Microstructural evolution of cement paste under combined effect of carbonation and chloride salt

In order to clarify the microstructure of cement paste under combined effect of carbonation and chloride salt, based on X-ray computed tomography (X-CT) and electron probe micro analyses (EPMA) technologies, the impact of chloride salt on carbonation rate of cement paste was investigated and element concentration distributions of Cl, S and Na in cement paste under the effect of carbonation were measured. The results show that the chloride salt can refine the pore structures of cement paste, improve the degree of density and slow down the carbonation rate of the cement paste with maintenance period of 28 d. The cracks in carbonation district of cement paste are easily generated under the effect of carbonation, and carbon dioxide gas diffuses into the inside of cement paste by those cracks to carbonize, leading to the uneven carbonation depth of cement paste. During carbonation process, Cl, S and Na elements migrate and concentrate to the non-carbonation area, the contents of elements which are distributed in initial state uniformly decease in carbonation area and increase in non-carbonated area, thus provide the scientific basis for durability design of concrete construction.