This paper presents test data on the development of the microstructural porperties of concrete containing 50% replacement of cement by ground granulated blast furnace slag of varying fineness. The following characteristics and properties were investigated: pore structure; hydration and rate of heat evolution; microstructure by scanning electron microscopy and water permeability. It is shown that the use of very fine slag of about 1200 sq m/kg specific surface area results in accelerated hydration, high early strength of 30 to 50 MPa at 3 days, high 28-day cylinder compressive strength of 100 to 110 MPa, very low total pore volume and fine pore sizes, a highly densified microstructure and very low water permeability diffusion coefficient.