"Concrete" is a generic term for a composite material made up of aggregates and a glue to ensure cohesion. Concrete, in which the glue is Portland cement and the aggregates are a mixture of sand and pebbles, is undoubtedly the best-known of all, and is generally referred to as "concrete". It is also the most widely used industrial material in the world, with almost one cubic metre used per inhabitant per year. Bituminous concrete - the material that currently covers our roads in a near-monopolistic fashion - vies with the former for the position of king material in our infrastructures. Together, they form the backbone of our territory.
There is a third concrete, totally natural and at least as widespread as the other two on a planetary scale, yet still largely ignored. This is simply "raw" (unbaked) earth, which in the form of dried mud bricks or compacted earth provides shelter, social buildings and places of worship for a third of humanity.
Add to this wet sand, the material that has made us all great builders, and you have a collection of granular building materials whose cohesion is highly variable: barely sufficient to build a doll's shelter in the case of sand, and more than sufficient to build a kilometre-high skyscraper in the case of cement concrete. Raw earth is intermediate. The forces that hold it together are just as variable. Water is the main source of cohesion, through capillary depression, in damp sand. In raw earth, which can be considered as clay concrete, capillary forces also play an important role, essential even when the clay particles are electrically neutral. However, when the particle surface is electrically charged, more intense forces come into play, involving either mobile ions and concentration fluctuations, or localized ions and strictly electrostatic interactions. This evolution towards stronger, shorter-range attractive forces continues in cement "hydrates", the colloidal minerals that result from the reaction of Portland cement with water and ensure the cohesion of concrete.
What is the source of progress that will further improve concrete's performance and limit its defects (such as theCO2 production associated with cement manufacture)? We have tried to show that hybridization using polymers - whether this hybridization is weak, by adsorption or intercalation, or strong, by grafting - represents a credible avenue.
