for HCFA20. An increase in FA content from 10% to 20% improves compressive strength. However, replacing more than 20% of cement with FA leads to a decline in compressive strength. This finding corroborates the work of Hemalatha et al. (2017), who identified 20% FA as the optimal replacement level for enhancing concrete strength. The improvement is attributed to the enhanced formation of calcium silicate hydrate (C-S-H), which contributes to the strength and durability of concrete (Sun et al., 2019; Mohsen et al., 2023).

The results of this study indicate that the optimum FA replacement level lies within 10-20%, where compressive strength is maximized. This finding is consistent with previous investigations on self-compacting concrete and semi-flowable self-compacting concrete incorporating FA, SCBA, and RHA, which reported that such pozzolanic materials enhance strength development, particularly at later curing ages, due to their gradual pozzolanic reactivity (El-Mir et al., 2022; Kannur & Chore, 2021; 2023a; 2023b; 2025a). Specifically, studies on pavement concrete and slip-form paving applications have highlighted that an FA content of around 20% represents an optimum range for balancing strength and durability. The present results corroborate these findings, despite the differences in constituent materials; river gravel residue and MRWR were adopted in this study. These factors emphasize the compatibility of FA with alternative aggregates and admixtures for sustainable HSC applications.

While concrete specimens without FA (HCFA0) exhibits stable compressive strength beyond 28 days, specimens incorporating FA (e.g., HCFA20) demonstrate continuous strength development due to the pozzolanic reaction of FA. This reaction, which progresses more slowly than cement hydration, contributes to forming additional calcium silicate hydrate (C-S-H) over time. Consequently, at 56 days, the compressive strength of HCFA20 approaches that of HCFA0, highlighting the long-term benefits of using FA at an optimal replacement level of 20%. These findings align with studies such as Hemalatha et al. (2017), which emphasize the potential of FA in enhancing long-term strength and durability. Replacing more than 20% of the HC with FA significantly reduces the tricalcium silicate (C₃S) content in the cement, thus slowing the hydration process and affecting compressive strength development at all curing times.

Fig. 5 illustrates the relationship between compressive strength and curing age for concrete specimens with varying MRWR contents of 1.5%, 3.0%, 4.5%, and 6.0%, designed with an optimum w/b of 0.308 and incorporating a 20% FA replacement ratio. The results demonstrate that MRWR dosages of 1% and 1.5% significantly enhance compressive strength at all curing ages. The specimen with a 1% MRWR content achieves a maximum 7-day compressive strength of 50.8 MPa, demonstrating that this dosage effectively promotes the early hydration process. This behavior can be attributed to the efficient dispersion of cement particles, which enhances the initial formation of C-S-H. However, at curing ages > 7 days, the specimen with a 1.5% MRWR content exhibits the highest compressive strength, reaching 64.6 MPa at 56 days of curing, surpassing the target compressive strength. The superior performance of the 1.5% dosage at later ages can be explained by its ability to retain moisture within the concrete matrix, ensuring prolonged hydration and the continued formation of hydration products over time.

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Fig. 5.

Compressive strength of samples with various MRWR contents at different curing times (w/b = 0.308, FA = 20%).

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In contrast, when the MRWR content was increased beyond 1.5%, a noticeable decline in compressive strength was observed across all curing periods. This reduction can be attributed to an imbalance in the w/b ratio, which may hinder proper hydration and negatively affect the microstructure of the concrete. These findings highlight the importance of optimizing MRWR content to achieve the desired balance between early-age strength and long-term strength development.

The results of this study demonstrate that the HSC, utilizing river gravel residue as a substitute for traditional coarse aggregate (limestone), combined with HC and FA as a binder, can be produced with the incorporation of MRWR (the combination of Type D and Type G). Type D acts as a water-reducing and set-retarding agent, while Type G provides high-range water reduction with similar set-retarding properties. However, using excessive amounts of MRWR can lead to an excessively low water content in the concrete mix, which subsequently impairs the hydration reactions of both HC and FA. This reduced hydration activity negatively impacts the development of compressive strength over time (Jia & Wang, 2022; Ammar et al., 2024).

In addition, excessive use of MRWR can disrupt the delicate balance between water reduction and concrete workability. While reducing water content generally improves early compressive strength, overuse of MRWR can lead to the mix becoming too dry or cause uneven dispersion of cement particles. This negatively impacts the hydration process, ultimately hindering the development of compressive strength (Dong et al., 2020; Al Haffar et al., 2021). Based on these observations, the optimal MRWR dosage for enhancing the compressive strength of HSC under the tested conditions should not exceed 1.5%. This dosage maintains a balance between compressive strength improvement and workability without compromising the mechanical properties of the concrete.

3.2 Role of river gravel residue in enhancing concrete performance

Fig. 6(a) shows the naturally rounded shape and smooth surface of river gravel residue, which allows for better rolling and flow during mixing, hence facilitating the uniform dispersion of binder within the cement paste. This uniformity leads to a more efficient and consistent hydration process, ultimately contributing to the steady development of compressive strength throughout the curing period. Additionally, the smooth surface of river gravel has a direct impact on the behavior of the interfacial transition zone (ITZ), which serves as the boundary between the cement hydration products and the aggregates. As highlighted by Lyu et al. (2019), the surface texture of aggregates can significantly affect both the porosity and the continuity of the ITZ. This observation helps explain why the surface condition of aggregates, even when smooth, still plays a meaningful role in the microstructural development of concrete.

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Fig. 6.

(a) River gravel residue and (b) Failure of concrete specimens.

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Meanwhile, Alanazi et al. (2022) reported that hydration products within the ITZ strongly influence the overall strength of concrete. Although the smooth surface of river gravel may reduce mechanical interlocking, the uniform dispersion of binder, together with the use of MRWR, enables a lower w/b ratio while maintaining good workability. The result leads to a denser cement paste, reduced porosity and discontinuity in the ITZ, and promotes the continuous formation of C–S–H around the gravel surface. These observations are consistent with the findings of Li et al. (2012), who demonstrated that effective improvement of the ITZ is directly associated with the development of compressive strength over curing time. Therefore, under appropriate proportions of HC, FA, and MRWR, this mechanism supports the formation of a dense microstructure and the continuous development of strength in concrete throughout the curing period.

In addition to enhancing the concrete’s mechanical properties, river gravel residue offers environmental benefits. By minimizing the need for energy-intensive extraction processes associated with quarrying traditional aggregates, using river gravel residue reduces the overall consumption of energy and resources. This makes river gravel residue a more sustainable and eco-friendly alternative, aligning with the construction industry’s ongoing efforts to mitigate environmental impacts.

The interparticle bond plays a vital role in increasing concrete strength over time (Schneider et al., 2011; Prusty et al., 2016). While river gravel residue has a smooth, rounded surface, using a suitable binder and MRWR can improve its performance. This strategy ensures adequate adhesion between the cement paste and aggregates, which helps in achieving the desired compressive strength. Fig. 6(b) illustrates the failure pattern of concrete specimens under compressive load. The failure develops within the cement paste and shears along the surface of the river gravel residue. These observations confirm that river gravel residue is an effective coarse aggregate for producing HSC, meeting the 60 MPa design target. Moreover, they demonstrate that river gravel residue works well with HC and FA.

Importantly, river gravel residue also improves the workability of concrete, even when using low w/b ratios. The addition of river gravel residue facilitates better flowability of the concrete, enhancing the workability of the mixture while reducing the water content. This leads to denser concrete, which is essential for achieving both higher compressive strength and long-term durability. The ability of river gravel residue to maintain good workability at low w/b ratios makes it particularly advantageous in the production of high-performance and sustainable concrete.

3.3 Temperature of chemical reaction

Fig. 7 illustrates the temperature variations within cement paste containing varying percentages of FA replacement at different curing times. The purpose of monitoring temperature variations was to better understand the chemical mechanisms associated with increasing FA content. Specifically, the objective was to examine whether FA influences the activation of the hydration process or alters the formation of hydration products. Such measurements are important because they reflect the rate of chemical reactions and the associated heat release, both of which are directly linked to the development of compressive strength and the microstructural evolution of concrete. This approach underscores the connection between chemical behavior and mechanical performance, while also providing a means to evaluate the role of pozzolanic materials and admixtures in enhancing overall concrete performance. FA replacement at levels between 10% and 20% leads to an increase in the internal temperature of the cement paste across all curing ages. This rise in temperature occurs due to the hydration reaction between cement and water, which produces C-S-H and Ca(OH)₂. This process generates heat, with calcium oxide present in FA accelerating the hydration reaction, resulting in an overall increase in the internal temperature of the mixture.

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Fig. 7.

Temperature change of cement paste at different FA contents.

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The results demonstrated that the maximum recorded temperature remained below the engineering threshold of approximately 70°C. This limit is generally recommended to prevent delayed ettringite formation and to reduce the risk of thermal cracking (Giatec Scientific Inc., 2020). In addition, the measured temperature differential (ΔT) within the concrete mass did not exceed 20-25°C. This range is considered safe to avoid thermally induced stresses that could otherwise lead to structural cracking (National Ready Mixed Concrete Association [NRMCA], 2020). These findings confirm that the hydration process in this study posed no risk of thermal cracking. More importantly, they highlight the role of FA in modifying hydration mechanisms. This agrees with previous studies that have established correlations between time–temperature profiles and the strength and durability of high-performance and mass concrete (El-Mir, Assaad, Nehme, & El-Hassan, 2022). Prior work has also addressed the influence of coarse aggregates on hydration heat-release behavior (Tan et al., 2024). The temperature trends observed here are consistent with the long-term development of compressive strength in mixtures incorporating FA. This further validates the beneficial role of FA in sustainable HSC.

Moreover, the silicon dioxide (SiO₂) present in FA reacts with Ca(OH)₂, which is a by-product of the cement hydration, to produce C-S-H further, contributing to the continuous heat generation throughout the hydration process (Bourchy et al., 2020; Chai et al., 2021). These compounds play a crucial role in enhancing the hydration process, particularly in cases where the w/b ratio is low, as in the case of HCFA20 (w/b = 0.308). Under such a condition, the hydration reaction intensifies, resulting in greater heat generation. The limited water content allows the chemical components in HC and FA to react more efficiently and intensively, thereby increasing the heat released during the entire curing period (Chai et al., 2021; Ding et al., 2024).

This finding is consistent with the study by Hu et al. (2014), which affirmed that a low water-to-cement (w/c) ratio accelerates the release of heat from hydration reactions, particularly during the initial hours. However, the rate of hydration decreases thereafter. HCFA30 exhibits a reduction in internal temperature compared to HCFA20, while HCFA10 consistently demonstrates the lowest internal temperature across most curing periods. Although FA contributes additional calcium oxide (CaO) and silicon dioxide (SiO₂) to the binder matrix, the significantly low w/b ratio may result in insufficient water for the hydration reaction to proceed completely. This deficiency leads to a reduction in heat generation (Sun et al., 2019; Ding et al., 2024; Huang et al., 2024), which in turn impacts the development of compressive strength.

The test results reveal that the compressive strength of HCFA30 is lower as compared to that of HCFA20, indicating that the primary cause is the high replacement level of HC with FA at 30%. This high FA content significantly reduces the amount of cement, which is the primary binder contributing directly to strength development. Although the inclusion of FA introduces additional CaO and SiO₂ into the binder matrix, the excessive replacement level also results in the predominance of the pozzolanic reaction, which proceeds at a slower rate compared to the cement hydration. This slower reaction cannot fully compensate for the reduction in HC content within the specified curing period. Furthermore, a low w/b ratio may limit the availability of water sufficient for the hydration of both HC and FA, leading to incomplete reactions and reduced strength development. In addition, it is plausible that a portion of the available water is consumed in reactions with FA. Consequently, incorporating a very high FA content (30%) may result in inadequate water availability for the hydration process, leading to incomplete hydration reactions and a corresponding reduction in compressive strength (Chai et al., 2021; Huang et al., 2024).

Fig. 8 demonstrates the temperature variation in cement paste incorporating a 20% FA replacement (HCFA20), with varying dosages of MRWR at 1%, 1.5%, 3%, 4.5%, and 6%. Increasing the MRWR dosage from 1% to 1.5% significantly enhances the heat released during the hydration process. This increase in heat generation is attributed to the improved dispersion of cementitious materials, which facilitates more efficient hydration. At the 1.5% dosage level, the cement paste recorded the highest average temperature of 31.3°C (±0.90°C), compared to 30.0°C (±0.98°C) at the 1% dosage. This finding suggests that the hydration process at the 1.5% dosage is more complete and results in a higher rate of exothermic reaction during early hydration (Wu et al., 1983; Poon et al., 1999; Gong et al., 2020). However, as the dosage of MRWR increases beyond 1.5%, the average temperature begins to decline. Specifically, at dosages of 3%, 4.5%, and 6%, the temperatures drop to 29.5°C (±0.82°C), 29.5°C (±0.79°C), and 29.3°C (±0.82°C), respectively. This decline is indicative of reduced hydration activity, likely due to the excess MRWR causing a delay in the cement setting process.

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Fig. 8.

Temperature change of HCFA20 cement paste at different admixture contents.

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The retardation effect of the MRWR appears to surpass its hydration acceleration benefits when the dosage exceeds 1.5%, leading to a decline in compressive strength development. The 1.5% dosage represents the optimal balance between improving hydration and achieving compressive strength. In contrast, higher dosages disrupt the hydration process, leading to incomplete hydration and lower heat evolution, ultimately resulting in reduced strength gain. This observation aligns with the findings of Mitchell & Margeson (2006), who demonstrated that the curing temperature directly influences the efficiency of set-retarding admixtures. Their study reported that at higher curing temperatures (40°C), retarders delayed the setting by approximately 5 h, whereas at lower temperatures (25°C), the delay extended to 20 h, further influencing the hydration kinetics (Feng et al., 2020; Kim et al., 2022; Huang et al., 2024).