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The understanding of the diffusion process and mechanisms of harmful species (e.g. chlorides) in porous cementitious materials is important to control and improve the material durability under harsh environments. In this paper, fractal analysis on the pore structure of porous cementitious materials was conducted and involved in a diffusion model. Macro material geometric parameters were considered in the model to avoid the difficulties in the measurements of microscopic pore parameters. The deformations of porous cementitious materials under the uniaxial elastic loads were considered to correct the diffusion model. The stress-affected diffusivity was displayed in an elegant expression involving some macro material parameters (e.g. total porosity, elastic modulus of solid skeleton, Poisson ratio). Results show that the effective diffusivity is greatly influenced by the porosity and stress ratio. The uniaxial elastic loads decrease the pore areas but increase the lengths of the pore channels for mass diffusion, which eventually causes the decrease of the effective diffusivity. The plots of the relative diffusivity against the stress ratio follow linear forms. The developed fractal diffusion model may help better understand the diffusion process in complex porous cementitious materials under elastic loads. Going beyond this, the fractal diffusion model may provide a new tool to predict the diffusivity of porous building materials under complex mechanical and environmental loads.