Från restmaterial till resurs: Potentialen för invasiva ostronskal som bindemedelsersättare

Sammanfattning

The construction and real-estate sector is highly resource-intensive, underlining the need for sustainable material choices that can substitute finite natural resources. One viable pathway is to transform by-products and waste streams from other sectors into useful construction materials. One such potential resource is the shell of the Pacific oyster (Magallana gigas), an invasive species that has spread uncontrollably along the Swedish west coast over the past two decades, posing a significant threat to local biodiversity. Management and mitigation of this ecological challenge generate large quantities of biological residual material, which currently lacks practical application and ends up in landfills. Since both conventional limestone and oyster shells consist primarily of calcium carbonate, there is a sound chemical basis for using oyster shells as a substitute for conventional construction lime. The overarching aim of this study was to evaluate, through experimental investigations, the potential of using oyster shells as a sustainable alternative to conventional lime in plaster and mortars. More specifically, the study examined how the use of oyster shell lime affects the early-age E-modulus (up to 42 days), workability, and consistency in both lime-cement mortar and air lime mortar, in comparison with conventional reference mortars. Laboratory work formed the core of the study and was conducted in accordance with applicable European standards to ensure reliable and comparable results. The oyster shells were calcined to produce quicklime, which was subsequently slaked and used in the production of plaster and mortar. The mechanical properties of the specimens were evaluated weekly up to 28 days and once again at 42 days of curing, in order to assess the influence of oyster shell lime on mortar performance. This included measurements using the Free-Free Resonance method (FFR), through which mortars containing oyster shell lime were compared with conventional reference mortars. As FFR is a non-destructive testing method, continuous monitoring of the same specimens over time was possible, reducing the total number of specimens required compared to destructive testing methods. These measurements were complemented by destructive mechanical testing at the end of the study period to verify and cross-validate the results obtained by the two methods. The results from the calcination trials demonstrated that complete calcination of the oyster shells required a temperature of 900 °C for 3 hours, which resulted in a brittle material structure compared to burning at 800 °C for 2 hours. During mortar production, the oyster-shell lime resulted in a more grainy consistency and an increased tendency towards water separation. This was attributed to the material’s coarser particle size and consequently lower specific surface area, which shifted the effective mixing ratio toward a theoretically leaner mix. The E-modulus development up to 42 days displayed two opposing patterns for the two investigated mortar types. For the lime-cement mortar, the water separation led to a retarded cement hydration and approximately a halved mechanical strength. For the air-lime mortar, however, the coarser grain structure provided a mirrored, positive effect. The oyster-shell lime generated a more open pore structure that facilitated carbon dioxide diffusion and accelerated the curing via carbonation, resulting in a nearly doubled strength compared to the reference mortar. The study concludes that shells of the invasive Pacific oyster possess a high potential as a circular binder in the construction sector, provided that their application is directed towards pure air-lime mortars where the physical properties of the material constitute a mechanical advantage.