Effects of pore structure and chloride binding capacity on chloride diffusion in limestone cement paste

The limestone powder has been a preferred admixture in cement-based materials due to its abundant availability and potential to reduce carbon emission. The purpose of this study is to clarify the key influencing factors for chloride diffusion in cement paste as limestone is added. The pore structure, chloride binding capacity and chloride diffusivity of cement paste with different limestone addition and fineness were measured. The deconvolution of physically and chemically bound chloride was achieved by combining X-ray diffraction, energy dispersive spectroscopy and the Rietveld method. The results show that the addition of fine limestone increases the number of 50-100 nm pores while the coarse limestone leads to more pores in the size range of 100-1000 nm. As the limestone content increases, less chloride is bound by physical adsorption while more Friedel's salt is formed. The effective chloride diffusivity is almost unaffected when less than 10 % fine limestone is added because of increased pore tortuosity. The changes of chloride binding capacity and chloride diffusivity are equally important for the chloride diffusion when 20 % fine limestone is added. On the other hand, the reduction of chloride resistance of cement paste containing coarse limestone is mainly due to the increase of the chloride diffusivity.