Sulfates in solution in contact with concrete can cause chemical changes to the cement, which can cause significant microstructural effects leading to the weakening of the cement binder (chemical sulfate attack). Sulfate solutions can also cause damage to porous cementitious materials through crystallization and recrystallization (salt attack). Sulfates and sulfites are ubiquitous in the natural environment and are present from many sources, including gypsum (calcium sulfate) often present as an additive in blended cements which include fly ash and other sources of sulfate. With the notable exception of barium sulfate, most sulfates are slightly to highly soluble in water.  These include acid rain where sulfur dioxide in the airshed is dissolved in rainfall to produce sulfurous acid.  In lightning storms, the dioxide is oxidized to trioxide making the residual sulfuric acid in rainfall even more highly acidic. Concrete sewage infrastructure is most commonly attacked by sulfuric acid and sulfate anions arising from the oxidation of sulfide present in the sewage. Sulfides are formed when sulfate-reducing bacteria present in sewer mains reduce the ubiquitous sulfate ions present in water drains into hydrogen sulfide gas ().  is volatile and released from water in the sewage atmosphere. It dissolves in a thin film of water condensed onto the wall of the sewer ducts where it is also accompanied by hydrogeno-sulfide () and sulfide () ions. When  and  anions are further exposed to atmospheric oxygen or to oxygenated stormwater, they are readily oxidized and produce sulfuric acid (in fact acidic hydrogen ions accompanied by sulfate and bisulfate ions) according to the respective oxidation reactions: 
or,
The corrosion often present in the crown (top) of concrete sewers is directly attributable to this process – known as crown rot corrosion.