ABSTRACT
Monitoring of chemical and mechnical development of high performance concretes (HPCs) during the first days enables the properties already found in building applications to be explained. The chemical reactions nevertheless continue, as long as the hydric state of the concrete allows it. Setting time and hardening rate are dependent on many hydration reactions, and on the establishment of chemical links between particles. Mechanical resistance will be produced as a result of all these phenomena, and it seems that during the setting period, the time when the crystals growth is at its fastest, the speed of healing is high enough for no effect on the rise in resistance level to be noticeable. Setting reactions are set off by a "chemical trigger", which is the precipitation of portlandite. This takes place when the level of concentration of silicates of the aqueous phase has dropped below a threshold of around a few micromoles per litre.
