A model for the response of concrete to the combined effects of drying and sustained load is discussed with respect to the behaviour of concrete in which blastfurnace slag was partially substituted for Portland cement. Differential sorption of fluids was used to identify the quantity of water residing in the gel structure. The results indicated that this quantity increased with the proportion of blastfurnace slag and that such increase was associated with greater creep and shrinkage as commonly experienced with concrete containing blastfurnace slag. Differences in the amount of water entering spaces of restricted adsorption were considered to arise from either: (a) variations in the shapes of gel pores; or (b) differences in free lime capable of being precipitated in such pores. The magnitudes of increased creep and shrinkage in concrete containing 50 per cent substitution of blastfurnace slag for Portland cement were such that they could be compensated by adjustments in the mix proportions.