In this paper, a probabilistic method is developed for estimating the effective diffusivity of a composite material. The method is simple to code, stable and suitable for parallel computing. Furthermore, the application of the method does not require scale separation. Numerical results are based on models of a porous binder and non-porous aggregates in two dimensions to reduce computation demands. Based on the experimental results of a reference mortar and the corresponding neat cement paste, the effective diffusivity of another mortar produced from the same cement paste with a different sand content is estimated. The resulting estimate agrees with the experimental result. © Civil-Comp Press, 2013.