Concrete is formed by providing a wet layer of a first concrete 4, applying a second wet layer 6 of concrete on the first layer 4 of wet concrete and setting the layers 4 and 6 to provide a composite concrete structure, wherein at least one of the layers comprises, AACM (Alkali-Activated Cementitious Material). An ionic bond 2 is formed between the two layers. The AACM layer may comprise a reinforcement structure and cathodic protection.