The near field of the planned deep geological repository for low- and intermediate-level radioactive waste (L/ILW) in Switzerland will consist of different waste materials with different reactivities that are conditioned in cementitious matrices. Geochemical modelling was applied to predict the temporal evolution of the chemical conditions in a cement-stabilised model waste form by simulating the degradation of the waste materials and the alteration of the cementitious matrix. The model waste form contains large amounts of metallic waste and a low amount of organic waste. The geochemical modelling considered the use of either siliceous or calcareous aggregates for fabrication of the cementitious matrix used to condition the waste form. The results show that the type of aggregates has a major effect on the temporal evolution of the chemical conditions in the waste form. The use of calcareous aggregate instead of the commonly used siliceous aggregate is expected to maintain highly alkaline conditions of the waste form over the period of concern for the L/ILW repository. The latter conditions prevent accelerated iron corrosion and, related to that, accelerated H₂ production from occurring due to the high pH. The evolution of the waste form is decisively controlled by the availability of water. In the absence of water ingress from the near field, the degradation processes cease as free water in the waste package is exhausted. In the case of saturation achieved by water ingress with time, the degradation processes continue over the entire period of concern for the L/ILW repository. The present study aims to illustrate the potential of geochemical modelling for predicting the temporal evolution of the chemical conditions of a cement-stabilised waste form.