Assessment of mussel shells for the development of different bio-based building materials

Abstract

Assessment of mussel shells for the development of different bio‐based building materials Cultured mussels are produced worldwide, and Spain is one of the top producers, being the shores of Galicia the centre of this industry, with an annual production in Galicia of about 200,000 tonnes. These mussels are sold in fresh although a large fraction is processed and commercialized canned (approximately 38%). This canning industry sells only the flesh discarding the shells that subsequently become a by‐product. The estimation of waste generation of this industry only in Galicia is of 25,000 annual tonnes. The management of this waste is not solved and nowadays some of the waste is dumped in uncontrolled landfills. This is also a global problem and it was approached in numerous publications since the 1990s. This doctoral thesis analyses the use of mussel shells as bio‐based building material in different applications such as aggregates for the manufacture of concrete, innovative coating mortars and as loose‐fill insulation. The mussel shell was heat‐treated at 135 °C for 30 minutes and then crushed and sieved to produce gravel and sand. The aggregate are composed mainly of calcium carbonate (>95%) with traces of organic compounds and small amounts of sulphate and chloride. The first application considered was the production of plain concrete. The results lead to establish that mussel shell replacement should be limited to 25% of fine or coarse aggregates, or 12.5% of both fine and coarse aggregates. With these percentages the concrete displays a correct behaviour and an interesting aesthetic finishing with fragments of seashells. The second application was mortars for coatings, in these, the 25%, 50% and 75% of the aggregate was replaced by shell aggregates. The binders considered were cement and lime that was studied as sustainable alternative. The cement‐based mortars agreed with the results in concrete with a recommended replacement rate of 25%. This limit is also the recommendation for the lime mortars, although with larger quantities the changes are not severe, and the finishing could be appealing. Mussel shell aggregates affect workability, density, and mechanical properties of coating mortars with both binders. Also, the capillary uptake was reduced alongside with the enhancement of water retention. This was attributed mainly to the organic matter content and flaky shape. This two parameters also affect the carbonation of lime mortars, at young ages it was decreased while the total carbonated area was increased at old ages when compared with baseline. The third application was to use mussel shell aggregate as loose fill material. This mussel shell material has been found suitable for using as building insulation material. Mussel shell confined inside an enclosed space (e.g. a wooden box) has a thermal conductivity similar to that of a light conifer wood, so it can be considered a material with low thermal conductivity. In addition, the material displays an acoustic behaviour comparable to others commercialised insulation materials. This research work was part of the Biovalvo Project "Assessment of Galician bivalve shell in the construction sector ". The last step of this project was the erection of an experimental building that includes all the building materials described in this research: roof, walls, floor and foundation included mussel shells with very good outcomes. The results of this experience will be published in scientific papers after this document although a description of the building and the preliminary results are presented in annex A of this thesis.