Ettringite Swelling in the Treatment of Sulfate-Containing Soils Used as Subgrade for Road Constructions

In earthworks and road construction, several cases of damage caused by the formation of the mineral ettringite are known. If soils with natural sulfate content were treated with calcium-based binders, the binder may react under certain conditions with sulfate ions to cause the mineral ettringite. Ettringite has a very high crystal water content and is therefore highly voluminous and lightweight. Due to this increase in volume, the mineral reaction may cause uplift damage. Although factors are known for this mineral reaction in the soil, quantitative testing methods and standardized prevention strategies are currently not yet available. The aim of this study is to develop the basis for a practical test specification. With the help of adequate research and testing, in the future it should be possible to avoid damage cases. First, a literature study was conducted. It became clear that the mineral content, the water supply, the chemical environment, the temperature, the pore structure and the overburden pressure in the soil are key factors. In the second step, comprehensive powder-swelling-tests have been carried out on different materials. There is a relation between the sulfate content and the volumetric expansion due to ettringite formation. Critical sulfate contents are strongly dependent on the soil and can be influenced by the choice of the binder. It could be confirmed that treatment with burnt lime caused more heaving under the same boundary conditions, than the use of cement binder.Indications to evaluate the risk of ettringite swelling were developed for engineering practice and recommendations for test methods are proposed.