Limestone together with calcined clay are unique among sup- plementary cementitious material (SCMs), as they not only offer excellent properties when used in combination with cement, but conversely to other SCM’s their availability is unlimited. Limestone calcined clay cements (LC3) are blended cements that combine clinker, calcined clay, limestone, and gypsum. They take advantage of the high reactivity of calcined clay and the synergistic reaction between limestone and clay, offering equivalent mechanical performance to Ordinary Portland Cement (CEM I/OPC), with the benefit of decreasing clinker factors to 50%. LC3, while retaining the mechanical behaviour of OPC, also significantly improves some relevant properties such as resistance to chloride ingress and alkali silica reaction as compared to other cements. Furthermore, limestone and calcined clays are among the few raw materials available in the quantities required to constitute a technology suitable for coping with the projected worldwide demand for cement.