Sulfide soils are commonly encountered along the coastal regions between Sweden and Finland in post-glacial lowland areas. These silty and clayey soils are characterized by high natural water content, low undrained shear strength, high compressibility and significant organic content. Due to their poor geotechnical properties, sulfide soils are frequently excavated during infrastructure development projects. While sulfide minerals remain stable under waterlogged conditions, once excavated and exposed to atmospheric oxygen, they are subjected to oxidation which generates acidity and creates significant environmental challenges. As a result, excavated sulfide soils are often transported to specially designed landfills where they are either maintained under saturated conditions or treated with chemical buffering agents to minimize oxidation. These management approaches require strict environmental control and long-term maintenance, making sulfide soil disposal technically challenging and expensive. One alternative is the reuse of excavated sulfide soils in construction applications to reduce the transportation costs, landfill demand and environmental impact. However, these soils often do not possess adequate engineering strength for direct reuse and therefore require stabilization to improve their mechanical performance. In addition, as sulfide soils are susceptible to acidification, stabilization must also address the chemical behavior of the soil to ensure long-term performance. Consequently, understanding the interaction between soil chemistry and binder performance is essential for successful stabilization. This thesis investigates the strength development of sulfide soils with a focus on the factors controlling stabilization performance. 

The initial part of the work examines the behavior of cement-stabilized sulfide soils under varying sulfur content, organic matter, water content, oxidation state and mixing conditions. Unconfined compressive strength (UCS) testing demonstrated that water content is one of the most critical parameters affecting stabilization efficiency. High sulfur-high organic content soil (HS_HLOI) continuously shows less strength than low sulfur-low organic content soil (LS_LLOI) even at the similar water content. This indicates that sulfur compounds and organic matter interfere with cement hydration and contribute to strength reduction. High water content soils exhibited very low strength development whereas reducing water content significantly improved UCS. For HS_HLOI, UCS increased from 10 kPa at 70% water content to 65 kPa after reducing the water content to 50%, using 10% cement after 7d of curing. In contrast, LS_LLOI achieved strength exceeding 300 kPa and 800 kPa at 60% and 45% water content respectively after 7d of curing.  Oxidation increased the UCS of LS_LLOI under dry mixing conditions but caused only minor changes in HS_HLOI. The results further indicate that dry mixing of cement produced higher strength than wet mixing under laboratory conditions for most cases. SEM-EDS and XRD analysis provide further understanding of hydration products like C-S-H gels, ettringite, and gypsum formation, voids and particle bonding mechanisms. 

The second part of the study evaluated the acidification behavior of sulfide soils collected from multiple locations in Sweden using incubation and accelerated cyclic leaching methods. The results showed that acidification behavior depends on the balance between sulfide content and buffering capacity rather than sulfur concentration alone. Soils with low Ca/S ratios exhibited high acidification while soil with high buffering capacity maintained stable pH values during oxidation cycles. The accelerated oven-dry method produced rapid acidification whereas incubation and air-dry methods better represented gradual field oxidation conditions. These findings indicate that oxidation behavior and pH evolution are important factors that reduce overall pH and influence binder performance during stabilization.

Based on the limitations identified in cement stabilization and to reduce the environmental footprints associated with cement use, this thesis introduces alkali-activated binders derived from industrial by-products such as ground granulated blast furnace slag (GGBS) and calcium-based fly ash as potential sustainable alternatives to conventional cement-based binder systems. A preliminary screening experimental program was therefore developed to evaluate the feasibility of cement-free alkali-activated stabilization of sulfide soils. The screening study provides a starting point for future research focused on binder optimization, reaction mechanisms, durability and long-term performance. This thesis shows that successful stabilization of sulfide soils requires an integrated understanding of geotechnical behavior, soil chemistry and binder interactions while highlighting the potential of sustainable alkali-activated binders for future sulfide soil stabilization and its applications.