What Does Gypsum Mean?
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate. It forms as an evaporite mineral and as a hydration product of anhydrite. It occurs chiefly in sedimentary deposits and is used to make plaster of Paris and fertilizers, as well as in the building industry. Its chemical formula is CaSO4·2H2O. Gypsum contains calcium, sulfur bound to oxygen and water. It is an abundant mineral and widely mined.
In structural engineering, from a corrosion-technical point of view, contacts of metals and mineral building materials that are produced with cement and gypsum play an important role. Gypsum is a major constituent of backfill materials in cathodic protection.
Corrosionpedia Explains Gypsum
Gypsum consists of hydrated calcium sulfate. It is moderately water soluble and, in contrast to most other salts, it exhibits retrograde solubility, becoming less soluble at higher temperatures. Gypsum crystals are found to contain anion water and hydrogen bonding.
Although gypsum is available in nature, it is also formed as a byproduct of sulfide oxidation, amongst others, by pyrite oxidation. Under reducing conditions, the sulfates it contains can be reduced back to sulfide by sulfate-reducing bacteria. Electric power stations burning coal with flue gas desulfurization produce large quantities of synthetic gypsum as a byproduct from the scrubbers. It can be used interchangeably with natural gypsum in some applications.
In cathodic protection of metals, gypsum is used as backfill materials. The chemical backfill used with galvanic anodes provides an environment which is conducive for anode dissolution. A typical mixture is 75% powdered gypsum (calcium sulfate), 20% granular bentonite and 5% sodium sulfate. This mixture has a resistivity of 50 W cm and is suitable for use in high resistivity soils.
Gypsum/gypsum mortar attacks zinc and iron (steel) very strongly in combination with humidity. Steel pipes and galvanized steel pipes that are in contact with gypsum, which was moistened long-term, are attacked by thick rust products and can be destroyed after only a few years. Aluminum materials and lead in general are not likely to be attacked by the more neutrally reacting gypsum building materials. In aluminum that is free of copper, moist gypsum promotes a limited pitting corrosion. Aluminum alloying, that contains copper, sometimes corrodes quite intensively in wet gypsum.