Long-Term Hindrance Effects of Algal Biomatter on the Hydration Reactions of Ordinary Portland Cement

Adding chlorella or spirulina above 5wt% in ordinary Portlandcement induces a long-term hindrance of the hardening hydration reactions. Theincorporation of carbon-fixing materials such as photosyntheticalgae in concrete formulations offers a promising strategy towardmitigating the concerningly high carbon footprint of cement. Priorliterature suggests that the introduction of up to 0.5 wt % chlorellabiological matter (biomatter) in ordinary Portland cement inducesa retardation of the composite cement's strength evolutionwhile enabling a long-term compressive strength comparable to purecement at a lower carbon footprint. In this work, we provide insightsinto the fundamental mechanisms governing this retardation effectand reveal a concentration threshold above which the presence of biomattercompletely hinders the hydration reactions. We incorporate Chlorella or Spirulina, two algal specieswith different morphology and composition, in ordinary Portland cementat concentrations ranging between 0.5 and 15 wt % and study the evolutionof mechanical properties of the resulting biocomposites over a periodof 91 days. The compressive strength in both sets of biocompositesexhibits a concentration-dependent long-term drastic reduction, whichplateaus at 5 wt % biomatter content. At and above 5 wt %, all biocompositesshow a strength reduction of more than 80% after 91 days of curingcompared to pure cement, indicating a permanent hindrance effect onhardening. Characterization of the hydration kinetics and the curedmaterials shows that both algal biomatters hinder the hydration reactionsof calcium silicates, preventing the formation of calcium hydroxideand calcium silicate hydrate, while the secondary reactions of tricalciumaluminate that form ettringite are not affected. We propose that thealkaline conditions during cement hydration lead to the formationof charged glucose-based carbohydrates, which subsequently createa hydrogen bonding network that ultimately encapsulates calcium silicates.This encapsulation prevents the formation of primary hydrate productsand thus blocks the hardening of cement. Furthermore, we observe newhydration products with composition and micromorphology deviatingfrom the expected hardened cement compounds. Our analysis providesfundamental insights into the mechanisms that govern the introductionof two carbon-negative algal species as fillers in cement, which arecrucial for enabling strategies to overcome the detrimental effectsthat those fillers have on the mechanical properties of cement.