Clay in cement-based materials: Critical overview of state-of-the-art

•Critical overview of effects of clay in cement-based materials is provided.•Dimensional stability of aggregates bearing clay minerals is examined.•Treatment of clays to mitigate their detrimental effects in concrete is outlined.•Research on nano-clays in cement-based materials is highlighted.•How to resolve ambiguity related to clays in cement-based materials is discussed. Limitations in guidelines and standards on the amount of clay in sand and coarse aggregate micro-fines and the influence of such micro-fines on fresh and hardened concrete properties is often ambiguous for practitioners and quality control professionals. This is compounded by conflicting related data in the open literature and the inadequacy of some standard test procedures for capturing the real problems associated with the presence of clays in cement-based materials. This paper examines the various types of clay, limitations on clays in aggregates in various standards, and the test methods used to assess the presence of clays in aggregates. A critical overview of literature on the possible effects of clay in cement-based materials is provided, including effects on water demand, workability, mechanical strength, dimensional stability and chemical admixtures dosage. The problem of dimensional stability of aggregates bearing clay minerals is examined. Possible chemical treatment of clays to mitigate their detrimental effects in concrete is also outlined. Finally, emerging research on using nano-clays in cement-based materials is highlighted and recommendations to resolve ambiguity related to the presence of clays in cement-based materials are proposed.