'The Built Environment's Glue'
Consider for a moment the materials in the structures around you: in the buildings, roads, bridges, dams, tunnels, etc. There is a lot of cement. In fact, the quantity of Portland cement used is enormous, with annual world production rapidly approaching three billion tonnes. The volume of concrete produced – by mixing cement with aggregates and water – is truly vast; about one cubic metre for every person in the world, every year! The material that binds all this concrete together is clearly important stuff; but what is it? A number of solid products are formed when the cement reacts with water, some of which are crystalline and so are easily identified. But, the main binding phase is nearly amorphous, highly variable in chemical composition, nanostructure and morphology, and it is mixed on a very fine scale with microcrystals of other phases; establishing its exact nature has consequently proved somewhat difficult. To add further complication, the Portland cement in concrete is now often partially replaced with various mineral admixtures, which results in significant changes. In this lecture, the nature of the binding phase in a range of cements will be described in detail, including the most comprehensive compilation of compositional data to date, and a nanostructural model will be presented that accounts for the experimental data. Examples will be given that impact upon a number of concrete durability issues and on the encapsulation of radioactive waste.