Bacteria have been cementing sand into rock for a very long time, at ambient temperature and with no kiln. From that fact it is tempting to conclude that Bio-Cementation must be a low-impact alternative to industrial binders. A paper published on 15 December 2021 in the open-access journal Sustainability (volume 13, article 13878) by Hannah Porter, Abhijit Mukherjee and Navdeep Kaur Dhami of Curtin University and Rabin Tuladhar of James Cook University asks whether that inference survives an accounting of what engineers actually pour into the ground. Its answer is conditional: some microbial routes come out ahead of factory-made calcium carbonate, the most popular route does not, and much of the result turns on how pure the purchased chemicals are.
The argument: engineered biocement is not the natural process
The authors do not dispute that natural mineralisation in corals, stromatolites and beach rock is sustainable. Their point is that it runs over geological time on dissolved carbon and calcium present at micromolar concentrations. A construction project wants a result in under 15 days, so both the bacteria and their feed are enriched, and researchers have typically used purified laboratory-grade reagents at concentrations far above anything found in nature. Producing those reagents costs energy.
The paper's background figures show why this matters. It cites production energy of about 6.21 MJ/kg for ordinary Portland cement against 30.54 MJ/kg for urea, and field-scale work indicating that 0.6 kg of urea and 1.1 kg of calcium chloride are needed for each kilogram of calcium carbonate made by the ureolytic route. It also cites an earlier costing of biocement as a surface treatment for building materials, in which the urea and calcium chloride solution made up roughly 47% of a total of 23–28 EUR per square metre.
What was compared, and how
The study models six metabolic routes to Microbially Induced Calcite Precipitation (MICP): four heterotrophic (urea hydrolysis, denitrification, ammonification by myxobacteria, and methane oxidation) and two autotrophic (carbonic anhydrase-producing bacteria and photosynthesis by cyanobacteria). Each is set against the conventional industrial carbonation process. The unit of comparison is 1 kg of precipitated Calcium Carbonate (Calcite, CaCO₃), and four things are tallied for it: carbon footprint, eutrophication potential, embodied energy and raw-material cost. Every route is run twice, once with laboratory-grade inputs and once with commercial-grade substitutes.
This is a desk calculation, not an experiment. It was built in SimaPro 8.0 with the Ecoinvent V2.2 database, the Australian AUSLCI inventory and the Cumulative Energy Demand 2.01 method, and the paper says it is based on laboratory-scale data. The authors describe it as "a high-level assessment" meant to steer research, and say that a full life cycle analysis is prevented by the lack of industrial-scale data.
What the numbers say
Carbon footprint. With laboratory-grade inputs, the paper reports that every route except denitrification has a lower carbon footprint than carbonation, because nothing has to be calcined. The two autotrophic routes are additionally credited with drawing 0.44 kg of carbon dioxide per kilogram of CaCO₃ from the air. The best performer, the carbonic anhydrase route, is given as 0.681 kg CO₂ per kg CaCO₃, which the authors put at 70% below carbonation.
Eutrophication. Here the common routes do badly. Ureolytic MICP has the highest figure (0.24 kg SO₄/kg CaCO₃ in the paper's units), followed by myxobacteria (0.065) and denitrifying bacteria (0.02); all three exceed carbonation, and the paper attributes this to their nitrogen-bearing by-products. The other routes are described as having minimal eutrophication potential.
Embodied energy. With laboratory-grade chemicals, every microbial route is reported to need significantly more energy than carbonation, ranging from 12.9 MJ (carbonic anhydrase) to 28.4 MJ (ureolysis). Laboratory-grade calcium chloride alone accounts for between 44 and 98% of the total.
The effect of cheaper chemicals. Swapping in a less pure commercial calcium chloride, and a nitrate fertiliser in place of sodium nitrate for denitrification, changes the picture sharply. Embodied energy falls by between 43 and 95% in every route but denitrification, and the carbonic anhydrase route drops to 0.619 MJ, reported as 91% below carbonation. Carbon footprints fall too, although the paper gives two different ranges for the reduction: between 18% and 49.62% in one passage and between 26.7 and 82% in another.
Cost. Laboratory-grade inputs make the microbial product enormously more expensive than carbonated calcium carbonate, which the paper prices at 0.4 USD/kg: the stated range runs from 62 USD/kg (cyanobacteria) to 138 USD/kg (ammonification or methane oxidation). Commercial-grade inputs are said to bring costs "into the same range" as carbonation while still remaining higher.
The verdict on ureolysis
The paper's overall ranking places carbonic anhydrase bacteria first, then methane oxidation, then cyanobacteria; with laboratory-grade inputs these three already beat carbonation on overall environmental impact. The route that dominates practice, Ureolysis-Driven Carbonate Precipitation, is judged to have poor sustainability on two counts: the carbon footprint and embodied energy of the urea itself, and the eutrophication potential of the ammonium it releases. In discussing the commercial-grade results, the paper says that for ureolysis and denitrification the supplied urea or nitrate makes a greater contribution to the carbon footprint than the calcium chloride does; a later passage words this differently, describing the contribution of the urea, nitrate or glucose in the ureolysis, denitrification and ammonification routes as equal to that of the calcium source.
The remedies proposed are commercial-grade or waste-derived reagents, by-product nutrients such as corn steep liquor and lactose mother liquor (which the authors' earlier work tested as growth media for Sporosarcina pasteurii), and treatment or reuse of the waste stream, for example as fertiliser. Not every substitute helps: acetic acid with limestone as a calcium source is noted to carry a higher footprint (1.556 kg CO₂/kg and 53.35 MJ/kg) than laboratory calcium chloride (0.854 kg CO₂/kg and 10.96 MJ/kg), and the benefit of eggshell calcium could not be assessed because the cost of collecting it is unknown.
What the study does not establish
The model's assumptions all favour the microbial routes or leave gaps. Every reaction is taken to be 100% efficient, with all supplied calcium ending up as carbonate. Differences in how fast each metabolism works are ignored. Transport of materials and the energy to run a fermenter are excluded. Waste products are counted as impacts, but the cost of treating or recycling them is not; the effort that implies in practice is the subject of Rinsing Out the Ammonium: A 3.7-Metre Test of By-product Removal After Ureolytic Bio-Cementation (Lee et al., 2019). Water use, land use and resource depletion are outside the scope.
The comparison is also kilogram for kilogram against manufactured calcium carbonate. The paper does not measure how much precipitate is needed to reach a given strength in soil or concrete, so it does not directly compare a biocemented structure with a Portland cement one. There are internal inconsistencies as well: the two carbon-reduction ranges noted above, and cost figures quoted in USD where the methods section states that costs were compiled in AUD. Detailed inputs and unit rates are placed in supplementary tables.
The authors declare no conflict of interest; the work was funded by Curtin University.
Source notes & attribution
- https://researchonline.jcu.edu.au/72839/1/Life%20cycle%20assessment%20of%20biocement%20an%20emerging%20sustainable%20solution.pdf
- https://doi.org/10.3390/su132413878