This chapter examines the materials found in modern concretes in terms of their impact on the durability of concrete. Prior to the early 1930s, concrete was a relatively simple, straightforward material, consisting only of "pure" Portland cement, aggregate, and water. In most cases, not much can be done with the cementing materials themselves to eliminate leaching and efflorescence, or chemical attack from acids or from water containing significant amounts of industrial or agricultural wastes. These types of problems are governed by the chemistry of the cements and of the deleterious chemicals. The mechanisms involved in sulfate attack are complex, and have been described in detail by Skalny Concrete will be attacked by acids, the severity of the attack depending on the type of acid and its concentration. As with chemical attack, the first line of defense against physical attack is also to use as low a w/b ratio as is practicable for any individual structure.