Calcium is the great structural element. It is the basis of much of the great architecture in nature as well as many of the incredible structures made by man. Presenter Justin Rowlatt hears from chemistry supremo Andrea Sella at the Royal Institution in London, where calcium was first isolated two centuries ago. He visits the obscure birthplace of the biggest modern-day use of calcium - cement - and sees that use in action at London's giant Crossrail construction project. And, if that weren't enough, we also hear from Professor Serena Best of Cambridge University about how she is trying to replicate the way the human body uses calcium to construct bone.