Stabilization of sulfide soil with by-product originated hydraulic binder in a region with seasonal frost – A field investigation

Fine-grained soils often show a low bearing capacity as well as a high frost susceptibility. These aspects are a challenge for the needs of infrastructure. The addition of hydraulic binder to fine-grained soils is common worldwide to improve the soil properties for engineering purposes. The classical hydraulic binders are lime and cement, but nowadays more and more by-producs are used as well like e. g. fly ash, slag or filter dust. The binders are called “hydraulic” because they react with water. By this reaction new minerals are formed, connecting the soil particles together. This improves the properties of the soil: A strength increase is even visible when the curing takes place in cold environment or after freezing and thawing cycles. Another challenge that occurs in fine-grained sulfidic soils is their possible acidification, when aerated due to e. g. excavation or drainage. Sulfide minerals in contact with oxygen produce sulfuric acid. The low pH caused by this oxidation can mobilize metals from the soil minerals, with harmful consequences for the environment. The addition of lime or calcite is one possible action to improve acid sulphate soils for agricultural and aqua-cultural purposes. The high pH of the lime and calcite increases the buffer capacity of the soil. However, the addition of hydraulic binder to a fine-grained sulfide soil in order to improve both the engineering properties and to buffer the potential acidification is sparsely investigated. In the present publication a field investigation is described where a cement mixture with cement kiln dust (CKD) is used alone and in combination with a calcite-rich by-product from the paper industry to improve a fine-grained sulfide soil for possible usage in earthworks. Samples taken from the surface after one year show a buffering of the potential acidification. Additionally, a strength increase can be seen in the stabilized soil when compressed and stored in a tube in field conditions.