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Article

Evaluating Sustainable Colored Mortars Reinforced with Fly Ash: A Comprehensive Study on Physical and Mechanical Properties under High-Temperature Exposure

by
Zehra Funda Akbulut
1,*,
Soner Guler
1,
Faruk Osmanoğlu
2,
Mehmet Rıza Kıvanç
3 and
Mehran Khan
4,*
1
Faculty of Engineering, University of Van Yüzüncü Yıl, Van 65080, Turkey
2
Department of Architecture, Faculty of Architecture, Van Yüzüncü Yıl University, Van 65080, Turkey
3
Department of Health of Science, Van School of Health Sciences, Van Yüzüncü Yıl University, Van 65080, Turkey
4
School of Civil Engineering, University College Dublin, D04V1W8 Dublin, Ireland
*
Authors to whom correspondence should be addressed.
Buildings 2024, 14(2), 453; https://doi.org/10.3390/buildings14020453
Submission received: 15 January 2024 / Revised: 1 February 2024 / Accepted: 5 February 2024 / Published: 6 February 2024
(This article belongs to the Special Issue Advanced Research on Construction Materials for Sustainable Built Environment)
Browse Figures
Versions Notes

Abstract

:
This research primarily delves into a comprehensive investigation concerning the synergistic effects of fly ash (FA) with yellow pigment (YP) and red pigment (RP) in the workability, physical characteristics, and mechanical properties of colored mortars, both pre-and post-exposure to high temperatures. Within the experimental design, FA was employed as a 20% substitute for cement, while YP and RP were systematically incorporated into the cement mixtures at varying concentrations (1%, 3%, and 5% by weight). The specimens underwent controlled exposure to high temperatures, ranging from 300 °C to 800 °C. This study’s outcomes unveiled that while the introduction of FA positively influenced mortar workability, including YP and RP adversely impacted spreading diameters (SD), resulting in a discernible reduction in overall workability. Despite these effects, FA emerged as a pivotal factor to enhancing the residual compressive strength (RCS) and residual flexural strength (RFS) of the colored mortars. For instance, after 90 days at 800 °C, the control concrete (R0) exhibited a notable 66.13% decrease in RCS, and the sample solely incorporating FA (R1) demonstrated a reduced reduction of 55.39%. Similarly, mortars with YP additives (R2–R4) and RP additives (R5–R7) showcased RCS reductions within the range of 53.32% to 55.12% and 54.51% to 56.04%, respectively.

1. Introduction

Colored concrete (CC) is a remarkable innovation in the construction realm, offering a visually captivating permanence that demands meticulous attention in the material selection, shaping, placement, and finishing to achieve the desired architectural allure. By infusing liquid, granular, and powdered pigments into mortar mixtures, CC attains its distinctive hues and textures, opening doors for its widespread application in diverse settings, from plazas, shopping malls, and parks to car parks and industrial floors [1,2,3]. Despite the remarkable advantages of CC, challenges persist, notably in its elevated production costs and the preservation of chromatic properties over time. Pigments employed in CC generally comprise particles smaller than 90 μm. The coloring ability within the cement matrix elevates as pigment size diminishes. However, reducing pigment grain size escalates the concrete’s water demand, reducing its workability [4,5].
The spectrum of pigments produced in CC encompasses organic and inorganic variants [6,7,8]. Organic pigments from animal or vegetable sources exhibit high tinting strength but incur elevated expenses due to intricate production methods [9,10,11]. Contrarily, inorganic pigments, primarily metal oxides, assure exceptional lightfastness and durability. Yet, their color strength might fluctuate due to varying impurity levels, often posing challenges for high-standard applications in the concrete industry [12,13,14]. This perspective, coupled with the burgeoning demand for quality, led to the advent of synthetic inorganic pigments. These meticulously engineered pigments result from precise manufacturing processes, preventing color tone discrepancies and ensuring consistency across various concrete applications [15,16,17].
Aesthetic considerations in CC extend beyond standard concrete properties, encompassing factors like the influence of pigments on cement setting, color stability, environmental resistance, mechanical properties, and resilience against high-temperature effects. High temperatures represent a critical physical impact that jeopardizes strength and color stability in CC. Such detrimental effects can render structures inoperable, leading to potential risks to life and property. Concrete degradation under high temperatures stems from factors including shrinkage-induced cracks in the cement paste, thermal expansion of aggregates, and thermal incompatibility between these constituents [18,19]. Additional influences such as applied loads, humidity levels, and the duration and severity of high-temperature exposure further contribute to concrete degradation [20]. At temperatures above 300 °C, the dehydration of calcium silicate and aggregate thermal expansion initiate stresses within the matrix, instigating micro-sized cracks in the material. Beyond 500 °C, the decomposition of C–S–H bonds crucial for cement paste strength is accelerated. At 800 °C, a substantial portion of these bonds disintegrate, significantly compromising the concrete’s integrity. Consequently, severe microstructural transformations transpire, substantially losing strength [21,22,23,24].
Recent studies in the literature spotlighting conventional and CC behavior under diverse conditions and with various additives have emerged [25,26,27,28]. These researchers obtained different results under different load effects on samples with other components. Mazzuca et al. [21] investigated how exposure to elevated temperatures impacts the mechanical properties of polypara-phenylene-benzo-bisthiazole (PBO) fiber-reinforced cementitious matrix (FRCM) composites, a prevalent choice for reinforcing deteriorated reinforced concrete structures. The composite system integrates PBO fiber meshes into an inorganic mortar matrix, forming a robust reinforcement mechanism. This study meticulously examines the enduring tensile characteristics of PBO FRCM composites and the sustained bond performance between PBO FRCM and concrete. Conducting experimental investigations, specimens undergo heating–cooling cycles over a temperature spectrum from 20 °C to 300 °C, subsequently tested at an ambient temperature (20 °C). Direct tensile (DT) tests on PBO FRCM coupons and direct single-lap shear (DS) tests on PBO FRCM-concrete elements are executed, incorporating varying target temperature values of 20 °C, 100 °C, 200 °C, and 300 °C. The comprehensive results encompass parameters such as cracking stress, tensile strength, ultimate tensile strain, uncracked and cracked elasticity moduli, failure mode, bond strength, and corresponding slip values. The findings revealed a nuanced and parameter-specific response to exposure at different temperatures, with substantial reductions in values noted primarily at 300 °C compared to those measured at ambient temperature. This study establishes temperature-dependent relationships for both tensile and bond mechanical parameters, providing valuable insights applicable to thermo-mechanical simulations and fire design considerations for reinforced concrete structures fortified with PBO FRCM composites. A thorough comparative analysis is conducted between the experimental results presented in this study and the existing literature, contributing to a holistic understanding of the material’s performance under diverse thermal conditions. Jesus et al. [25] delve into producing low-carbon cement for CC from ternary mixtures, emphasizing reduced CO2 emissions and enhanced pigmentation capabilities. Jesus et al. [25] stated that in developing countries with high housing deficits, building social housing using multi-purpose, colored concrete coatings produced from low-carbon types of cement, which do not contain mortar and paint, can provide a new short-term solution. The availability of light and kaolinite-rich tailings in the Amazon make it possible to produce low-carbon cement (LCC) from ternary blends. The most crucial aim of their study was to evaluate the performance of colored concretes obtained from LCC against the weather conditions of the rainy equatorial climate of the Amazon, which promotes the formation of biofilm films that deteriorate the aesthetic quality of facades. LCC has provided colored concretes with good pigmentation capacity and significantly reduced CO2 emissions. However, they stated that its use compromises the surface quality of concrete due to low chromatic stability. The climatic conditions of the Amazon and the low alkalinity of LCC favor the growth of microorganisms, leading to premature darkening of concrete surfaces, lowering their albedo, and consequently increasing the surface temperature of the substrate, compromising the thermo-energetic performance of concrete. However, they noted that the results were promising and justified continued research using new limestone-calcined clay cement compositions in conjunction with more efficient surface protection systems. Jang et al. [26] used White Portland Cement (WPC) and inorganic pigment in colored concrete mixtures. In addition, the color expression properties and physical properties of the colored mortar using Granular Ground Blast Furnace Slag (GGBFS) were investigated and generally compared with the properties of colored mortars with WPC additives. The results showed that the GGBFS addition rate significantly affected the color value efficiency of colored mortar. They stated that the total portlandite (CH) amount in the colored mortar used in GGBFS was smaller than in the WPC colored mortar, and this contributed to the reduction of efflorescence on concrete surfaces and gave the mortar mixtures a more visible color. They noted that the flow of colored mortar decreased with mixing inorganic pigments but increased proportionally with the addition rate of GGBFS. They also stated that although the strength of GGBFS-colored mortars in long-term aging (after 28 days) was generally higher than that of WPC-colored mortar samples, its strength developed more slowly at early ages.
Similarly, Junak et al. [27] explored water absorption and compressive strength changes in CCs incorporating yellow, red, and brown pigments with fly ash, observing altered properties compared to control concrete. This study aims to obtain information about the effects of pigment color on selected properties (total water absorption and compressive strength) of concrete samples. The initial intention has been extended by replacing cement with biomass FA. These researchers noted that using pigment caused a slight increase in the total water absorption of the tested samples. Of the three colors used, brown pigment had the most minor effect on increased water absorption. They also stated that the brown pigment positively impacted the development of the compressive strength of the samples and reached the highest strength among all the samples. The results also showed that the combination of colored pigment and FA had an interesting synergistic effect, as evidenced by a compressive strength of 45 MPa and a 15% reduction in water absorption.
Pyeon et al. [28] investigated high-strength colored mortars based on silica fume with green and red liquid pigment additives, noting variations in compressive strength and color stability with different pigment contents and ages. Pyeon et al. [28] tried to predict the mechanical properties and pigment-induced changes in the physical properties of pigment-containing ultra-high strength cement composites (UHSCCs) by thermal and X-ray diffraction analyses. Hydrates in the samples were analyzed using thermogravimetry. Additionally, the change in color expression with UHSCC age was examined by L*a*b* analysis. Correlation analysis was performed to determine linear relationships between experimental factors by calculating R2. A change in hydrate expression was confirmed as potency increased with age. The pigment used has been shown to affect the shift in hydrate expression and color development. Correlation analysis of the results for all ages revealed that the 5% red pigment mixture gave the highest R2 value of 0.9858 in density-a*. The 10% red pigment mixture gave the lowest R2 value of 0.5229 as density-b*. It was concluded that, depending on the amount of pigment used, quantitative results can be obtained by considering L* (contrast) rather than the relationship between intensity and color components. They stated that the appropriate mixing ratio based on the density expression of the red pigment varies between 3% and 8% and is inversely proportional to the green pigment density and strength.
The prevalent use of Portland cement (PC) in traditional concrete poses significant environmental challenges, contributing to a substantial portion of global CO2 emissions. To address this, incorporating mineral additives like fly ash (FA) is crucial for sustainable concrete formulations. FA, known for its pozzolanic activity, fills voids in the concrete matrix and enhances strength, offering a sustainable alternative to PC. While CC is widely used for its aesthetic appeal and practicality, potential changes in its properties due to high-temperature exposure, especially in fire incidents, require thorough investigation. This multifaceted investigation aims to fill knowledge gaps related to mineral additive integration, particularly the combined use of FA and pigments. This study seeks to understand the influence on CC’s workability, resilience, and structural robustness across various environmental conditions. By utilizing experimental methodologies, this study strives to reveal the intricate changes occurring within the concrete matrix, providing insights for informed decisions in sustainable construction practices with an aesthetic focus.

2. Research Significance

Within the sustainability framework, the prevalent use of PC as the primary binding agent in CC poses significant environmental challenges, contributing to 5–7% of global CO2 emissions from PC production [29]. Integrating mineral additives like fly ash (FA) becomes imperative in cement formulations for sustainable practices to counteract the adverse environmental impacts associated with PC. FA, with its inherent pozzolanic activity and filler effect, not only fills voids in the concrete matrix but also enhances its strength [30]. Its spherical grain structure notably augments the workability of freshly blended concrete. The undeniable aesthetic appeal and necessity of CC, especially for exterior applications in architecturally striking designs, are evident. However, the potential alterations in surface attributes and structural integrity of CC due to high-temperature exposure, particularly in fire incidents, warrant thorough investigation [31]. More comprehensive research is needed to investigate the holistic changes in the physical and mechanical properties of CC integrating mineral additives under ambient conditions and after exposure to elevated temperatures. Hence, this study aims to capitalize on the beneficial properties of FA in tandem with yellow pigment (YP) and red pigment (RP), assessing their combined impact on the physical and mechanical traits of CC before and after exposure to high temperatures. The initial phase delineates how FA influences spreading diameters, thereby evaluating mortar workability when paired with YP and RP. Subsequent stages involve a detailed analysis of surface damages, color shifts, mass losses, and alterations in residual compressive strength (RCS) and residual flexural strength (RFS) capacities of samples subjected to high temperatures under ambient conditions.
Moreover, the study progresses to a deeper analytical phase, employing X-ray diffraction (XRD) and scanning electron microscopy (SEM) on select samples. These advanced analytical techniques aim to elucidate alterations in crystal structures and internal properties triggered by high-temperature exposures, providing nuanced insights into the evolving nature of CC under extreme thermal conditions. This multifaceted investigation seeks to bridge knowledge gaps surrounding mineral additive integration, particularly FA in conjunction with pigments, and its multidimensional influence on CC’s workability, resilience, and structural robustness across varied environmental settings. By employing cutting-edge analytical methodologies, this study endeavors to unravel the intricate changes occurring within the CC matrix; thus, empowering informed decisions in sustainable construction practices with an aesthetic edge.

3. Experimental Program

3.1. Materials Used in the Colored Mortar Mixtures

Yellow and red inorganic synthetic iron oxide pigments, supplied in powder form, were used as pigment materials. YP and RP grain sizes varied between 0.1–0.8 μm and 0.3–1 μm, respectively. A CEM I 42.5 R PC with high early strength was used as a binder in the mortar mixtures. F-class FA material, obtained from İsken Sugözü Thermal Power Plant, was employed as the pozzolanic additive material, and standard sand with 0–2 mm grain size was used as aggregate. The Blaine-specific surface areas of CEM1 42.5 R and FA are 3448 cm2/g and 3820 cm2/g, respectively. Brunauer–Emmett–Teller (BET)-specific surface areas of YP and RP were determined as 57.2 m2/g and 35.4 m2/g. The superplasticizer (SP) used in this study to achieve better workability is a new-generation polycarboxylate-based material. The view of the FA, YP, and RP used in the colored mortar mixtures is shown in Figure 1. An XRD graph provides crucial information for determining the crystal structural characteristics of a material. XRD peaks signify the presence of crystal order and atomic arrangement. In the XRD graph of FA, distinct peaks often indicate diffractions from minerals such as aluminosilicate phases, quartz, and mullite. In addition, it is seen that YP has goethite, and RP mostly has hematite peaks. The positions of these peaks reveal the mineralogical composition and crystal structures of the material.
The FA, YP, and RP images were obtained with an XRD and SEM analysis under ambient conditions, as shown in Figure 2 and Figure 3, respectively.
The chemical properties of PC and FA were specified with the help of X-ray fluorescence (XRF) analysis and are given in Table 1.
The chemical properties of YP and RP obtained from XRF analysis are presented in Table 2. The physical properties of YP and RP obtained from the manufacturer are given in Table 3.

3.2. Preparing Colored Mortar Mixtures

A total of eight mixtures, including one control and seven FA, YP, and RP-added mixtures, were prepared by EN 196-1 [32]. FA was used in all mixtures except the control mixture, replacing it with cement at a rate of 20% [33,34,35]. YP and RP synthetic inorganic pigments were used at 1%, 3%, and 5% of the cement by weight. The mixtures are R0, the control mixture; R1, with 20% FA substituted with cement; mixtures with YP pigment additives are named R2–R4; and RP pigment additives are called R5–R7. The names of the mixtures and the quantities of materials used in their preparation are presented in Table 4.
A total of three samples from each sample were tested, and the average was taken. The preparation of the mixtures was conducted in an automatic cement mixer. First, PC and FA were mixed dry in the mixtures for 2 min. After SP was added to the mixing water and mixed, half of the mixing water was put into the mixture and stirred for another 1 min. Then, YP and RP synthetic inorganic pigments were added, and the mixtures were stirred for another 2 min. In the last stage, the other half of the mixing water and SP was put into the mixtures and mixed for another 1 min. SP was used at 0.5% of cement by weight in all mixtures. This ratio was determined with trial and error methods to obtain good workability at the experimental stage. After mixing the mortars, spreading experiments were carried out on fresh mortars on the mini-spreading table. Following this, fresh mortars were filled into prismatic molds of 40 × 40 × 160 mm3 size. The samples were removed from the mold after 24 h and kept in the curing pool, saturated with water, at ambient conditions for 28 days. The samples taken from the curing pool were held in the oven at 105 °C for another 24 h and then dried in the oven. Then, the first measurements were made under ambient conditions. After being kept in a high-temperature oven at 300 °C, 500 °C, and 800 °C for 2 h, the samples were removed, and the relevant tests were repeated. The temperature increase rate in the automatic high-temperature oven was adjusted to 5 °C/min.

3.3. Performing Tests of Mortars

In the experimental phase, the spreading diameters (SD) values of the fresh mixtures were first measured on the automatic mini-spreading table. During the experiment, mortar mixtures were filled into the truncated cone on the flow table. The filling process was implemented in 2 layers and by applying 25 compression operations in each layer. After the truncated conical mold was slowly lifted, it was struck 60 times by rotating the shaking device per EN 1015-3 [36]. Finally, the SD values in both directions of the spread mortar on the table were measured, and the average of these values was taken. Then, under ambient conditions and after high-temperatures effect, surface and color changes, mass loss, compressive, and flexural tests were carried out on the 40 × 40 × 160 mm3 prismatic specimens. Although the samples’ color changes, surface damages, and mass losses were determined on 28-day samples, the RCS and RFS capacities were determined at 28 and 90 days. We carried out a single-point bending test on the 40 × 40 × 160 mm3 hardened samples following the EN 1015-11 standard [37]. We kept applying force steadily at 50 ± 10 N/s until the samples broke. This helped us understand how well the material could withstand bending stress. After that, we performed a compressive strength test on the identical specimens, splitting them into two parts based on the earlier flexural test. Using 40 × 40 mm wide caps, we applied a load at a rate of 500 N/s until the samples broke during the compressive strength test by the EN 1015-11 standard [37]. To obtain a more detailed look at the compressive strength, we performed a pressure test on six half samples from each series that resulted from the flexural test. We calculated the compressive strength value by averaging the results of these pressure tests. This straightforward testing approach, following standard procedures, gives us a clear picture of how the material behaves under bending and compression, offering valuable insights into its mechanical performance under specific loads. XRD and SEM analyses of the samples were carried out on 90-day samples. The samples’ mass losses (ML) were obtained using the relevant equation in Equation (1). Here, K0 and Ki are the measured weights of the examples under ambient conditions and after high temperatures, respectively:
M L = K i − K 0 K 0   × 100
Color measurements of the examples were performed according to ASTM D2244-21 [38] standard. The color changes in the samples were obtained using a PCE-brand calorimeter test device. The color properties of the samples are expressed as L*, a*, b* and E* in the CIELAB color system. A total of five color measurements were made for each sample, and three were used for each mixture. A mask with horizontal and vertical quadrants was used to guarantee that measurements routinely occurred at the same points on the specimen surface. L parameter; brightness (L: 0 black, L: 100 white), parameter a indicates redness–greenness (a > 0 red, a < 0 green), parameter b shows yellowness–blueness (b > 0 yellow, b < 0 blue). Color measurement values of the samples were obtained using the relations in Equations (2)–(5) as follows:
∆ E * a b = ∆ L * 2 + ∆ a * 2 + ∆ b * 2
∆ L * = L * 1 − L * 0
∆ a * = a * 1 − a * 0
∆ b * = b * 1 − b * 0
here, L*1, a*1 and b*1 are the values obtained from the samples after heat treatment, and L*0, a*0 and b*0, are the reference values obtained from the samples before the heat treatment. A schematic view of the CIELAB color space system is shown in Figure 4.
In the next step, XRD and SEM images of some of the selected samples were obtained under ambient conditions and at 300 °C, 500 °C, and 800 °C temperatures. SEM images of FA, YP, and RP samples were taken under 1 × 5000 magnification under ambient conditions. SEM images of R0, R1, R4, and R7 samples were taken under ambient conditions and after each temperature value under 1 × 1000 magnification. XRD analyses of R0, R1, R4, and R7 samples were performed to determine the mineral composition of the selected samples before and after the impact of high temperature. The prepared powder samples mineralogical analysis and phase contents were determined with Cu Kα radiation at 2°/min in the angle range 2–70°. A schematic representation of the preparation and testing of samples is displayed in Figure 5.

4. Results and Discussion

4.1. Spreading Diameters of Colored Mortars

The SD values of the fresh mortars play a crucial role in understanding their workability. Higher SD values typically indicate easier handling and placement of mortars. In our study (Figure 6), the control mixture (R0) exhibited an SD value of 203 mm. Adding 20% FA (R1) increased this value to 225 mm, demonstrating an improved workability attributed to FA’s spherical grain size [39,40,41]. However, observations differed with the inclusion of pigments. For mixtures with 1%, 3%, and 5% YP (R2–R4), SD values decreased to 214, 202, and 184 mm, respectively, compared to R0 and R1. Similarly, mixtures with 1%, 3% and 5% RP (R5–R7) showed SD values of 220 mm, 209 mm, and 195 mm, respectively. These values were lower than R0 and R1 but slightly higher than R2–R4. YP and RP additions, alongside FA, reduced the SD values of fresh mortars. The substantial specific surface areas of YP (57.2 m2/g) and RP (35.4 m2/g) significantly impacted mortar cohesion, reducing fluidity and subsequently decreasing SD values [42,43,44]. With its higher specific surface area than RP, YP notably led to even lower SD values in the mortars. Furthermore, particle shape and fineness influenced SD values. RP’s spherical shape positively impacted workability compared to YP’s needle shape. Additionally, the finer grain size of YP (1 mm) increased water demand and absorption, resulting in lower SD values compared to RP (3 mm) in the mortars [45,46].

4.2. Surface Damages and Color Changes of Samples

Examining samples before and after exposure to high temperatures (Figure 7) revealed notable observations. Despite an 800 °C impact, no fragmentation or rupture was evident. Instead, as seen in Table 5 and Figure 8, a rough, porous outer structure with visible skinny cracks formed on most of the samples, accompanied by substantial color changes observable through the CIELAB system [47,48,49].
Post-high-temperature exposure, the color shifts in mortar samples offer insights into the thermal influence on mechanical properties like compressive strength [50,51]. Specifically, in our study, samples with YP additives (R2–R4) exhibited heightened sensitivity to high temperatures, displaying more significant color shifts than the FA-only R0 and R1 and RP-added R5–R7 samples. At 300 °C, R0 and R1 transitioned from gray to yellowish–gray, hinting at temperature exposure. At 500 °C, they showed a slight pinkish hue, and by 800 °C, they moved toward a whitish–gray shade. The YP-added R2–R4 turned yellow at 300 °C, intensifying to pinkish–reddish at 500 °C, and whitish–gray at 800 °C. Similarly, the RP-added R5–R7 turned slightly yellow and pinkish at 300 °C, evolving to darker red at 500 °C, and eventually to a whitish–gray surface at 800 °C. Table 5 and Figure 8 demonstrate a surge in ∆E, denoting overall color changes as the temperature increased. For instance, at 300 °C, R0 displayed an ∆E of 4.62, whereas the YP-doped R2–R4 ranged between 3.74 and 5.49, and the RP-doped R5–R7 varied between 4.53 and 5.37. At 500 °C, these values dramatically escalated, especially in the YP-doped samples, showcasing substantial color transitions.
Interestingly, the YP-doped mortars showed more significant color variations than the RP-doped or control samples. YP additives, particularly at 500 °C, led to a drastic transition from yellow to red, while at 800 °C, both YP and RP samples exhibited a whitish tone, although the YP-doped samples had higher ∆E values than the RP-doped ones. This study highlights YP’s pronounced impact on color alterations in high-temperature environments, especially evident at 500 °C, although at 800 °C, differences persist, and the intensity slightly reduces compared to 500 °C.

4.3. Mass Losses of Specimens

The primary reason for mass loss (ML) in mortar samples under high temperatures is water vaporization within the mortar. At 300 °C, there’s relatively less vaporization of free and chemically bound water, but this intensifies notably at 500 °C and especially at 800 °C, leading to increased ML in the samples [52,53,54]. Figure 9 displays the ML values post-high-temperature exposure, illustrating an apparent rise in ML with escalating temperatures. At 300 °C, the ML values for R0, R1, YP-added R2–R4, and RP-added R5–R7 ranged between 2.08% and 2.31%. These values increased notably at 500 °C (5.42% and 5.58%) and further at 800 °C (9.52% and 9.82%). Interestingly, incorporating FA, YP, and RP did not mitigate ML values in the samples. Moreover, increasing YP and RP from 1% to 5% by weight did not reduce ML. Additionally, using differently shaped and structured pigments (YP and RP) did not demonstrate superiority over one another in improving ML values in the mortar samples.

4.4. Residual Compressive Strength

The mortar samples’ 28- and 90-day RCS capacities before and after exposure to high temperatures are depicted in Table 6 and Table 7, and Figure 10a,b. The impact of high temperatures, specifically at 300 °C, 500 °C, and 800 °C, on mortar samples’ RCS was meticulously analyzed. The observations highlighted significant reductions in RCS following exposure to elevated temperatures. Noteworthy is that select samples exhibited marginally higher RCS after exposure to 300 °C compared to their ambient counterparts. Comparative analysis of the 28-day RCS capacities unveiled a trend where the control specimen (R0) experienced relatively lesser reductions than the FA-added specimens (R1–R7). This disparity can be attributed to the inclusion of FA in the mortar, which notably filled internal structural gaps, imparting enhanced impermeability, and strength. However, the sluggish pozzolanic activity of FA significantly curtailed additional strength gains, primarily influencing the early-age compressive strengths of the mortar samples, while offering marginal enhancements to the 28-day strength.
Further examination at the 90 day mark demonstrated more pronounced reductions in RCS for the control sample (R0) than the FA-added samples (R1–R7). The delayed pozzolanic reaction of FA, reliant on free lime formation, initially contributed to slower strength development but eventually matched or exceeded the control specimen’s strength at later stages. Notably, at 90 days, the FA-added samples exhibited less significant reductions in RCS following exposure to high temperatures, especially at 500 °C and 800 °C, compared to the control sample (R0). Comparative analysis of the samples integrated with yellow and red pigments (YP and RP) against the control (R1) revealed a positive influence on enhancing compressive strengths, notably at the 90-day interval, with YP exhibiting a more substantial impact. This discrepancy in effectiveness stems from YP’s finer grain size, allowing for better micro-filling effects within the mortar structure than RP. However, the efficacy diminished notably when YP and RP were used in higher 5% ratios.
In summary, the study underscores the positive impact of YP and RP on compressive strengths, particularly at later stages. YP demonstrates more promising effects owing to its finer grain size. This finer granularity enabled YP to contribute significantly to improved structural integrity and strength of the mortar post-exposure to high temperatures.

4.5. Residual Flexural Strength

The mortar samples’ 28- and 90-day RFS capacities before and after exposure to high temperatures are depicted in Table 8 and Table 9, and Figure 11a,b. Figure 11a,b illustrates a notable reduction in RFS capacities as the temperature increased. While no significant decreases were observed at 300 °C, substantial reductions occurred at 500 °C and 800 °C. Referring to Table 8 and Figure 11a, the 28-day RFS values of samples R1–R7 with FA, YP, and RP additives exhibited a slightly higher decrease than the R0 control sample. Specifically, after exposure to 800 °C, the 28-day RFS decreased by 64.57% in the R0 control example, 68.85% in the R1 sample, between 65.18% and 71.26% in the YP-doped R2–R4 samples, and between 72.68% and 75.86% in the RP-doped R5–R7 samples [55,56,57]. This can be attributed to the delayed pozzolanic reaction of FA, hindering the formation of new C–S–H bonds crucial for additional flexural strength within 28 days. Moreover, as observed in Table 8 and Figure 11a, the RFS reduction in the R0 control example after high-temperature influence is less pronounced than in the FA-added R1–R7 samples. This phenomenon is linked to the completion of FA’s pozzolanic reactions at later stages, fostering increased RFS capacities by forming new C–S–H bonds via chemical transformations with portlandite (CH) [58,59,60].
In examining Table 9 and Figure 11b, the 90-day RFS values post-exposure to 800 °C exhibited a decrease of 72.04% in the R0 control example, 61.46% in the R1 sample, between 60.25% and 61.84% in the YP-doped R2–R4 examples, and between 61.20% and 63.75% in the RP-doped R5–R7 examples. Comparing the YP and RP-added mortar examples, YP demonstrated a superior filler effect due to its finer grain size, marginally enhancing RFS capacities compared to RP [61,62,63]. Furthermore, akin to the RCS results, utilizing YP and RP at 1% and 3% levels enhanced the RFS capacity of the samples by over 5%.

4.6. XRD Analysis

The alterations in crystal phases of the R0, R1, R4 and R7 samples under ambient conditions and subsequent exposure to high temperatures were determined through X-ray diffraction (XRD) analysis, as illustrated in Figure 12a–d. The primary phases in all samples under ambient conditions were identified as portlandite (CH), calcium–silicate–hydrate (C–S–H), and quartz (Q) [64,65]. However, distinct from the R0 control sample, the FA-doped R1, R4, and R7 samples exhibited crystal phases of calcite (CaCO3) and mullite (Al6Si2O13) in addition to these primary phases [66,67]. Further, the YP and RP-doped R4 and R7 samples showcased crystal phases of goethite (FeOOH) and hematite (Fe2O3), respectively, alongside the earlier mentioned phases [68,69]. Following exposure to 800 °C, a substantial decrease in C–S–H peaks was evident across all samples, while quartz and mullite peaks exhibited relatively higher intensities. The diminishment of C–S–H phases emerged as a significant factor contributing to the decreased RCS and RFS capacities at 800 °C compared to 300 °C or 500 °C. Specifically, the notable prevalence of mullite peaks at 300 °C and 500 °C, significantly higher than 800 °C, is noteworthy. Mullite is acknowledged for contributing to superior mechanical properties, stability, low thermal conductivity, and limited expansion. This might elucidate the comparatively low RCS and RFS capacities observed in FA-doped R1, R4, and R7 samples at 800 °C [70,71]. Furthermore, a discernible decrease in the goethite and hematite crystal phases was observed in YP and RP-added samples under the influence of 800 °C. This decline significantly contributed to the diminished RCS and RFS capacities, particularly noticeable in the YP and RP-added R4 and R7 samples, which originally possessed goethite and hematite crystal phases, respectively, especially after exposure to 800 °C [72,73].

4.7. SEM Analysis

When subjected to high-temperature conditions, mortar samples incorporating PC and FA as binders undergo substantial external and internal damage. Thermal expansion of aggregates within the mortar mix, triggered by water vaporization due to high temperatures [74,75,76], induces shrinkage in the cement paste [77,78,79]. Consequently, water vaporization leads to critical cracks within the cement paste due to the pressure exerted. An elevated temperature intensifies water vaporization, thereby increasing water vapor pressure and widening cracks within the sample’s internal structure, spreading throughout the matrix [80,81,82,83,84]. SEM analysis captured images of R0, R1, R4, and R7 samples under ambient conditions and subsequent exposure to high temperatures, depicted in Figure 13.
The images illustrate a progressive increase in internal structural damage with rising high-temperature effects, accompanied by an escalation in the number and size of cracks. Samples, initially stable under ambient conditions, succumbed to damage and structural instability with increased temperature. Cracks, initially micro-sized and fewer under ambient or 300 °C conditions, amplified significantly in number and macro-size under the influence of 500 °C and notably 800 °C. Micro-sized cracks initially formed and increased within the matrix, coalescing into more prominent macro-cracks that appeared elongated, more comprehensive, and deeper, significantly compromising integrity [85]. This loss of structural integrity and the emergence of macro-cracks post-high-temperature exposure markedly reduced the residual compressive and flexural strengths, particularly noticeable after exposure to 500 °C and 800 °C.
In contrast to the R0 control sample, FA exhibited a filling effect in the R1, R4, and R7 samples, ameliorating matrix gaps, and slightly reinforcing internal structural integrity. Similarly, YP and RP in the R4 and R7 samples exhibited a filling effect akin to FA, leading to fewer and more minor cracks after high-temperature exposure. Consequently, the R1, R4, and R7 samples showcased relatively diminished crack sizes and numbers compared to the R0 control sample post-high-temperature exposure, aiding in the preservation of their structural integrity to some extent.

5. Conclusions

This study investigated the impact of fly ash (FA) alongside two pigment types, yellow pigment (YP) and red pigment (RP), on the workability, physical, and mechanical properties of mortars. The findings yield several key observations:
1.
Using FA in conjunction with YP and RP proves efficient, primarily enhancing the workability of freshly mixed mortars. Subsequently, after exposure to elevated temperatures, a modest improvement was observed in the RCS and RFS of samples.
2.
FA’s spherical grain structure contributed to the improved workability of fresh mortars, resulting in higher spreading diameter (SD) values. However, the addition of YP and RP, despite enhancing mortar cohesion, led to a decreased workability, evidenced by reduced SD values. Despite its needle-like structure, YP affected workability more adversely than RP, which possesses a naturally spherical grain structure.
3.
After being subjected to elevated temperatures, the mortar specimens displayed noticeable surface degradation, characterized by the emergence of cracks. It is essential to highlight that despite this degradation, there was no observed fragmentation or rupture in the specimens. The manifestation of cracks suggests a susceptibility of the mortar to thermal stress, leading to structural changes on the surface.
4.
The ML observed in concrete exposed to high temperatures can be attributed to several factors. Firstly, the water content within the concrete evaporates under elevated temperatures, separating it from the concrete matrix and resulting in ML. Additionally, the thermal decomposition of organic and inorganic components present in the concrete can induce changes in the material’s structure, contributing to ML. The combined application of FA, YP, and RP exhibited limited efficacy in attenuating the ML values of the specimens after exposure to elevated temperatures.
5.
Prominent alterations in color were discernible in the specimens, particularly following exposure to temperatures of 500 °C and 800 °C. Remarkably, specimens with the addition of YP exhibited more pronounced total color changes (ΔE) than those with RP.
6.
The influence of FA on the mortar samples’ RCS and RFS properties following exposure to elevated temperatures is particularly noteworthy. This enhancement can be attributed to FA’s unique filler effect and ability to form new calcium–silicate–hydrate (C–S–H) bonds due to its high pozzolanic activity. The positive impact of FA on the mechanical properties becomes even more pronounced in the 90-day samples, where the completion of pozzolanic reactions is more apparent compared to the 28-day samples. The extended duration allows for the full utilization of FA’s pozzolanic potential, resulting in a more substantial improvement in the compressive and flexural strengths of the mortar. This observation underscores the long-term effectiveness of FA in enhancing the structural resilience of mortar, particularly under high-temperature conditions, offering insights into the temporal evolution of its beneficial effects on the material’s properties.
7.
The YP and RP played a crucial role as fillers, effectively addressing voids within the internal structure of the mortar samples. This filler function significantly enhanced RCS and RFS capacities. With its finer grain size, YP demonstrated a marginal superiority over RP in augmenting these mechanical properties. The finer particles of YP exhibited a more efficient filling of voids, leading to a more pronounced improvement in compressive and flexural strengths compared to the RP.
8.
The utilization of YP and RP at varying ratios, precisely at 1%, 3%, and 5%, has been examined regarding their effectiveness in restoring RCS and RFS capacities following exposure to high temperatures. The findings indicate that employing YP and RP at the 1% and 3% ratios resulted in more favorable outcomes in restoring RCS and RFS capacities when compared to the 5% ratio. This observed trend suggests that the lower percentages of YP and RP exhibit a more efficient contribution to the post-high-temperature resilience of the mortar. Although not devoid of benefits, the higher concentration of pigments at the 5% ratio appears to reach a threshold beyond which the stimulating effects on compressive and flexural strengths diminish. This nuanced understanding emphasizes the importance of optimizing pigment ratios to achieve an optimal balance between enhancing mechanical properties and mitigating the adverse effects of high-temperature exposure. Further exploration into the intricate relationships between pigment ratios and the restoration of strength capacities provides valuable insights for developing resilient concrete formulations in the face of elevated temperature challenges.
9.
This study explores the performance of colored mortars, blending YP and RP with FA, especially when exposed to high temperatures. While offering valuable insights, the study has room for improvement and suggests future research directions. Additional properties like thermal conductivity and tensile strength should be considered to provide a more comprehensive understanding. Diverse combinations of FA types and pigment ratios could enrich this study’s scope. A closer look at workability, application aspects, and sustainability considerations, including environmental impacts and recyclability, is crucial. Future studies may also investigate the influence of different temperature ranges, not limited to high temperatures. Addressing these aspects in future research will enhance our knowledge and contribute to developing sustainable and high-performance colored mortars in the construction industry.

Author Contributions

Conceptualization, Z.F.A.; methodology, Z.F.A., S.G., M.R.K. and M.K.; software, Z.F.A. and F.O.; validation, M.K.; formal analysis, S.G. and M.R.K.; investigation, S.G.; resources, F.O.; data curation, F.O.; writing—original draft, Z.F.A.; writing—review and editing, S.G., F.O. and M.K.; visualization, M.R.K.; supervision, S.G., M.R.K. and M.K.; project administration, M.R.K. All authors have read and agreed to the published version of the manuscript.

Funding

This study benefited from the backing of the Van Yüzüncü Yıl University BAP unit, under project ID FCD-2023-10468. Their support was instrumental in facilitating and advancing the research conducted in this investigation.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The view of (a) FA, (b) YP and (c) RP.
Figure 1. The view of (a) FA, (b) YP and (c) RP.
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Figure 2. The XRD patterns of (a) FA, (b) YP and (c) RP.
Figure 2. The XRD patterns of (a) FA, (b) YP and (c) RP.
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Figure 3. The SEM images of (a) FA, (b) YP and (c) RP.
Figure 3. The SEM images of (a) FA, (b) YP and (c) RP.
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Figure 4. CIELAB color space and sample color measurement view. (a) CIELAB, (b) mask and (c) color measurement.
Figure 4. CIELAB color space and sample color measurement view. (a) CIELAB, (b) mask and (c) color measurement.
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Figure 5. Schematic representation of the stages of preparation and testing of samples.
Figure 5. Schematic representation of the stages of preparation and testing of samples.
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Figure 6. SD diameters of R0, R1, R4 and R7 fresh mortars.
Figure 6. SD diameters of R0, R1, R4 and R7 fresh mortars.
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Figure 7. Surface damages and color changes of R0–R7 samples at ambient and after high temperatures.
Figure 7. Surface damages and color changes of R0–R7 samples at ambient and after high temperatures.
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Figure 8. CIELAB parameters obtained for R0–R7 samples.
Figure 8. CIELAB parameters obtained for R0–R7 samples.
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Figure 9. Mass loss values of R0–R7 samples at 28 days.
Figure 9. Mass loss values of R0–R7 samples at 28 days.
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Figure 10. RCS values of R0–R7 samples (a) 28 days (b) 90 days.
Figure 10. RCS values of R0–R7 samples (a) 28 days (b) 90 days.
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Figure 11. RFS values of R0–R7 samples (a) 28 days (b) 90 days.
Figure 11. RFS values of R0–R7 samples (a) 28 days (b) 90 days.
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Figure 12. XRD patterns of R0, R1, R4 and R7 samples at ambient and after high temperatures.
Figure 12. XRD patterns of R0, R1, R4 and R7 samples at ambient and after high temperatures.
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Figure 13. SEM images of R0, R1, R4 and R7 samples at ambient conditions and after high temperatures.
Figure 13. SEM images of R0, R1, R4 and R7 samples at ambient conditions and after high temperatures.
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Table 1. The chemical, physical, and mechanical properties of PC and FA.
Table 1. The chemical, physical, and mechanical properties of PC and FA.
Chemical Properties
CompositionPC (%)FA (%)
SiO218.1262.30
Al2O34.8723.72
Fe2O33.278.10
CaO65.11.74
MgO3.122.43
TiO20.021.28
K2O0.580.51
Na2O0.390.10
Loss on ignition1.342.36
Physical properties
Blaine (cm2/g)34483820
Specific gravity (g/cm3)3.162.18
Compressive strength (MPa)
2 days25.1-
7 days43.7-
28 days58.6-
Table 2. The chemical properties of YP and RP pigments (%).
Table 2. The chemical properties of YP and RP pigments (%).
Fe2O3Cr2O3Al2O3CuOZnOSiO2SO3MnOCaONa2O
YP95.080.0270.0530.0140.080.0631.210.0190.0140.15
RP97.570.1410.0560.1570.0492.610.0770.1070.028-
Table 3. The physical properties of YP and RP pigments.
Table 3. The physical properties of YP and RP pigments.
ParametersYPRP
TypeIron oxide (FeOOH)Iron oxide (Fe2O3)
Pigment colorYellowRed
Particle shapeNeedleSpherical
Density (g/cm3)3.75
Particle size length (μm)0.3–0.8-
Particle size diameter (μm)0.05–0.20.3–0.8
Specific surface area (m2/g)57.235.4
Table 4. Mix design of R0–R7 mortar mixtures.
Table 4. Mix design of R0–R7 mortar mixtures.
Mix CodeMixesCement (g)Fly Ash (g)Sand (g)Pigment (%)Water (g)SP (%)w/b
R0Control450-135002250.50.45
R1FA2036090135002250.50.45
R2FA20-YP136090135012250.50.45
R3FA20-YP336090135032250.50.45
R4FA20-YP536090135052250.50.45
R5FA20-RP136090135012250.50.45
R6FA20-RP336090135032250.50.45
R7FA20-RP536090135052250.50.45
Table 5. ΔL*, Δa*, Δb* and ΔE* parameters of R0–R7 samples after high-temperature effects.
Table 5. ΔL*, Δa*, Δb* and ΔE* parameters of R0–R7 samples after high-temperature effects.
Mix CodeTemperatureΔL*Δa*Δb*ΔE*
R0300 °C−4.180.731.824.62
500 °C−0.651.61.392.22
800 °C7.03−0.220.067.04
R1300 °C−4.181.854.334.87
500 °C−2.421.170.542.74
800 °C7.06−0.43−0.587.1
R2300 °C−3.041.912.654.46
500 °C−9.344.19−10.5714.72
800 °C0.164.420.237.31
R3300 °C−1.232.652.343.74
500 °C−8.076.51−8.3213.29
800 °C1.983.39−5.076.42
R4300 °C−4.932.041.35.49
500 °C−9.88.14−10.4716.69
800 °C−6.472.73−7.2310.08
R5300 °C−4.180.493.345.37
500 °C−3.12−1.180.243.34
800 °C2.43−1.791.763.5
R6300 °C−4.42−0.382.495.09
500 °C−2.03−0.81−1.042.42
800 °C0.86−2.483.094.06
R7300 °C−4.18−0.451.684.53
500 °C−1.77−1.27−1.112.45
800 °C6.06−6.1−2.338.91
Table 6. 28-day RCS values of R0–R7 samples at ambient conditions and after high-temperature effects.
Table 6. 28-day RCS values of R0–R7 samples at ambient conditions and after high-temperature effects.
Mix CodeAmbientSt.Dev.300 °CSt.Dev.Decrease (%)500 °CSt.Dev.Decrease (%)800 °CSt.Dev.Decrease (%)
R048.320.7547.610.82−1.4835.210.94−27.1417.121.06−64.57
R144.330.6342.390.77−4.3829.270.75−33.9713.810.85−68.85
R243.190.6841.490.83−3.9330.491.06−29.4013.881.11−67.86
R341.760.2740.190.98−3.7530.131.17−27.8414.540.97−65.18
R440.850.4439.010.96−4.5026.941.25−34.0511.740.57−71.26
R542.450.3440.570.90−4.4428.891.38−31.9511.340.55−73.29
R639.930.4338.270.76−4.1628.140.98−29.5310.910.86−72.68
R737.820.1935.840.92−5.2424.210.77−35.999.131.08−75.86
Table 7. 90-day RCS values of R0–R7 samples at ambient conditions and after high-temperature effects.
Table 7. 90-day RCS values of R0–R7 samples at ambient conditions and after high-temperature effects.
Mix CodeAmbientSt.Dev.300 °CSt.Dev.Decrease (%)500 °CSt.Dev.Decrease (%)800 °CSt.Dev.Decrease (%)
R053.170.7551.181.02−3.7437.451.13−29.5718.011.01−66.13
R158.210.6356.181.31−3.4946.672.10−19.8225.971.31−55.39
R259.020.6857.011.39−3.4148.111.39−18.4926.670.98−54.81
R359.750.2757.781.22−3.3049.130.92−17.7727.890.89−53.32
R457.770.4455.661.53−3.6546.540.87−19.4425.931.29−55.12
R558.880.3456.661.44−3.7747.290.76−19.6826.381.24−55.20
R659.110.4357.031.11−3.5248.211.51−18.4426.891.26−54.51
R757.210.1954.670.73−4.4445.320.90−20.7825.150.97−56.04
Table 8. 28-day RFS values of R0–R7 samples at ambient conditions and after high-temperature effects.
Table 8. 28-day RFS values of R0–R7 samples at ambient conditions and after high-temperature effects.
Mix CodeAmbientSt.Dev.300 °CSt.Dev.Decrease (%)500 °CSt.Dev.Decrease (%)800 °CSt.Dev.Decrease (%)
R06.480.116.460.19−0.315.090.18−21.452.240.23−65.43
R16.520.166.330.13−2.914.940.19−24.232.010.13−69.17
R26.440.106.280.12−2.485.010.22−22.202.120.19−67.08
R36.410.136.260.14−2.345.010.13−21.842.220.22−65.37
R46.350.176.120.21−3.624.880.19−23.151.880.19−70.39
R56.530.136.350.12−2.765.010.20−23.282.030.13−68.91
R66.470.106.30.13−2.635.260.22−18.702.150.14−66.77
R76.410.136.140.15−4.214.880.19−23.871.890.20−70.51
Table 9. 90-day RFS values of R0–R7 samples at ambient conditions and after high-temperature effects.
Table 9. 90-day RFS values of R0–R7 samples at ambient conditions and after high-temperature effects.
Mix CodeAmbientSt.Dev.300 °CSt.Dev.Decrease (%)500 °CSt.Dev.Decrease (%)800 °CSt.Dev.Decrease (%)
R07.010.136.780.13−3.285.010.15−28.531.960.11−72.04
R17.110.187.010.15−1.415.480.17−22.932.740.20−61.46
R27.130.187.040.17−1.265.510.13−22.722.770.20−61.15
R37.170.137.110.11−0.845.610.10−21.762.850.18−60.25
R47.050.156.910.15−1.995.440.16−22.842.690.22−61.84
R57.090.136.950.19−1.975.470.13−22.852.710.20−61.78
R67.140.167.010.12−1.825.560.12−22.132.770.19−61.20
R76.980.186.820.17−2.295.370.17−23.072.530.13−63.75
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MDPI and ACS Style

Akbulut, Z.F.; Guler, S.; Osmanoğlu, F.; Kıvanç, M.R.; Khan, M. Evaluating Sustainable Colored Mortars Reinforced with Fly Ash: A Comprehensive Study on Physical and Mechanical Properties under High-Temperature Exposure. Buildings 2024, 14, 453. https://doi.org/10.3390/buildings14020453

AMA Style

Akbulut ZF, Guler S, Osmanoğlu F, Kıvanç MR, Khan M. Evaluating Sustainable Colored Mortars Reinforced with Fly Ash: A Comprehensive Study on Physical and Mechanical Properties under High-Temperature Exposure. Buildings. 2024; 14(2):453. https://doi.org/10.3390/buildings14020453

Chicago/Turabian Style

Akbulut, Zehra Funda, Soner Guler, Faruk Osmanoğlu, Mehmet Rıza Kıvanç, and Mehran Khan. 2024. "Evaluating Sustainable Colored Mortars Reinforced with Fly Ash: A Comprehensive Study on Physical and Mechanical Properties under High-Temperature Exposure" Buildings 14, no. 2: 453. https://doi.org/10.3390/buildings14020453

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