Locally available materials, like clays, present a potential to meet the growing concrete demand and to produce more sustainable material. This paper reviews the behaviour of common clay minerals (kaolinite, illite and montmorillonite) giving an insight into the complexity of clay soils that usually appear as mixtures of minerals and impurities. It is therefore crucial to perform reactivity study when considering clay as supplementary cementitious material (SCM). The paper also shows major clay excavation sites in Croatia highlighting those which will be a part of the Swiss-Croatian research project ACT.