Stabilisation of Sulfide‐Bearing Clays as a New Building Ground

ABSTRACT Fine‐grained, sulfide‐bearing marine clays are found along the Finnish coastline. When excavated and exposed to air, the sulfides within these soils react with oxygen to form sulfuric acid. Such soils are classified as hypersulfidic soils. The acid can then lower the soil's pH, causing harmful elements to be released into the environment through runoff water. Once the oxidation process has been initiated, these soils are classified as sulfuric soils, and together with hypersulfidic soils, they form subgroups of acid sulfate (AS) soils. Stabilising the soils (i.e., hypersulfidic soils) can prevent the leaching of metals and metalloids. Clay‐rich AS (AS clay) and non‐AS (non‐AS clay) soil materials were collected from different areas in the city of Turku in Southwestern Finland. The aim was to find solutions for stabilising urban fine‐grained sulfide‐bearing clays to enable a safe beneficial use. Another objective was to study possible differences in strength development between AS clays and non‐AS clays. Stabilisation formulations for the soils were systematically developed using cement, blast furnace slag, quicklime and ash. Different levels of binders mixed with the clays were pressed into cylindrical samples, and the unconfined compressive strength was measured after 7, 28 and 91 days. The leaching of elements was determined on the stabilised samples. The morphology of the stabilised samples was analysed using SEM/EDS. Industrial side streams were successfully utilised for the stabilisation of the clays. Up to 65% of cement could be replaced with blast furnace slag without decreasing the long‐term strength values, suggesting a significant reduction in carbon footprint, calculated as CO 2 emissions. The compressive strength was significantly lower for the stabilised AS clay samples, implying more binders are needed to achieve the same strength as for the non‐AS clays. It should be noted that water content and particle size also influenced the compressive strength. When the AS soil samples were incubated, harmful metals were leached out as expected. However, stabilising the soils prevented the decrease in pH, effectively immobilising harmful elements in AS soils and making them suitable as building ground material.