Cementitious materials are widely used in the concepts of radioactive waste disposal facilities. During the lifetime of these disposals, those materials will undergo physicochemical degradations. To assess their impacts, reactive transport modellingisused.Reactivetransportcodesmodifythetransportpropertiesbasedonthemodelledporosityevolutionby usingArchie’slawasafeedbackbetweenporosityanddiffusiveproperties.Theselawsarenotsuitedtocementitiousmaterials, whose pore structure is complex and expands over a wide range of pore sizes. The ultimate goal of this research is about developing a microstructure-based feedback relation for the diffusive properties of complex porous structures such as cementitious ones. Therefore, we developed an algorithm designed to generate numerical microstructures representative of simplified cement pastes and performed an experimental campaign consisting of dedicated experiments. Arandom-walkalgorithmisusedtocomputetheeffectivediffusioncoefficientsofournumericalmicrostructures.This paper investigates the description of the initial numerical microstructure and how transport properties are sensitive to different microstructural features that can be controlled from the designed algorithm. Simulations both on the experimental microtomograph and the generated microstructures allow to show that our models are complete to describe the microstructure and diffusion transport property of simplified cementitious materials. Sensitivity analysis is also provided, whose results show that a simple feedback relation cannot properly describe these transport properties. This gives confidence in our approach and its future extension toward the description of cementitious material degradations.