In general terms cement products have good durability, but their strength will decrease or even damage structures with the role of some erosive media (such as soft water, acid water), known as the erosion of cement paste. The main reasons for corrosion are: Soft-water Corrosion (Dissolution Corrosion) Rain, snow, distilled water, industrial condensate water, and the river water and lake water with low bicarbonate content, all of them belong to soft water.
When hardened cement paste has contacted with these kinds of water for a long time, calcium hydroxide in the cement paste dissolves first (per liter water can dissolve calcium hydroxide of 1. 3g). Under the role of still water or zero-pressure water, the dissolution will stop because the surrounding water is easy to get saturated due to the dissolved calcium hydroxide, and dissolution only occur on the surface, little impact.
But if the cement paste is in fluid water or pressure water, the dissolved calcium hydroxide is easy to be washed away, the density of gypsum keeps decreasing, and even other hydrates will dissolve. The corrosion gets to the inner part to enlarge the cement pores and lower the strength, which will destroy the structures of cement paste to totally collapse. When the environmental water contains bicarbonate, bicarbonate will react with the calcium hydroxide in cement paste and generate the insoluble calcium carbonate: Ca (0H) 2+Ca (HC03) 2=2CaC03+2H20.
The generated calcium carbonate accumulates in the pores of hardened cement paste, forming dense protective layer to prevent the infiltration of water outside and the solution of calcium hydroxide, thus preventing the erosion. In many projects, the cement structures which will contact with soft water should get hardened in the air at first to form a layer of calcium carbonate, which may protect the structures from dissolution corrosion. Acid Corrosion •Carbonate Corrosion When carbon dioxide dissolving in industrial effluent and groundwater.
The carbon dioxide in water reacts with calcium hydroxide in cement paste and generates calcium carbonate. If continuing to react with phenolated water, it will change into soluble calcium bicarbonate. The structure of cement paste will be destroyed because of the dissolution of calcium bicarbonate and the decomposition of the other products in cement paste. •Normal Acid Corrosion There are inorganic and organic acids in industrial effluent, groundwater and marsh water; there is sulphur dioxide in the gas from industrial furnaces which generates sulfurous acid by contacting with water.
Various acids have different degrees of corrosive effect on cement paste. They react with calcium hydroxide in cement paste and generate chemical compounds which can dissolve in water or can expand in volume, leading to the damage of cement paste. Among all these acids, the ones with the fastest corrosive effect are hydrochloric acid, hydrofluoric acid, nitric acid, and sulfuric acid (which are inorganic acids) and acetic acid, formic acid and lactic acid (which are organic acids).