THE EFFECTS OF A PHOSPHONATE INHIBITOR ON THE GROWTH OF ETTRINGITE IN IOWA HIGHWAY CONCRETE: PRELIMINARY STUDIES

Early deterioration of Iowa highways is due, in part, to the formation of expansive minerals such as brucite, portlandite, and ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂.26H₂O], in air voids and pore spaces in highway concrete. Ettringite is produced as a result of sulfate being added to the cement paste to help speed up concrete curing. It also forms later (sometimes years later) as the roadway ages. This "delayed" ettringite, as well as sulfates intermediate between ettringite and monosulfate [Ca₄Al₂(SO₄)(OH)₁₂.16H₂O], forms either as a rim lining the margins of air voids and pores, or it completely fills the void or pore. Cracks develop in the cement paste around these openings after they are completely filled. The reason for the formation of delayed ettringite is, in large part, due to the release of sulfur formed by the conversion of pyrite to goethite [FeO(OH)], and through the incorporation of sulfur impurities in highway deicing salts. We are investigating ways to inhibit the growth of delayed ettringite and monosulfate so as to minimize the number of completely filled air voids and pores, which, in turn, should reduce concrete damage. This is being done by evaluating the effect of addition of different concentrations of a phosphonate, Dequest® 2060S, which has proven to inhibit or modify the growth of ettringite crystals in controlled laboratory experiments. Slabs of highway concrete constructed in 1992 and 1997, which show variable degrees of deterioration, were treated weekly with NaCl deicer and inhibitor and left outside to be exposed to Iowa weather conditions for 1 year. In addition, wet-dry and freeze-thaw experiments were performed by placing highway samples in an oven and a refrigerator over the same period of time. These experiments mimic extremes in Iowa weather conditions. SEM images of ettringite crystals at the surfaces of treated samples, when compared to untreated samples, were stunted, rounded or completely corroded and, in places, converted to intermediate ettringite-monosulfate compositions. These experiments reaffirm that the growth of "delayed" ettringite can be controlled by chemical inhibitors. We anticipate being able to provide the Iowa DOT with a set of road maintenance guidelines it can use to minimize the effects of delayed ettringite on highway deterioration. In-situ experiments of highway US-20 with Dequest® 2060S are currently underway.