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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 …