Cold-bonded artificial aggregate: Processing, performance and perspectives

This article reviews cold-bonded artificial aggregates (CBAAs) produced from industrial wastes such as bottom ash, fly ash, slags, sludges, and other waste-derived precursors. It provides a detailed examination of the factors influencing their performance and the treatments used to enhance their properties. CBAAs are broadly classified into two groups based on binder type: cement-based aggregates and alkali-activated aggregates (AAAs). Cement-based CBAAs develop their required properties through hydration and pozzolanic reactions. Reported compressive strengths typically range from 1 to 12 MPa, while water absorption ranges from 10 % to 25 %. These properties vary significantly with precursor type, mix proportions, curing conditions, and surface treatments. In AAAs, cement is replaced by an alkaline binder to provide a more environmentally sustainable alternative. The properties of AAAs result from the formation of N-A-S-H or C-A-S-H gels, with commonly reported compressive strengths between 2 and 8 MPa. Performance can be improved by selecting the appropriate precursor type and optimizing the ratio of precursor to alkaline activator. Carbonation further enhances the properties of CBAAs by reducing porosity by up to 30 %, increasing compressive strength by a factor of two to three, and capturing CO₂ through the formation of CaCO₃. Environmental assessments indicate that converting industrial waste into aggregates can lower greenhouse gas emissions and keep leaching parameters within regulatory limits. This review also identifies ongoing challenges, such as precursor variability, energy demand, and scaling production, and outlines future directions to support the commercial deployment of CBAAs.