The authors report on concrete incorporating ground pelletized blast-furnace slag evaluation findings after being exposed to a marine tidal zone environment for 25 years. In order for laboratory evaluation to occur, two large concrete blocks (305 x 305 x 915 mm [1 x 1 x 3 ft]) were sampled after being retrieved from the exposure site. Three series of mixtures with 0.40, 0.50, and 0.60 water-cementitious material ratios (w/cm) supplied the specimens; concrete mixtures within each series contained 0, 25, 45, or 65% slag by total cementitious materials mass. For concretes containing higher levels of slag (45 or 65%) with a w/cm or 0.50 or 0.60, there was observation of severe surface erosion, increasing slag levels correlating with increasing levels of damage. For all concretes with a 0.40 w/cm, however, the surface condition was satisfactory. Determination of chloride ion penetration depth, hardened air-void parameters, chloride diffusion, chloride permeability (ASTM C1202) splitting strength, modulus of elasticity, and compressive strength were included in laboratory testing. After 25 years of exposure, chloride penetration depth for all control concretes (i.e., without slag) was greater than 100 mm (4 in.) regardless of the w/cm. Significantly greater chloride ion penetration resistance (generally <50 mm [2 in.] for 45 and 65% slag) was shown by slag concretes, with penetration depth decreasing with decreasing w/cm and increasing slag content. In terms of mass transportation reduction, laboratory tests confirmed slag's beneficial effect with more than tenfold reductions in chloride permeability and diffusion coefficients shown by the slag concretes. That slag use, provided the w/cm is kept low (i.e., w/m is less than or equal to 0.40), at relatively high replacement levels by North American standards (i.e., 45 to 65%) results in a significant concrete performance increase in a very aggressive marine environment is indicated by the data.