Clay Soil Heave Caused by Lime-Sulfate Reactions

Reactions between lime, alumina released from clay during pozzolanic reactions, and sulfates present in some soils (causing the formation of the highly expansive crystalline mineral ettringite) have been responsible for the deterioration and ultimate failure, by expansion, of several lime stabilization projects. The mechanisms for these reactions were hypothesized, and a laboratory research program using lime-treated artificial soil samples of compacted kaolinite-sand and montmorillonite-sand mixtures, incorporating known amounts of sulfates, was designed and undertaken. The strength, swelling, compositional, and micromorphological characteristics of the treated samples were determined following different curing times and soaking conditions. Ettringite formation was evidenced in all lime-treated samples, whenever sulfates were present. A non-expansive monosulfate calcium-aluminum-hydrate forms first in the high alumina content lime-treated kaolinite-sand mixes. The monosulfate converts to an expansive trisulfate form (ettringite), after a period of a few months. Conversely, ettringite starts to form at the early curing stages (after a few days) in the low alumina lime-treated montmorillonite-sand mixes. It was found that the amount of heave following ettringite hydration and growth is a function of the amount and rate of release of alumina into solution. The amount and type of sulfates present is also a factor influencing the quantity and crystal morphology of ettringite formed. Moreover, temperature and humidity fluctuations were also found to play an important role in the overall ettringite-related heave mechanism, as they affect reaction rates, solubilities of species, and the overall stability fields of a soil system's components. Continuing research will extend the results reported here to a more quantitative specification of conditions leading to deleterious reactions in lime stabilization applications. Concurrently, treatment methods that may prevent such adverse reactions are being explored.