A model describing the gain in the unconfined compressive strength (UCS) with curing time (applicable also to soil stabilisation with lime or cement) is introduced and relevant derived equations are given. This model is based on the assumption that the responsible mechanism for strengthening is the formation of cementitious material at sites where fly ash particles are in contact with each other. The variation of the UCS with temperature, the mean size of the constit­uent element, the compaction effort and the rate of transformation of a site into a cementitious material are derived. It is shown that the proposed model, in addition to predicting the experi­mental results, demonstrates the inherent limitations of the existing empirical equations.