Reactivity of the surfaces of concrete

The reactivity of concrete surfaces is a complex subject that can be defined by the interactions at the interface between the microstructure of a hydraulic binder-based material and the external environment, the latter of which can consist of various gases (CO ₂ , NO ₂ ), liquid water (more or less loaded with salts), stripping products (or residues) and even micro-organisms (molds, algae...).

The aim of this article is to describe the variability of possible reactions between the concrete surface and its immediate environment, whether during hydration and hardening, or carbonation and ageing. Various examples will be presented to explain the type of reaction taking place at the surface, with the interactions detailed in this article separated into three groups:

• those resulting from reactions with liquid water or ions present in solution: hydration phenomena at an early age, efflorescence formation after formwork stripping, leaching and decalcification following recurrent exposure to water, acid attack, interactions with sulfates and penetration of chloride ions ;

• those resulting from interactions with formwork components, such as moulds and stripping agents: influences of stripping oils, contamination and functionalization with certain moulds, portlandite growth favoured by certain surfactants ;

• those resulting from interactions with the environment: carbonation in the presence of CO ₂ , depollution of certain irritating or toxic gases, growth of micro-organisms.

Finally, the last chapter will describe the functionalities provided by the various types of protective coatings potentially applicable on concrete, and their implications with regard to the interactions previously described.