A discussion of the paper “Scanning electron micrographic studies of ettringite formation” by P. K. Mehta

Abstract

Some Iowa highway concrete constructed of coarse carbonate aggregate exhibits premature deterioration, which is, in part, caused by the growth of secondary minerals, including ettringite. Petrographic scanning electron microscope (SEM) and energy-dispersive analytical X-ray (EDAX) studies were conducted to determine the abundance, spatial location, and morphology of ettringite and the spatial relationship of ettringite to the occurrence of oxidized pyrite and coarse/fine carbonate aggregate.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.