During the construction phase of oil wells, a cement slurry is pumped into the annular space between the casing and the rock. The cement sheath aims to support the casing, provide zonal isolation in the well and reservoirs of CO2 storage and protect the casing against corrosion. During the life of the well, the cement is submitted to various thermal and mechanical solicitations along the well that can modify its mechanical properties and damage its sealing performance. The aim of this thesis is to study the effect of temperature and stresses on the mechanical properties of cement paste during hydration and in the hardened state. The used approach is based on an experimental study that combine the calorimetric tests, waves velocity measurement and oedometric tests in STCA system (Slurry To Cement Analyzer) on cement paste during hydration as well as the uniaxial and triaxial compression tests on hardened cement paste. The experimental results showed that temperature and pressure accelerate the kinetics of hydration. Temperature affects significantly the elastic properties of the material whereas the pressure does not modify them for a range of pressure limited to 20MPa. A hydration kinetics modelling associated to a homogenization method is used to interpret the tests. It is shown that for hydration under stress, the irreversible strains can evolve in the cement paste and conduct to the formation of a micro-annular between the cement sheath, the casing and geological formation. The time-dependent behaviour of hardened cement paste is studied using creep tests under uniaxial loading and also from the results of isotropic compression tests. The results show that the uniaxial creep is more important for cements hydrated at higher temperatures, which is attributed to a more heterogeneous microstructure. A visco-damaged model allowing to reproduce the primary creep and tertiary creep is developed and calibrated. Isothermal isotropic drained and undrained compression tests show a time-dependent behaviour with hysteresis during unloading-reloading cycles. These tests are analyzed on the basis of a poro-visco-plastic model. The elastoplastic behaviour in short terms is studied from triaxial tests under drained deviatoric loading. These tests bring to light that the test temperature affects highly the initial yield surface and the strains but it does not significantly modify the failure surface. A model of plasticity with a closed yield surface and hardening, depending on the accumulated plastic strains is developed to describe these tests. Finally, a preliminary study on the effect of mechanical and thermal loading cycles is performed. The thermal loading cycles with temperatures lower than the hydration temperature seem to do not affect the mechanical properties of the material. The mechanical loading cycles show a rapid degradation with the number of loading cycles when the axial stress exceeds 60% of the uniaxial strength
