This article outlines an experimental and numerical study of the long-term interaction between silica fume and Portland cement in concrete subjected to air, water or sealed curing. For this purpose about 2000 kg of eight qualities of concrete were studied at 4 different ages each over a period of 90 months. Half of the concretes contained silica fume. Parallel studies of strength, hydration and internal relative humidity were carried out. The article contains a great deal of valuable data based on comprehensive testing and data analysis. New and original results and analyses of the interaction between Portland cement and silica fume related to compressive strength, splitting tensile strength, hydration and internal relative humidity are presented. The project was carried out between the years 1989 and 1996.