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

Introduction Given the critical role of cement in the construction industry and its significant impact on global climate change, there is an urgent need to develop alternatives to the traditional clinker-based cement. In this study, the Eggshell Powder (ESP), used as a partial cement replacement in mortar, with an emphasis on its thermal behavior, passive indoor temperature regulation, and synergistic interactions with multiple supplementary cementitious materials, namely, Fly Ash (FA), Silica Fume (SF), and Blast Furnace Slag (BFS), was investigated. The effects of ESP on shear strength were also investigated, as they have not been extensively documented. Distinctively, this research implemented a large-scale screening of 60 mix designs (360 specimens) to maximize cement reduction while maintaining, or even improving, mechanical performance relative to the control. Methods Mortars with ESP at 0, 10, 15, and 20 wt.% were tested for flow (EN 1015-3) and flexural and compressive strengths at 7 and 28 days (EN 196-1). Shear strength at 28 days was measured using the Z-push-off method. Blended (ternary and quaternary) mixes replaced 30 wt.% of cement through combinations of ESP, FA, SF, and BFS. Thermal behavior was monitored using chambers (7 × 15 × 15 cm; wall thickness = 4 cm) made with 0, 15, 30, and 50 wt.% ESP over 30 days, with temperature readings taken at 09:00, 12:00, 15:00, 19:00, and 00:00 via type-K thermocouples. Results were summarized as average values. Results Workability decreased beyond 15 wt.% ESP. The best blended mix (70% cement, 10% ESP, 10% FA, 5% SF, and 5% BFS) exhibited a 12% increase in 28-day compressive strength and a 17.1% increase in 28-day flexural strength compared to the control, while the mix containing 10 wt.% ESP reached a 28% shear strength in 28 days. ESP-modified mixes exhibited an improved passive thermal response, with peak internal temperature reductions of up to ≈ 7°C during hot periods (≈ 12:00– 15:00) and comparable or slightly higher internal temperatures during cooler periods. Discussion ESP dosages of 5-10 wt% can enhance mechanical performance if they are combined with FA, SF, and BFS. This is likely due to filler densification and nucleation effects in addition to limited pozzolanic activity. Thermal monitoring showed that ESP improves passive regulation under variable ambient conditions. However, the practicality of higher ESP levels (> 15 wt.%) decreases due to increased water demand and reduced workability. Conclusion Within SCM-blended systems, ESP provides a cost-effective and eco-efficient approach to reducing cement while maintaining or even enhancing mechanical strength and thermal comfort. However, the findings are limited to laboratory-scale and short-term curing conditions (7–28 days). Therefore, future research should extend to long-term durability assessments, such as freeze–thaw resistance, chloride ion penetration, and sulfate attack, along with microstructural validations (SEM, XRD, TGA), building-scale thermal analyses, and techno-economic evaluations to establish the practical feasibility of ESP-modified mortars for sustainable construction applications.