Sustainable Mortar Incorporating Treated Eggshell Powder and GGBS: Microstructure and Performance Assessment

Eggshells contain a thin organic inner membrane that adheres to the CaCO 3 crystals, forming a weak interfacial transition zone (ITZ) between the eggshell powder (ESP) and the cement paste, limiting its use as a cement replacement in mortar mixes. This study investigates the effect of phosphoric acid treatment of eggshell and the combined use of phosphoric acid‐treated ESP (PA‐ESP) and ground granulated blast‐furnace slag (GGBS) as partial cement replacements in mortar. Eggshells were treated with 5% phosphoric acid and calcined at 500–800°C at 100°C intervals, then characterized using X‐ray fluorescence (XRF), X‐ray diffraction (XRD), Fourier‐transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM)/energy dispersive X‐ray (EDS) to evaluate chemical composition, crystalline phases, functional groups, and microstructural features. Mortar mixtures incorporating 0%–7.5% ESP and 0%–10% GGBS were prepared and tested for bulk density, compressive strength, flexural strength, and water absorption. Results indicate that ESP calcined at 500°C exhibited optimal characteristics, including a high calcium oxide (CaO) content (~68.5%), a preserved calcite phase, and a porous morphology that enhances particle packing and hydration. The combined use of ESP and GGBS significantly improved compressive strength, with the optimal mix (5% ESP + 10% GGBS) achieving 28‐day strength that is 46.4% higher than the control. This improvement is attributed to the synergistic effects of ESP acting as a micro‐filler and nucleation site, alongside the pozzolanic reactivity of GGBS. Flexural strength showed minimal variation, while water absorption results suggested improved matrix densification in selected mixes. Overall, the findings demonstrate that PA‐ESP, in combination with GGBS, is a viable and sustainable cement replacement. However, limitations related to long‐term durability and the extent of chemical modification warrant further investigation.