Nano-Silica-Enhanced Binder Synergy and Multi-Index Performance of Resource-Efficient Alkali-Activated Composites for Sustainable Infrastructure Applications

Abstract

This study presents a multi-index performance approach that moves beyond the conventional reliance on compressive strength, offering a more holistic evaluation of nano-silica-enhanced binders in resource-efficient alkali-activated composites. Based on the Strength Activity Index (SAI) framework described in ASTM C618, the method integrates fresh state flowability with mechanical strength indices to capture the overall binder synergy. High-calcium fly ash (HCFA) and low-calcium fly ash (LCFA) were used with fine aggregate replacement, the level of which was kept constant at 20% by mass, and nano-silica was incorporated at 0, 1, 2, and 3 wt% of the binder to prepare alkali-activated slag fly ash composites. The fresh-state performance was assessed using the Initial Flow Index (IFI) and Flow Retention Index (FRI), while the mechanical performance was evaluated using the compressive, tensile, and flexural indices (SAI, TSI, and FSI). These results indicate that with an increase in nano-silica content, flowability and workability retention reduce systematically, with LCFA-based mixtures always exhibiting higher fresh-state retention than HCFA systems. Optimal mechanical performance was achieved with an intermediate nano-silica concentration of about 2 wt%, with consequent maximum SAI performance of about 120% at 28 days with HCFA-based mixtures and 118% at 28 days with LCFA-based mixtures, as well as a uniform improvement in TSI and FSI. Correlation analyses between SAI and tensile and flexural indices revealed clear linearity (R² of about 0.91–0.95), which indicated that compressive strength is not a sufficient measure of total mechanical performance. The mineralogical and microstructural analyses assisted by X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the performance trends observed depend on the interactions of the calcium supply, amorphous aluminosilicate and the nucleation effects of nano-silica. Therefore, the proposed multi-index framework offers a robust and practical tool for quantifying binder synergy and optimizing nano-silica dosage, advancing the understanding and development of sustainable alkali-activated composites for infrastructure applications.

1. Introduction

The construction industry worldwide is experiencing a paradigm shift toward resource-efficient and low-impact material systems, with growing environmental limitations, resource depletion, and performance requirements of the contemporary infrastructure [1,2,3]. Traditional substances that use Portland cement are still considered to be among the most energy-consuming construction products, which also leads to a substantial contribution to global carbon emissions and the exhaustion of natural resources [2,4,5]. These challenges have increased research on alternative binder systems that would provide high mechanical performance with minimum environmental burdens [6,7].

Alkali-activated materials have been considered an exciting category of cementitious composites since they can use industrial by-products like slag and fly ash as the main precursors [8,9,10].

In this study, alkali-activated composites refer to mortar-scale systems designed to isolate and quantify binder synergy and nano-silica modification effects under controlled conditions. The use of mortar mixtures enables systematic evaluation of fresh-state behavior and mechanical performance without the additional variability introduced by coarse aggregates while providing a rational basis for subsequent extension to concrete-scale applications.

Alkali-activated composites have significantly lower carbon footprints, greater chemical performance, and superior durability in hostile conditions compared to normal Portland cement systems [2,11]. Alkali-activated materials are frequently associated with reduced environmental impact relative to Portland cement-based systems; however, such advantages are highly formulation-dependent. In particular, the use of sodium silicate activators can significantly influence the overall carbon footprint, and not all alkali-activated systems necessarily outperform conventional cementitious materials from a life-cycle perspective. As demonstrated by Ouellet-Plamondon and Habert [12], substantially lower carbon footprints are consistently achieved only for specific formulations, such as one-part alkali-activated systems without sodium silicate. Since the present study does not include a life-cycle assessment (LCA), sustainability-related discussions are framed in terms of material efficiency and performance optimization rather than quantified environmental impact. The fact that they are versatile in the sense of adapting to various sources of aluminosilicate further increases their applicability to the practice of constructing resources efficiently [7]. The behavior of alkali-activated systems is, however, quite sensitive to the binder chemistry, precursor activity, and microstructural growth, which requires special attention to the optimization of the mixture [13].

The role of fly ash in alkali-activated composites is important because of its aluminosilicate composition and the morphology of the particles in the form of a sphere [14,15]. The chemical and mineralogical properties of fly ash, most notably the content of calcium, affect the kinetics of the reaction, as well as mechanical performance, to a significant extent [16,17]. The early-age reactivity of a high-calcium fly ash is generally higher, and these fly ashes assist in the creation of products rich in calcium, contributing to higher rates of strength development, whereas fly ashes with low calcium levels can enhance the base of polymerization-related processes that help to enhance stability over time [18,19,20]. Although fly ash tends to be used as a precursor to a binding agent, its application in the form of a partial fine aggregate replacement has been of interest recently because it could enhance the properties of particle packing and minimize the use of natural sand resources [16,21].

Although these are the benefits, the balancing of the binder synergy in alkali-activated slag fly ash systems is still not easy to achieve [7,22]. Over-incorporation can cause dilution effects, low load-bearing capability, and deteriorated fresh-state properties, and, under incarceration, can restrict sustainability advantages [23,24]. Additionally, it has been established that alkali-activated systems are sensitive to the parameters of the mixture, like water demand, activator concentration, and size distribution, which have a direct influence on the workability and development of strength [7,25]. To overcome these issues, advanced modifiers that can be used to refine the microstructure and increase reaction efficiency to the detriment of fresh-state performance are necessary.

Nano-silica has also drawn much interest as a nano-scale cementitious and alkali-activated system nano-additive because of its tremendously high specific surface area and chemical reactivity [26,27,28]. As a physical filler and a chemically active nucleation agent, nano-silica can hasten reaction kinetics and enhance the development of extra binding phases, as well as refit pore structure [9,28]. With alkali-activated composites, nano-silica has also been cited to help in increasing the interfacial transition zone, better packing density of particles, and raising the degree of polymerization, which results in better mechanical performance [29,30]. Nevertheless, nano-silica is highly dosage-sensitive, and its over-incorporation can lead to particle agglomeration, an increase in water requirement, and low workability [26,31].

Recent multiscale investigations have further highlighted that the performance of nano-silica-modified alkali-activated systems is highly sensitive to dispersion quality. Proper dispersion governs nano-silica participation in nucleation processes, microstructural refinement, and stress transfer efficiency, whereas inadequate dispersion promotes agglomeration and localized heterogeneities that limit mechanical performance gains [24,29]. These findings emphasize the importance of controlled dispersion strategies when evaluating nano-scale modifiers in alkali-activated composites.

Although many research works have analyzed nano-silica-modified cementitious and alkali-activated systems, most tests have been largely based on compressive strength as the key outcome of performance [10]. This method will give little information on tensile and flexural behavior, which will be critical in determining cracking resistance, redistribution of stress, and the overall structural reliability [12,30]. Furthermore, the interplay of nano-silica and various types of fly ash, under regulated alkali-activated circumstances, has not been assessed in systematic appraisals of combined performance-based systems [9].

Recently, it has been highlighted that there is a necessity to have a multi-indexed performance measurement system that goes beyond compressive strength to reflect the intricate mechanical action of advanced cementitious composites [31,32]. Normalized performance indices consisting of tensile and flexural performances give a better measure of material efficiency, particularly under systems where microstructural refinement and interface bonding are of primary importance [12]. The use of fresh-state performance measures combined with mechanical indexes also increases the trustworthiness of mixture optimization, as it ensures constructability as well as mechanical efficiency [33].

From an infrastructure engineering perspective, these performance-based evaluation systems are becoming increasingly demanded to ensure that advanced cementitious composites not only meet the strength requirements but also meet the constructability, crack resistance, and long-term service performance requirements under a realistic loading environment [34,35,36,37].

In recent infrastructure usage, complex stress conditions, time-dependent loading and aggressive service conditions to which materials are commonly subjected make durability and mechanical reliability equally important. As a result, assessment systems that combine fresh-state behavior and multi-directional mechanical performance give a more realistic assessment of in-service behavior when compared to strength-based measures alone. With these approaches, careful selection of materials, and optimization of mixtures can be made in a way that addresses the needs of infrastructure systems where consistency in performance, resilience, and sustainability are critical design goals.

Despite extensive research on nano-silica-modified alkali-activated materials, existing studies predominantly assess performance using isolated mechanical metrics, most commonly compressive strength, without integrating fresh-state constructability and multi-directional mechanical response within a unified evaluation framework. Furthermore, the combined influence of nano-silica dosage and fly ash calcium content under controlled activator and mixture conditions remains insufficiently explored. The present study addresses these gaps by proposing a holistic multi-index performance framework that simultaneously evaluates fresh-state flow behavior and normalized compressive, tensile, and flexural performance, enabling a rational assessment of binder synergy and optimal nano-silica dosage for infrastructure-oriented applications.

In line with this, the current study aims to examine the synergistic effect of nano-silica to control the interaction between the binder and the development of the resource-efficient alkali-activated composite using a holistic multi-index performance platform, which is based on the Strength Activity Index (SAI) as a reference index according to ASTM C618 [38].

In the context of this study, binder synergy is defined as the simultaneous enhancement in compressive, tensile, and flexural performance indices while maintaining acceptable fresh-state flowability and workability retention. This synergy is evaluated through integrated performance indices, correlation analysis, and supporting mineralogical and microstructural evidence.

The composites were prepared with slag and fly ash as alkali-activated slag and with low-calcium and high-calcium fly ash, with a fixed degree of fine aggregate replacement, and nano-silica added at different dosages. Flow-based indices were used to measure fresh-state behavior, and normalized compressive-, tensile-, and flexural-based indices were used to measure mechanical performance. An integration of these indicators provides this study with the complete perspective of nano-silica-enhanced synergy and establishes a rational base for the creation of high-performance, resource-effective alkali-activated composite systems with clear perspectives in terms of sustainable infrastructure applications.

It is essential to note that the proposed framework is intended to be a performance-integration methodology, rather than a mechanistic approach or a predictive modeling approach. Unlike the classical study of compressive strength as the selection of a performance metric, the current study involves an integrative multi-index design that integrates fresh-state indices (IFI, FRI) and normalized hardened-state indices (SAI, TSI, and FSI). To make such a performance-based measure, mineralogical (XRD) and microstructural (SEM) characterizations are given to decode the phase evolution, amorphous phase, and microstructural refinements of the evident fresh-state and mechanical performance of the identified materials.

While durability is a critical requirement for infrastructure materials, the present study focuses on establishing a performance-based framework integrating fresh-state and mechanical indicators; direct durability testing was not included and is therefore beyond the scope of the current investigation.

Durability related indicators such as shrinkage, permeability, chemical resistance, and long-term deformation are therefore recognized as essential extensions of the proposed framework and are identified as priority directions for future investigations.

The experimental and characterization methodology facilitates the study of the alkali-activated slag fly ash composites more comprehensively and in a performance-based way, particularly systems where constructability and mechanical response are highly conditional on the nano-scale adjustment and interaction of binders.

The novelty of this work lies in the development of a unified normalized indicator system integrating fresh-state flow behavior with multi-directional mechanical performance, the controlled comparative assessment of high-calcium and low-calcium fly ash-based systems under identical mixture and activator conditions, and the identification of an optimal nano-silica dosage supported by consistent trends across flowability, compressive, tensile, and flexural performance.

2. Experimental Program and Methodology

As depicted in Figure 1, the sequence of actions followed in the experimental outline applied in this study includes the selection of raw materials and mix proportions prior to the testing procedure, which was conducted with the aim of studying nano-silica adjusted synergy in alkali-activated, resource-saving composites.

The framework also involves the mineralogical and microstructural characterization using X-ray diffraction (XRD) and scanning electron microscopy (SEM) to give a fundamental insight into the mechanics of the interaction between the binder system and nano-silica incorporation. This combined description allows for relating phase composition and microstructural refinement with reaction kinetics, matrix densification and fresh-state and mechanical performance of alkali-activated composites.

The workability and flowability retention properties were quantitatively measured by the use of the Initial Flow Index (IFI) and Flow Retention Index (FRI) in assessing fresh-state quantitative performance.

Mechanical performance was measured using a multi-index of the Strength Activity Index (SAI), which is defined in ASTM C618 [38] and complemented by tensile-based and flexural-based indices, the Tensile Strength Index (TSI) and Flexural Strength Index (FSI). The synergistic method enables the comprehensive evaluation of the influence of nano-silica inclusion on fresh and hardened properties that act as a performance-driven basis to the knowledge of the synergy of the role of binder and mixture optimization in the alkali-activated composite systems.

2.1. Materials and Constituent Properties

Slag cement (SC), high-calcium fly ash (HCFA), low-calcium fly ash (LCFA), nano-silica (NS), an alkaline activator (AL), and fine aggregate (S) were used as the main constituents of this study. Their combination in the simultaneous application permitted the performance of the systematic analysis of the synergetic effect of slag binders, alkali-activated fly ash, and nano-silica activeness on the formation of resource-efficient alkali-activated composite systems.

The slag cement (SC) is a by-product of municipal waste, and it contains high proportions of amorphous materials. It was selected as a green option since it can minimize the environmental impact that is usually associated with the production of Portland cement.

The sources of the high-calcium fly ash (HCFA) and low-calcium fly ash (LCFA) were coal power plants, which utilize a wide variety of fuels. The HCFA is rich in CaO, and LCFA is rich in amorphous silica and alumina.

The nano-silica (NS) is recycled waste glass as a nano-scale modifier to bring synergy in binders and micro-structure fineness in the alkali-activated system. The nano-silica is largely amorphous, as shown by its chemical composition and physical characteristics (

Table 1. Chemical compositions of slag cement (SC), high-calcium fly ash (HCFA), low-calcium fly ash (LCFA), and nano-silica (NS).

Chemical Compositions (%)	CaO	SiO2	Al2O3	MgO	Fe2O3	Na2O	TiO2	P2O5	LOI
Slag Cement—(SC)	43.1	32.5	13.5	2.9	2.7	1.8	1.3	0.8	1.4
High-Calcium Fly Ash—(HCFA)	18.8	48.8	19.8	1.5	3.8	1.2	3.9	0.5	1.7
Low-Calcium Fly Ash—(LCFA)	6.3	57.6	26.5	1.2	4.2	0.5	1.9	0.3	1.5
Nano-Silica—(NS)	0.6	98.2	0.4	0.1	0.2	0.3	<0.05	<0.05	0.2

and

Table 2. Physical properties of slag cement (SC), high-calcium fly ash (HCFA), low-calcium fly ash (LCFA), and nano-silica (NS).

Physical Properties	Specific Gravity (g/cm3)	Specific Surface Area, Blaine (cm2/g)	Specific Surface Area, BET (m2/g)	Average Particle Size D50 (μm)
Slag Cement—(SC)	2.80	3750	---	6.48
High-Calcium Fly Ash—(HCFA)	2.80	3780	---	16.25
Low-Calcium Fly Ash—(LCFA)	2.14	3630	---	18.35
Nano-Silica—(NS)	2.20	---	85	0.04

) and a very small particle size and very high specific surface area.

These properties make nano-silica a useful physical filler and chemically active moiety, inducing nucleation of reaction products and increasing the density of particle packing and the interfacial bonding of the alkali-activated matrix.

This paper used nano-silica in the percentage of total binder to provide a systematic assessment of the dosage-related impact of the nano-silica and fly ash types on the fresh-state behavior and hardened mechanical performance.

Although fly ash was incorporated as a partial replacement for fine aggregate, its partial reactivity under the applied alkaline activation conditions cannot be neglected. Accordingly, the binder system in this study should be considered as a composite of slag and reactive fly ash phases, rather than a purely slag-based binder, and mixture proportions are interpreted within this composite binder context.

Mixed care was taken to eliminate agglomeration of nano-silica and to achieve even distribution of the nano-silica so that its full involvement in the alkali-activation process could be achieved.

Due to the potential health risks associated with nanoparticle exposure, specific safety and handling procedures were adopted during nano-silica preparation and mixing. Nano-silica was handled in well-ventilated laboratory conditions, and appropriate personal protective equipment (PPE), including gloves and protective masks, was used throughout weighing and mixing operations. Measures were taken to minimize airborne particle release during handling. These precautions are consistent with established recommendations on nanoparticle toxicity and occupational exposure control [39,40].

The alkaline activator (AL) is sodium silicate (Na₂SiO₃) with a molar ratio of SiO₂/Na₂O = 1.0 (50% SiO₂ and 50% Na₂O), which was chosen as an activator of slag-fly ash binders. Activation dosage was kept unchanged in all mixtures (AL/SC = 20% by mass) in order to isolate the effect of the addition of nano-silica.

The fine aggregate (S) was used in the aggregate phase and consisted of crushed sandstone with specific gravity of 2.58 and fineness modulus of 2.83; fly ash (FA/S = 20% by weight) was incorporated in the aggregate phase to partially substitute the fine aggregate, enhancing the ability of the particle to pack efficiently and decreasing the use of natural sand.

X-ray fluorescence (XRF) analysis was used to establish the chemical composition of slag cement, types of fly ash, and nano-silica, and loss on ignition (LOI) was determined according to the standard procedures. The specific surface area and particle size distribution of slag cement and fly ash were calculated using the Blaine permeability technique and laser diffraction method, respectively. Conversely, the Brunauer–Emmett–Teller (BET) method was used to determine the specific surface area of nano-silica because it is nano-scale in nature.

Table 1 shows the chemical compositions of slag cement (SC), high-calcium fly ash (HCFA), low-calcium fly ash (LCFA), and nano-silica (NS), whereas Table 2 presents the physical properties of these materials in a summary.

These characterizations form the requisite background on understanding the effect of nano-silica on the interaction of binders, the fresh-state behavior and the mechanical performance of the alkali-activated composites.

2.2. Mixture Design and Proportioning

The experimental program was intentionally conducted at the mortar scale to allow for an accurate assessment of nano-silica dosage effects and binder interactions under controlled conditions. While coarse aggregates were not incorporated, this approach enables isolation of binder chemistry and nano-scale modification effects without additional variability. The resulting insights provide a fundamental understanding of material behavior that can be transferred to alkali-activated concrete systems through appropriate adjustment of aggregate grading and mixture proportions.

Table 3. Mix proportions of nano-silica-modified alkali-activated composites.

Mix ID	Fly Ash Type	NS (%)	FA/S (%)	W/SC (%)	AL/SC (%)	Mix Proportioning (kg/m3)
						Slag Cement (SC)	Water (W)	Alkali-Activator (AL)	Fly Ash (FA)	Sand (S)	Nano-Silica (NS)
HCFA-NS0—Ref.	HCFA	0	20	50	20	600	300	120	200	1000	0
HCFA-NS1		1				594					6
HCFA-NS2		2				588					12
HCFA-NS3		3				582					18
LCFA-NS0—Ref.	LCFA	0	20	50	20	600	300	120	200	1000	0
LCFA-NS1		1				594					6
LCFA-NS2		2				588					12
LCFA-NS3		3				582					18

summarizes the mix proportions used in this work. Each of the alkali-activated composite mixtures was prepared to examine the effects of the incorporation of nano-silica and fly ash types on binder synergy and performance at constant baseline mixture parameters. The primary binder in all mixtures was slag cement (SC) and fly ash was added as a partial replacement of fine aggregates in all mixtures at a 20% fly ash–sand (FA/S) proportion by weight. High-calcium fly ash (HCFA) and low-calcium fly ash (LCFA) were used to investigate the relationship between fly ash types and nano-silica dosage.

Despite its use as a fine aggregate replacement, fly ash exhibits partial reactivity under alkaline activation, and thus contributes to the effective binder phase alongside slag cement.

To determine the impact of nano-silica and fly ash types, the water-to-slag ratio (W/SC), alkali activator-to-slag ratio (AL/SC) were kept constant throughout the mixtures at 50% and 20%, respectively. The experimental program was conducted using a single alkaline activator formulation and a fixed activator dosage to isolate the effects of nano-silica addition and fly ash type under controlled conditions. While this approach enables direct comparison among mixtures, the resulting performance trends are inherently specific to the selected activator chemistry and curing regime. Nano-silica was added in 0, 1, 2, and 3 wt% to the total binder content. The selected nano-silica dosage range (0–3 wt% of binder) was chosen based on prior studies reporting effective dispersion and performance enhancement at low-to-moderate nano-silica contents while also highlighting agglomeration tendencies and diminishing mechanical returns at higher dosages. This range enables identification of an optimal nano-silica content that balances microstructural refinement and fresh-state stability without introducing excessive surface area demand or dispersion inefficiencies. The reference mixtures containing no nano-silica (NS = 0%) were used as control mixes to normalize the performance and make a comparative evaluation.

Fine aggregate was a mixture of crushed sandstone, which was controlled in terms of grading and was partially substituted by fly ash to increase the efficiency of particle packing and minimize the use of natural sand. To compare reliably and consistently different dosages of nano-silica and fly ash types, the mixtures were prepared under the same conditions in terms of mixing, casting, and curing conditions.

Nano-silica was incorporated following a controlled mixing sequence to promote uniform dispersion and minimize particle agglomeration. The nano-silica was gradually introduced during mixing under continuous shear to ensure homogeneous distribution within the binder matrix. Although no chemical dispersants were employed, careful control of mixing time and sequence was adopted to reduce particle clustering. It is acknowledged that dispersion efficiency remains a critical factor influencing nano-silica performance and may affect the generalization of the identified optimal dosage.

2.3. Experimental Procedures and Testing

The mineralogical properties, fresh-state behavior and mechanical performance of nano-silica-modified alkali-activated composites were compared through an organized experimental program that was carried out under controlled and consistent testing conditions. The experimental processes were planned to allow for the comparison of the various nano-silica dosages and fly ash types to be made in a reliable way so that they are relevant to the infrastructure-based performance assessment.

2.3.1. Mineralogical Characteristics of Constituent Materials

Mineralogical characterization of the raw materials was made to provide a basic platform on which the reactivity and interaction mechanisms of the raw materials in the nano-silica-modified alkali-activated composite system could be understood. The analysis by X-ray diffraction (XRD) was used to determine crystalline and amorphous phases in slag cement (SC), high-calcium fly ash (HCFA), low-calcium fly ash (LCFA), and nano-silica (NS). The resulting patterns of diffraction were qualitatively evaluated to determine the presence of crystalline phases containing calcium and amorphous aluminosilicate components, which are important in regulating reactivity and reaction between alkali activation and other components.

2.3.2. Fresh-State Flow Assessment

Flow-based indices were used to quantify the fresh-state performance of nano-silica-modified alkali-activated composites to measure flowability and the retention of workability. Immediately after mixing, the Initial Flow Index (IFI) was obtained in a standard flow-table test following the guidelines of JIS A1150-JSA 2014 [41], wherein the spread diameter of each mixture was normalized against the respective reference mixture devoid of nano-silica (NS = 0%). Flow Retention Index (FRI) was established after casting for 15 min to evaluate the potential of the mixtures to retain their flowability. The measures of both indices were based on standardized procedures that made it possible to compare mixtures with varying nano-silica dosages and types of fly ash.

2.3.3. Mechanical Performance Evaluation

The nano-silica-modified alkali-activated composites were tested in mechanical performance with respect to compressive, splitting tensile, and flexural strength. A multi-index methodology was used to assess mechanical performance. The compressive strength was measured in terms of Strength Activity Index (SAI), which, according to ASTM C618 [38], is determined as the ratio of compressive strength of the nano-silica-modified mixtures to the compressive strength of the corresponding reference mixtures without nano-silica (NS = 0%). Although SAI was originally defined for Portland cement-based systems, it is adopted here strictly as a normalized comparative indicator rather than a compliance metric, enabling consistent evaluation of relative compressive strength development under identical binder chemistry and mixture parameters. The tensile and flexural performance was determined by testing in terms of Tensile Strength Index (TSI) and Flexural Strength Index (FSI), respectively. Both indices were determined by dividing a measured splitting tensile and flexural strength of each mixture by the relevant reference mixtures, using the same normalization strategy as SAI. Compressive, splitting tensile and flexural strength tests were performed based on JIS A1108-JSA 2006a [42], JIS A1113-JSA 2006c [43], and JIS A1106-JSA 2006b [44], respectively. All of the tests were conducted at curing ages of 1, 3, 7 and 28 days of specimens exposed to a controlled steam-curing regime.

All specimens were subjected to a controlled steam-curing regime to ensure consistent alkali activation and early-age strength development. Steam curing was applied at a constant temperature under saturated humidity conditions for a fixed duration, after which specimens were demolded and stored under controlled laboratory conditions until the designated testing ages. This curing protocol was applied uniformly to all mixtures to isolate the effects of nano-silica dosage and fly ash type on fresh-state behavior and mechanical performance. Cylindrical specimens (50 × 100 mm) were used to measure compressive as well as splitting tensile strengths, and prismatic specimens (40 × 40 × 160 mm) were used to measure flexural strength. Each mixture was tested on three specimens at each curing age.

2.3.4. Microstructural Characterization (SEM)

To support the interpretation of the trends of fresh-state and mechanical performance, scanning electron microscopy (SEM) was used to scan the microstructural characteristics of the chosen alkali-activated composites. At 28 days of curing, representative specimens were made of reference mixtures and mixtures containing nano-silica. The morphology of the matrices, the distribution of the particles, the structure of the pore, and the relationship between the product of the reactions and the particles that had not reacted were observed using SEM.

3. Results

3.1. Flow-Based Performance Indicators (IFI–FRI)

Quantitative evaluation of fresh-state behavior of nano-silica-modified alkali-activated composite was done by Initial Flow Index (IFI) and the Flow Retention Index (FRI), as shown in Figure 2 and Figure 3.

These indices have been used to measure the initial flowability immediately after mixing and the capacity of the mixtures to keep the workability with time as a function of the dosage of nano-silica and the types of fly ash.

In both HCFA-based and LCFA-based systems, there was a steady decrease in fresh-state performance as nano-silica content was increased. The reference mixtures without nano-silica (NS0) had defined values of IFI and FRI of 100% and were used as normalized baselines to be used in comparative evaluation instead of reflecting the absolute performance of the flow. Initial flow and 15 min post-cast reference mixtures were normalized based on corresponding average flow diameters (220 mm and 190 mm) for HCFA, while diameters of 240 mm and 210 mm were used for LCFA as the basis of flowability normalization across all mixtures.

With a dosage of nano-silica of 1 wt% (NS1), the values of IFI showed a moderate reduction of about 98% with HCFA mixtures and 96% with LCFA mixtures (Figure 2). This reduction at the beginning is indicative of the effect of the specific surface area and fine particle incorporation that is related to the addition of nano-silica. With further addition of nano-silica, a further reduction in initial flowability was observed. At a 2 wt% nano-silica (NS2) concentration, IFI reduced to almost 91% in the case of HCFA and about 88% in the case of LCFA, which indicates that there is increased internal friction and low particle movement within the fresh matrix. In the highest doses of nano-silica 3 wt% (NS3), IFI was found to have minimum values of about 82% and 79% of HCFA and LCFA, respectively, which indicate a severe loss of original flowability.

The same, but even more, noticeable trend was reflected in flow retention behavior (Figure 3). With nano-silica at 1 wt% concentration, FRI values were relatively high, with HCFA and LCFA mixtures maintaining about 95% and 93% of the initial flowability, respectively. However, when nano-silica was added at 2 wt%, FRI decreased further to about 87% in HCFA and 83% in LCFA, indicating the increasing susceptibility of the blends to the effects of time in stiffening. At the maximum level of nano-silica concentration (3 wt%), HCFA mixtures lost the ability to retain workability noticeably, with FRI values decreasing to almost 76%, whereas LCFA mixtures retained a slightly better stability, with FRI values being about 74%.

The IFI-FRI indices show that an increase in dosage of nano-silica has a negative result on the fresh-state flowability and workability retention of the mixtures containing the two types of fly ash.

The reduction in IFI and FRI with increasing nano-silica dosage may be partially attributed to the fixed water-to-slag and activator-to-slag ratios adopted in this study. As nano-silica increases surface area and accelerates reaction kinetics, additional water or activator demand may arise, contributing to fresh-state performance loss beyond the intrinsic nano-silica effect.

However, mixtures using LCFA were always slightly more stable at high contents of nano-silica than their HCFA counterparts.

This observation indicates that the reduced calcium level and delayed early-age reactivity of LCFA system schemes partially counterbalances the rapid hardening caused by nano-silica add-in, but mixtures of HCFA, with a higher calcium content, are more inclined to increased rate-build and lose its operating ability more rapidly at higher levels of nano-silica incorporation.

3.2. Strength-Based Performance Indicators

A multi-index based on strength was used to assess the mechanical performance of the nano-silica-modified alkali-activated composites based on compressive, tensile, and flexural outputs.

The methodology allows for the study of mechanical behavior in a comprehensive way to characterize the load-bearing capacity and mechanisms of resistance.

Compressive strength response was mainly measured by the Strength Activity Index (SAI), which used tensile- and flexural-based indices to give more details on the effect of nano-silica on the overall mechanical performance. These findings are addressed in the subsequent subsections against nano-silica dosage and the types of fly ash.

3.2.1. Compressive Strength Activity Index (SAI)

The Strength Activity Index (SAI) was used to assess the development of compressive strength of the nano-silica-modified alkali-activated composites with nano-silica, as demonstrated in Figure 4 and Figure 5, respectively, of the systems based on the HCFA and LCFA.

The compressive strength results were normalized with respect to the corresponding reference mixtures without nano-silica (NS0), which were defined as 100% for each fly ash system and curing age.

The absolute compressive strength of the reference mixtures at 28 days was 47.2 MPa for the HCFA mixture and 46.4 MPa for the LCFA mixture, providing the basis for comparative normalization across all nano-silica dosages.

In HCFA-based mixtures (Figure 4), the addition of nano-silica led to a progressive increase in compressive strength activity at all curing ages.

SAI values at 1 day increased above the level at the reference level to about 104% in NS1 and about 110% in NS2, which is a sign of faster early-age development of strength. There was a slight decrease at the highest nano-silica dosage (NS3), which indicates that diminishing returns were beginning at high levels of nano-silica content.

The same tendencies were observed at 3 and 7 days, with NS2 having the largest SAI values at both points, 112% and 116%, respectively.

At 28 days, HCFA mixtures with NS2 gave peak SAI values near 120% and NS3 mixtures continued to perform well in comparison to the reference at a lower level.

A similar improvement trend was noted on LCFA-based mixtures (Figure 5), but the magnitudes of SAI were generally lower than those of HCFA systems at the same dose of nano-silica. This is because the calcium content in LCFA is low, and it moderates the reaction kinetics of early age and restricts the degree of strength gains compared to the calcium-enriched mixtures of HCFA.

At early curing ages, NS2 showed the greatest improvement and SAI values of about 108 and 110 percent at 1 and 3 days, respectively.

LCFA-based mixtures with NS2 achieved an SAI of almost 115% and 118 at 7 and 28 days, respectively, which confirms the positive effect of nano-silica in enhancing strength increases over time.

As in the case of HCFA systems, addition of more nano-silica to 3 wt% led to slightly lower SAI values in comparison to NS2, still higher than the reference mixtures.

Comprehensively, the results of the SAI show that the incorporation of nano-silica improves compressive strength activity in both HCFA-based and LCFA-based alkali-activated composites with an optimum dosage of nano-silica of about 2 wt% (NS2).

The steady performance peak of NS2 during curing ages and fly ash types indicates a compromising nucleation performance and dispersion efficiency of particles.

Excessive nano-silica concentration appears to limit additional strength enhancement, and this might be due to the surface area demand and reduced workability that may prevent effective transfer of stress in the hardened matrix.

3.2.2. Tensile Strength Performance Index (TSI)

The tensile performance of alkali-activated composites that were modified with nano-silica was measured based on the Tensile Strength Index (TSI), as shown in Figure 6 and Figure 7, respectively, of the HCFA- and LCFA-based systems.

The tensile strength results were normalized with respect to the corresponding reference mixtures without nano-silica (NS0), which were defined as 100% for each fly ash system and curing age. The absolute splitting tensile strength of the reference mixtures at 28 days was 3.38 MPa for the HCFA mixture and 3.35 MPa for the LCFA mixture, providing the basis for comparative normalization across all nano-silica dosages.

With mixtures based on HCFA (Figure 6), tensile strength activity was evidently increased with the addition of nano-silica at all ages of curing. It was found that the values of TSI at 1 day were higher than the reference value (around 102 percent in NS1 and around 108 percent in NS2), indicating faster tensile strength at an early age. At NS3, there was a small but significant decrease of approximately 104%, which indicates the development of diminishing returns to increasing nano-silica contents. The same pattern was observed at 3 days, with NS2 giving the highest TSI values of about 108%, with 103% and 105% by NS1 and NS3, respectively. The improvement was more pronounced at later ages at 7 days; TSI values at 7 days in NS2 were approximately 109% and at 28 days, and TSI values in NS2 were approximately 110%.

Although NS3 mixtures retained better tensile properties compared to the reference, they had lower values of TSI compared to those of NS2 at all ages. Such a pattern indicates that excessive nano-silica levels might be restrictive to the tensile strength improvement in comparison to the optimum intermediate level.

The same trend of gradual improvement was found in LCFA-based mixtures (Figure 7). The value of TSI at 1 day was slightly higher, approximately 102% in NS1 and 106% in NS2, which is an indication of slower reaction kinetics using low-calcium systems.

At 3 days, NS2 once again showed the most notable improvement, with a TSI value of around 106%, with slightly lower values in NS3. LCFA-based mixtures of NS2 obtained at 7 and 28 days gave TSI values of about 107% and 108%, which corroborates the long-term time-dependent contribution of nano-silica to tensile strength development, whereas NS3 maintained lower TSI values than NS2.

The TSI findings provide evidence that incorporation of nano-silica produces noticeable tensile performance improvement to both the HCFA- and LCFA-based systems, with an optimum nano-silica dosage of about 2 wt% (NS2). The high tensile performance peak over the constant NS2 indicates the positive effect of nano-silica in fine-tuning the matrix microstructure, interfacial bonding, and the efficiency of crack-bridging. Higher dosage seems to be accompanied by higher surface area and particle aggregation requirements, which limit additional tensile performance improvement, as with the compressive development trends.

3.2.3. Flexural Strength Performance Index (FSI)

The flexural performance of the nano-silica-modified alkali-activated composites was evaluated by the Flexural Strength Index (FSI), which is shown in Figure 8 and Figure 9, respectively, in HCFA-based and LCFA-based systems.

The flexural strength results were normalized with respect to the corresponding reference mixtures without nano-silica (NS0), which were defined as 100% for each fly ash system and curing age. The absolute flexural strength of the reference mixtures at 28 days was 3.82 MPa for the HCFA mixture and 3.73 MPa for the LCFA mixture, providing the basis for comparative normalization across all nano-silica dosages.

In the case of HCFA-based mixtures, as shown in Figure 8, the flexural strength of the mixtures was systematically increased with the addition of nano-silica at all curing ages. At 1 day, the FSI values were greater than the reference value and were about 102% under NS1 and about 105% under NS2, and at the same time, the values of NS3 were slightly lower, at about 103%.

The improvement was more noticeable at 3 days, as NS2 had an FSI value of about 107% versus 104% in NS1 and 105% in NS3. A further improvement in NS2 was also demonstrated at 7 days, with an FSI value of approximately 110%, and at 7 days, NS3 was also lower, with an FSI of about 106 percent. At 28 days, HCFA mixtures with NS2 recorded the highest FSI values of around 112%, which confirms the ongoing role of nano-silica in the development of flexural resistance. With LCFA-based mixtures (Figure 9), the strength enhancement trend in flexural strength was moderate but still followed the same trend. On day 1, the FSI values increased by a few percentage points (102% NS1, 104% NS2) because the low-calcium systems accreted their structure more slowly. At 3 days, the highest FSI values were seen in NS2, at a range of about 106%, although the improvement in NS3 was slightly less. The FSI was noticeably increased in 7 and 28 days by LCFA mixtures with NS2 and yielded a more stable but less pronounced improvement as compared to HCFA systems, 108% and 110%, respectively.

The FSI measurements indicate that the flexural performance both HCFA- and LCFA-based alkali-activated composites is improved by incorporating nano-silica, and the optimal performance is always recorded at around 2 wt% nano-silica.

This enhanced flexural response at a range of intermediate nano-silica dosages can be explained by the increased continuity of the matrix, finer pore structure, and increased stress transfer across microcracks. High nano-silica concentrations, though, seem to have a restrictive effect on additional flexural gains because of dispersion inefficiencies and surface area demand, which were observed to follow similar trends in compressive and tensile performance development.

At a nano-silica dosage of 3 wt%, further improvement in mechanical performance becomes limited despite increased binder reactivity. This behavior can be attributed to the excessive surface area demand associated with high nano-silica contents, which promotes particle agglomeration and reduces effective dispersion within the matrix. As a result, localized heterogeneities and inefficient stress-transfer zones may develop, limiting additional gains in compressive, tensile, and flexural performance.

3.2.4. Integrated Multi-Index Performance Assessment

Normalized performance indices were employed in this study to isolate the influence of nano-silica dosage and binder composition while maintaining consistent baseline mixture parameters. Unlike simple normalization of individual strength values, the proposed multi-index framework integrates fresh-state flow indices with normalized compressive, tensile, and flexural performance indicators within a unified assessment approach. This enables a comparative evaluation of binder synergy, stress transfer efficiency, and material performance beyond compressive strength alone. The use of reference mixtures without nano-silica (NS = 0%) provides a consistent baseline for normalization, allowing the framework to emphasize relative performance evolution induced by nano-scale modification rather than absolute strength levels.

The multi-index evaluation is an integrated assessment of the mechanical performance of nano-silica-modified alkali-activated composites through correlation of compressive, tensile, and flexural strength responses through a single performance framework. Instead of focusing only on compressive strength, the joint analysis of the Strength Activity Index (SAI), Tensile Strength Index (TSI), and Flexural Strength Index (FSI) was applied to make a more reliable evaluation of the binder synergy, stress transfer efficiency, and redistribution of loads induced by the introduction of nano-silica.

The compressive, tensile, and flexural performance trends are generally in line with the prior research carried out on the alkali-activated slag-fly ash systems, where moderate levels of supplementary aluminosilicate materials are reportedly found to increase the homogeneity and mechanical efficiency of the matrix, with higher rates found to substitute the reactive phases negatively on the strength development process [7,16,17,19]. Consistent with the previous results on fly ash reactivity and calcium effect [4,10,22], the current experiment findings reveal that the two HCFA-based and the LCFA-based systems exhibit an optimum mechanical capability at the middle nano-silica concentrations.

From a mechanical performance perspective, the current findings reveal that compressive strength is not sufficient to reflect the important changes in tensile and flexural behavior, especially in nano-silica-modified alkali-activated materials. This finding supports previous results, which show that, to estimate cracking resistance, redistribution of stress, and efficiency of load transfer in advanced alkali-activated composites, overall mechanical assessment is necessary [2,30,32].

In this regard, the suggested multi-index performance framework adds further analytical value in this aspect by allowing for a more consistent and performance-based explanation of binder synergy and nano-scale effects of modification.

Therefore, to support this performance-based interpretation, the relationships between compressive strength development and the associated tensile and flexural performance were quantitatively determined using correlation analysis. Figure 10 and Figure 11 show the SAI-TSI of the HCFA-based and LCFA-based systems, respectively. In the case of HCFA mixtures, a good linear relationship was achieved, with a coefficient of determination of R² = 0.91 (Figure 10).

This means that the increases in compressive strength are successfully converted into tensile resistance but with certain dispersion explained by the calcium-rich reaction environment and localized cracking behavior Conversely, LCFA mixtures had a somewhat more consistent correlation, with a larger value of R² = 0.94 (Figure 11), indicating a more homogeneous stress transfer under the control of aluminosilicate-dominated reaction products. The same pattern was observed in flexural performance.

Figure 12 and Figure 13 demonstrate that mixtures of HCFA provided a good relationship between SAI and FSI with R² = 0.93 (Figure 12), which verified a high level of coupling between compressive strength improvement and flexural resistance in calcium-rich mixtures.

An even greater correlation was observed with LCFA mixtures, with a notably high R² = 0.95 (Figure 13), indicating that compressive strength is translated into flexural performance coherently because of geopolymeric gel formation, which was gradual and homogeneous.

In both fly ash systems, the multi-index analysis has always determined mixtures with intermediate nano-silica contents (NS2 level) to exhibit the most balanced mechanical behavior, with corresponding improvements in SAI, TSI and FSI.

This action proves that nano-silica is not just effective in increasing compressive strength but also crack-bridging capacity and flexural load resistance, which contribute to the structural efficiency of the composite.

These observed performance trends are consistent with recent multiscale studies reporting that optimized nano-silica dispersion enhances mechanical efficiency through improved stress transfer and microstructural continuity, whereas insufficient dispersion promotes particle agglomeration and limits further performance gains.

This agreement reinforces the role of dispersion quality as a governing factor in achieving effective nano-silica-induced synergy.

The SAI-TSI-FSI correlation analysis represents an empirical observation of trends in performance, but not a predictive relationship, which means that compressive strength is not a reliable predictor of tensile and flexural performance, especially in alkali-activated systems whose reactivity is governed by differing calcium and alumino-silicate. The specific correlation tendencies of HCFA and LCFA mixtures indicate the determining role of the binder chemistry and nano-silica-induced refinement of the microstructure in regulating mechanical synergy. The partial participation of fly ash in the alkali-activation reaction contributes to the observed mechanical response and microstructural evolution and should therefore be considered when interpreting the influence of nano-silica dosage and binder synergy within the proposed multi-index framework. The results confirm the proposed multi-index framework as an effective performance-based methodology for evaluating nano-silica-modified alkali-activated composites in the context of infrastructure applications. Within this framework, synergistic behavior is identified when concurrent improvements in SAI, TSI, and FSI are achieved without disproportionate deterioration in IFI and FRI, indicating efficient stress transfer, refined microstructure, and balanced reaction kinetics.

It should also be emphasized that the environmental implications of alkali-activated composites are strongly dependent on precursor selection, activator chemistry, and processing routes. Previous life-cycle assessment studies have demonstrated that only specific formulations, particularly one-part systems without sodium silicate, consistently achieve substantially lower carbon footprints than Portland cement-based materials. Accordingly, the present work does not claim universal environmental superiority but focuses on performance-driven material optimization within the investigated formulation scope.

Although the proposed multi-index framework provides an integrated assessment of fresh-state constructability and multi-directional mechanical performance, it remains sensitive to mixture design parameters, activator chemistry, and curing conditions. The proposed multi-index framework provides an integrated assessment of fresh-state constructability and multi-directional mechanical performance; it remains sensitive to mixture design parameters, activator chemistry, and curing conditions. In addition, the use of constant water-to-slag (W/SC) and activator-to-slag (AL/SC) ratios represents a design constraint that may influence fresh-state behavior, particularly at higher nano-silica dosages, and should be considered when interpreting the observed flowability and workability trends.

Moreover, the present evaluation is based on laboratory-scale mortar specimens and a single alkaline activator formulation with fixed dosage; therefore, the generalizability of the reported optimal nano-silica dosage and performance trends to other activator systems, molar ratios, or curing regimes requires further experimental validation.

These aspects should be considered when interpreting the reported results and when extending the framework to broader material systems and field applications.

The present evaluation is based on laboratory-scale mortar specimens and a single alkaline activator formulation with fixed dosage; consequently, the applicability of the identified optimal nano-silica content and associated performance behavior to alternative activator chemistries, molar compositions, or curing conditions requires further experimental verification and should be considered when interpreting the present findings and extending the framework to broader material systems and practical applications.

3.2.5. Performance Implications for Infrastructure Applications

The findings obtained at the mortar scale are directly relevant to concrete applications, as the governing mechanisms related to binder chemistry, nano-scale modification, and microstructural refinement remain valid in concrete systems. Nevertheless, the extension of the proposed framework to full-scale alkali-activated concrete incorporating coarse aggregates is identified as a necessary step for field implementation and practical validation.

As observed, the combined multi-index evaluation focuses on the functionality of nano-silica-modified alkali-activated composites to infrastructure-based composites, where fresh-state constructability and multi-directional mechanical strength are required. It has been established through a combination of the flow-based indices (IFI, and FRI) and strength-based indices (SAI, TSI, and FSI) that enhancement in compressive strength alone cannot result in optimal material performance, but that controlled synergy between binder chemistry, nano-scale modification, and reaction kinetics is required, as has been shown in recent performance-based infrastructure material studies [34,35].

This interaction between fresh-state behavior, mechanical indices, and infrastructure performance requirements is schematically shown in Figure 14.

In the case of an intermediate nano-silica dosage of approximately 2 wt% (NS2), both HCFA-based and LCFA-based systems had a favorable balance of maintaining workability and forming compressive strength, tensile resistance, and flexural capacity.

Balanced performance applies particularly to those infrastructure elements under more complicated loading circumstances, such as cracking, bending, and stress redistribution, such as pavements, precast structural members, and repair overlays. The tensile and flexural performance achieved at this dosage is high, suggesting that there is high efficacy of crack-bridging and stress transmission, most critical to infrastructure design focused on durability [35,36].

In addition to this, the difference between the HCFA and LCFA systems provides feasible tips on how mixtures should be customized according to the usage requirements. HCFA-based composites showed higher early-age strength development as compared to LCFA-based systems, albeit with poorer fresh-state stability, and a slower strength development, which is more constructable and long-term stable.

These contrary steps may be associated with the recent findings, which claim that the infrastructure materials should be selected based on the performance criteria and not on the optimization of a single parameter [34,37].

Thus, the findings indicate that the proposed multi-index system offers a rational and performance-driven basis of the process of translating the laboratory-scale maximization of the materials to the actual infrastructure implementation.

The framework enables the establishment of resource-efficient alkali-activated composites that can fulfil the mechanical, constructability, and sustainability demands of modern infrastructure systems, based on fresh-state behavior, mechanical synergy, and nano-scale modification mechanisms [35,36,37].

From a practical implementation perspective, the application of nano-silica-modified alkali-activated composites at the infrastructure scale requires careful consideration of large-scale mixing procedures, nano-silica dispersion efficiency, curing strategies, and associated cost implications. While the laboratory-scale results demonstrate clear performance benefits and material efficiency, field implementation will require validation under realistic production and exposure conditions, as well as assessment of economic feasibility and long-term durability performance.

Although durability testing was not experimentally investigated in the current study, the proposed framework provides a rational basis for selecting material systems that balance constructability and mechanical performance, which are prerequisites for long-term durability.

The integration of durability indicators within the same multi-index framework represents a logical continuation of the present work and will be necessary to fully validate the applicability of nano-silica-modified alkali-activated composites in infrastructure environments.

3.3. Mineralogical and Microstructural Interpretation of Performance Trends

3.3.1. XRD-Based Mineralogical Interpretation

The XRD patterns shown in Figure 15 provide a fundamental indication of the mineralogical properties of the constituent materials, which are utilized to interpret the trends of fresh-state and mechanical performance of the nano-silica-modified alkali-activated composites.

The assemblage of crystalline phases and amorphous content of slag cement (SC), high-calcium fly ash (HCFA), low-calcium fly ash (LCFA) and nano-silica (NS) directly determine the reaction kinetics, binder synergy and load-transfer efficiency of the composite matrix.

The XRD pattern of slag cement represents pronounced crystalline peaks with the main participants as quartz and calcium-based phase, which are superimposed over a partially amorphous background.

This mineralogical reagent is the reason the rapid early-age response in HCFA-based systems is achieved, whereby the presence of calcium enhances faster dissolution–precipitation reactions during alkaline activation. This action is in line with the stiffer losses in IFI and FRI with progress in dosage of HCFA mixtures of nano-silica, as early-reaction products form faster, leading to increased particle interlocking and lowered retention in workability.

On the contrary, both HCFA and LCFA show a wide amorphous aluminosilicate hump centered in the 2θ range of about 25–35°, which is more intense and broader in LCFA. This increased amorphous aluminosilicate is responsible for slower, more controlled reaction kinetics and the better flow retention behavior (FRI) that is always witnessed in LCFA-based mixtures. Lower LCFA early-age reactivity reverses the premature stiffening and therefore increases the fresh-state stability relative to the systems based on HCFA.

Nano-silica has a predominant hump of amorphous type and small crystalline peaks, indicating that it is highly reactive and has a glassy structure. This mineralogical property is the basis of the abovementioned improvement in mechanical indices at an intermediate dose of nano-silica of about 2 wt% (NS2). The amorphous silica offered by NS helps to enhance the way the particles are packed, the speed at which the reaction products are nucleated, and the optimization of the interfacial transition zones.

The same effects manifest in the parallel increases in SAI, TSI, and FSI in each of the two systems based on the incorporation of nano-silica (HCFA- and LCFA-based).

The mineralogical description is also supported by correlation analysis of SAI-TSI and SAI-FSI Figure 10, Figure 11, Figure 12 and Figure 13.

Close linear correlations with large coefficients of determination (R² of approximately 0.91–0.95) are a sign that the development of compressive strength is closely associated with tensile and flexural performance when the balance of mineralogical reactivity is achieved.

In mixtures made with LCFA, these correlations are slightly more coherent, which can be explained by the fact that it is the reaction products that are based on aluminosilicate, which facilitates more coherent transfer of stress and crack-bridging.

On the other hand, the HCFA-based systems have a larger scatter, indicating the effects of calcium-enriched crystalline phases, which are characterized by fast strengthening but add heterogeneity at a microscopic level of development.

At increased dosages of nano-silica (NS3), the mineralogical balance is less desirable. Amorphous silica in excess causes demand on surface area and insufficient binder packing, resulting in marginal mechanical gains and noticeable loss in fresh-state performance. This action proves that it is mineralogical synergy, or complete amorphous content, and not absolute amorphous content, that governs performance of composites.

Altogether, the mineralogical support of the trends of the performance of IFI–FRI and SAI–TSI–FSI tests is presented in Figure 15.

The optimal performance of the middle doses of nano-silica is an indication of a balanced reaction among the availability of calcium, amorphous aluminosilicate content and nano-scale reactivity.

The findings verify that the multi-index framework provided is a mineralogically based, performance-based approach to create resource-efficient alkali-activated composites to be utilized in infrastructure applications.

3.3.2. SEM-Based Microstructural Interpretation

Microstructural observations support this interpretation, as excessive nano-silica incorporation is expected to increase localized densification and microstructural non-uniformity, particularly in calcium-rich regions. Such localized reactions may promote stress concentration, reduce effective crack-bridging efficiency, and hinder load redistribution, thereby explaining the diminishing mechanical returns observed at the highest nano-silica dosage.

Scanning electron microscopy (SEM) was used to investigate the microstructural features of the alkali-activated composites to justify the mineralogical interpretation as well as elucidate the fresh-state and mechanical performance tendencies as discussed previously.

SEM micrographs of an HCFA- and LCFA-based reference system (nano-silica free) and the systems with 2 wt% nano-silica at 28 days are shown in Figure 16 in order to assess the effect of calcium content and nano-scale modification on the morphology of the matrix, particle interaction, and microstructural homogeneity.

In reference mixtures, that is, mixtures that do not contain nano-silica (NS0), the microstructure differs with the fly ash chemistry. In the HCFA-based reference system (Figure 16a), the microstructure is characterized by partially reacting fly ash spheres and embedded in a calcium-based reaction matrix. Even though a relatively thick binder phase is formed, distinctive heterogeneities, micro-voids, and localized microcracks are observed, particularly with regard to unreacted or partially reacted particles. These properties indicate a rapid dissolution precipitation reaction aided by heavy calcium contents, which leads to premature strength increase and formation of disproportionate distribution of stress and localized regions of reduced mechanical integrity in the matrix. These microstructural anomalies are also in line with an increased dispersion of tensile and flexural performance of HCFA mixtures.

On the other hand, the reference system of LCFA (Figure 16b) shows a rather homogenous and continuous structure of silicate aluminosilicate gels. SEM images reveal less product reaction microcracks, smoother product, and particle connectivity. The LCFA systems are low in calcium, which leads to slower and more controlled geopolymerization, less premature precipitation, and slower formation of binding steps. It is this microstructural homogeneity that contributes to the enhanced flow retention and more homogeneous mechanical behavior of LCFA-based mixtures even without nano-silica.

Addition of nano-silica at 2 wt% (NS2) causes both systems to be refined to a remarkable degree. In the case of the HCFA-based system, which contains nano-silica (Figure 16c), all SEM images depict a general densification of the matrix, where micro-voids are filled with nano-scale reaction products. Nano-silica is an active filler and nucleation agent that is capable of speeding up gel formation and interparticle bonding. Nevertheless, localized heterogeneities and microcracks can be seen, especially in the calcium-rich regions, which suggest that the combined presence of large quantities of calcium and nano-silica can enhance local reactions and stress concentration. This finding compares with enhanced compressive behavior but comparatively dispersed tensile and flexural response in the systems of HCFA.

The LCFA-based system with the additive of nano-silica (NS2) Figure 16d has a comparatively refined and uniform microstructure. SEM micrographs indicate that reaction products are well-distributed with lower pore connectivity and are better packed in. The aluminosilicate-based gel creates a continuous matrix that plays a good role in bonding fly ash particles and reducing microstructural defects. The reduced reaction kinetics of LCFA enable the nano-silica to be more favorably involved in the nucleation and microstructural refining without precipitating early stiffening. This improved morphology is aligned with the increase in flow stability and the enhanced correlation among compressive, tensile, and flexural performance indices of LCFA composites.

Thus, when comparing reference and nano-silica-modified systems, it can be concluded that nano-silica can substantially enhance the densification of the matrices, interfacial transition zones, and microstructural coherence when introduced at an optimal dosage.

The results of the SEM observations confirm that performance growth is burdened by harmonious mineralogical interaction rather than the excessive nano-scale supplementation. Such microstructural observations offer direct physical support for the multi-index performance framework and the reasons behind the optimum mechanical and fresh-state performance of intermediate doses of nano-silica when applied to alkali-activated composites to be used in sustainable infrastructure applications.

3.3.3. Integrated Mineralogical–Microstructural Interpretation

The combined explanation of mineralogical and microstructural features provides an explanation of the observed fresh-state and mechanical performance tendencies of the nano-silica-modified alkali-activated composites in a unitary manner.

While XRD analysis elucidates the phase constituent, amorphous content, and dominating reaction mechanisms that govern the systems founded on calcium and silicate alumina, SEM observations represent the actual findings of the microstructural organization, particle packing, and interface properties. The enhanced amorphous aluminosilicate hump localization is seen in the LCFA-based systems in the XRD sense, which is translated into SEM in the form of a more homogenized, compact, and well-linked structure to enhance the capacity of holding flow and transferring stress in an efficient way.

On the other hand, the localized densification, high microstructural heterogeneity, and vulnerability to premature stiffening, which are seen in the SEM micrographs, are related to the presence of localized densification and crystalline phases rich in calcium, as revealed by XRD.

At the intermediate dosage of nano-silica (2 wt%), both techniques consistently demonstrate that balanced mineralogical synergy rather than the highest proportion of amorphous content regulates the process of microstructural refinement and overall performance enhancement.

This integrative mineralogical–microstructural analysis is conducive to the validity of the proposed multi-index framework as a viable, performance-based approach methodology to the creation of resource-efficient alkali-activated composites with specific prospects to utilize them in the context of sustainable infrastructural applications.

4. Discussion

The results of the experiment demonstrate the need to consider a performance-based approach to the assessment of nano-silica-modified alkali-activated composites. Instead of relying upon the measures of isolated strength, the current results indicate that the relationship between fresh-state behavior, binder chemistry, and multi-directional mechanical response dictates the overall efficiency of nano-scale modification.

This supports the assumption that compressive strength alone cannot be used to characterize the functional performance of alkali-activated systems, especially systems that use nano-sized modifiers and heterogeneous precursors.

The gradual decrease in flowability and workability retention with the rise in nano-silica is evidence of the dual physical and chemical functions of nano-silica in alkali-activated matrices.

Physically, the high specific surface area of nano-silica is very high and thus promotes internal friction and water demand, which restricts the mobility of the particles in the fresh state. In chemistry, nano-silica enhances reaction kinetics at the early-stages of reaction because it offers large nucleation sites, which creates gels quickly and stiffens rapidly. These effects are also exacerbated by constant water-to-slag and activator-to-slag ratios and, thus, the constraints of mixture design appear to be of paramount importance in the regulation of fresh-state performance at high nano-silica dosage.

The relative enhanced flow stability of low-calcium fly ash systems demonstrates that the decreased reactivity at early ages can be partially compensated by the nano-silica effect on the stiffening propensity, and the controlling factor is the availability of calcium in fresh-state.

This progressive decrease in flowability and workability retention as the nano-silica content increases is indicative of the two physical and chemical functions of nano-silica in alkali-activated matrices. Physically, nano-silica, due to its extremely high specific surface area, causes internal friction and water demand, thus restricting the mobility of the particles in the fresh state. At the molecular level, nano-silica enhances the kinetics of the initial reaction by offering sufficient sites of nucleation, which allows for rapid gel formation and fast stiffening. When fixed water-to-slag and activator-to-slag ratios are used, these effects are intensified, which indicates that the limitations of mixture design are of vital importance to regulate fresh-state performance at high doses of the nano-silica. The comparatively higher stability of the flow of low-calcium fly ash systems transports the idea that the reduced reactivity at early ages may be offset to some extent by the tendency to stiffening of nano-silica, which emphasizes the dominating influence of calcium supply in fresh-state formation.

The relative performance of the high-calcium fly ash systems and low-calcium fly ash systems further demonstrates the sensitivity of the performance of nano-silica to the binder chemistry. Agreement systems with a high calcium content have a faster strength curve and are stiffer in early ages, which can be beneficial in early load-bearing applications but can cause microstructural heterogeneity and localized crack behavior. In comparison, the low-calcium systems promote slower reaction pathways dominating the system with the formation of aluminosilicate gels that contribute to the increased storage and homogenization of mechanical response. There is a higher interrelationship between compressive and tensile and flexural indices in low-calcium systems, indicating a more coherent load-transfer network, indicating that reaction kinetics rule the mechanical synergy.

The multi-index structure used in this study offers a powerful way of measuring these interactions with fresh-state indicators and normalized mechanical indices. The assessment using correlation indicates that compressive performance gains do not always result in proportional tensile or flexural gains, especially in a system where there are varying reaction mechanisms. This observation has been consistent with the larger observations in more complex cementitious materials in which crack resistance and flexural strength is more sensitive to microstructural continuity and interfacial quality compared to compressive strength of the bulk. The framework achieves this by focusing on the progression of relative performance, but not on absolute levels of strength, allowing for a useful comparison between material systems, taking into consideration inherent variations in precursor reactivity.

In spite of these insights, there are a few shortcomings that are worth considering. The experiment was carried out on a mortar scale with one alkaline activator formulation and constant proportions of mixture, and this limits the extrapolation of the results to concrete-scale applications. Moreover, there were no experimentally investigated properties of durability, and it has not been established that long-term performance is also experimental when subjected to aggressive exposure conditions. Although the suggested scheme offers a logical foundation to material optimization, the ultimate applicability on the infrastructure systems would be on the premise of validation by durability testing, other activator chemistries, and field-relevant curing regimes.

5. Conclusions

This study revealed that the proposed multi-index performance is a comprehensive and systematic framework through foundational measurements of the synergies of nano-silica incorporation into the resource-efficient alkali-activated composites. The proposed framework facilitates the use of performance-based assessment, integrating the fresh-state flow indicators and strength-based mechanical indicators, thereby providing material assessment other than the conventional compressive strength-based techniques.

The performance of fresh-state systems based on the Initial Flow Index (IFI) and the Flow Retention Index (FRI) gradually declined as the dosage of the nano-silica increased in both HCFA-based systems and LCFA-based systems. This behavior is attributed to the increase in the surface area and the rate of reaction in the initial stage. Nevertheless, LCFA systems never showed limited flow retention or fresh-state stability as compared to HCFA systems because of the influence of low calcium concentrations and low reaction rates in inhibiting quick stiffening.

An intermediate nano-silica dosage of about 2 wt% was optimum, yielding the enhanced compressive, tensile, and flexural response with both fly ash systems. At this dosage concentration, the Strength Activity Index (SAI), Tensile Strength Index (TSI) and Flexural Strength Index (FSI) showed improved development of strength. An increase in the nano-silica content led to a decrease in the performance gains, and the results illustrated the need to optimize dosage to prevent the negative effects of excessive fineness and particle agglomeration.

Correlation analyses of Strength Activity Index (SAI) and Tensile and Flexural Strength Index (TSI and FSI) revealed that it is not possible to reliably estimate tensile and flexural performance based only on compressive performance, particularly in nano-silica, fly ash, and alkali-activated composites, which react in different ways. Such findings suggest that mechanical synergy and the load-transfer properties of alkali-activated composites require a multi-index evaluation.

Mineralogical and microstructural analyses (XRD and SEM) revealed that the effectiveness of the reaction, as well as the densification of the matrix and stress transfer, is determined by the interaction of amorphous aluminosilicate components, the availability of calcium, and the reactivity of nano-silica. An intermediate dosage of nano-silica (2 wt%) was favorable to balance the nucleation and refinement of the microstructure, but a high dosage favored agglomeration, non-homogeneous nucleation, and low performance. These values agree with the experimentally determined patterns of fresh-state behavior and mechanical performance.

Collectively, the results endorse that the proposed multi-index framework is an effective methodological approach in the optimization of nano-silica dosage and directing the design of resource-efficient alkali-activated composites, which have proven to be relevant in sustainable infrastructure applications.

Although the proposed multi-index framework has its benefits, this study has its constraints because it was carried out on mortar-scale systems, and its formulation used only a single activator and did not involve direct testing of durability. More research is recommended to check how well the framework works with different types of activators, curing conditions, and mixture recipes to see if it can be used more broadly. It will be extended to concrete-level systems using coarse aggregates, will include durability parameters like shrinkage, permeability, and chemical resistance, and will also be tested under field-relevant exposure parameters to support the practical applications. Also, the issue of sustainability here is talked about in qualitative terms; environmental assessment and the LCA of activator chemistry should be added to the proposed multi-index in the future to support and enhance the suggested framework.