Comportement des materiaux cimentaires: Actions des sulfates et de la temperature (French text)

The research work presented in this Ph.D. thesis is related to the nuclear waste underground repository concept. Concrete could be used in such a repository, and would be subjected to variations of temperature in presence of sulfate, a situation that could induce expansion of concrete.; The research was lead in three parts: an experimental study of the possibility of an internal sulfate attack on mortars; an experimental study and modeling of the chemical equilibriums of the CaO-SiO₂-Al₂O ₃-SO-H₂O system; and a modeling of the mechanisms of internal and external sulfate attacks, and the effect of temperature.; The results show that mortars can develop expansions after a steam-cure during hydration, but also when a long steam-cure is applied to one-year-old mortars, which is a new point. Ettringite precipitation can be considered as responsible for these expansions.; The experimental study of the CaO-SiO₂-Al₂O ₃-SO₃-H₂O system clarified the role of Calcium Silicate Hydrates (C-S-H) on chemical equilibriums of cementitious materials. Sulfate sorption on C-S-H has been studied in detail. The quantity of sulfate bound to the C-S-H mainly depends on the sulfate concentration in solution, on the Ca/Si ratio of the C-S-H and is not significantly influenced by temperature. Aluminium inclusion in the C-S-H seems to be a significant phenomenon. Temperature increases the calcium sulfoaluminate solubilities and thus increases sulfates concentration in solution. A modeling of the chemical system is proposed.; Simulations of external sulfate attack (15 mmol/L of Na₂SO ₄) predict ettringite precipitation at 20 and 85°C.; Simulation of internal sulfate attack was performed at a local scale (a hydrated cement grain). An initial inhomogeneity can lead, after a thermal curing at 85°C, to ettringite precipitation in zones originally free from ettringite. This new-formed ettringite could be the origin of the expansions.