The denser and the more waterproof a concrete is, the higher is its resistance against attack by sulphates. The addition of fly ash to a concrete increases its sulphate resistance. The fly ash reduces the porosity of concrete, and its spherical grain shape densifies the internal structure of concrete. The sulphate resistance plays an important role, for example, in the production of sewage pipes and in foundation construction, because these objects are often exposed to water or soils that contain sulphate.