Seeding improves the strength of bio-tiles grown with microbially induced calcium carbonate precipitation

Bio-tiles are a biobased alternative to conventional tiles that utilise a promising technology called microbially induced calcium carbonate (CaCO₃) precipitation (MICP). This technology has low energy requirements and also sequesters carbon. Bio-tiles have been made in previous work using a submersion method, however, the process required additives such as 0.3 M magnesium chloride to achieve bio-tiles that meet international standards. The current study aimed to improve the bio-tile strength properties with CaCO₃ crystal seeding and a pumping method instead of the use of magnesium that also increases ionic strength. With this technique, cementation solution containing the required calcium and urea for the MICP reaction was pumped through a sealed mould in a series of programmed treatments. The highest concentration of ureolytic Sporosarcina pasteurii with an effective urease activity of 40 mmol NH₄-N/L·min was found to be most beneficial to the breaking strength of the bio-tiles, as were the shortest retention times of 1 h between treatments. Seeding with CaCO₃ crystals offered significant benefit to the MICP process. Pre-seeding of the geotextiles was explored and the mass of seeds initially present on the geotextiles was found to have a direct improvement on the breaking strength of 21-82 %, increasing with seed loading. The highest CaCO₃ seed loading tested of 0.072 g seeds/cm² geotextile resulted in bio-tiles with a breaking strength of 940 ± 92 N and a modulus of rupture of 16.4 ± 1.7 N/mm², meeting international targets for extruded tiles with 6-10 % water absorption. When a seed loading of 0.021 g/cm² was used instead, bio-tiles meeting targets for tiles with a water absorption of >10 % were produced at 628 ± 18 N and 10.5 ± 1.1 N/mm².