State-of-the-art review on mechanisms and microstructural insights of eggshell powder for clay soil stabilization

Innovative, cost-effective, and sustainable soil stabilization approaches are increasingly needed to address the challenges posed by problematic clay soils in geotechnical engineering. Eggshell waste, generated globally in millions of tons each year, has emerged as a promising bio-based stabilizing agent due to its high calcium content and cementitious potential. Although numerous experimental studies have reported improvements in strength and index properties of clay soils treated with eggshell powder (ESP), existing research remains fragmented, with limited synthesis of stabilization mechanisms and insufficient linkage between microstructural evolution and macroscopic soil behavior. This state-of-the-art review critically examines the role of ESP in clay soil stabilization by integrating reported changes in key geotechnical performance indicators with microstructural evidence from advanced characterization techniques. Emphasis is placed on understanding the interactions governing clay–ESP bonding and the development of soil strength. The review indicates that ESP contents up to approximately 10 % by dry soil weight can enhance soil strength and stability; however, effectiveness varies significantly with soil mineralogy, ESP processing methods, and curing conditions. This review also identifies critical research gaps, including the lack of long-term durability assessments, limited field-scale validation, and incomplete environmental and life-cycle evaluations. Addressing these gaps is essential for advancing the reliable and sustainable application of ESP in geotechnical engineering practice.