The high purity of calcium carbonate content (95–97%) of mussel shells makes this filler, when powdered, a suitable component in cement, and in particular as the replacement for aggregate. In this sense, mussel shells represent one of the so-called “aquaculture modifiers”, together with, e.g., oyster shells or scallop shells, which are globally presented in [25]. In the case of polymers, calcium carbonate is frequently used to reduce their cost, improve their properties (in particular hardness), and better control their rheology during molding. As a substitute for extracted limestone, mussel shells are lately in competition with eggshell powder (ESP), which also has around 95% calcium carbonate content [26]. The two fillers have their origin as food industry waste in common. However, it can also be observed that calcium carbonate is prevalently formed by aragonite crystals, a high-density and -hardness material; thence it is very adapted as a bio-filler for polymer matrices [27]. Some uncertainty remains about whether the few percentages of proteinaceous matter present in both of them would effectively link with the polymer or should be removed, possibly by thermal degradation, as they might harm the performance of the manufactured materials [28].

The use of seashells in thermoplastic polymers requires more sophisticated methods, such as directly blending in polymer pellets before twin-screw extrusion, such as in [29] where proportions of seashells (SS) of up to 18% were introduced in a nylon-6 matrix. Seashell content of up to 15% increases the composite crystallinity, leading to higher storage modulus, and at 18% content, SS leads to a more pronounced elastic behavior, as indicated by the lower loss modulus, though the temperature for degradation onset is lower for higher oxidative stability.

Another possibility regarding the use of calcium carbonate from seashells is its introduction in a bio-based polymer, as is the case for bio-epoxy. The work from Fombuena et al. evidenced as two main points the significant increase in flexural properties, up to 50%, achievable through the introduction of 30 wt.% seashell calcium carbonate, and, once again, the higher thermal stability subsequently reached, with an increase in glass transition temperature from 80.6 °C to 91 °C [30]. A similar work, though focusing on bio-polyurethane with 25% castor oil resin content, was carried out in [31] to improve the sound absorption characteristics of polyurethane foam, namely below 500 Hz, and at the same time to enhance its compression strength. To try to standardize test results, especially in regards to thermal characterization, it is essential to offer indications about the exact species from which the shells are obtained, as was the case for one of the first studies in the sector, concerning Rapana Thomasiana, a type of sea-snail [32]. To continue with the specific applications of seashell powder obtained from selected species encountered in sea waste, the use of cowrie shell powder in polypropylene-acrylonitrile injection-molded composites has also been reported [33].

1.2. Cellulosic Fillers