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

This study investigates eggshell powder (ESP) 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). The effects of ESP on shear strength are also examined, as they remain sparsely documented in existing literature. Distinctively, this research implements a large-scale screening of 60 mix designs (360 specimens) to maximize cement reduction while maintaining or improving mechanical performance relative to the control.

Mortars with ESP at 0, 10, 15, and 20 wt.% were tested for flow (EN 1015-3) and for 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 are summarized as average values.

Workability decreased beyond 15 wt.% ESP. The best blended mix (70% cement, 10% ESP, 10% FA, 5% SF, 5% BFS) achieved +12% higher 28-day compressive strength and +17.1% higher 28-day flexural strength compared to the control; the 10 wt.% ESP mix reached +28% higher shear strength at 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.

Moderate ESP dosages (≈ 5–10 wt.%) can enhance mechanical performance when combined with FA, SF, and BFS, likely through filler densification and nucleation effects in addition to limited pozzolanic activity. Thermal monitoring indicates that ESP contributes to improved passive regulation under variable ambient conditions. However, the practicality of higher ESP levels (> 15 wt.%) decreases due to increased water demand and reduced workability.

Within SCM-blended systems, ESP provides a cost-effective and eco-efficient approach to cement reduction 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.