Reducing CO₂ Emissions in Urban Infrastructure: The Role of Siliceous Fly Ash in Sustainable Mortar Design

Abstract

The incorporation of industrial by-products such as fly ash (FA) into cementitious materials plays a vital role in promoting environmental sustainability during cement production. This study evaluates the feasibility of using siliceous fuel fly ash, sourced from thermal power stations in the Rhenish region of Germany, as a partial cement replacement in mortar formulations. Mortar specimens with FA replacement levels ranging from 5 wt% to 30 wt% were prepared and tested. Data were collected through standardized laboratory testing of mechanical properties (compressive and flexural strength), physical characteristics (porosity, sorptivity) and microstructural analysis via SEM and XRD. The results showed that increasing FA content generally led to reductions in strength and increases in porosity and sorptivity, due to the mineralogical composition and higher water demand linked to the porous FA structure. However, when FA was used at levels not exceeding 10 wt%, the physical and mechanical properties remained within acceptable limits for construction applications. Additionally, the use of plasticizing admixtures proved effective in mitigating workability and strength issues by reducing the water-to-binder ratio. These findings highlight that, despite certain limitations, siliceous FA can be safely and effectively used in low percentages, contributing to sustainable mortar production and reducing reliance on Portland cement. In addition, the use of fly ash contributes to reduced CO₂ emissions and lower production costs, promoting sustainable and cost-efficient construction solutions.

1. Introduction

Thermal power plants generate harmful residues, such as bottom ash and fly ash. The latter is a generally fine-grained material (particle size diameter ranging from <1 μm up to 150 μm) with a Blaine specific surface area between 130 m²/kg and 600 m²/kg, depending on its type, which can influence the characteristics and workability of cement-based materials when employed as a partial cement substitute [1,2]. As a consequence, the additions of SCMs such as slags (steel or blast furnace) or pozzolans (fly ash and silica fume) may enhance the durability of cement-based composites [3,4,5,6]; the aforementioned materials exhibit pozzolanic and/or filling properties when added in cementitious blends. Fly ash produced from coal-fired power stations has been extensively and successfully used in the production of cementitious composites, leading to better resistance against chloride ingress and thus enhanced durability [7,8,9], as well as in higher long-term compressive strength [10,11], owing mostly to its high contents of silicon oxide (total and amorphous) and alumina, and its particle size [12]. Conversely, siliceous fuel-derived fly ash (FA) remains significantly underutilized, mostly due to the lack of extensive relevant research. FA differs in composition and physical characteristics compared to fly ash produced from coal-fired power stations. It is rich in carbon (ca. 50% wt.), sulfur, cadmium, vanadium, magnesium and nickel [13,14,15,16] and generally contains coarser grain particles than fly ash produced from coal firing. Furthermore, FA is not abundant in silica/aluminum cenospheres and has a low Si/Al ratio [17,18]; this raises concerns regarding its suitability for cementitious applications, despite the fact that the presence of even low amounts of glassy shallow particles may potentially improve its pozzolanic reactivity and hence the properties of hardened cementitious composites. As a result, engineers worldwide face the challenge of developing high-quality cementitious composites with FA as a cement-replacing additive, especially since the latter is also not classified in international standards, as opposed to fly ash produced from coal firing.

Camilleri et al. [19] investigated the use of FA for the production of low-grade concrete. These researchers suggest the use of FA as a replacement for cement in amounts not exceeding 20% wt. to avoid strength reduction. Another study [20] recommends the use of FA, either as a pigment or cement admixture, to produce ornamental bricks with acceptable engineering properties; According to previous studies, low amounts (2–5% wt.) of FA addition do not appear to negatively affect the compressive strength of the end-product, while the product is likely to remain environmentally benign with minimal trace element and metal leaching into underground water. [21]. At the same time, siliceous FAs may improve the Ca²⁺ ion leaching resistance, leading to increased strength; FA addition possibly closes the pores and cracks in the cement mortars preventing C-H (Ca(OH₂)) release [22]. Al-Osta et al. [23] produced asphalt concrete using 3% wt. FA as a filler replacement and modifier; their results show an improvement in the stiffness and fatigue of the end-products. Other researchers [24] evaluated the performance of FA mortars through a series of physico-mechanical tests. Their results show that the use of FA leads to increased water:cement ratios. Reduced setting times were also observed due to the presence of carbonates in the FA used. Furthermore, increased strength was achieved when FA was used without any pre-treatment (grinding and/or sieving); however, the strength of the end-product was reduced with increasing the percentage of FA.

The main objective of the present study is to investigate the potential exploitation of FA produced in electricity-generating thermal powers stations in western Germany (Rhenish area) in cement mortar production for use by the construction industry. At the same time, this study aims to contribute to the scarce global literature on FA, in an effort to assist engineers worldwide to design cementitious composites with FA as a cement replacement material. The urgent need for the management of this solid waste material renders the present study timely and highlights its significance and contribution to environmental sustainability, since the potential use of FA as a cement replacement material will inevitably reduce the use of the latter, leading at the same time to the reduction in CO₂ emissions in the environment from cement manufacturing plants.

It is generally accepted that the cement industry contributes 5–7% of total worldwide CO₂ emissions [25,26,27], while the global emissions in 2016 only amounted to 1.45 ± 0.20 Gt CO₂, which corresponds to approximately 4% of the emissions generated from fossil fuel combustion in that year [28,29,30]; therefore, by replacing cement with fly ash during concrete production, a lower amount of cement is used and this results in lower carbon dioxide emissions.

2. Materials and Methodology

2.1. Raw Materials and Mix Design/Preparation

In this study, mortar specimens were fabricated using CEM I 32.5 N cement, finely crushed calcareous aggregates (0–4 mm), and water from the local municipal supply. Fly ash (FA), sourced from a German thermal power plant and characterized by a maximum particle size of ≤500 μm, was used to partially replace cement in proportions ranging from 5% to 30% by weight.

To evaluate the influence of FA incorporation, four mortar formulations were developed and cast under laboratory conditions: 5FA, 10FA, 20FA, and 30FA. These designations correspond to FA replacement levels of 5%, 10%, 20%, and 30% by weight of cement, respectively; a reference mix without FA (designated as REF) was also produced to provide a baseline for comparison. A total of 27 cubes (50 × 50 × 50 mm³) and 9 prisms (160 × 40 × 40 mm³) were cast per group; the quantities in kg/m³, the ratios of raw materials and the slump of fresh mortar are summarized in

Table 1. Mix proportions of each group (kg/m³), ratios of raw materials and workability of fresh mortars.

	Proportions (kg/m3)				Workability (mm)
Group	Cement	Fly Ash	Sand	Water
REF	35.22	0.00	88.04	22.44	166.7
5FA	33.46	1.76	88.04	25.90	167.4
10FA	31.70	3.52	88.04	26.41	168.5
20FA	28.18	7.04	88.04	33.50	169.7
30FA	24.65	10.57	88.04	36.45	175.6
Water/cement + FA	0.64
Sand/cement	2.5

. Fresh mortar was poured into steel molds of different geometries (cubes and prisms). The freshly mixed mortar was introduced into these molds in two successive layers to ensure proper filling. Each layer was compacted with a jolting table to ensure homogeneous distribution and minimize trapped air voids that could compromise the structural integrity of the specimens. After casting, specimens were allowed to set undisturbed for 24 h. After this period, the hardened mortar samples were carefully demolded to prevent any damage to their surfaces or edges. After demolding, the specimens were submerged in water and cured under controlled conditions. This curing process was maintained consistently until the commencement of testing to ensure optimal hydration and strength development in the mortar samples.

The binder-to-aggregate (cement + FA to sand) weight ratio was uniformly set to 2.6 for all mixes to ensure uniformity in composition. To achieve comparable workability among the different mixtures, flow table tests were conducted. The target consistency for all mixes was controlled within a range of 165 ± 10 mm. A constant water-to-binder ratio of 0.64 was maintained across all groups, facilitated by the use of a low dosage of superplasticizer during mixing. The experimental methodology adopted in this study is summarized in the research flow chart presented in Figure 1.

In the present study, three specimens from each experimental group and age were subjected to testing under controlled and identical conditions. The data obtained from these tests were subsequently processed utilizing Microsoft Excel 2021 and OriginPro 8.5.1 software. To represent the average performance of each group in the tests, the arithmetic mean was calculated. Additionally, the standard deviation (SD) was employed to quantify the variability observed among the three specimens within each group.

Both Microsoft Excel and OriginPro facilitated the creation of graphical representations of the data, including bar charts that incorporated error bars (±SD). These visual aids were instrumental in enabling comparative analysis between different groups and across various age categories. The analytical approach adopted was primarily descriptive statistics; inferential statistical tests were not applied, as the objective was to identify trends and differences within the experimental results. All measurements were conducted under consistent experimental conditions to ensure reliability and validity of the findings.

Properties of Siliceous Fuel Fly Ash

The siliceous fuel-derived fly ash [31] examined in this research displays a complex mineralogical profile, mainly consisting of mullite, quartz, sulfur, iron oxides such as hematite and magnetite, anhydrite, and carbonaceous residues. Minor amounts of feldspars, calcite, and clay minerals were also detected. Table 2 presents a semi-quantitative overview of the mineralogical composition derived from X-ray diffraction (XRD) analysis (Figure 2), highlighting the relative abundance of the identified mineral phases.

Chemical analysis further indicated that the fly ash is notably enriched with hazardous trace elements, including arsenic (As), chromium (Cr), nickel (Ni), zinc (Zn), and lead (Pb). These observations are consistent with earlier studies showing that fly ashes from siliceous fuel oil combustion typically contain high concentrations of hazardous metals [32,33]. The occurrence of these toxic elements highlights the considerable environmental hazards linked to inadequate management or disposal of fly ash. Considering its chemical makeup and enrichment in potentially harmful elements, the implementation of appropriate handling strategies is critical to minimize ecological risks [34,35,36]. Uncontrolled disposal of raw fly ash may result in the contamination of soil and aquatic environments, posing threats to ecosystems and public health.

Thus, a thorough understanding of the mineralogical and chemical characteristics of fly ash is vital for formulating effective approaches to its safe use or disposal. The oxide composition presented in Table 2, along with the XRD pattern in Figure 2, provides valuable insights into the structural characteristics of the FA material.

Table 2. Oxide and trace element [37,38,39,40] compositions of the fly ash used in this study.

Oxide (% wt)			Trace Element
SiO2	~52.0%	Arsenic (As)	47–160 mg/kg
Al2O3	~24.0%
Fe2O3	~8.0%	Chromium (Cr)	159–300 mg/kg
SO3	~2.4%
CaO	~3.0%	Nickel (Ni)	80–250 mg/kg
C	~3.0%
MgO	~1.2%	Zinc (Zn)	190–700 mg/kg
Na2O + K2O	~1.0%
TiO2	~2.0%	Lead (Pb)	160–400 mg/kg
Other	~3.4%

Table 2. Oxide and trace element [37,38,39,40] compositions of the fly ash used in this study.

Oxide (% wt)			Trace Element
SiO2	~52.0%	Arsenic (As)	47–160 mg/kg
Al2O3	~24.0%
Fe2O3	~8.0%	Chromium (Cr)	159–300 mg/kg
SO3	~2.4%
CaO	~3.0%	Nickel (Ni)	80–250 mg/kg
C	~3.0%
MgO	~1.2%	Zinc (Zn)	190–700 mg/kg
Na2O + K2O	~1.0%
TiO2	~2.0%	Lead (Pb)	160–400 mg/kg
Other	~3.4%

Table 3. Physical properties of raw fly ash used in the present study.

Property	Value
Specific gravity (estimated by pycnometer)	2.53
Loss on ignition (LOI)	59.40%
Reactivity index (%)	6%
LOI-adjusted index (%)	14.85%
Moisture content	13.80%
Fineness (by particle size analysis)	4.12

provides a detailed summary of the physical characteristics of the fly ash (FA) employed in this investigation. The findings reveal that the material, when introduced as a partial replacement for cement, exhibits a moisture content exceeding 10%. This relatively high moisture level is of particular importance, as it may influence the hydration kinetics and adversely affect the workability of cementitious mixtures. Moreover, the loss on ignition (LOI) analysis revealed that the organic matter content surpasses 50%, indicating a substantial presence of unburned carbon or other organic compounds. Such a high LOI value can significantly impact the pozzolanic activity of the fly ash and, consequently, its performance in construction-related applications.

The particle size distribution of the fly ash (FA) examined in this study is presented in Figure 3. Based on the corresponding distribution curve, two key parameters were calculated: the coefficient of uniformity (C_u) and the coefficient of gradation or curvature (C_c), using standard formulas (Equations (1) and (2)). The analysis yielded a C_u value of 12.7 and a C_c value of 1.6.

These findings suggest that the FA particles possess a relatively wide particle size range, as indicated by the elevated C_u value, pointing to a non-uniform distribution. In parallel, the intermediate C_c value denotes a moderately well-graded material, with a relatively balanced gradation curve. These granulometric characteristics are of particular significance, as they directly influence the packing efficiency, compaction behavior, and interparticle interactions of FA when incorporated into composite materials or applied in geotechnical contexts. Consequently, such properties are pivotal in assessing the material’s suitability for applications including soil stabilization, embankment construction, or as a filler component in cementitious mixtures.

$${\mathrm{C}}_{\mathrm{u}}={\displaystyle \frac{\mathrm{D}60}{\mathrm{D}10}}$$

(1)

$${\mathrm{C}}_{\mathrm{c}}={\displaystyle \frac{\mathrm{D}{30}^2}{\mathrm{D}60\times \mathrm{D}10}}$$

(2)

where D₆₀, D₃₀, and D₁₀ are the particle diameters corresponding to 60%, 30% and 10% fineness on the cumulative particle size distribution curve.

The findings of this investigation suggest that the examined fly ash (FA) can be classified as a “well-graded” material based on its particle size distribution characteristics [41]. This classification is supported by the calculated values of the coefficient of uniformity (C_u ≥ 4) and the coefficient of curvature (1 ≤ C_c ≤ 3), which fall within the standard thresholds for well-graded granular materials. Furthermore, the FA sample is identified as “coarse” in nature, as indicated by its median particle diameter (D₅₀) of 0.88 mm. This observation is further corroborated by microscopic analysis, which confirms the presence of relatively large and heterogeneous particles.

The underutilization of siliceous fuel fly ash (FA) in construction applications stems from a combination of unfavorable physical and chemical characteristics that limit its pozzolanic performance and compatibility with cementitious systems. The FA analyzed in this study is notably coarse (D₅₀ = 0.88 mm), which reduces its specific surface area and likely contributes to a low pozzolanic reactivity compared to conventional fly ash from coal combustion. Chemically, the material contains elevated levels of sulfur and unburned carbon (LOI > 50%), along with significant moisture (>10%), all of which interfere with cement hydration and may hinder the development of long-term strength. The presence of anhydrite (CaSO₄) further complicates its behavior in mortar systems, as it can lead to excessive sulfate availability, promoting the formation of ettringite during early hydration stages. This may affect dimensional stability, setting time, and durability, especially in uncontrolled curing conditions. Moreover, the enrichment of FA in toxic trace elements (e.g., As, Cr, Ni, Pb) raises serious environmental and health concerns related to leaching. Taken together, these issues, along with the absence of international standardization for such materials, contribute to the industry’s hesitancy to adopt siliceous FA as a reliable supplementary cementitious material.

In conclusion, the physical characterization of the investigated fly ash highlights its potential suitability as a partial cement replacement in construction-related applications. Nevertheless, the notably high moisture content and significant presence of organic residues call for meticulous handling and processing to ensure consistent performance and reliability in practical implementations.

The siliceous fly ash (FA) examined in this study demonstrated a notably high loss on ignition (LOI) value of 59.4%, indicating a considerable amount of unburned carbon. Given that carbon is inert in pozzolanic reactions, its presence effectively dilutes the proportion of reactive mineral phases. To evaluate the impact of this characteristic, an LOI-adjusted pozzolanic reactivity index was calculated, which normalizes the reactivity to the non-carbon fraction of the material. This adjusted index was determined to be 14.85%, emphasizing the pronounced inhibitory effect of unburned carbon on the pozzolanic activity of the FA. These findings suggest that, without appropriate pre-treatment or beneficiation, high-LOI fly ash is unlikely to contribute significantly to cement hydration.

Microstructural analysis of the sample (Figure 4), conducted at various magnifications, revealed several morphological features relevant to its composition and hydration behavior. At ×800 magnification, the FA appears as agglomerated masses with a coarse surface texture. Irregular dark areas and hollow structures—likely representing porous unburned carbon—are dispersed throughout the matrix. Scattered spherical particles (cenospheres) are also faintly visible. At ×1000 magnification, the surface becomes more granular, with the appearance of flaky or plate-like structures, potentially attributable to carbonates or aluminosilicates. In this view, faint bright needle-shaped formations suggest the early development of ettringite, a common hydration product formed in sulfate-rich environments. At ×2000 magnification, distinct needle-like crystals—presumably ettringite—are more clearly defined, embedded in a matrix filled with calcium silicate hydrate (C–S–H) gel and unreacted FA particles. Finally, at ×4000 magnification, the carbon-rich porous structure is more apparent, characterized by micropores and voids consistent with unburned carbon residues. The surrounding smooth and fibrous matrix is likely composed of C–S–H gel, encapsulating both residual FA particles and carbonaceous fragments.

2.2. Measurements and Tests

2.2.1. Assessment of Capillary Absorption and Porosity

The durability of construction materials is intrinsically linked to their porosity and capillary absorption (sorptivity) [42,43,44]. In this study, the porosity of laboratory-produced cement mortars was evaluated using cubic specimens with dimensions of 50 × 50 × 50 mm. The porosity measurements were conducted through the vacuum saturation method, as described in previous studies [45,46,47]. Notably, closed pores do not contribute to the transport of water or aggressive agents (e.g., chlorides, CO₂), and therefore have negligible influence on durability-related degradation mechanisms or reinforcement corrosion. Thus, the measured open porosity is more relevant for evaluating the potential of FA-modified mortars to resist transport-driven deterioration.

The experimental testing program was carried out at curing ages of 28, 56, and 90 days to monitor the evolution of mortar properties over time. Before each testing phase, the specimens were oven-dried to a constant mass, a critical step for eliminating all free water from the pore structure. This ensured that the baseline measurements reflected only the solid matrix of the mortar, thereby improving the accuracy of subsequent porosity assessments.

Once completely dried, the specimens were placed in a vacuum desiccator and subjected to a reduced pressure of approximately 10⁻¹ bar for 2 to 2.5 h. This vacuum treatment was implemented to extract air trapped within the pore network, enabling uninhibited water ingress during the next phase. Immediately after vacuuming, the samples were immersed in water at atmospheric pressure for 24 h to fully saturate the pore system. This dual-step approach—vacuum evacuation followed by water saturation—allowed for comprehensive and accurate characterization of the pore volume.

By adopting this standardized and rigorous methodology, the study produced reliable data on the porosity of mortar samples across three distinct curing ages. These results provided critical insights into the durability and long-term performance of cement mortars, especially in applications where resistance to moisture ingress and material aging is essential.

Using the liquid-saturated mass (w_sat), the immersed liquid-saturated mass (w_a) and the dry mass (w_d), key parameters such as bulk density (ρ_b), bulk volume (V_b) and porosity (f_o) were determined. These calculations employed the following equations:

$${\mathsf{\rho }}_{\mathrm{b}}={\displaystyle \frac{{\mathrm{w}}_{\mathrm{d}}{\mathsf{\rho }}_{\mathrm{w}}}{{\mathrm{w}}_{\mathrm{sat}}-{\mathrm{w}}_{\mathrm{a}}}}$$

(3)

$${\mathrm{V}}_{\mathrm{b}}={\displaystyle \frac{{\mathrm{w}}_{\mathrm{sat}}-{\mathrm{w}}_{\mathrm{a}}}{{\mathsf{\rho }}_{\mathrm{w}}}}$$

(4)

$${\mathrm{f}}_{\mathrm{o}}={\displaystyle \frac{{\mathrm{w}}_{\mathrm{sat}}-{\mathrm{w}}_{\mathrm{d}}}{{\mathrm{w}}_{\mathrm{sat}}-{\mathrm{w}}_{\mathrm{a}}}}$$

(5)

where ρ_w is the density of water (998 kg/m³).

To evaluate the capillary absorption, or sorptivity, of the mortar specimens, cubic samples with dimensions of 50 × 50 × 50 mm³ were subjected to systematic testing. Prior to the experiment, the specimens were oven-dried to ensure consistent initial conditions and eliminate any residual moisture that might interfere with accurate absorption measurements. Once dried, the samples were placed on spacers inside a shallow container, which was partially filled with methanol (Figure 5). The use of spacers ensured that only one surface of each specimen was exposed to the liquid, thereby allowing unidirectional flow during the test.

The methanol level was carefully maintained at a height of 2–3 mm above the bottom surface of each specimen to ensure precise control over the liquid absorption process. The quantity of methanol absorbed by the specimens was measured at regular intervals using a high-precision balance with an accuracy of ±0.1 mg, allowing for the detection of even the smallest changes in mass due to liquid uptake. Methanol was selected as the testing medium instead of water to prevent any potential chemical reactions between water and cementitious materials, which could alter or bias the results.

Temperature control was another critical factor in the experiment. The temperature of the methanol was continuously monitored and recorded throughout the test to ensure uniform conditions. By minimizing temperature fluctuations, external factors that could influence the sorptivity measurements were effectively controlled, ensuring reliable data collection.

The sorptivity (S) of the specimens was estimated as the slope of an i vs. t^1/2 graph, in accordance with Equations (6) and (7):

$$\mathrm{i}={\displaystyle \frac{\mathsf{\Delta }w}{a{\rho }_m}}$$

(6)

$$\mathrm{i}=\mathrm{S}\sqrt{\mathrm{t}}+\mathrm{b}$$

(7)

where Δw is the mass of liquid absorbed (g), A is the absorbing surface area (mm²), ρ_m is the density of methanol (g/mm³), i is the cumulative absorption per unit surface area (mm), t is the time (min), S is the sorptivity (mm/min^1/2) and b is the y-intercept.

2.2.2. Mechanical Properties

The performance of cementitious composites is primarily evaluated based on their mechanical properties, specifically tensile and compressive strength [48,49]. The most accurate method for determining the tensile strength of cement-based materials involves directly applying uniaxial tensile stress. However, this method requires careful preparation and execution, making it less commonly used in practice. As a result, indirect methods are often preferred due to their practicality and ease of implementation. Among these methods, splitting (Brazilian) tests and bending tests are widely used. These approaches generate tensile stresses along the failure surfaces [50], allowing for an indirect estimation of the material’s resistance to tension [51].

In this study, the flexural strength of cement mortars, prepared under controlled laboratory conditions, was evaluated using a three-point bending test. The specimens were standardized to dimensions of 160 × 40 × 40 mm to ensure uniformity across all tests. For each mortar mixture being investigated, three specimens were tested at predetermined intervals. This methodology ensured the collection of reliable and reproducible data by minimizing variability in the testing process.

After completing the flexural strength tests, compressive strength assessments were conducted on half-beam specimens derived from the fractured beams used in the bending tests. This approach maximized material usage while maintaining consistency in specimen preparation and testing protocols. A total of six samples per mixture were subjected to compressive strength testing at each designated time interval.

Both flexural and compressive strength evaluations were carried out at three specific curing periods: 28 days, 56 days, and 90 days after casting the specimens. These time intervals were chosen to monitor the development of mechanical properties over time, providing valuable insights into both the early and long-term performance characteristics of the cement mortars.

2.2.3. X-Ray Diffraction (XRD) and Microscopical Analysis (SEM)

X-ray powder diffraction (XRD) is a widely used analytical technique, renowned for its ability to quickly and accurately identify crystalline phases in various materials. In this study, XRD was employed to characterize the crystalline phases present in cementitious mortars prepared under strictly controlled laboratory conditions. To prepare the mortar specimens for analysis, they were initially fragmented into smaller pieces. Samples were carefully extracted from the interior of the specimens at a depth of approximately 1 cm below the surface to avoid contamination or any effects from surface exposure. The extracted samples were then thoroughly dried to remove any residual moisture, ensuring that no water content interfered with subsequent analyses. After drying, the samples were ground into a fine powder with particle sizes below 0.063 μm. This step was critical for achieving homogeneity and optimizing diffraction results by minimizing variability in particle size.

The XRD analyses were conducted 28 days after casting the mortar specimens. A Bruker D5000 X-ray Powder Diffraction System diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) (Figure 6—left) equipped with Cu Kα radiation (λ = 0.15418 nm) was used for these measurements. To ensure comprehensive phase identification and improve data accuracy, the samples were subjected to continuous rotation during scanning. This rotation minimized preferred orientation effects, which could otherwise distort diffraction patterns and reduce analytical precision. The scans covered a diffraction angle range of 10–80° (2θ) and were performed at a scan rate of 1°/min. These parameters ensured high-resolution data acquisition, suitable for both qualitative identification and semi-quantitative analysis of crystalline phases.

For phase identification, diffraction patterns obtained from the samples were qualitatively compared with entries in the International Centre for Diffraction Data (ICDD) PDF2 database. This comparison facilitated accurate recognition of the crystalline phases present in the mortar specimens. Additionally, semi-quantitative phase analysis was conducted using Bruker’s TOPAS software V4,2, which utilizes advanced Rietveld refinement techniques to estimate phase proportions with high precision.

The microstructural characterization of the mortar samples was performed using a JEOL JSM-6610 LV Scanning Electron Microscope (JEOL Ltd., Tokyo, Japan) (Figure 6—right). This advanced instrument offers a maximum resolution of 3.0 nanometers when operated at an accelerating voltage of 30 kilovolts under high vacuum conditions. It provides magnifications ranging from as low as 5× to as high as 300,000×, making it suitable for detailed imaging and analysis. For microstructural observations, Secondary Electron Imaging (SEI) was employed to capture detailed surface morphology. To ensure optimal imaging quality and enhance electrical conductivity during SEM analysis, the sample surfaces were sputter-coated with a thin gold layer, approximately 80 nanometers thick. This step was crucial for obtaining clear and accurate imaging results.

3. Experimental Results

For the experimental set-up, the tests were conducted under controlled laboratory conditions to ensure the reliability and repeatability of the results. Prior to testing, all equipment (e.g., strength testing machines, capillary absorption setups and XRD instruments) were calibrated according to the manufacturers’ specifications. Samples were handled with care to avoid microstructural damage, especially before mechanical and microstructural analyses. Environmental conditions such as temperature and humidity were monitored and maintained stable, particularly during tests sensitive to these variables (e.g., capillary absorption and strength tests).

3.1. Porosities and Bulk Densities

Figure 7 and Figure 8 present the average values of porosities and bulk densities (based on three samples) for the cement mortars analyzed in this study. Testing was performed at 28, 56, and 90 days after casting. The results clearly demonstrate an inverse relationship between porosity and bulk density; as porosity increases, bulk density decreases. This trend is particularly evident when comparing the properties of samples at different curing ages. Furthermore, a strong correlation was found between the porosity and/or bulk density of the tested samples and the fly ash (FA) content in the mortars. Specifically, higher FA content resulted in increased porosity and decreased bulk density. This relationship becomes especially prominent when comparing properties across different curing ages.

A notable correlation was also observed between the porosity and/or bulk density of the tested samples and the fineness of the fly ash (FA) incorporated into the mortars. Higher FA content led to an increase in porosity while simultaneously decreasing bulk density. This behavior can be attributed to various chemical and physical mechanisms related to the pozzolanic activity and particle characteristics of fly ash. The inverse relationship observed between porosity and bulk density is primarily influenced by the packing efficiency of particles within the mortar matrix. As porosity increases, the material contains more voids or air pockets, which reduces its overall mass per unit volume (bulk density). Conversely, a denser packing arrangement minimizes void spaces, thereby increasing bulk density.

Additionally, the observed results can also be explained by the increased water demand during sample preparation, which arises from the unique morphology and chemical composition of fly ash. Specifically, FA is characterized by porous carbon particles and ettringite content. These features significantly influence the hydration process and water absorption behavior. Porous carbon particles increase the surface area available for water interaction, while ettringite—a hydrous calcium aluminum sulfate mineral—further contributes to water retention due to its expansive crystalline structure formed during hydration reactions.

The relationship between the water-to-binder ratio (w/b) and porosity in cementitious materials is well established in prior research [52,53]. A higher w/b ratio typically results in increased porosity, as the excess water creates voids within the hardened matrix when it evaporates during the curing process. This effect directly influences the microstructure of the material, leading to a reduction in its density and mechanical strength. In this study, the samples containing 20% and 30% Fly Ash (FA) (denoted as 20FA and 30FA) demonstrated porosity levels greater than 35%, which is notably higher than the optimal thresholds for durable cementitious composites.

3.2. Compressive Strength

Figure 9 illustrates the compressive strength results of mortar samples tested at various curing ages. The reference mixture consistently demonstrates superior compressive strength across all curing periods. However, a noticeable decline in compressive strength is observed with the incorporation of high free lime fly ash (FA), particularly in samples containing 20% FA (denoted as 20FA) and 30% FA (denoted as 30FA). These samples exhibit compressive strengths below 15 MPa, indicating a significant reduction in mechanical performance. This reduction in compressive strength can be attributed to the delayed hydration of tricalcium aluminate (C₃A), which is caused by the formation of ettringite needles during the early stages of curing. Ettringite forms as a result of the reaction between calcium sulfate (gypsum), water, and C₃A present in cement (Equation (8)). In this reaction, ettringite (C₆AS₃H₃₂) is produced as a crystalline structure that occupies a larger volume than its reactants. This expansive nature disrupts the microstructure of the cement matrix, leading to increased porosity and reduced compactness. Consequently, these structural changes hinder the development of early-age strength. The presence of high free lime content in FA exacerbates this issue by increasing the alkalinity of the pore solution. Elevated pH levels accelerate the dissolution of alumina and silica phases from FA particles, promoting secondary reactions that further contribute to ettringite formation. Additionally, excessive free lime reacts with water to form calcium hydroxide (Ca(OH)₂), which does not contribute significantly to strength development but instead increases porosity.

The microstructural analysis (Figure 10) supports these findings, revealing an abundance of needle-like ettringite crystals within samples containing higher proportions of FA. These crystals disrupt the continuity and density of hydration products such as calcium silicate hydrate (C-S-H), which is primarily responsible for imparting strength to mortar. Moreover, prolonged exposure to high free lime content may lead to carbonation reactions over time (Equation (9)). This carbonation process results in the precipitation of calcium carbonate (CaCO₃), which further alters the pore structure and reduces long-term durability.

In conclusion, while fly ash can enhance certain properties when used judiciously, excessive replacement levels—particularly with high free lime content—adversely affect both early-age and long-term compressive strength due to mechanisms involving ettringite formation, increased porosity, and secondary chemical reactions that disrupt the microstructural integrity.

C₃A + 3CaSO₄·2H₂O + 26H₂O → C₆AS₃H₃₂

(8)

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(9)

3.3. Microstructural Analyses

Figure 10 provides compelling evidence of the presence of ettringite needles in samples 20FA and 30FA, which correspond to their higher proportions of fly ash (FA) in the mix design. In contrast, only a limited number of ettringite crystals are observed under scanning electron microscopy (SEM) in samples 5FA and 10FA. These latter samples also display hydration products such as portlandite (calcium hydroxide, C-H) and calcium silicate hydrate (C-S-H), resulting in a microstructure that closely resembles that of the reference mixture (REF). This structural similarity accounts for the acceptable physico-mechanical properties observed in samples containing ≤10% wt. FA addition. Furthermore, fly ash demonstrates significant potential to enhance mortar durability by consuming calcium hydroxide (C-H) during pozzolanic reactions. This process promotes the formation of secondary hydration products such as calcium silicate hydrate (C-S-H), which contribute to improved long-term performance characteristics [54,55].

The increased visibility of ettringite needles in samples with higher FA content (20FA and 30FA) can be attributed to the elevated availability of sulfate ions from the fly ash. These ions react with calcium aluminate phases during hydration to form ettringite crystals. Conversely, in samples with lower FA content (5FA and 10FA), the reduced sulfate ion concentration limits ettringite formation. Instead, these mixtures exhibit a predominance of primary hydration products like portlandite and C-S-H gels, which are characteristic of conventional cementitious systems. The improved physico-mechanical properties observed in mixtures with ≤10% FA replacement compared to those of 20FA and 30FA groups can be explained by their balanced microstructure. The limited addition of FA ensures sufficient pozzolanic activity without compromising the integrity or cohesiveness provided by primary hydration products. This balance results in enhanced compressive strength and durability compared to mixtures with higher FA content.

Quantitative analysis of the SEM micrographs, performed using ImageJ 1.54g software, offers a deeper understanding of the microstructural variations associated with different fly ash (FA) contents. The reference sample (REF), which contains no FA, exhibited a compact and homogenous microstructure characterized by dense calcium silicate hydrate (C-S-H) formations and minimal porosity, serving as a baseline for comparison. In contrast, samples incorporating FA displayed progressively greater structural heterogeneity. The fiber area fraction increased markedly from 5.2% in 5FA to 51.3% in 30FA, reflecting the extensive formation of needle-like ettringite crystals at higher FA levels. Similarly, the average pore area expanded from 2.3 μm² in REF and 5FA to 12.5 μm² in 30FA, indicating internal expansion caused by sulfate-driven ettringite crystallization. The average fiber diameters measured in fiber-rich specimens (20FA and 30FA) were 0.92 μm and 0.85 μm, respectively, suggesting relatively uniform ettringite morphology. Conversely, REF and low-FA samples (5FA and 10FA) demonstrated higher agglomerate densities, reaching up to 15.4 agglomerates per 100 μm², indicative of a denser, more cohesive microstructure dominated by primary hydration products such as portlandite and C-S-H. These quantitative findings align with the visual SEM observations and confirm that elevated FA dosages, due to their high sulfate content, promote the proliferation of expansive ettringite crystals and increased porosity.

3.4. Modulus of Rupture

The flexural strength of the mortars exhibits a decreasing trend with the incorporation of fly ash (FA), as depicted in Figure 11. The most significant reductions are observed in samples containing 20% FA (20FA) and 30% FA (30FA). These results are consistent with the findings from compression tests, which also indicate a decline in mechanical performance at these substitution levels. However, it is noteworthy that the flexural strength of all mortar samples improves over time. This improvement can be attributed to the substantial silicon oxide (SiO₂) content in FA, approximately 30%, which reacts with cement hydration products to form calcium silicate hydrate (C-S-H) over an extended curing period.

Furthermore, the elevated moisture content of FA, as detailed in Table 3, plays a critical role by providing additional curing water. This supplementary water acts as an internal curing agent, facilitating prolonged hydration reactions and enhancing the microstructure of the mortar matrix. The internal curing mechanism reduces heat generation during hydration and mitigates drying shrinkage, both of which contribute to improved long-term flexural strength [56]. These combined effects underscore the dual role of FA as both a pozzolanic material and a moisture reservoir within the mortar system. The reduction in drying shrinkage positively influences flexural strength more than compressive strength since flexural strength is more sensitive to shrinkage-induced cracks in cement mortars. From the experimental results, it is evident that the flexural strength (f_r) does not fully correlate with the compressive strength (f_ck) measurements. In addition to its dependence on drying shrinkage, the flexural strength of cement-based materials is influenced by supplementary cementitious materials (SCMs), which significantly affect water demand and, consequently, workability. Despite these variations, the √f_c/f_r ratio ranges from 0.65 to 1.14 (

Table 4. Square root of compressive strength (f_ck), modulus of rupture (f_r), and corresponding k coefficient values for each mortar mixture and curing age.

	28 Days			56 Days			90 Days
	fr	$\sqrt{{\mathit{f}}_{\mathit{c}\mathit{k}}}$	k	fr	$\sqrt{{\mathit{f}}_{\mathit{c}\mathit{k}}}$	k	fr	$\sqrt{{\mathit{f}}_{\mathit{c}\mathit{k}}}$	k
REF	5.80	5.76	1.01	6.03	6.03	1.00	6.04	5.88	1.03
5FA	4.58	5.08	0.90	5.48	5.36	1.02	5.59	5.31	1.05
10FA	4.49	4.77	0.94	5.07	5.16	0.98	5.81	5.09	1.14
20FA	2.70	3.31	0.82	2.98	3.84	0.78	3.59	3.76	0.95
30FA	1.77	2.22	0.80	1.79	2.74	0.65	2.02	2.52	0.80

) aligning well with findings reported by other researchers. Mohd et al. [57] report coefficient values between 0.3 and 1 for predicting the flexural strength of concrete elements. Similarly, other studies [58] and guidelines from the American Concrete Institute (ACI) suggest k values within the ranges 0.68 ≤ k ≤ 1.2 and approximately 0.67, respectively.

$$f_r=k\sqrt{f_c}$$

(10)

where f_r is the modulus of rupture (measured from the three-point bending test), f_c is the compressive strength (MPa) and k is a coefficient calculated using Equation (8).

3.5. Capillary Absorption

The capillary absorption behavior was interpreted considering transport phenomena governed by advection–dispersion mechanisms. The generalized solutions proposed by other researchers [59] for systems with time- and space-dependent sources provide a theoretical framework supporting the observed trends.

Figure 12 illustrates the outcomes of capillary absorption tests performed on various mortar samples at different curing ages. The results demonstrate a clear linear relationship between the volume of liquid absorbed per unit surface area and the square root of time, which is consistent with theoretical predictions. Furthermore, the addition of siliceous fuel fly ash (FA) significantly influences sorptivity, as evidenced by its consistent increase across all curing ages. This effect is particularly pronounced in samples containing 20% and 30% FA (designated as 20FA and 30FA), where sorptivity values consistently exceed 1 mm/min^1/2. These findings underscore the impact of FA content on enhancing water transport properties in cement-based materials.

Previous studies [60,61,62] have demonstrated that increasing the content of fly ash in cementitious composites tends to enlarge pore diameters, particularly at early ages of hydration. This phenomenon provides a plausible explanation for the heightened sorptivity observed in our study with the incorporation of fly ash. Sorptivity, which refers to the ability of a material to absorb liquid through capillary action, is influenced by the size and distribution of pores within the composite. However, as time progresses, the sorptivity of these composites decreases. This reduction can be attributed to two primary processes: hydration reactions occurring within the cement and pozzolanic reactions facilitated by the fly ash. Hydration reactions involve the chemical interaction between water and cement particles, leading to the formation of various hydration products that fill voids and reduce pore sizes. Concurrently, pozzolanic reactions occur when silica present in fly ash reacts with calcium hydroxide produced during cement hydration, resulting in additional binding compounds that further enhance the microstructure of the composite. These combined processes contribute significantly to a reduction in pore size within hardened composites over time. As pore sizes diminish, the resistance of these materials to liquid absorption improves, thereby enhancing their durability and performance in various applications.

In summary, while initial increases in fly ash content may lead to larger pore diameters and increased sorptivity, long-term effects driven by hydration and pozzolanic reactions ultimately result in reduced porosity and improved resistance to liquid absorption.

3.6. X-Ray Difraction (XRD)

Figure 13 provides a comprehensive summary of the X-ray diffraction (XRD) analysis results for the samples evaluated after 28 days of curing. As illustrated in Figure 13, there is a noticeable reduction in the calcite content within mortars containing fly ash (FA). This reduction is primarily attributed to the partial substitution of cement with FA, which contributes minimally to the overall calcium content due to its inherently low calcium carbonate composition. In contrast, mortars without FA exhibit an increased portlandite content, indicative of a slower hydration process. This observation underscores the role of FA as a pozzolanic material, actively engaging in secondary reactions with calcium hydroxide to enhance the hydration mechanism.

The X-ray diffraction (XRD) analyses also reveal a predominant presence of silicate minerals across all examined mortar samples, which is primarily attributed to the incorporation of calcareous aggregates. Notably, despite the consistent use of identical quantities of aggregates in all mix designs, a reduction in silicate content is observed with the inclusion of fly ash. This decline can be attributed to the lower cement content in mixtures containing this additive. For clarity, Figure 6 consolidates silicate minerals into a single category; however, mullite—a critical phase that significantly contributes to the pozzolanic activity of fly ash—is distinctly presented as an individual component.

Ettringite was identified as a prominent phase within the analyzed mortars, with its concentration increasing proportionally to the amount of fly ash incorporated. This trend is consistent with observations derived from scanning electron microscopy (SEM) analysis. The presence of ettringite can be attributed not only to its occurrence in the raw FA material but also to chemical interactions between sulfur compounds present in FA and tricalcium aluminate (C₃A) within the cement matrix. Additionally, water availability during the curing process significantly facilitates ettringite formation, further influencing the microstructural development of cementitious composites [63].

4. Discussion

This study provides valuable insights into the impact of incorporating siliceous fly ash (FA) on the properties of cement mortars designed for urban infrastructure applications, with particular attention to sustainability through cement reduction. The results highlight the intricate interrelationships among FA content, porosity, bulk density, mechanical strength, water absorption, and microstructural characteristics.

The SEM micrographs (Figure 3) reveal the morphological characteristics of the siliceous fly ash sample at various magnifications (×800 to ×4000). The images clearly show heterogeneous surface textures and irregular particle morphologies, typical of fly ash with high sulfur content. Spherical and angular particles are present, suggesting incomplete combustion processes. Notably, the structure includes visible carbonaceous materials and porous regions, indicating the presence of unburned carbon. These findings suggest that the fly ash sample contains significant amounts of free carbon, which may influence its reactivity and behavior in applications such as cement replacement or pozzolanic reactions. The presence of sulfur-rich particles further supports the classification of this fly ash as siliceous, potentially impacting its environmental and structural performance.

The observed inverse relationship between porosity and bulk density (Figure 7 and Figure 8) aligns with a well-established principle in cementitious materials: an increase in void spaces correlates with a decrease in mass per unit volume [64]. The significant association between higher FA content and increased porosity, coupled with a reduction in bulk density, indicates that the inclusion of FA disrupts the packing efficiency of particles within the mortar matrix. This disruption may lead to alterations in the mechanical properties and durability of the mortars, which are critical factors for their performance in urban infrastructure applications.

Understanding the relationships between fly ash (FA) characteristics and cement mortar properties is crucial for optimizing its utilization as a supplementary cementitious material. Precise control over the proportion of FA incorporated into cementitious systems allows for the targeted enhancement of specific performance attributes while simultaneously advancing sustainability objectives through a reduced reliance on traditional Portland cement.

The intrinsic characteristics of FA particles, particularly the presence of porous carbon and their relatively high surface area, contribute to an increased water demand during the mixing process [65]. Consequently, the evaporation of this excess water during the curing phase promotes enhanced pore formation within the hardened matrix, resulting in higher porosity and a corresponding reduction in density. Notably, experimental observations revealed porosity levels exceeding 35% in mortars formulated with 20% and 30% FA replacement levels, indicating potential implications for the long-term durability of these specific compositions.

The results of the compressive strength tests, as illustrated in Figure 9, indicate a significant decline in mechanical performance with increased levels of fly ash (FA) replacement, particularly at 20% and 30% substitution rates. At these elevated FA levels, the observed strength values fall below 15 MPa. This reduction in strength is primarily linked to the formation of expansive ettringite crystals during the initial stages of hydration, as represented by Equation (8) [66].

Further analysis using scanning electron microscopy (SEM), depicted in Figure 8, supports this conclusion by revealing a higher density of needle-like ettringite structures within samples containing high FA content. The presence of these ettringite formations disrupts the development of the calcium silicate hydrate (C-S-H) gel matrix, which is essential for achieving optimal strength gain in cementitious materials.

Analysis of flexural strength data (Figure 11) revealed trends consistent with those observed for compressive strength, specifically a reduction in strength with increasing fly ash (FA) replacement levels. Notwithstanding this trend, a significant improvement in flexural strength was observed over time across all mix proportions. This temporal enhancement is attributable to the pozzolanic activity of the siliceous fly ash. The pozzolanic reaction involves the consumption of calcium hydroxide (CH), a byproduct of cement hydration, by the amorphous silica (SiO₂) present in fly ash, resulting in the formation of additional calcium silicate hydrate (C-S-H) gel [67]. Furthermore, the relatively high moisture content of the fly ash (Table 3) may contribute to this long-term strength development by facilitating internal curing [68,69]. This internal moisture supply can prolong the hydration process and mitigate drying shrinkage, a phenomenon known to negatively impact the tensile and flexural performance of mortar. A consistent correlation between compressive and flexural strength was not observed, as indicated by the variability in the calculated √f_c/f_r ratios (Table 4). This finding aligns with previous studies that acknowledge that compressive and flexural strengths are governed by distinct failure mechanisms within the matrix [70].

Capillary absorption tests, as depicted in Figure 12, revealed a notable increase in sorptivity corresponding to higher fly ash (FA) content across all evaluated curing ages. This observed relationship between increased FA content and elevated sorptivity is primarily attributed to the presence of larger pore diameters within mortar mixes incorporating higher proportions of FA. This phenomenon aligns with findings previously documented in the academic literature [71]. However, a subsequent and consistent reduction in sorptivity was observed over time across all tested samples, irrespective of their FA content. This temporal decrease in sorptivity indicates that ongoing hydration of cementitious materials and the progressive pozzolanic reactions involving fly ash contribute to a refinement of the pore structure. This pore refinement process effectively reduces the connectivity and size of capillary pores, thereby leading to decreased permeability of the mortar matrix. Consequently, these findings underscore the dynamic and dualistic influence of fly ash on the transport properties of mortar, demonstrating an initial impact related to pore size distribution and a long-term effect driven by microstructural development through pozzolanic activity.

XRD analysis (Figure 13) provided valuable insights into the mineralogical transformations resulting from the incorporation of fly ash (FA). The observed reduction in calcite content within FA-blended mortars corresponds with the anticipated decrease in cement content. This finding suggests that the presence of FA may facilitate a more efficient use of materials, leading to lower calcite levels. Furthermore, the increased portlandite content noted in the reference mix may indicate a less effective early-stage pozzolanic reaction compared to the FA-modified mixes, where calcium hydroxide is actively consumed as part of the pozzolanic process [72]. The persistence of silicate phases derived from calcareous aggregate aligns with theoretical expectations regarding their stability under these conditions. These findings confirm the presence of ettringite, as previously discussed in relation to mechanical performance. Lastly, the diminished carbon content in composite samples relative to raw FA is likely attributable to carbon dissolution and leaching that occur during both mixing and curing processes.

In summary, the integration of siliceous fly ash as a partial substitute for cement emerges as an effective approach to mitigate CO₂ emissions in urban construction. This study underscores the critical importance of meticulously regulating the dosage of fly ash, particularly concerning its free lime content. Elevated replacement levels, specifically in the range of 20% to 30%, were associated with increased porosity, diminished bulk density, and significant declines in mechanical strength. These adverse effects can primarily be attributed to excessive ettringite formation, which is exacerbated by heightened concentrations of sulfate and free lime. Conversely, lower replacement levels (10% or less) achieved a more advantageous equilibrium, preserving adequate mechanical performance while simultaneously promoting environmental sustainability.

Last but not least, the gradual substitution of cement with fly ash offers a notable reduction in CO₂ emissions associated with cement production. Utilizing fly ash, a byproduct of coal combustion, not only reduces the demand for Portland cement but also promotes the recycling of industrial waste. This contributes to a lower environmental impact by decreasing the embodied carbon of the mortar. Moreover, the use of fly ash results in cost savings, as it is typically less expensive than cement. Therefore, the incorporation of fly ash provides both environmental and economic advantages in sustainable construction practices.

Future research should focus on enhancing the long-term durability of cement mortars incorporating fly ash, especially by utilizing alternative FA sources with lower free lime and sulfur content. In addition, exploring the synergistic effects of combining fly ash with other supplementary cementitious materials, such as ground granulated blast-furnace slag (GGBS), may offer improved mechanical and durability properties [73]. These strategies could further optimize the performance of blended binders while maximizing environmental benefits and ensuring structural reliability in urban infrastructure applications.

5. Conclusions

This study assessed the feasibility of utilizing siliceous fuel fly ash (FA) from the Rhenish Power Station as a partial cement replacement in mortar production, targeting its practical use in the construction industry. The experimental results showed that increasing FA content leads to reduced compressive and flexural strength, higher porosity, and increased sorptivity—mainly due to its mineralogical characteristics (e.g., ettringite formation) and its high-water demand linked to particle morphology and porosity.

Nevertheless, it was demonstrated that incorporating FA at replacement levels of up to 10% by weight of cement can yield mortar with acceptable engineering properties for practical use. To optimize performance in real-world applications, practitioners are advised to combine FA with water-reducing admixtures (e.g., plasticizers) to lower the water-to-binder ratio, enhance workability, and partially compensate for strength losses. Moreover, in applications such as non-structural masonry, pavements, or mortar layers with moderate mechanical requirements, FA-modified mixtures may offer a viable and cost-effective alternative.

For engineering implementation, it is recommended that FA undergoes pre-treatment (e.g., drying or sieving) and is used in combination with standardized SCMs to stabilize its performance variability. Guidelines for safe use should also include leaching assessments, especially due to the presence of trace toxic elements such as Ni, Cr, and Pb, to ensure compliance with environmental standards.

Future research should investigate the long-term durability of FA-modified composites under various exposure conditions (e.g., sulfate attack, carbonation, freeze–thaw cycles) and explore hybrid systems combining FA. Additional studies should also focus on enhancing the pozzolanic reactivity of FA through thermal or mechanical activation and understanding its interactions in blended binder systems.

The partial replacement of cement with up to 30% fly ash in mortar mixtures resulted in satisfactory performance while offering clear environmental and economic benefits. This approach contributes to CO₂ emission reduction and lowers production costs, supporting the development of more sustainable construction materials.

Finally, this study contributes to the broader effort toward sustainable construction by promoting the reuse of industrial by-products and supporting the reduction in CO₂ emissions from cement manufacturing, a sector responsible for approximately 7% of global anthropogenic emissions.