EFFECT OF COMPOSITION, ENVIRONMENTAL-FACTORS AND CEMENT-LIME MORTAR COATING ON CONCRETE CARBONATION

The paper describes the physicochemical processes of concrete carbonation and presents a simple mathematical model for the evolution of carbonation in time, applicable under constant relative humidity higher than 50%. The model is based on fundamental principles of chemical reaction engineering, and uses as parameters the ambient concentration of CO2, the molar concentrations of the carbonatable constituents, Ca(OH)2 and CSH, in the concrete volume, and the effective diffusivity of CO2 in carbonated concrete. The latter is given by an empirical function of the porosity of hardened cement paste and of relative humidity, derived from laboratory diffusion tests. The validity of the model for OPC or pozzolanic cement concretes and mortars is demonstrated by comparison of its predictions with accelerated carbonation test results obtained in an environment of controlled CO2 concentration, humidity and temperature. The mathematical model is extended to cover the case of carbonation of the coating-concrete system,for concrete coated with a cement-lime mortar finish, applied either almost immediately after the end of concrete curing or with a delay of a certain time. Parametric studies are performed to show how the evolution of carbonation depth with time is affected by cement and concrete composition (water/cement or aggregate/cement ratio, percentage OPC or aggregate replacement by a pozzolan), environmental factors (relative humidity, ambient concentration of CO2), the presence and the time of application of a lime-cement mortar coating and its composition (water/cement, aggregate/cement and lime/cement ratios of the mortar, percentage OPC or aggregate replacement by a pozzolan).