Experimental optimization of sustainable fine-grained concrete with marine quartz sand, fly ash, silica fume, and ground granulated blast slag

Abstract

This study proposes an optimized fine-grained concrete incorporating marine quartz sand (FGCMQS) and blended mineral admixtures to enhance mechanical performance while promoting sustainability. Marine quartz sand (MQS) was combined with crushed sand at a 30:70 ratio, resulting in an optimal gradation that meets the requirements of ASTM C33. Using response surface methodology (RSM), this study developed four statistical models to optimize the proportions of fly ash (22%), silica fume (9%), ground granulated blast-furnace slag (32%), superplasticizer (0.85%), and a water-to-cement (W/C) ratio of 0.32. The optimized FGCMQS achieved a slump of 4 cm, a compressive strength of 65.1 MPa, a chloride ion permeability of 812 C (Coulombs), and a sulfate-induced length change of 0.04%. Compared to conventional fine-grained concrete using river sand, the FGCMQS exhibited a 10.5% improvement in compressive strength but slightly higher chloride permeability and sulfate expansion. SEM analysis confirmed a denser microstructure with well-developed C-S-H and C-(A)-S-H gels. Despite durability trade-offs, the optimized FGCMQS presents a viable, eco-friendly alternative to traditional concrete, reducing cement consumption while offering enhanced strength. This study provides a foundational approach for developing high-performance, low-carbon fine-grained concrete, with potential applications in sustainable constructions. Future research should focus on durability enhancement under aggressive environmental conditions.

Graphical abstract

Keywords

marine quartz sand / fine-grained concrete / mineral admixtures / concrete durability / response surface method / CO₂ emissions / sustainable concrete

1 Introduction

Concrete remains the most widely used construction material globally due to its high compressive strength, durability, and cost-effectiveness [1,2]. However, conventional concrete production heavily relies on river sand and Portland cement, contributing to significant environmental concerns. The global construction sector is responsible for approximately 8% of total CO₂ emissions, with cement production accounting for nearly 7% of anthropogenic CO₂ emissions [3]. Additionally, the excessive extraction of river sand has led to severe ecological impacts, including riverbank erosion, habitat destruction, and depletion of natural resources, especially in rapidly urbanizing regions.

To address these challenges, research has increasingly focused on the development of sustainable fine-grained concrete (FGC). This material offers improved microstructural density and enhanced mechanical and durability performance, particularly when optimized with supplementary cementitious materials (SCMs) such as fly ash (FA), silica fume (SF), and ground granulated blast-furnace slag (GGBS). The combined use of SCMs reduces clinker content, mitigates carbon emissions, refines pore structure, and improves both long-term strength and resistance to sulfate and chloride attack [4–8].

Among potential alternatives to river sand, marine quartz sand (MQS) is abundant and geographically accessible, especially for coastal regions. MQS is chemically characterized by a high SiO₂ content and mineralogical uniformity, providing potential pozzolanic benefits when combined with SCMs [9]. However, its use in reinforced concrete presents significant challenges. Due to its smoother and more rounded morphology compared to river sand, MQS may impair particle packing, reduce interlock between particles, and negatively influence workability [10]. Furthermore, marine aggregates often contain chloride ions (Cl⁻), which pose serious durability risks by promoting reinforcement corrosion through the depassivation of steel bars [11,12]. According to ACI 318-19, the maximum allowable chloride ion content in concrete for exposure category C2 is 0.15% by cement mass, equivalent to approximately 0.4  kg/m³ of concrete [13]. Exceeding this threshold accelerates corrosion initiation, jeopardizing structural integrity. Despite the development of several desalination techniques, such as multi-stage washing and electrochemical processes [14], the availability of practical and effective solutions for large-scale application remains constrained.

Despite the availability of MQS and advancements in SCM technologies, the combination of MQS with ternary or quaternary SCM systems in FGC has not been thoroughly investigated, especially regarding long-term durability, chloride ion permeability, and sulfate resistance. Previous studies have either focused on the mechanical performance of MQS concrete or explored SCM-blended systems independently [15,16]. There is a critical lack of integrated research that addresses the synergistic effects of MQS and SCMs on both mechanical and environmental performance, particularly when optimized using advanced statistical methods.

This study aims to systematically develop and optimize an FGCMQS incorporating FA, SF, and GGBS. A multi-objective optimization approach using RSM is employed to balance compressive strength, chloride ion penetration, and sulfate resistance. Additionally, scanning electron microscopy (SEM) is used to analyze microstructural evolution, while carbon footprint assessment provides insights into the environmental impacts compared to conventional concrete.

Unlike previous works, this research proposes an integrated strategy combining electrochemical desalination of MQS, SCM-based mix design, and holistic performance evaluation. This comprehensive framework offers a practical and sustainable solution for coastal regions experiencing river sand scarcity. It also aligns with global low-carbon concrete initiatives.

2 Materials and methods

2.1 Materials

2.1.1 Cement and supplementary cementitious materials

Ordinary Portland Cement (OPC), type PC50, produced by Ha Tien Cement Company, was used as the primary binder (Fig. 1), conforming to ASTM C1157 [17]. The SCM included FA class F, 940U-type SF from Eklem silicon materials, and GGBS supplied by Hoa Phat Group (Fig. 1). The detailed chemical composition, physical properties, and pozzolanic activity indices of all binders are summarized in Table 1.

2.1.2 Fine aggregates

MQS and crushed sand (CS) were used as the fine aggregates. The MQS, obtained from the South Central Coast of Vietnam, is characterized by a high silica (SiO₂) content. However, its fineness modulus of 1.4 does not meet the minimum threshold set by ASTM C33 [4]. CS, with a fineness modulus of 3.7, was blended with MQS to adjust particle gradation.

A three-step pretreatment process was implemented for the MQS to ensure its chloride ion content met the requirements of ASTM C33 [4]. Initially, mechanical sieving was conducted to eliminate shells, coral fragments, and other foreign materials, thereby improving the physical cleanliness of the aggregate. Subsequently, electrochemical desalination was performed by immersing the MQS in an electrolyte solution and applying a direct current to promote the migration of chloride ions. This process followed specific electrochemical reactions, as represented by Eqs. (1) and (2).

(1)\mathrm{A}\mathrm{n}\mathrm{o}\mathrm{d}\mathrm{e}:2\mathrm{C}{\mathrm{l}}^{-}{\to }_{}\mathrm{C}{\mathrm{l}}_2\left(\mathrm{g}\right)+2{\mathrm{e}}^{-}

(2)\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{d}\mathrm{e}:2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2\left(\mathrm{g}\right)+2\mathrm{O}{\mathrm{H}}^{-}

Finally, the MQS underwent hot water washing at 90–95 °C in a 1:1 mass ratio for 10 min, followed by thorough rinsing with clean water to remove residual salts. This integrated pretreatment method successfully reduced the chloride content of MQS to levels below the permissible threshold specified by ASTM C33, ensuring the suitability of the treated sand for use in all concrete mixtures of this study. Figure 1 shows the treated MQS sample. Additionally, the specific surface area and water absorption of MQS at particle sizes of 4.75–2.36 mm, 2.36–1.18 mm, 1.18–0.6 mm, 0.6–0.315 mm, 0.315–0.14 mm, and 0.14–0.075 mm are illustrated in Fig. 2.

To determine the optimal blending ratio, successive volumetric compaction tests were performed using varying MQS:CS ratios (90:10 to 10:90). The highest bulk density was achieved at a 30:70 ratio of MQS to CS, corresponding to a fineness modulus of 2.4 and a particle size distribution compliant with ASTM C33 (Fig. 3).

2.1.3 Chemical additives

The selected superplasticizer (SP) is Sika Viscocrete 8565, a polymer-based superplasticizer. This admixture has a high water-reducing capacity, retains slump for over 90 min, contains no chlorides or corrosive substances, and features a density of 1.04–1.07 g/cm³ with a pH between 3.8 and 5.7.

2.2 Calculation and optimization of component material mass

The mix design of FGCMQS was developed using the absolute volume method, with the sand void content used to determine the required paste volume. The CS-to-MQS ratio was optimized for maximum packing density, whereas the W/C ratio was determined in compliance with ACI 363.2R [18] to provide a balance between workability and strength. To optimize this multi-variable system, RSM with a Central Composite Design (CCD) was employed due to its ability to capture nonlinear interactions and reduce experimental runs [19]. CCD includes factorial, axial, and center points, enabling precise modeling of both linear and quadratic effects while estimating experimental error. This study considered five key design factors: FA (10%–40%) [20], SF (5%–10%) [21], GGBS (15%–50%) [22], W/C ratio (0.25–0.45) [23–25], and SP dosage (0.5%–2%) [25]. Three optimization models were constructed to evaluate the combined effects of SCMs and SP on mechanical and durability properties, as outlined in Tables 2 and 3. This approach provides an efficient pathway to develop a high-performance, low-carbon FGCMQS mixture.

A quadratic regression model was employed to define the relationship between design variables and the mechanical properties of FGCMQS, as expressed in Eq. (3):

(3)R_n=b_0+\sum _i^k{b}_i{X}_i+\sum _i^k{b}_{ii}{X}_i^2+\sum _{i\ne j}^k{b}_{ij}{X}_i{X}_j+\epsilon ,

where b₀ is the constant, b_i is the linear regression coefficient, b_ii is the quadratic coefficient, b_ij is the interaction coefficient between the variables X_i and X_j, ɛ is the model error.

Analysis of variance (ANOVA) was used to assess model validity, with key indicators including R², adjusted R², and p-values for model terms and the lack-of-fit test. An R² value approaching 1.0 reflects a strong correlation between experimental and predicted data, while p-values below 0.05 indicate statistically significant effects. Residual analyses confirmed that the model assumptions, including normality, homoscedasticity, and randomness, were satisfied, thereby ensuring the validity of the optimization process. Section 3 outlines the detailed results of the statistical analysis.

2.3 Experimental method

2.3.1 Workability

The slump test was performed in accordance with ASTM C143 [17] to evaluate the workability of the fresh concrete mix. Given that the aggregate diameter was less than 25 mm, an Abraham cone with a base diameter of 200 mm, a top diameter of 100 mm, and a height of 300 mm was utilized (Fig. 4). The slump value, measured in centimeters (cm), was determined as the difference between the height of the mold and the height of the highest point of the tested concrete specimen.

2.3.2 Compressive strength

Compressive strength tests were conducted by ASTM C39 [26] on cylindrical concrete specimens measuring 100 mm in diameter and 200 mm in height. The specimens were cured as per ASTM C192 [27] and tested at 28 d using a universal testing machine (UTM), ensuring full compliance with the specified technical standards. Furthermore, after completing the 28-day compressive strength tests, the fractured fragments of the specimens were collected for microstructural characterization using SEM.

2.3.3 Chlorine ion permeability

The resistance of the concrete to chloride ion penetration was evaluated at 28 d using the Rapid Chloride Permeability Test (RCPT) in accordance with ASTM C1202-19 [28]. Three cylindrical specimens (100 mm in diameter and 50 mm in thickness) were sectioned from the mid-height of standard 100 mm × 200 mm cylinders. The specimens were vacuum-saturated prior to testing. During the test, one side of each specimen was exposed to a 3.0% NaCl solution (negative terminal), and the other side to a 0.3 M NaOH solution (positive terminal). A constant 60 V DC voltage was applied across the specimen for 6 h. The total charge passed, measured in C (Coulombs), was recorded automatically.

2.3.4 Length variation in expansion test

The sulfate resistance of the concrete was assessed by measuring the length change of mortar bars exposed to a sulfate solution, following the procedures outlined in ASTM C1012/C1012M-18b [29]. For each mixture, three prism specimens with dimensions of 25 mm × 25 mm × 285 mm were cast. The specimens were cured in water until they reached a compressive strength of at least 20 MPa, at which point an initial length reading was taken. Subsequently, the specimens were fully immersed in a 5.0% sodium sulfate (Na₂SO₄) solution. The solution was refreshed every 20 d to maintain a constant sulfate concentration. Length change measurements were taken periodically, and the results reported in this study correspond to the expansion after 180 d of immersion. The length changes were assessed using Eq. (4).

(4)\mathrm{\Delta }L=\frac{L_{\mathrm{x}}-L_{\mathrm{i}}}{L_{\mathrm{g}}}\times 100,

where ∆L is the percentage variation in specimen length (%), L_x is the comparator measurement of the specimen, L_i is the initial comparator measurement of the specimen, L_g is the comparator measurement of the reference bar.

2.3.5 Carbon footprint analysis

The CO₂ emission assessment evaluates the carbon footprint of concrete mixtures incorporating MQS, FA, SF, and GGBS. The total CO₂ emissions are quantified and expressed in Eq. (5) [30].

(5)\mathrm{C}{\mathrm{O}}_2=\mathrm{\Sigma }\left(M_i\cdot E{F}_i\right),

where CO₂ is total CO₂ emission (kg CO₂/m³ concrete), M_i is mass of component i in concrete (kg/m³), EF_i is CO₂ emission factor of component i (kg CO₂/kg).

3 Results and discussion

3.1 Assess the impact of research variables

To determine the optimal proportions of mineral and chemical additives for producing fine-grained concrete incorporating marine quartz sand, a comprehensive screening study was conducted using three models (1, 2, and 3), as summarized in Table 3.

In model 1, the dosages of FA, SF, and GGBS were fixed at 10%, 5%, and 15%, respectively. In contrast, the W/C ratio and SP content were systematically varied to evaluate their influence on workability, mechanical performance, and durability (Table 4). The results demonstrated that increasing the W/C ratio from 0.25 to 0.45 significantly reduced the compressive strength from 65.2 to 50.8 MPa, primarily due to increased porosity and weaker internal bonding, in agreement with prior studies [31]. Simultaneously, chloride ion permeability rose from 1050 to 1300 C, indicating diminished durability at higher W/C ratios.

The addition of SP significantly enhanced workability, as evidenced by a slump increase from 1.5 to 7.0 cm when the SP content increased from 0.5% to 2.0%. However, higher SP dosages resulted in a reduction in compressive strength. At a constant W/C ratio, increasing the SP content from 0.5% to 2.0% led to a decrease of approximately 4 to 5 MPa in compressive strength, possibly due to interference with cement hydration and a loss of interparticle cohesion, as reported in previous studies [32,33].

Among the tested mixtures, the KZ6 specimen, designed with a W/C ratio of 0.35 and a SP dosage of 2.0%, exhibited the lowest chloride permeability at 950 C, while maintaining a compressive strength of 58.8 MPa. In contrast, the KZ1 mixture, incorporating a W/C ratio of 0.25 and 0.5% SP, achieved the highest compressive strength (65.2 MPa); however, its low slump value of 1.5 cm may limit its practical applicability. Taken together, the optimal FGCMQS design necessitates a balanced approach among workability, mechanical performance, and durability. A W/C ratio between 0.30 and 0.35 combined with approximately 1.0% SP appears most effective in achieving sufficient strength while minimizing chloride ion permeability (Fig. 5).

In model 2, the W/C ratio and SP dosage were maintained at constant levels, while the proportions of FA (10%–40%), SF (5%–10%), and GGBS (15%–50%) were systematically adjusted to identify the optimal combination for achieving a balance between strength and dimensional stability (Table 5). The results revealed that increasing SF from 5 to 10% enhanced compressive strength from 46.2 to 55.4 MPa, confirming its role in refining the microstructure and improving packing density [34]. Conversely, increasing FA to 40% reduced strength to 43.8 MPa due to insufficient Ca²⁺ availability, which delayed pozzolanic reactions and hindered hydration [35,36].

For GGBS, at 50% replacement, strength decreased to 41.9 MPa, but length change dropped to 0.035%, indicating improved volume stability through a void-filling mechanism [22,37]. Sample KZ20 (25% FA, 10% SF, 15% GGBS) exhibited superior performance, achieving a compressive strength of 57.4 MPa and a length change of 0.089%. The recommended ranges for FGCMQS mixtures include 10%–25% FA, 7.5%–10% SF, and 15%–32.5% GGBS, as illustrated in Fig. 6. This well-proportioned ternary blend produces a synergistic effect, significantly enhancing both the mechanical properties and long-term durability of the composite.

In model 3, the influence of SF, GGBS, and the W/C ratio on the compressive strength and length change of FGCMQS mixtures was investigated, with FA and SP contents maintained at fixed levels (Table 6).

Increasing SF from 5% to 10% enhanced compressive strength from 52.3 to 58.1 MPa and reduced length change from 0.072% to 0.048%, reflecting the densifying role of SF. In contrast, increasing GGBS from 15% to 50% decreased compressive strength (from 52.3 to 42.7 MPa) but significantly improved dimensional stability (length change reduced to 0.029%) due to slower hydration and pore-filling effects [38]. Lowering the W/C ratio from 0.4 to 0.3 improved compressive strength (from 48.5 to 62.4 MPa) and reduced shrinkage (from 0.084% to 0.038%) in samples KZ30 and KZ29. The optimal blend was identified as 10% SF, 32.5% GGBS, and a W/C ratio of 0.3, yielding a high compressive strength of 62.4 MPa and minimal length change of 0.038% (Fig. 7). These findings underscore the importance of optimizing the proportions of SF, GGBS, and W/C to achieve both mechanical strength and sulfate resistance. A combined assessment of FA, SF, and GGBS revealed that SF promotes the formation of C-S-H gel due to its high silica content. In contrast, FA contributes to long-term strength through secondary pozzolanic reactions. GGBS enhances durability by reducing porosity and permeability. This well-balanced ternary blend improves both mechanical performance and resilience under aggressive conditions.

3.2 Mix design optimization and laboratory testing

Based on the statistical analysis of the first three models, model 4 was designed to incorporate optimal material proportions, including FA (15%–30%), SF (7.5%–10%), GGBS (15%–32.5%), W/C (0.30–0.35), and SP (0.8%–1.2%). These proportions are selected to enhance compressive strength, reduce chloride ion permeability, and control length change under sulfate exposure. Such adjustments aim to ensure a well-balanced combination of mechanical performance and long-term durability in FGCMQS.

3.2.1 RSM model optimization and predictive results

As shown in Fig. 8, the optimized results for mineral–chemical admixture combinations using RSM require experimental validation to confirm predictive accuracy. Table 7 outlines the key parameters for evaluating the model’s performance in determining the optimal FGCMQS composition.

The comparison between the RSM-predicted values and experimental results confirmed the high accuracy of the model in determining the material composition parameters (FA, SF, GGBS, SP, and W/C), with deviations of less than 0.3%, indicating a reliable prediction of mix proportions. However, minor discrepancies were identified in some performance indicators. The experimental compressive strength reached 65.1 MPa, exceeding the predicted 60.6 MPa by 7.4%, while the chloride ion permeability was 812 C, slightly higher than the predicted 777.3 C (4.5% error). The length change also differed, with an experimental value of 0.04% compared to the expected 0.02%. These variations may stem from factors such as curing conditions, compaction quality, or environmental fluctuations. Despite these differences, the RSM model effectively captured the optimization trend, particularly in maximizing compressive strength and minimizing chloride ion permeability and dimensional instability. The findings confirm the model’s applicability to FGCMQS mix design, although further calibration may be necessary to enhance prediction accuracy under practical construction conditions.

To better understand the advantages and limitations of MQS, a comparison with river sand-based fine-grained concrete (FGCRS) is essential. Previous studies have shown that river sand, with its higher fineness modulus (typically 2.6–3.1), provides better workability and lower chloride ion permeability compared to MQS. A control FGCRS mixture was developed using the same optimized binder composition as FGCMQS to ensure consistency across all parameters, except for the fine aggregate type, which was intentionally varied to evaluate its influence on performance. This design facilitated a direct and meaningful comparison between the two mixtures. The key performance metrics of both mixtures are presented in Table 8.

As summarized in Table 8, the optimized FGCMQS exhibited a 10.5% increase in compressive strength compared to FGCRS (65.1 vs. 58.9 MPa). This improvement likely results from the high silica content and uniform grading of the MQS blend, which, as confirmed by SEM analysis, facilitates the formation of a denser C-S-H matrix. However, durability trade-offs were evident. FGCMQS exhibited a 5.6% higher chloride ion permeability (812 vs. 769 C) and a 25% greater sulfate-induced expansion (0.04% vs. 0.032%). These results suggest that residual chlorides in MQS, while not adversely affecting compressive strength, may accelerate ionic ingress and promote expansive reactions such as ettringite formation. Therefore, further improvement in MQS pretreatment or the incorporation of corrosion inhibitors is essential to balance strength and durability, enabling the sustainable application of FGCMQS in aggressive environments.

3.2.2 Validation of the RSM models

ANOVA results for all response variables (compressive strength, chloride permeability, length variation, and slump) confirmed the statistical robustness of the developed RSM models, as summarized in Table 9.

The ANOVA results (Table 9) confirm the statistical robustness and adequacy of the developed polynomial models. All models yielded highly significant p-values (p < 0.0001) across the evaluated response variables, including compressive strength, chloride ion permeability, length variation, and slump. These results indicate strong correlations between the experimental responses and the input parameters, namely FA, SF, GGBS content, W/C ratio, and SP dosage.

Models for compressive strength, chloride ion permeability, and length variation exhibited excellent fit, with R² values exceeding 0.96 and adjusted R² values above 0.93. These results indicate that more than 93% of the response variability was captured, with no evidence of overfitting. Despite its lower R² value of 0.741, the statistically significant slump model still captured key patterns in workability, likely influenced by external conditions such as temperature or mixing. All models passed the Lack-of-Fit test (p > 0.05) and demonstrated strong signal-to-noise ratios (Adequate Precision ranging from 18.2 to 28.3), confirming their reliability for optimization. Diagnostic plots verified normality, homoscedasticity, and independence of residuals, with only minor deviations observed in the slump model, which were consistent with expected variations in fresh-state behavior.

These findings collectively suggest that the developed RSM models are both statistically sound and practically applicable for optimizing the mechanical performance and durability of FGCMQS mixtures.

3.3 Scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis

As shown in Fig. 9, the dominant hydration products observed in both the FGCRS and FGCMQS mixtures include calcium silicate hydrate (C-S-H) gel, calcium aluminosilicate hydrate (C-(A)-S-H), and Portlandite [Ca(OH)₂] crystals. In the FGCRS specimen incorporating natural river sand, the SEM image (Fig. 9(a)) reveals a relatively heterogeneous matrix with abundant pores and microcracks, indicating suboptimal packing and low compactness. At high magnification (20000 ×), large hexagonal or lamellar Portlandite crystals are evident, typically associated with poor mechanical integrity due to their high solubility and propensity to form weak zones. The C-S-H gel in this matrix appears loosely interconnected and porous, while the C-(A)-S-H phase, though present, is sparsely distributed, resulting from partial pozzolanic activity. The EDS area scan (Fig. 9(b)) indicates an average atomic Ca/Si ratio of 1.07, consistent with conventional Portland cement systems containing siliceous aggregates.

In contrast, the optimized FGCMQS mixture, incorporating mechanically washed MQS, displays a markedly denser and more uniform microstructure (Fig. 10(a)). The SEM images reveal reduced microcrack density and enhanced particle packing, with an almost complete absence of visible Portlandite crystals. The observed microstructural refinement is attributed to the pozzolanic reactivity of the blended SCMs (FA, SF, and GGBFS), which consume Ca(OH)₂ and promote the generation of secondary C-S-H and C-(A)-S-H phases. This transformation not only strengthens the matrix but also reduces the formation of weak ITZs. The dominant microstructural phase is a compact and interwoven C-S-H network enriched with C-(A)-S-H, as evidenced by the strong aluminum signal in the EDS spectrum (Fig. 10(b)). Notably, the average Ca/Si atomic ratio in FGCMQS is significantly lower, at approximately 0.81, indicating the predominance of a low-calcium C-S-H gel, a phase widely recognized for its improved intrinsic strength and chemical stability.

These microstructural observations align with the enhanced compressive strength of FGCMQS reported in Subsection 3.2. The improvement in mechanical performance can primarily be ascribed to: 1) the formation of a more compact low-Ca/Si C-S-H gel; 2) the space-filling effect of C-(A)-S-H; 3) the near-complete consumption of Portlandite; and (4) the refinement and densification of the ITZs between the binder and aggregate phases.

However, despite the denser matrix, FGCMQS exhibits higher chloride ion permeability and greater dimensional instability under sulfate exposure. A plausible explanation for these unexpected results lies in the microchemical interactions occurring during early hydration. Specifically, although MQS underwent a controlled washing process before mixing, residual chloride ions may have remained adhered to the particle surfaces. During early hydration, these ions can migrate into the pore solution, increasing the internal ionic strength and promoting the ingress of external {\mathrm{S}\mathrm{O}}_4^{2-} ions upon exposure. Such conditions suggest that chloride ions may partially replace OH⁻ in the pore solution, thereby disrupting the electrochemical balance and facilitating deeper penetration of sulfate ions into the cement matrix. Second, chloride ions may chemically interact with hydration products, particularly the low-Ca/Si C-S-H gel. As illustrated in Eq. (6), Cl⁻ can destabilize the silicate chains in C-S-H, diminishing long-term durability. Wherein, C-S-H* represents the C-S-H structure affected by Cl⁻ ions, leading to reduced chemical stability. Moreover, Cl⁻ is known to accelerate the dissolution of tricalcium aluminate (C₃A) and reactive alumina from GGBFS, promoting the formation of expansive ettringite phases through reactions described in Eqs. (7) and (8). This reaction sequence offers a mechanistic rationale for the pronounced sulfate-induced expansion observed in FGCMQS, even in the presence of a refined microstructure.

(6)\mathrm{C}-\mathrm{S}-\mathrm{H}+\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{C}-\mathrm{S}-{\mathrm{H}}^{\ast }+{\mathrm{C}\mathrm{a}}_2^{+}+\mathrm{O}{\mathrm{H}}^{-}

(7){\mathrm{C}}_3\mathrm{A}+{\mathrm{H}}_2\mathrm{O}+\mathrm{C}{\mathrm{l}}^{-}\to {\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H})}_4^{-}+{\mathrm{C}\mathrm{a}}_2^{+}+\mathrm{C}{\mathrm{l}}^{-}

(8)\begin{array}{rl}& {\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H})}_4^{-}+{3\mathrm{C}\mathrm{a}}_2^{+}+3{\mathrm{S}\mathrm{O}}_4^{2-}+32{\mathrm{H}}_2\mathrm{O}\\ \to & \mathrm{C}{\mathrm{a}}_6\mathrm{A}{\mathrm{l}}_2{\left(\mathrm{S}{\mathrm{O}}_4\right)}_3{\left(\mathrm{O}\mathrm{H}\right)}_{12}\cdot 26{\mathrm{H}}_2\mathrm{O}\end{array}

3.4 Assessment of CO₂ emissions

This study employed model 2 to investigate the relationship between compressive strength and carbon dioxide emissions, with a focus on the partial replacement of cement with FA, SF, and GGBS. In comparison, model 1 primarily investigates the influence of the W/C ratio and the use of superplasticizer on workability and strength development. The model 3 does not include FA, which plays a critical role in reducing carbon emissions. In contrast, the model 4 incorporates all variables simultaneously, resulting in excessive complexity that limits its suitability for analyzing emission reduction strategies.

As illustrated in Fig. 11, CO₂ emissions varied significantly across mixtures, ranging from 119.88 kg (KZ17) to 448.88 kg (KZ14), with cement content being the dominant factor. Mixtures rich in FA and GGBS (e.g., KZ17, KZ19) exhibited notably lower emissions, confirming their efficacy in reducing environmental impact. Conversely, SF contributed less to emission reduction due to its limited dosage. Samples with high FA content (e.g., KZ12, KZ13, KZ17) aligned with the regression trend, showing lower CO₂ emissions, owing to FA’s low emission factor (~0.02–0.05 kg CO₂/kg) [39]. Similarly, GGBS-rich mixtures (e.g., KZ15, KZ17, KZ21) also yielded reduced emissions (~0.07–0.15 kg CO₂/kg) [40], though excessive use may compromise strength, as seen in KZ17 (35.5 MPa). In contrast, SF primarily improved mechanical properties (e.g., KZ11, KZ13, KZ20) but had minimal impact on emissions (~0.01–0.05 kg CO₂/kg) [41].

Optimized combinations of FA, SF, and GGBS (e.g., KZ22, KZ23) achieved a balance between reduced CO₂ emissions and adequate strength, underscoring the importance of admixture optimization. This strategy supports sustainable concrete development and encourages the use of locally available materials, such as MQS, in regions with limited natural sand resources.

To contextualize the sustainability credentials of the optimized FGCMQS mixture, a comparative analysis with various low-carbon concrete systems reported in the literature is presented in Table 10. The FGCMQS mixture, incorporating supplementary cementitious materials (SCMs), achieves a notable 28-day compressive strength of 65.1 MPa. This performance is not only highly competitive but also exceeds that of several established high-volume SCM systems. For instance, the ternary blend developed in this study outperforms both high-volume fly ash (HVFA) concrete [42] and optimized binary GGBS systems [22].

Notably, the compressive strength achieved by the FGCMQS mixture is comparable to high-performance ternary systems incorporating conventional aggregates [44], indicating that the use of MQS does not compromise the mechanical performance of concrete formulated with multi-component binders. From a sustainability standpoint, the estimated CO₂ emission of 251.8 kg/m³ is significantly lower than that of conventional Portland cement concrete and is also competitive with emerging low-clinker technologies such as limestone calcined clay cement [43]. These comparative advantages affirm the potential of FGCMQS as a technically robust and environmentally preferable alternative for structural applications in the context of low-carbon construction.

This study developed a concrete mixture incorporating SCMs and MQS as a sustainable fine aggregate. The mixture outperforms many previously reported binary and ternary systems in terms of mechanical properties. The combined use of FA, GGBS, and SF promotes synergistic pozzolanic and hydraulic reactions, enhancing performance while substantially reducing carbon emissions.

4 Conclusions

This study investigated the use of MQS as a fine aggregate and the partial replacement of cement with FA, SF, and GGBS in FGC. The main conclusions are as follows.

1) The RSM was employed to optimize the mix design, achieving a balance among compressive strength, workability, and durability. The resulting mixtures demonstrated improved overall performance in FGC.

2) The incorporation of mineral admixtures significantly improved compressive strength and durability. SEM analysis confirmed the formation of C-S-H and C-(A)-S-H phases, along with a marked reduction in free calcium hydroxide, which contributed to a denser microstructure and enhanced resistance to deterioration.

3) FA and GGBS effectively reduced carbon dioxide emissions, while SF notably enhanced strength. The combined and optimized use of these materials enables the development of concrete that offers both technical efficiency and environmental benefits.

4) Future research should pursue multi-objective optimization approaches that incorporate durability, erosion resistance, and cost-effectiveness, in addition to strength and emissions. Particular emphasis should be placed on life cycle assessment and comprehensive environmental evaluation to support the development of sustainable fine-grained concrete technologies.

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