A model for the direct aqueous carbonation of steel slag (a calcium rich solid industrial waste) into calcium carbonate is developed. The model is based on a two-step aqueous carbonation reaction mechanism. A novel two-layer diffusion model, with two moving boundaries, that accounts for the secondary phases formed during slag carbonation, namely, ash (insoluble residue) and product (calcium carbonate), is formulated. The kinetic resistance of leaching of calcium from slag is also incorporated into the model. Closure of pores, to account for the precipitation of calcium carbonate crystals within the pores of the ash layer, is also modelled. This model accurately represents the dynamics of carbonation of steel slag and may be used to predict the rate of carbonation as a function of various process variables. It can be generalized to describe the carbonation of other minerals of interest. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved.