Environmentally sustainable concretes containing the waste-replacements pulverised fly ash (PFA) and ground blast furnace slag (GGBS) as direct replacements for Portland cement (PC), are widely regarded as important carbon-reducing materials.
This is because they are by-products that would otherwise be thrown away, so their use in concrete matrices is both economically and environmentally beneficial in both marine and terrestrial infrastructure.
During my master’s thesis ‘Implications of waste-aggregate cement replacements on biofilm formation and potential impacts on fisheries and human health’, I tested whether waste materials or aggregates such as those from the iron and coal industry, impacted young biofilm composition when added to concrete mixtures.
I was particularly interested in its comparisons to standard grade concrete in varying areas of salinity, as the addition of these waste replacements change trace metal concentrations availability to the wider environment and can influence the microbial colonisers found. I was looking to assess the settlement of toxin producing phytoplankton within the microphytobenthos and its primary productivity. Harmful algal bloom (HAB) species such as Pseudo-nitzschia spp. (image above) encourage the build-up of toxins in shellfish when consumed by them, which can lead to increased illness (and in extreme cases, death) among consumers.
The sample collection consisted of deploying concrete settlement tiles along the salinity gradient in three estuaries in Devon. After three days submerged samples were collected, preserved in glutaraldehyde and catalogued.
Half the samples were then chemically fixed and processed in a lab using a variable pressure-scanning electron microscope to enumerate and identify the phytoplankton present, with the remaining half assessed for photosynthetic productivity using fluorimetry, a process that uses high frequency light pulses to measure chlorophyll a florescence.
The three concrete treatments and three salinity regimes were found to have a significant impact on the primary productivity of the biofilm. The highest quantum yields (Fv/Fm) were observed on GGBS (0.0215) in marine water, PFA (0.0205) in brackish and PC (0.0155) in a freshwater environment.
However, the settlement of harmful algal species was not observed to be impacted by these variables, suggesting that these sustainable concretes do not increase the potential for fisheries disruption but do have the potential to influence biofilm photosynthesis.
My findings suggest that their uses could be adjusted accordingly if there is a requirement to improve the productivity of an aquatic ecosystem, with no adjustments needed in regard to HAB species settlement as this was not influenced, further research should be conducted to test these findings.
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