Blast furnace slags (BFS) and pulverised fly ash (PFA) have been extensively used as additives to Ordinary Portland Cement (OPC) to make low permeability pastes with adequate long-term strengths. These properties are a consequence of phase development in the matrix which proceeds non-unformly because the OPC clinker and blending agent react at different rates. Also, sheaths of hydration products forming around anhydrous grains inhibit reaction. This complicates our interpretation of the properties of blended cement systems because phases which are observed as products on laboratory timescales are not necessarily representative of the steady state assemblages. The aqueous chemistry is also subject to time dependent changes since solution composition is related to that of the coexisting solids, In some applications, e.g. radioactive waste immobilisation, it is necessary to predict long term physico-chemical properties. This can only be achieved through modelling, based on sound scientific principles and using as much information as it is realistic to obtain from immature systems. The present paper describes progress in respect of model development and verification.