Urban Mining of Bivalve Shell Waste as a Sustainable Alternative to Limestone Exploitation: A Review on Alkali-Activated Cements and Mortars

The concept of urban mining refers to the recovery and valorization of valuable resources from urban and industrial waste, contributing to circular economy principles. Within this framework, the present study provides a critical review of alkali-activated binders incorporating bivalve mollusk shells as alternative calcium sources. Shells from oysters, scallops, mussels, clams, cockles, and periwinkles were examined, either in their natural or calcined forms, for use as calcium sources, alkaline activators, or fillers in low-carbon binders. The review evaluates key processing parameters, including precursor composition, type and concentration of alkaline activators, curing conditions, and calcination temperatures, and compares the resulting mechanical, chemical, and microstructural properties. In addition, several studies report applications of these binders in soil stabilization and heavy metal immobilization, demonstrating performances comparable to Portland cement. The findings confirm the technical potential of mollusk shell residues and their contribution to the circular economy by diverting aquaculture waste from landfills and marine environments. Nonetheless, significant knowledge gaps persist, including the limited investigation of non-oyster species, the absence of field-scale studies, and the lack of resource mapping, life cycle, or economic assessments. This synthesis highlights preliminary insights, such as optimal calcination temperatures between 700 and 900 °C and effective combinations with silica and alumina-rich residues. Overall, it outlines a pathway toward transforming an underutilized waste stream into sustainable and technically viable construction materials.