Relationship Between Neutralization Depth and Concentration Distribution of CaCO3 -Ca(OH)2 in Carbonated Concrete

Abstract

The relationship between the neutralization depth determined by a phenolphthalein 1% ethanol solution and the concentration distribution of CaCO3-Ca(OH)2 in carbonated concrete is discussed, based upon accelerated carbonation and outdoor exposure tests, and field survey research by making use of powder X-ray diffraction and thermal analytical methods. It was found that the neutralization depth exists in the partly carbonated zone of concrete where both CaCO3 and Ca(OH)2 are observed, and that carbonation front depth from which CaCO3 is not detected, exists much deeper in concrete. Further, it was confirmed that the neutralization depth is about half of the carbonation front depth. This fact is interpreted by theoretical research of unsteady state dynamic analysis for the diffusion of CO2 from the surface inwards into concrete accompanied by carbonation reaction with Ca(OH)2. Computer simulation was done for the converted CO2 concentration in carbonated concrete by using the effective diffusion coefficient estimated as a function of water cement ratio. If the converted CO2 concentration in the neutralization depth is assumed to be 10% of the surface concentration, the neutralization depth is almost the same as the depth calculated using Hamada's law which is considered to be adequately applicable for the progress of neutralization of concrete with a water cement ratio of 60% exposed outdoors in the rain. It is concluded that the relationship between the neutralization depth (X sub n) and the carbonation front depth (X sub f) is expressed by the following equation: X sub n = (1/2) X sub f.