The mechanics and rheology of cement pastes are largely determined by the Calcium-Silicate-Hydrate (C-S-H) gel, which is normally the main binding phase. The C-S-H gel is an amorphous material with a nanoscale pore network. Here we consider a model structure of the gel, where a continuous molecular model of non-porous C-S-H is combined with a colloidal model that incorporates the porosity of the gel. Numerical simulations predict mechanical properties and microscopic processes associated with yielding under shear. The proposed model can now be used to extract further properties and to contribute to a broader effort aimed at describing and engineering the multi-scale structure and mechanical behaviour of the cement paste. We finally discuss the opportunities offered by our model with respect to the scientific and technological challenges of modelling and controlling ageing and creep over long time scales.