Utilization of Eggshell Powder in Cement Mortars: Enhancing Mechanical and Thermal Properties for Sustainable Construction

4.2.1. Compressive Strength

The findings of the compressive strength tests demonstrated how ESP content and its combination with Fly Ash (FA), Silica Fume (SF), and Blast Furnace Slag (BFS) affect the hydration kinetics, microstructural development, and overall mechanical performance of the mortar. This provides useful insight into how Eggshell Powder (ESP) interacts synergistically with supplementary cementitious materials (SCMs).

We tested the compressive strength of all 60 mix designs at both 7 and 28 days. A summary of the results is presented in Table 4, and a visual illustration is shown in Fig. (11).

In our first set of mixes (#1–5), we replaced cement entirely with Eggshell Powder (ESP) at varying rates (5–20%). As shown in Fig. (12), modest replacements of 5% and 10% boosted compressive strength by roughly 11% and 14%, respectively, at 28 days compared to the control. These gains were attributed to the fine ESP particles acting as fillers, which help densify the cement matrix. However, raising the replacement levels to 15% and 20% resulted in significant reductions in strength (14% and 32%, respectively). This decline occurs mainly because there is less cement to react, and ESP offers limited pozzolanic activity at such high dosages.

The average 28-day compressive strength of the control mix was 19.54 MPa. Among the 60 mixes, 17 exceeded this baseline value, 9 of which contained ESP. Mix #48 was the strongest, reaching 21.93 MPa, representing a 12% increase over the average and a 20% improvement compared with the weakest control sample. Notably, this mix contained 70% cement, 10% ESP, 10% FA, 5% SF, and 5% BFS. This demonstrates that a 30% reduction in cement content can be achieved without compromising, and even enhancing, mechanical performance.

The incorporation of additional pozzolanic materials, such as FA, SF, and BFS, alongside ESP, contributed significantly to strength development. The pozzolanic activity of FA and SF, particularly their capacity to generate secondary C–S–H gel and fill capillary pores, led to improved matrix densification. BFS, known for its latent hydraulic properties, further supported strength gain over time. For instance, mixes #48 and #52, which contained all three SCMs, exhibited superior performance despite their relatively high ESP levels.

From a microstructural perspective, the combined presence of FA, SF, and BFS with ESP promotes both pozzolanic and filler synergy [8, 11]. The amorphous SiO₂ and Al₂O₃ in FA and SF react with Ca(OH)₂ released during cement hydration to form secondary C–S–H and C–A–H gels. This refines the pore network and strengthens the interfacial transition zone (ITZ) [25]. At the same time, BFS contributes additional CaO and latent hydraulic phases, further enhancing long-term strength and microstructural densification [19]. The fine CaCO₃ particles in ESP complement these mechanisms by filling micro-voids, acting as nucleation sites for hydration products, and stabilizing early-age C–S–H formation [12, 48]. As a result, the blended systems exhibit a denser, more continuous matrix with fewer interconnected pores. This is consistent with SEM observations from previous studies that reported improved packing density and reduced microcracking at 5–10 wt.% ESP replacement levels [8, 11].

Previous studies are consistent with these findings. They demonstrated that the incorporation of pozzolanic materials along with ESP can markedly enhance compressive strength through complementary microstructural mechanisms [8, 11]. For instance, Teara et al. incorporated up to 30% FA by weight of cement with 0–15% ESP and observed a 16.8% increase in compressive strength for mixes with 5% ESP compared with the control [53]. This enhancement was attributed to secondary C–S–H gel formation and microvoid filling provided by the amorphous silica in FA, which promoted matrix densification and reduced pore connectivity. When higher levels (10-15%) were used, the dilution of reactive clinker phases outweighed the benefits, resulting in lower strength [53].

The existing research reinforces the mechanisms observed in the present experimental program. We found similar trends in our study, where compressive strength decreased by 15–49% in mixes with high ESP and pozzolan content, with the notable exception of mixes #48 and #52. These specific mixes achieved significantly higher strengths due to the balanced synergy among ESP, FA, SF, and BFS. The micro-filler and nucleation effects of ESP, combined with the pozzolanic reactions of the SCMs, produced a denser and more cohesive microstructure that helped mitigate dilution effects [11, 12]. Consequently, low-to-moderate ESP levels (≈ 5–10 wt.%) led to optimized hydration, reduced porosity, and stronger ITZs, which explains the enhanced mechanical performance [48].

While excessive ESP alone may reduce strength due to cement dilution, the addition of supplementary cementitious materials effectively counteracts this limitation, yielding mortars with superior mechanical properties [19]. The combined use of ESP with FA, SF, and BFS presents a promising pathway toward sustainable and high-performance cementitious systems [8, 11]. In the final analysis, the results confirmed that ESP primarily acts as a micro-filler and hydration accelerator at low dosages. Its combination with SCMs transforms it into a synergistic component, enhancing both early- age and long-term performance [8, 12].

4.2.2. Flexural Strength

Flexural strength tests were conducted at 7 and 28 days for all 60 mortar mixes to evaluate the tensile performance of the composites. The results are summarized in Table 4 and illustrated in Fig. (13).

In samples #1–5, cement was partially replaced with Eggshell Powder (ESP), as shown in Fig. (14). Mortars containing 5% and 10% ESP (samples #2 and #3) exhibited flexural strength increases of 23% and 35%, respectively, at 28 days compared with the control sample (#1). However, mixes with 15% and 20% ESP (samples #4 and #5) showed corresponding reductions of 7% and 19%, respectively.

The average 28-day flexural strength of the control mortars was 5.89 MPa. When all 60 mixes were evaluated, 14 surpassed this benchmark, including 8 that contained ESP. Sample #24 achieved the highest flexural strength at 6.93 MPa. This corresponded to a 17.6% improvement over the control mortars and a 38% increase compared with the weakest control sample (#1). This mix consisted of 80% cement, 10% ESP, and 10% FA, achieving a 20% reduction in cement content while yielding a notable enhancement in flexural strength. Sample #48 had the second-highest value (6.90 MPa), attaining a 17.1% improvement over the control. This mix contained 70% cement, 10% ESP, 5% SF, 10% FA, and 5% BFS, and demonstrated that a 30% reduction in cement content can be achieved without compromising strength.

Furthermore, 22 of the 60 samples exhibited higher 28-day flexural strength than the control. Of these, 12 contained ESP. This indicates that low-to-moderate ESP content has a positive effect on tensile performance. The most significant strength gains were observed in blended systems that combined ESP with supplementary cementitious materials (SCMs). These results confirmed a synergistic effect between ESP and SCMs in enhancing tensile capacity, especially when FA and SF were incorporated with moderate ESP levels.

We attribute this synergy to the microstructural refinement and pozzolanic reactivity of the blended systems, which significantly improve load distribution and crack resistance. FA contributes to tensile strength primarily due to enhanced interfacial bonding and a refined pore structure, while SF improves matrix density and crack propagation resistance. This is consistent with trends in compressive strength, demonstrating the effectiveness of blended pozzolanic systems in producing high- performance mortars.

It is worth noting that mixes containing high ESP content alone exhibited poor performance. However, we largely mitigated this limitation by combining ESP with appropriately proportioned SCMs [53, 54]. These experimental observations are consistent with previous studies that have investigated how ESP affects flexural performance across diverse cementitious systems.

From a microstructural perspective, the improvement in flexural strength observed in mixes containing 5–10% ESP is largely due to the formation of a denser, more cohesive matrix. The fine CaCO₃ particles within the ESP act as nucleation sites for early hydration. This accelerates the formation of C–S–H gel and promotes a more uniform distribution of hydration products. Such refinement strengthens the interfacial transition zone (ITZ) between the paste and aggregates, which helps delay microcrack initiation and reduces crack propagation under flexural loading. Similar mechanisms were reported by Gowsika et al. [31] and Parthasarathi et al. [40], who observed that incorporating 5–10 wt. % ESP enhanced flexural strength through improved C–S–H gel development and pore refinement within the ITZ.

When ESP is combined with supplementary cementitious materials (SCMs), such as FA and SF, the amorphous SiO₂ and Al₂O₃ components react with Ca(OH)₂ released during hydration to form secondary C–S–H and C–A–H gels. These products fill capillary voids, refine the pore structure, and enhance the compactness of the matrix. This synergistic reaction is consistent with findings reported by Hama [39] and Pliya and Cree [45], who reported that low-to-moderate ESP replacement levels improve tensile and flexural strength through enhanced particle packing and the formation of additional hydration products, whereas higher ESP levels lead to dilution effects and reduced cohesion.

The micro-filler and nucleation effects of ESP, coupled with the pozzolanic reactivity of SCMs, generate a more homogeneous and well-bonded microstructure. These combined effects explain the superior flexural strength observed in ESP–SCM blended mortars in this study, aligning with SEM- based evidence from prior research [8, 11, 12, 23, 40, 48].

In contrast, Binici et al. [44] and Bhuvaneswari [55] reported reductions in flexural strength at high ESP replacement levels, which they attributed to weaker aggregate bonding and limited CaO activity. Conversely, Ing and Choo [43] demonstrated that ESP with higher calcium content (~95%) can offset this limitation, yielding up to 22.9% higher flexural strength due to accelerated hydration and enhanced microstructural bonding. These comparative findings highlight that ESP performance is strongly influenced by its chemical composition, fineness, and replacement ratio.

Overall, both the literature and the present experimental results indicate that ESP acts as a micro-filler and nucleation enhancer at low dosages, promoting a denser ITZ and improved crack resistance, while its synergy with SCMs enables further microstructural refinement. This consistent evidence across multiple studies confirms that the observed mechanical enhancements are primarily driven by hydration acceleration, secondary C–S–H formation, and uniform stress transfer through a densified matrix.

4.2.3. Shear Strength

For the shear strength tests, seven cement mortar samples were prepared: one control mix without Eggshell Powder (ESP) and six containing ESP at 5%, 10%, 15%, 20%, 25%, and 30% by weight of cement. The shear strength tests were conducted on the 28^th day using the Z-push-off method, and the results are presented in Fig. (15).

The shear strength of the control sample without ESP was 2.01 MPa. The sample containing 5% ESP exhibited a strength of 2.21 MPa, while the 10% ESP specimen achieved the highest value at 2.58 MPa. Specimens with 15%, 20%, 25%, and 30% ESP recorded strengths of 2.51 MPa, 2.32 MPa, 1.98 MPa, and 1.75 MPa, respectively. Notably, the 10% ESP mix demonstrated a 28% increase in shear strength compared with the control [56]. Similarly, the 15% and 20% ESP mixes showed 24% and 15% higher shear strength values, respectively.

The decline in shear strength observed in the 25% and 30% ESP mixtures can be attributed to the excessive dilution of cement within the mortar matrix, which reduces binder efficiency. These results indicate that partial cement replacement with ESP enhances shear performance up to a 20% substitution level, whereas higher replacement ratios lead to diminished strength. This reduction likely stems from the insufficient availability of reactive cementitious components necessary for proper hydration and bonding.

Microstructurally, the improved interfacial adhesion and matrix cohesion are responsible for the enhancement in shear strength at 10 to 20 wt.% ESP. The fine CaCO₃ particles in ESP act as nucleation centers during early hydration, promoting the generation of additional C–S–H gel and strengthening the bond between the cement paste and sand grains. This mechanism results in a denser interfacial transition zone (ITZ), enhancing shear load transfer capacity. Similar microstructural phenomena were reported by Hamada et al. [11], Sathiparan [19], and Nandhini and Karthikeyan [8], who associated ESP’s filler effect and calcium-rich composition with improved bonding in cementitious systems.

At higher ESP levels (> 20 wt.%), excessive CaCO₃ content can increase porosity and hinder the formation of a continuous C–S–H gel network, thereby reducing cohesion. Amin et al. [7] and Tchuente et al. [57] similarly reported that excess calcium-based fillers disrupt gel continuity and weaken the microstructural framework, findings consistent with the reductions observed in this study at 25–30 wt.% ESP.

The results of this study confirmed that ESP improves shear strength up to a 20% replacement ratio, whereas higher dosages lead to dilution-induced strength loss. These outcomes, consistent with previous studies, indicate that low-to-moderate ESP levels (≈10–15 wt.%) are optimal for enhancing shear performance [48, 56-58].

4.2.4. Thermal Analysis

We conducted the thermal performance analysis using cement-based mortar samples containing Eggshell Powder (ESP) at replacement levels of 0%, 15%, 30%, and 50% by weight. These samples were used to construct closed chambers measuring 7 cm × 15 cm × 15 cm with a wall thickness of 4 cm. We recorded the long-term thermal measurements at five specific times of the day (09:00, 12:00, 15:00, 19:00, and 00:00) using type-K thermocouples. During each test, ambient temperature, relative humidity, and internal chamber temperatures were monitored continuously.

To evaluate thermal performance systematically, we divided the experimental period into three ambient temperature categories: hot days, mild days, and cold days, based on temperature readings recorded at the specified intervals over a 30-day period.

The categorization criteria were as follows:

• Hot Days (Above 28°C): Days in which the ambient temperature was above 28°C, typically between 12:00 and 15:00. On these days, the thermal stress conditions are high and are suitable for evaluating the samples’ cooling capacity and their ability to maintain a comfortable temperature in spite of the heat.
• Mild Days (18–28°C): Days in which the ambient temperature was between 18°C and 28°C. This range falls within thermally comfortable conditions, and the demand for heating or cooling is minimal. These days are ideal for assessing the passive thermal regulation properties of mortars.
• Cold Days (Below 18°C): Days in which the temperature was below 18°C, with conditions suitable for evaluating the ability of the samples to retain heat and maintain comfortable temperatures inside when the ambient temperatures were low.

The temperature thresholds for these categories were based on distribution trends observed during the testing period. They are consistent with established thermal comfort models in the literature. This classification enables a structured and comparative analysis of ESP-containing mortars under varying climatic conditions.

Segmenting the data into these categories enabled the study to provide a comprehensive understanding of how ESP-modified mortars respond to different temperature regimes. It demonstrated their potential for enhancing indoor thermal comfort across diverse environmental settings.

We recorded the lowest ambient temperature as 4.2°C at 09:00, when solar radiation was minimal (Fig. 16). For the samples containing 0%, 15%, 30%, and 50% ESP, we measured corresponding internal temperatures of 2.9°C, 4.1°C, 3.9°C, and 4.0°C, respectively. It was observed that the control mix without ESP had an internal temperature 1.3°C lower than the ambient temperature, whereas the ESP-containing samples maintained temperatures closer to the ambient temperature, indicating improved thermal stability.

On a mild day with an ambient temperature of 20.1°C, varying internal thermal behaviors were observed across the samples. It was found that the control and 15% ESP samples recorded temperatures of 18.0°C and 19.6°C, respectively, remaining below the ambient value. In contrast, our measurements showed that the 30% and 50% ESP samples exhibited slightly higher internal temperatures of 20.9°C and 21.1°C. Notably, the 50% ESP mix showed an internal temperature 1°C higher than the ambient temperature, demonstrating its superior thermal retention.

On the warmest morning, as the ambient temperature reached 30.4°C, our measurements highlighted the effective cooling capacity of the ESP-modified samples. While the control sample recorded 26.9°C, we found that ESP inclusion significantly reduced internal temperatures. Specifically, the 15% ESP mix achieved the lowest reading at 24.2°C, followed closely by the 30% and 50% mixes at 24.9°C and 25.2°C, respectively. We observed that this cooling effect reached up to 6.2°C above ambient, demonstrating that ESP contributes substantially to improved thermal comfort under high-heat conditions.

By 12:00, we observed that solar radiation significantly influenced the environment, causing a noticeable rise in temperatures (Fig. 17). During this peak period, we recorded the lowest ambient temperature as 5.1°C. Our measurements showed that internal readings varied across the mix designs: the control sample remained coldest at 3.4°C, while the ESP-modified samples showed better heat retention. Notably, it was found that the 15% ESP sample demonstrated a slight advantage, maintaining an internal temperature of 5.5°C, which was 0.4°C higher than the ambient environment. During our tests under mild weather conditions (20°C ambient), we observed distinct thermal behaviors among the samples. While we recorded a cooler internal temperature of 19.0°C for the control sample, our data indicated that the inclusion of ESP generally improved heat retention. It was identified that the 50% ESP mix, in particular, exhibited the superior performance; we measured it at 20.3°C, maintaining a temperature 0.3°C higher than the surrounding environment.

On a warm day with an ambient temperature of 32.1°C, the thermal benefits of ESP were observed. While we measured an internal temperature of 27.6°C for the control sample, the ESP-modified samples remained significantly cooler, with readings ranging from 24.8°C to 25.7°C. Most notably, our data showed that the 15% ESP mix achieved an internal temperature 7.3°C lower than the outside environment, highlighting the material's exceptional capacity for passive cooling under heat stress.

At 15:00, when the ambient temperature reached its daily peak due to solar intensity (Fig. 18), we recorded a baseline ambient temperature of 5.5°C. Varying thermal responses were observed across the mix designs. It was found that the control sample remained significantly cooler at 3.8°C. In contrast, our measurements for the 30% and 50% ESP samples were 5.0°C and 5.4°C, respectively. Notably, we observed that the 15% ESP sample reached 5.5°C, perfectly matching the ambient temperature, whereas the other samples indicated limited heat retention under these conditions.

On a mild day with an ambient temperature of 20.4°C, we measured internal temperatures of 19.0°C, 20.4°C, 20.8°C, and 20.4°C for the 0%, 15%, 30%, and 50% ESP samples, respectively. The 30% ESP sample reached the highest internal temperature, exceeding ambient by 0.4°C, while the remaining samples had internal temperatures equal to or slightly below ambient.

On the warmest day, when the ambient temperature peaked at 32.4°C, we observed a distinct cooling effect in the ESP-modified samples compared to the control. We measured an internal temperature of 27.9°C for the control mix, which was 4.5°C lower than the ambient air. In contrast, our data revealed that all ESP-containing samples (15%, 30%, and 50%) maintained a consistent and significantly lower temperature of 25.6°C. This represents a reduction of up to 6.8°C relative to the surrounding environment, underscoring the superior passive cooling potential of ESP-modified mortars under high solar exposure.

By 19:00, as the sun's influence diminished and ambient temperatures began to decline (Fig. 19), we recorded the lowest ambient temperature at 5.0°C. Our measurements for the internal temperatures of the 0%, 15%, 30%, and 50% ESP samples were 3.5°C, 4.7°C, 5.2°C, and 5.0°C, respectively. We observed that the 30% ESP sample maintained an internal temperature 0.2°C higher than the ambient value, whereas the 50% ESP sample equalized with the ambient environment.

On a mild day with an ambient temperature of 19.9°C, we measured internal temperatures of 18.2°C, 19.8°C, 20.2°C, and 20.4°C for the 0%, 15%, 30%, and 50% ESP samples, respectively. Our data showed that the samples containing 30% and 50% ESP exceeded the ambient temperature by 0.3°C and 0.5°C, demonstrating enhanced heat-retention capability during evening conditions.

On a warm evening, as the ambient temperature reached 30.8°C, we observed notable differences in thermal comfort performance. We recorded an internal temperature of 27.6°C for the control mix.

However, our measurements for the ESP-containing mortars were significantly lower; we found values of 24.4°C, 24.5°C, and 25.3°C for the 15%, 30%, and 50% ESP mixes, respectively. This data indicates that ESP modification can maintain internal temperatures up to 6.4°C below ambient, offering improved thermal comfort as external temperatures decrease.

At midnight, the ambient temperature reached its lowest daily level (Fig. 20). On the coldest night, with an ambient temperature of 4.1°C, we measured internal temperatures of 2.7°C, 4.2°C, 4.0°C, and 4.0°C for the 0%, 15%, 30%, and 50% ESP samples, respectively. It was found that the 15% ESP sample exhibited an internal temperature 0.1°C higher than the ambient value, demonstrating a slight heat-retention advantage under low-temperature conditions.

On a mild night, when the ambient temperature was 20.4°C, we recorded internal temperatures of 19.2°C, 20.1°C, 20.9°C, and 20.7°C for the 0%, 15%, 30%, and 50% ESP samples, respectively. Our data indicated that the samples containing 30% and 50% ESP maintained internal temperatures 0.5°C and 0.3°C higher than the ambient value, confirming their enhanced thermal stability in moderate nighttime conditions.

On the warmest night, with an ambient temperature of 28.8°C, we identified distinct thermal advantages in the ESP-modified samples. While we measured an internal temperature of 27.9°C for the control group, our readings for the ESP samples were noticeably lower, ranging from 25.4°C (30% ESP) to 25.9°C (15% ESP). It was observed that these modified mortars sustained internal temperatures up to 3.4°C lower than the ambient environment, indicating a significant passive cooling effect and improved nighttime thermal comfort.

We recorded the humidity measurements at five distinct time intervals over a 30-day period, as shown in Fig. (21). This figure illustrates the temporal variation in ambient humidity, providing a clear visualization of daily trends and facilitating a comprehensive interpretation of moisture fluctuations within the testing environment.

In this study, we measured ambient humidity and temperature in closed chambers constructed using cement-based mortar samples containing different proportions of Eggshell Powder (ESP), along with a control sample without ESP. Our analysis indicated that ambient humidity had no significant influence on the observed temperature variations, confirming that material composition, rather than atmospheric moisture, primarily governed the thermal performance of the specimens. The relationship between ambient temperature and humidity is presented in Fig. (22).

Correlation analysis revealed a weak negative relationship between ambient humidity and temperature (r = -0.10). This low correlation coefficient suggests that fluctuations in ambient humidity exert a negligible influence on temperature variation. These results show that cement-based mortars inherently exhibit passive resistance to humidity-driven thermal changes. However, to gain a more comprehensive understanding, we recommend further investigations under diverse environmental conditions and extended exposure durations.

Our results clearly demonstrated that Eggshell Powder (ESP) markedly enhanced the thermal performance of cement-based composites [8, 11, 16, 59]. It was observed that mortars with higher ESP contents, particularly 30% and 50%, exhibited superior thermal regulation, maintaining internal temperatures closer to or below ambient levels under both hot and cold conditions. These outcomes highlighted the potential of ESP as a sustainable additive for improving the thermal efficiency of construction materials.

This improved thermal performance can be attributed to the porous morphology and high calcium carbonate (CaCO₃) content of ESP, which collectively reduce the effective thermal conductivity of the mortar matrix. The fine ESP particles introduce irregularities in heat-transfer pathways and increase phonon scattering, thereby restricting heat flow through the composite. In addition, closed micropores in ESP-modified mortars trap air pockets that act as natural thermal insulators. These combined effects result in lower heat conduction during hot periods and enhanced heat retention during cooler conditions.

Previous studies [11] [8] similarly confirmed that calcium-rich fillers, such as ESP, can decrease thermal conductivity and increase thermal inertia in cementitious systems, leading to slower heat transfer and greater heat-retention capacity. This dual effect, slower heat ingress and delayed heat release, accounts for the reduced diurnal temperature fluctuations observed in the ESP-modified chambers.

Furthermore, the influence of ambient humidity on internal temperature was found to be statistically insignificant, indicating that the thermal improvements in ESP-containing mortars are predominantly governed by their intrinsic microstructural and thermal properties, rather than by external environmental factors.

Nonetheless, it must be acknowledged that the current experimental setup employed small-scale laboratory chambers. While these results provide clear evidence of the passive thermal regulation potential of ESP-modified mortars, large-scale field investigations are necessary to confirm their practical applicability under real climatic conditions. Future studies should focus on long-term outdoor testing, building-scale prototypes, and dynamic thermal modeling to assess the scalability and durability of ESP-integrated systems.

In summary, ESP-based mortars enhance thermal regulation while remaining largely unaffected by humidity variations, offering a reliable solution for energy-efficient construction across diverse climatic zones. The findings confirm that ESP’s micro-filler action and pore-inducing behavior are key to its passive heat-moderation mechanism, validating its role as a sustainable and eco-efficient additive for next-generation cementitious materials.