Killing the Ettringite Reaction in Sulfate-Bearing Soils

The formation of ettringite in sulfate-bearing clay soils often results in rapid and significant expansion when calcium-based stabilizers are added to the soil. The researchers tested the hypothesis that ettringite crystal growth could be inhibited by adding a specific chemical that would prevent nucleation of ettringite crystallites, much like retarding the setting of portland cement by using organic additives. The researchers created ettringite in the laboratory by using clay mineral standards mixed with hydrated lime, gypsum, and water, with an ultrasonic homogenizer to catalyze the reaction. A natural soil from Texas (State Highway 289) that had caused sulfate-induced heave problems was also used. Diatomaceous earth, volcanic glass (amorphous silica), and calcium phosphate monobasic monohydrate were added in different proportions to see whether the formation of ettringite could be stopped. The diatomaceous earth and volcanic glass did not inhibit the growth of ettringite in any of the clay mineral standards or natural soil tested. However, the calcium phosphate monobasic monohydrate generated tricalcium aluminate monosulfate hydrate instead of ettringite in the kaolinite and in the natural soil, prevented the formation of both ettringite and tricalcium aluminate monosulfate hydrate. Therefore, phosphates may be a viable additive for prevention of heave in sulfate-bearing soils.