Dissolved sulfates react with aluminate phases and calcium hydroxide in hardened paste to form expansive ettringite and gypsum, which crack, soften and eventually crumble the concrete. Foundations in sulfate-rich soils of arid regions and wastewater structures are the classic victims. Defenses are a low w/cm, Type II or Type V cement and SCMs such as Class F fly ash or slag, chosen by exposure class from the soil test.
Geotech report shows high sulfates, so the footings need Type V and a 0.45 w/cm to hold off sulfate attack.
Also heard as
- sulfate exposure
- external sulfate attack
Related terms
Type I/II cement
The everyday portland cement, certified to meet both general-purpose Type I and moderate sulfate-resistant Type II limits.
Fly ash
Fine powder captured from coal-fired power plant exhaust, used to replace part of the cement in concrete.
Slag cement
Ground granulated blast-furnace slag, an ironmaking byproduct that replaces part of the portland cement in a mix.
w/cm
Water-to-cementitious-materials ratio: the weight of mix water divided by the weight of cement plus SCMs.