This paper investigates the applicability of nucleation and growth mechanisms to the hydration of alite. Various possible mechanisms of nucleation and growth were simulated using the recently-developed microstructural modelling platform mu ic. Comparison with the Avrami equation and the boundary nucleation model demonstrate the limitations of these equations. Experimental measurements of the rates of hydration of alite powders with different particle size distributions were then simulated with a boundary nucleation and growth model in mu ic. The results show that while the nucleation and growth of C-S-H having bulk densities in the currently accepted range can explain the acceleration during the first few hours of hydration, it cannot explain the later deceleration. It was also found that the resistance to flow of ions offered by the layer of hydrates forming over the surface of the alite particles (diffusion control) cannot explain the deceleration. The deceleration could be reproduced when C-S-H was assumed to be loosely packed in the beginning with its density of packing increasing with hydration. It is proposed that during the early hours of hydration a loosely-packed C-S-H fills a large fraction of the microstructure and the further development of its microstructure occurs due to an increase in its packing. (C) 2009 Elsevier Ltd. All rights reserved.