A cementitious binder comprises at least 90% by weight of a hydraulically-active material comprising ground granulated blast furnace slag (GGBS) and/or pulverised fuel ash (PFA), and at least 0.1% by weight of CaO in an activator composition for the hydraulically-active material. The cementitious binder does not comprise any Portland cement and is, therefore, more environmentally friendly. The binder further comprises a superplasticiser such as a polycarboxylate ether (PCE). A concrete, mortar, grout, screed or render may be formed from a mixture of the cementitious binder, aggregate particles, water and superplasticiser.