FROM NUISANCE TO RESOURCE: Sustainable Utilisation of Pacific Oyster (Magallana gigas) Shells in Swedish Coastal Ecosystems

Sammanfattning

The rapid expansion of the invasive Pacific oyster (Magallana gigas) along the Swedish west coast has created significant ecological and management challenges, resulting in a standing biomass of approximately 23,000 tonnes. While removal is essential for conservation, the resulting shell waste, constituting up to 70% of total mass, lacks a scalable utilisation pathway. This thesis investigates the feasibility of transforming this invasive biomass into value-added construction materials through a triple-substitution strategy: (i) as a filler material, (ii) as a partial aggregate replacement, and (iii) as a partial binder replacement within cementitious mortar systems. A multi-stage experimental programme was conducted to evaluate processed shell fractions of different particle sizes under various treatment conditions, including outdoor weathering and thermal calcination at 850−900∘C. Eleven mortar formulations were tested at 28 days for mechanical performance, comparing shell-inclusive mixes against high-performance reference controls. The results indicate that oyster shell-derived materials can be incorporated into mortar systems under selected processing and mix-design conditions. Small additions of fine shell powder were associated with increased compressive strength relative to the reference mix. The strongest shell-containing formulation (M4), which combined multiple material modifications, achieved a compressive strength of 77.25 MPa. At higher substitution levels, cohesion limits were observed, with some formulations failing during curing or demoulding. These findings provide an initial proof-of-concept for the use of Pacific oyster shells in cementitious materials. The results indicate that shell-derived materials may contribute to circular material strategies in coastal regions, particularly where invasive-species management generates large shell streams. However, because multiple variables were altered simultaneously, the results reflect formulation-level behaviour rather than independent material effects. Further work is required to evaluate long-term durability, isolate processing variables, and assess environmental and economic feasibility before large-scale applications can be recommended.