The formation and role of ettringite in Iowa highway concrete deterioration

In poorly performing concrete (<16-year service life), ettringite completely fills many small voids, occurs as rims lining the margin of larger air entrainment voids and as microscopic disseminations in the paste. Pyrite (FeS2) is commonly present in coarse aggregate, and goethite [FeO(OH)], one of its oxidation products, is observed in many concrete samples. Sulfate ions derived from pyrite oxidation apparently contribute to ettringite formation. The direct precipitation of ettringite from solution was responsible for most of the observed ettringite in voids and cracks. Microscopic ettringite, which commonly occurred in the paste, most likely was formed by the replacement of calcium aluminate. Severe cracking of cement paste is often spatially associated with ettringite, which strongly suggests that ettringite contributed to cracking and resultant deterioration.