Towards sustainable concrete pavements: a critical review on fly ash as a supplementary cementitious material

1.1 Methodology of the review

This review followed a structured and transparent approach to ensure a comprehensive evaluation of existing literature on the application of fly ash (FA) as a partial cement replacement in pavement concrete. A systematic search was carried out across major scientific databases, including Scopus, Web of Science, ScienceDirect (Elsevier), and Google Scholar, covering studies published between 2015 and 2025. The database search, completed in October 2025, employed Boolean keyword combinations such as “fly ash” OR “FA” AND “cement replacement” AND (“pavement concrete” OR “road concrete” OR “roller compacted concrete” OR “RCC”) AND (“mechanical properties” OR “durability” OR “hydration” OR “sustainability”). Additional search terms such as “CO₂ reduction,” “life-cycle assessment,” “abrasion resistance,” and “freeze–thaw durability” were used to identify studies addressing sustainability and performance aspects relevant to road infrastructure.

Only peer-reviewed journal articles, conference papers, and authoritative technical reports were considered eligible for inclusion if they focused on the use of FA in rigid or semi-rigid pavement concrete systems, provided quantitative data on strength or durability, included compositional or microstructural analysis of FA (e.g., oxide composition, fineness, SEM, or XRD results), or discussed environmental and life-cycle impacts. Studies were excluded if they examined soil stabilization or geopolymer systems without FA, asphaltic or non-cementitious materials, or lacked quantitative or comparative data. Short communications, editorials, and review notes without original analysis were also omitted.

The initial search retrieved approximately 160 publications. After screening titles, abstracts, and removing duplicates, 94 studies were selected for detailed examination. Among these, 62 studies provided sufficient experimental or analytical information for synthesis and discussion in this review. Data extraction emphasized key variables such as FA replacement levels, water-to-binder ratios, curing conditions, mechanical strength (compressive and flexural) at different ages, and relevant durability indicators including abrasion resistance, freeze–thaw performance, and sulfate attack resistance. The selected studies were then grouped according to the type of FA (Class F or Class C), combination with supplementary materials such as slag, silica fume, or metakaolin, and testing or curing regimes. Comparative evaluation focused on identifying consistent trends, mechanistic explanations, and divergences among studies rather than simple data aggregation. This structured methodology ensured a balanced and critical synthesis of the role of fly ash in enhancing the mechanical, durability, and environmental performance of pavement concrete. Although the dataset appears limited, this was intentional to ensure quality and comparability. Only peer-reviewed studies with complete quantitative data, such as mix details, ash classification, and curing parameters, were included. This selective approach prioritizes data reliability and clarity of trends over quantity, providing a strong and focused foundation for understanding fly ash performance in pavement concrete.

3.1 Chemical composition and classification

This research provides a comprehensive analysis of previous studies concerning the chemical and physical properties of fly ash used in concrete pavements. Understanding these properties is crucial, as it helps determine the material’s significance, potential benefits, and suitability for construction applications. FA is a finely divided powder generated from the combustion of pulverized coal in thermal power plants. Its chemical composition and particle morphology govern its pozzolanic reactivity and thus the performance of FA-blended concrete pavements. As shown in Table 3, the principal oxides in FA are silica (SiO₂), alumina (Al₂O₃), and ferric oxide (Fe₂O₃), together forming over 70 % of the total mass in most sources. The major oxides are silica (SiO₂), alumina (Al₂O₃), and ferric oxide (Fe₂O₃), accompanied by smaller fractions of calcium (CaO), magnesium (MgO), and alkalis (Na₂O + K₂O). The combined (SiO₂ + Al₂O₃ + Fe₂O₃) content and CaO proportion define two main ASTM C618 classifications: Class F (low-calcium, mainly pozzolanic, >70 % SiO₂ + Al₂O₃ + Fe₂O₃) and Class C (high-calcium, >10 % CaO, exhibiting both pozzolanic and self-cementing behavior) [28], 41].

Figure 2 summarises these compositional ranges across different ash sources, illustrating that fly ash occupies a distinct region characterised by moderate SiO₂ and Al₂O₃ and variable CaO contents. Compared with other industrial by-product ashes (e.g., rice-husk or bagasse ash), fly ash exhibits a more balanced oxide profile and greater consistency, which explains its widespread adoption in concrete pavements. Yoshitake et al. [28] reported that limestone-aggregate concrete containing Class F FA exhibited high abrasion resistance, emphasizing that its moderate lime index (≈0.1) and CaO < 10 % place it firmly in the pozzolanic domain. The lime-index relationship plotted in Figure 3 clarifies this classification as Class F fly ash falls within Zone I and behaves purely pozzolanically. Class C fly ash, with CaO >10 % and a higher lime index, occupies Zone II, exhibiting both pozzolanic and limited cementitious properties. This distinction is important for mixture design: low-calcium ashes contribute to long-term strength through secondary hydration reactions, while high-calcium ashes accelerate early-age strength gain.

Figure 2:

Triangular plot to differentiate different types of ash [28].

Figure 3:

Variation in the major chemicals in fly ash [28].

The ternary composition diagram in Figure 4 further demonstrates that most fly ash samples cluster near the SiO₂–Al₂O₃–Fe₂O₃ apex, confirming their dominance of silico-aluminous phases that promote C–S–H and C–A–H gel formation during hydration. The pozzolanic reaction mechanism of FA involves the dissolution of reactive silica and alumina from amorphous glassy phases, followed by their reaction with calcium hydroxide [Ca(OH)₂] liberated during cement hydration. This secondary reaction forms additional calcium-silicate-hydrate (C–S–H) and calcium-aluminate-hydrate (C–A–H) gels that refine pore structure and enhance long-term strength and durability [39], 113]. Shah and Wang [113] compared OPC and FA compositions and found that FA contained 46.8 % SiO₂, 23.9 % Al₂O₃, and 15.8 % Fe₂O₃, with only 4.7 % CaO, values typical of Class F ash capable of consuming portlandite to yield additional C–S–H. These reactions explain the lower early-age but higher later-age strength often observed in FA concrete.

Figure 4:

Variation in the lime index with CaO percentage [28].

Talkeri and Ravi Shankar [41] determined an average specific gravity of 2.30 and Blaine fineness of 345 m²/kg, confirming FA’s finer and smoother texture relative to Portland cement. Behera and Das [42] similarly observed spherical glassy particles of 10–100 µm with a maximum dry density around 1.2 g/cm³, characteristics that enhance particle packing and reduce bleeding and permeability in concrete. The fineness and spherical morphology contribute to improved workability and a denser interfacial transition zone (ITZ), which in turn strengthens the hardened matrix.

3.2 Micro- and elemental characterization

Energy-dispersive X-ray spectroscopy (EDS) in Figure 5 shows that FA typically contains low Ca and Al levels with dominant Si, O, and C peaks [43]. Trace amounts of K, Ti, and Fe are also present. The EDS and chemical data in Table 4 reaffirm that FA is richer in siliceous and aluminous phases than ordinary Portland cement (OPC). Shah and Wang [113] show a typical comparison: FA contains 46.8 % SiO₂, 23.9 % Al₂O₃, 15.8 % Fe₂O₃, and only 4.7 % CaO, whereas OPC has 64.3 % CaO and only 20.4 % SiO₂. These differences explain FA’s lower early-age reactivity but enhanced later-age strength through secondary pozzolanic hydration.

Figure 5:

Fly ash EDS analysis showing the proportions of different elements [43].

The hydration and pozzolanic reaction mechanisms of FA are illustrated in Figure 6. During cement hydration, calcium hydroxide (CH) is released, which subsequently reacts with amorphous silica and alumina from FA to generate secondary C–S–H and C–A–H gels. These products fill pore spaces and refine the matrix structure, thereby improving density and reducing permeability. This secondary reaction proceeds more slowly than cement hydration but contributes significantly to later-age strength and microstructural stability.

Figure 6:

Mechanism of fly ash hydration and pozzolanic reaction [114].

Color and alkalinity also vary with source. Zimar et al.[56] and Ahmaruzzaman [115] demonstrated that the color and alkalinity of FA vary with chemical make-up and unburned carbon content: high-Fe ashes appear tan, while high-carbon ashes are dark gray to black. Based on pH, FA is classified as acidic (pH ≈ 8), moderately alkaline (pH 8–9), or alkaline (pH 11–13). Its specific gravity ranges from 2.1 to 3.0, with density values ≈ 2.2–2.4 g/cm³ and Blaine fineness ≈ 345 m²/kg. The geotechnical parameters in Table 5 confirm that these ranges are lower than for most natural soils, supporting FA’s potential as a lightweight binder for pavement applications.

3.3 Reactivity and performance relevance

The pozzolanic mechanism involves the dissolution of amorphous Si and Al from FA, followed by reaction with Ca(OH)₂ from cement hydration to produce secondary C–S–H and C–A–H gels, which refine pore structure and enhance durability [39], 116]. Singh [39] showed that high reactive silica and alumina contents accelerate geopolymerization in alkaline environments, increasing early-age strength. Nayak et al. [106] and Kaewmanee, Krammart [117] further demonstrated that free lime and alkali content strongly influence setting behavior, while excess SO₃ can trigger expansive reactions if not controlled [116], 118].

Shah and Wang [113] and Kumar, Tike [119] reported that cement has a specific gravity of 3.15 and a 28-day strength ≈ of 57 MPa, whereas FA (45 µm sieve fraction) shows a specific gravity ≈ of 2.40 and a Blaine surface area ≈ of 0.35 m²/g. FA mixtures achieved a strength activity index of 120 % and a water requirement ≈ of 91 % of the control mix. Recent studies [111] conclude that replacing 15–20 % of cement with FA yields the best balance between strength gain, durability, and CO₂ reduction; higher dosages (>50 %) remain suitable for mass pavements where slow strength development is acceptable.

Collectively, the evidence from Tables 3–6 and Figures 1–6 confirms that fly ash’s balanced oxide composition, fine spherical particles, moderate alkalinity, and consistent silico-aluminous structure make it the most effective and globally available supplementary cementitious material for sustainable concrete pavements. Its application enhances workability, reduces the heat of hydration, and improves long-term mechanical and durability properties while supporting low-carbon construction objectives.

3.4 Microstructure and thermal properties

Figure 7 (adapted conceptually from multi-phase analyses of alkali-activated fly-ash–slag (AAFS) systems) illustrates the progressive refinement of the interfacial transition zone (ITZ) and paste microstructure during hydration. In AAFS and FA-blended cement concretes, the ITZ governs mechanical integrity because it represents the weakest link between aggregate and binder. The schematic depicts four chronological stages (a–d): (a) initial mixing, when unreacted fly-ash and slag particles are dispersed within a porous ITZ; (b) early hydration, in which partial dissolution of reactive Si and Al species initiates the formation of aluminosilicate gel; (c) continued pozzolanic and geopolymeric reactions, resulting in secondary C–S–H and C–A–S–H formation that fills microvoids and bonds aggregates to paste; and (d) late-age densification, where unreacted particles become coated by dense gel phases and microcracks begin to stabilize.

Figure 7:

Schematic representation of the microstructural evolution of the interfacial transition zone (ITZ) in fly-ash–based concrete [122].

The contour map beneath the schematic visualizes the spatial distribution of the elastic modulus, showing a gradual transition from the stiff aggregate core through the densified ITZ into the more compliant bulk paste. The accompanying modulus–time curve indicates that the ITZ achieves higher stiffness than the surrounding paste at intermediate hydration ages, reflecting the accelerated reaction of fine FA particles that act as nucleation sites.

Overall, this mechanism explains why fly-ash incorporation refines the ITZ, decreases microcrack density, and enhances the elastic compatibility between aggregate and paste. The resulting microstructure exhibits reduced porosity, improved load transfer, and superior long-term durability – outcomes consistent with the microstructural models and SEM observations summarized elsewhere in this review.

Scanning electron microscopy (SEM) of fly ash and plain concrete mixtures is necessary for proper knowledge of the microstructure of plain and high-volume fly ash (HVFA) concrete. SEM is a crucial and commonly used approach for material characterisation. Fly ash particles’ elemental distribution, exterior surface structure, and morphology can be studied using SEM, as shown in Figure 8 [43].

Figure 8:

Fly ash incorporated in a concrete mix, as viewed by SEM [43].

Figure 9(a–e) depict SEM images of different plain and fly ash concrete mixtures (h). The microstructural behaviour of concretes with 0 %, 25 %, 40 % and 60 % fly ash is shown in Figure 9(a–h), respectively. The concrete mix groups exhibit comparable microstructural changes. The OPC concrete mixtures contain few voids and cracks according to the SEM investigation and output photos.

Figure 9:

180-Day SEM pictures for mix groups 1 and 2: A and B represent plain concrete, C and D represent 25 % fly ash concrete, E and F represent 40 % fly ash concrete, and G and H represent 60 % fly ash concrete [43].

In plain concrete, the aggregates are tightly linked with the aid of Ca(OH)₂ and C–S–H gel and thus provide improved strength, as shown in Figure 9(b). In comparison with HVFA concrete, the plain and 25 % fly ash concrete mixes have higher calcium hydroxide concentrations. Figure 9(a) and (b) show the concrete’s ettringite and calcium hydroxide contents, respectively. Numerous voids and cracks are exhibited as the fly ash percentage increases, especially in the concrete mixes containing 40 % and 60 % fly ash (Figure 9(e)–(g)). The concrete specimens made with 25 % fly ash and ordinary concrete have similar microstructures. The percentage of Ca(OH)₂ and ettringite in the plain and 25 % fly ash concrete specimens is higher than that in the HVFA concrete specimens.

However, the addition of fly ash to concrete increases the production of C–S–H gels as a result of the chemical reaction between fly ash’s silica and Ca(OH)₂, but it decreases the amount of ettringite, which causes weak bonds and decreased strength in HVFA concrete specimens. Figure 9(h) illustrates the presence of considerable amounts of C–S–H gels in (60 % fly ash) concrete specimens in the form of white spots. Figure 9(e) and (f) reveal that in HVFA concrete, the structure of calcium hydroxide greatly decreases, whereas the development of C–S–H gel increases (Figure 9(h)). In all the combinations, C–S–H, calcium hydroxide, and ettringite can be seen through SEM examination. Meanwhile, the amount of C–S–H increases with the increase in the fraction of fly ash, and the amount of ettringite decreases as the concentration of fly ash rises.

4.1 Compressive strength

According to Pranav et al. [26], compressive failure in concrete pavements is exceedingly uncommon because compressive strength testing is frequently performed to evaluate the quality of pavement materials. Compressive strength closely correlates with tensile or flexural strengths, which are the most important factors to consider when designing concrete pavements. Additionally, determining concrete’s compressive strength is simpler than determining its tensile or flexural strength. The microstructure of hardened concrete is also closely related to its compressive strength.

High-strength concrete either maintains its homogeneity or forms microscopic fissures shortly before total breakdown. In other words, as the material’s compressive strength increases, it becomes harder and more brittle than before. A solid material’s compressive strength is influenced by its porosity. The interfacial transition zone (ITZ) and the porosity of the cement matrix, both of which affect concrete’s compressive strength, are strongly influenced by the water-to-cement (W/C) ratio [123]. However, various elements and their intricate relationships can affect how concrete actually reacts to applied compressive pressures. These variables are divided into three groups: material qualities and ratios (e.g., the W/C ratio, air entrainment, cement type, aggregate properties, mixing water, and admixtures); parameters for testing and curing conditions (e.g., duration, temperature, and humidity, and specimen and loading parameters) [26].

The variation in compressive strength reported across studies can be attributed to differences in fly ash class (F or C), replacement ratio, fineness, and curing regime. At moderate substitution levels (≈15–25 %), FA enhances strength due to the filler effect, improved particle packing, and the formation of additional C–S–H gels through pozzolanic reaction. However, at higher dosages (>40–50 %), dilution of clinker phases and slower pozzolanic kinetics reduce early-age strength. Class F ash, with low CaO, reacts more slowly but yields greater long-term gains, whereas Class C ash, containing more CaO, accelerates early hydration but can increase porosity when used excessively. Discrepancies among studies such as those by Xu et al. [108], Ahmad [124], and Kumar [119] largely stem from these mechanistic differences combined with variations in curing temperature, activator concentration, and water-to-binder ratio.

Singh [39] indicated that various factors, including the molarity of NaOH, SS/SH, alkaline liquid-to-binder ratio, and curing method, largely determine the compressive strength of GPC. With high NaOH content, fly ash achieves high compressive strength. At high NaOH concentrations, the polymerisation process is suppressed by the dissolution of the solid precursor material from the source material, leading to improved strength. Increased geo-polymerisation increases the leaching of alumina and silica, which in turn enhances the strength at ambient curing [125], [126], [127].

Rambabu [21] replaced OPC with various concentrations of fly ash and silica fume and found that the mixture with 10 % SF and 20 % fly ash replacement has the highest peak load-carrying capacity, followed by the mixture with 5 % SF and 10 % fly ash replacement. Therefore, the mixes with a fly ash: SF ratio of 2:1 have compressive strengths that are higher than (FA20 with 63.69 MPa) or comparable with (FA10 with 60.1 MPa) that of control concrete (61.7 MPa).

Xu et al. [108] reported that compressive strength is a vital condition of concrete pavement projects and applications and is normally achieved either with a cube or cylinder in terms of various standards and applications. Xu, Han [108] blended mixtures with a specific proportion and formed them into 150 mm square cubes in accordance with ASTM specifications. Afterwards, the cubes were transferred to a regular setting where they were allowed to cure for a number of days at a temperature of 23 °C or −2 °C before being tested. The dosage of fly ash at the beginning of the seven-day period decreased the compressive strength values. The specimens with fly ash produced smaller values than the control specimen. On the 14th day, the tested combinations showed the same pattern. However, the compressive strength increment rate of the specimens with Type I fly ash was exceptional, and their strength values were similar to the control value. Mixes with Type 2 fly ash had lower compressive strength values than those with Type I fly ash because of the difference in fineness [128].

In another study, the results of a 28-day compressive strength test showed that the majority of specimens with additional cementitious materials have strength values that are almost equivalent to or higher than those of the control specimen, with the exception of specimens with Type II fly ash. This result is primarily due to the action of extra cementitious materials, which may be compelled to react with cement hydration products to create new products and binding materials, thereby increasing the strength [108].

Jatale, Tiwari [129] used fly ash to replace some of the cement and discovered that the compressive strength of concrete decreases. Additionally, they discovered that the fly ash and W/C ratios in the concrete mix are related to the curing rates at various ages. At a fixed W/C ratio, fly ash concrete’s modulus of elasticity decreases as the fly ash ratio increases.

Ahmad, Malik [124] reported that compressive strength decreases by 20 % and 50 % at 7 and 28 days of age, respectively, when 10 % of cement is substituted with fly ash. With 20 % replacement, compressive strength increases by 7 % and 11 % at 7 and 28 days of age, respectively. With 30 % replacement, compressive strength increases by 23 % and 25 % at 7 and 28 days of age. Additionally, as concrete replaced by fly ash ages, its compressive strength increases [89].

The comparative mechanisms of Class C and Class F fly ash in pavement concrete are illustrated in Figure 10. As reported by Akbulut and Guler [121] and Akbulut et al. [109], Class C fly ash, characterized by high CaO (≈20–30 %) and moderate SiO₂, acts as a self-cementing binder that accelerates early hydration and improves initial strength and thermal resistance. In contrast, Class F fly ash, dominated by SiO₂ + Al₂O₃ (>70 %) and low CaO (<10 %), exhibits slower pozzolanic kinetics but refines pore structure, lowers permeability, and enhances freeze–thaw and sulfate resistance at later ages. Hybrid or blended systems containing both classes of FA yield a synergistic response, balancing early- and later-age performance and reducing embodied CO₂ emissions by up to 30 % compared with OPC-only concrete.

Figure 10:

Comparative performance of class C and class F fly ash in pavement concrete (data extracted from Akbulut and Guler [121] and Akbulut et al. [109]).

As summarized in Table 6, most studies indicate optimum compressive strength at 15–20 % FA replacement, consistent with enhanced packing and sustained pozzolanic reactivity. Mixes with higher ash contents (≥50 %) develop lower 7-day strength but often exceed the control by 90 days once the secondary hydration reaction progresses. The few reports of early strength loss are associated with coarse ash particles and insufficient Ca(OH)₂ for pozzolanic activation. Thus, discrepancies across studies arise primarily from differences in ash fineness, calcium availability, curing temperature, and replacement ratio, all of which influence the reaction kinetics of C–S–H gel formation.

4.2 Flexural strength

Vieira, Schiavon [82], Debnath, and Sarkar [130] indicated the need to establish a suitable w/b ratio that is neither too high nor too low in order to impair paste hydration or make the mixture inconsistent, resulting in the lack of cohesiveness between aggregate particles. In pervious concrete, the link between strength and water content is not as clear as it is in conventional concrete because of the varied requirements related to the material’s workability. If the dough absorbs a specific amount of moisture without becoming liquid, then the right amount of water has been added. Compared with the prior concrete, which sets very quickly, the application of a superplasticiser can improve workability qualities, and the use of retarding setting additives can be beneficial.

The abundant free water at a w/b ratio of 0.30 improves the formation of hydrated products and increases flexural and compressive strengths. According to Debnath and Sarkar [131], the amount of water required for cement hydration increases as the w/b ratio increases, thereby improving the cohesiveness between the aggregates and strength development. As a result, tensile strength increases, leading to a decline in flexural strength. The authors reported losses of 11.3 % and 6.6 % for concrete compositions with 10 % and 20 % porosity, respectively. Generally, the presence of fly ash enhances this feature. Similar results were obtained for concretes with 25 % and 50 % RCD, although a 21.56 % reduction in flexural strength was observed compared with the reference concrete, as shown in Figures 11 and 12.

Figure 11:

Comparison of the flexural tensile strengths for w/b 14 0.25 and 0.30 at 28 days [131].

Figure 12:

Comparison of tensile strength at 28 days for concretes containing 0.30 w/b 10 % fly ash and 0.30 w/b 10 % fly ash [131].

Chen and Wang [61] set up 34 mix designs with different fly ash and Portland cement concentrations [132], [133], [134]. Compressive strength, elastic modulus, and flexural strength were measured for every blend. The porosity of the mix designs ranged from 15 % to 34 %, which meant the designs were porous enough to allow surface runoff to pass through. Four categories (i.e., 15 %, 17 %, 30 % and 50 % fly ash) were used to categorise the fly ash-containing blends.

Xu, Han [108] showed that different supplemental cementitious materials have varying effects on specimens’ flexural strength values. From 7 days to 28 days, the strength values of specimens supplemented with Grade 2 fly ash increased swiftly and approached the reference value. At various ages, 15 wt% substitution of cement with Type I fly ash had the highest strength value amongst the dosages. A similar trend was observed for the mixes that included Type I fly ash as a single or compound additive. By 7 and 14 days of age, increasing the amount of fly ash resulted in a decrease in flexural strength, but at 28 days of age, the strength values of all the specimens exceeded the control group’s strength value. At the various ages, the specimens supplemented with single or compound slag had higher flexural strength values than those added with fly ash.

The analysis results of these studies demonstrate that when applying economic allocation, permeable pavements with PCF4 (50 % fly ash) in the surface layer consume more energy and emit more greenhouse gases than those with PCF3 (30 % fly ash), PCF2 (17 % fly ash), and PCF1 (15 % fly ash). The large layer thickness in this instance may jeopardise the environmental advantages of fly ash addition. The overall pattern is that as the fly ash content increases, surface course strength declines and layer thickness increases, thus requiring more materials and producing more greenhouse gas emissions. Given that mixes in the PCF2 group typically have high flexural strengths, PCF2 has the lowest life cycle energy consumption and GHG emissions amongst all mix groups. One of the reasons could be that mixes with low porosity rates typically perform well mechanically. PCF2 has an average porosity of 16 %, whereas other mixtures have an average porosity of 30 %. Without allocation, fly ash’s contribution to environmental consequences can be reduced greatly [61].

In the literature, flexural strength trends generally mirror compressive behavior, with moderate FA replacement (≈15–25 %) improving strength due to better ITZ bonding and microstructural refinement, while excessive replacement (>40 %) increases porosity and reduces bending resistance. The differences observed between Xu, Han [108], and Chen and Wang [61] can be explained by porosity variation and w/b ratio: higher porosity mixes (≥30 %) exhibit lower flexural strength even when compressive strength appears satisfactory. Class F FA, due to its fine, glassy texture, enhances ductility and flexural performance at later ages, but class C FA or poorly graded ashes may cause early cracking or increased brittleness. Figure 13 corroborates these findings, showing a consistent correlation between compressive and flexural strengths with increasing FA volume. Strength increases up to ∼20 % FA, then gradually decreases due to dilution and delayed hydration.

Figure 13:

Relation of flexural and compressive strengths to the volume of fly ash.

The results of a review of available studies on the flexural strength of concrete with fly ash replacement showed that the best values are obtained with a fly ash replacement percentage between 15 % and 20 %. The values appear to improve after 28 days, and in some cases, after 90 days. Some studies have also shown that the use of high percentages of fly ash, such as 50 % and 70 %, produces excellent results on flexural strength values after 28 and 90 days, as indicated in Table 7.

The mechanistic processes underlying the performance enhancement of fly ash (FA)-blended pavement concrete are illustrated in Figure 14. The incorporation of FA alters both the hydration kinetics and the phase evolution of Portland cement systems through a combination of dilution, nucleation, and pozzolanic effects. As shown in the calorimetric heat flow profile, increasing FA content suppresses the early hydration peak and delays the main exothermic reaction, indicating moderated heat release and a slower hydration rate [135]. This behavior reduces the risk of thermal cracking in large pavement sections, a critical advantage for field applications.

Figure 14:

Hydration, reactivity, and phase evolution in Portland cement–fly ash blended pavement concrete [135].

The cumulative heat–strength relationship confirms that compressive strength development is closely governed by hydration progress. Mixtures with FA exhibit slightly reduced early-age heat release but demonstrate comparable or higher strength at later ages due to secondary C–S–H formation [87]. The reactivity analysis reveals that FA with lower network polymerization reacts more efficiently, while highly polymerized ash particles contribute primarily as micro-fillers.

Phase evolution mapping further shows that, as the degree of FA reaction increases, portlandite and capillary porosity volumes decline, while C–S–H, ettringite, and monosulfoaluminate phases expand. These transformations densify the ITZ, enhance matrix continuity, and improve long-term durability. Collectively, the results in Figure 14 confirm that FA moderates early hydration, refines the pore structure, and sustains later-age strength gain – thereby providing a mechanistic explanation for the improved compressive, flexural, and abrasion performance discussed earlier.

5.1 Surface abrasion value

Xu, Han [108]mentioned that in accordance with ASTM C779M−2005 specifications, the abrasion resistance test can be used to examine how concrete’s surface abrasion properties are affected by surface treatment and finishing and mixing ratios. In the study by Xu, Han [108], an increasing replacement content corresponded to an increasing abrasion value for all concrete blends containing additional cementitious components. At 7 days, the abrasion values of mixes with additional cementitious components were higher than those of the control mix and the industry norm of 3.6 kg per square meter. At 14 days old, fly ash demonstrated a lower abrasion mass loss value than the standard value in the mixes made with other supplemental cementitious materials. At 28-day age, the combinations showed lower abrasion values than the normal value. However, compared with the other mixtures, the blends with Type 1 fly ash showed better abrasion resistance, and the mixes with a single slag had higher abrasion values. The results indicated that the initial abrasion values of all mixtures were relatively similar at age 7, but as the specimens aged from 14 to 28 days, those with fly ash had higher abrasion values than those with slag. These findings are consistent with those of other studies, including [144].

Kumar, Tike [119] reported that at all W/C ratios, the abrasion loss of concrete mixtures increases with the amount of fly ash. In all the mixtures in their study, it decreased as the W/C ratio increased. The abrasion loss in the control mixture, which did not contain fly ash, was 0.22 %, 0.18 % and 0.16 % at 0.F40, 0.34, and 0.30 W/C ratios, respectively. The relationship between the W/C ratio and the amount of fly ash in concrete had a similar effect on abrasion resistance as compressive strength. With an increase in the compressive strength of the concrete mixtures, the abrasion loss decreased, or the abrasion resistance improved. According to Indian Standard IS 9284 BIS 1959, the maximum permitted amount of abrasion loss for concrete pavements with pneumatic-tired vehicles is 0.24 %. In the test conducted by Kumar et al., the abrasion loss of HVFA concrete with 60 % fly ash and a 0.30 W/C ratio was much lower than the maximum permitted limit.

As shown in Figure 15, abrasion loss progressively decreases with FA addition up to approximately 20–25 % replacement, where the minimum loss (≈0.12–0.15 kg/m²) is achieved after 28 days. Beyond this range, the loss slightly increases but remains below that of the control mix (≈0.35 kg/m²). This trend demonstrates that moderate FA substitution improves abrasion resistance by refining pore structure and producing denser calcium–silicate–hydrate (C–S–H) gels, while excessive FA may delay early hydration and slightly reduce wear resistance.

Figure 15:

Variation of surface abrasion loss with fly-ash replacement (data extracted from [108]).

5.2 Surface freeze–thaw mass loss

Xu, Han [108] stated that the surface frost resistance test can be used to assess a PCC road surface’s capacity to withstand frost damage caused by water or compound de-icing salts, such as NaCl or CaCl₂. Xu, Han [108] also examined different mixes of specimen surface mass loss under freezing and thawing cycle effects with saturated sodium chloride solution and water, respectively. The different mixtures clearly differed from one another. Even in environments with saturated sodium chloride solution or water, the fly ash-prepared specimens showed large mass loss values because the concrete contained fly ash with poor susceptibility to portlandite migration from the dense C–S–H zones to the air voids. Additionally, the replacement cement content for all active materials increased the mass loss of the specimens. To decrease concrete’s resistance to surface scaling, fly ash was applied to increase the thickness and porosity of the surface layer. In terms of mass loss, the freezing–thawing cycle with saturated sodium chloride solution caused more harm than water. The types of supplementary cementitious materials and their contents supported the theory about the freezing resistance of cement concrete surfaces.

The durability enhancement mechanisms are summarized in Figure 16. The pozzolanic densification and refined pore network produced by FA significantly reduce permeability, moisture migration, and chloride penetration. These microstructural benefits limit freeze–thaw deterioration, sulphate attack, and abrasion under cyclic traffic loading. Consequently, FA incorporation extends pavement service life while lowering maintenance requirements and lifecycle carbon emissions.

Figure 16:

Schematic framework of durability improvement in fly-ash-blended concrete under road exposure conditions.