Recycling bio-waste into durable green mortars: Effects of eggshell powder on strength, microstructure, and durability

Calcium-rich eggshell powder (ESP) has gained attention as a sustainable alternative in construction. Similar to fly ash (FA), it can be incorporated into geopolymer systems. This study investigates the effects of replacing FA with ESP at 2.5 %, 5 %, 10 %, and 15 % on the workability, strength, and durability of FA-based geopolymer mortars. Mortars were cured at 70 °C for 24 h and stored at room temperature until day 28. Physical, mechanical, and durability tests were conducted along with regression analysis. Additionally, microstructure was examined using FE-SEM/EDX. Workability decreased with increasing ESP content, with up to a 19 % reduction observed at higher replacement levels. The results showed that 10 % ESP replacement enhanced flexural and compressive strengths by 14.2 % (from 7.87 MPa to 9 MPa) and 20 % (from 41.1 MPa to 49.3 MPa), respectively, while reducing abrasion loss by 32.8 % and sorptivity by up to 50 %. At elevated temperatures, the mixture containing 10 % ESP retained the highest compressive strength after 400 °C exposure (2.4 % loss), while the 2.5 % ESP series showed the lowest strength loss (35.1 %) at 800 °C. Under acid and sulfate exposure, the control (ESP-free) series exhibited the least strength losses (5.6 % and 18 %, respectively), while mixtures with higher ESP content showed greater degradation. Microstructural analysis showed that 10 % ESP improved Ca/Si, Na/Si, and Si/Al ratios, leading to a denser matrix. The results support the use of ESP as a promising bio-based additive in durable and sustainable alkali-activated materials.