Ettringite deposits in air voids

Investigations of pavements suffering from freeze-thaw damage in the presence of deicing salts have found significant deposits of ettringite in the air voids. Concern has been expressed that the deposition of ettringite in air voids reduces their effectiveness in protecting the concrete. A series of experiments intended to reproduce the process of deterioration in the laboratory allowed the authors to observe the sequence of events. A suite of cements was selected to provide a range of C(3)A contents, sulfate contents, and sources of sulfate within the cement (that is, either the clinker or the gypsum). Specimens were subjected to a modified version of ASTM C666 in which salt solutions with and without added gypsum were substituted for the water, and the specimens were allowed to dry periodically. Sacrificial companion specimens were examined petrographically, and the observations were correlated with mass loss, fundamental transverse frequency, and length change. The results demonstrate that the deposition of ettringite does not adversely affect the ability of the air voids to protect the concrete. The sequence of events (microcracks first, then ettringite deposits), the morphology of the ettringite (long, loosely packed needles), and the lack of association of air voids containing significant ettringite deposits with cracks all indicate that the deposition of ettringite is opportunistic. The observed large deposits of ettringite are a consequence of saturation and freeze-thaw damage, not a cause.