Microbial cementation and living concrete

Cement’s carbon problem is chemical before it is thermal. Calcining limestone — CaCO₃ → CaO + CO₂ at around 1450 °C — releases carbon dioxide from the rock itself, and that half of the emissions cannot be removed by changing the fuel. Microbial cementation forms the binder at ambient temperature instead.

Ureolysis, and why it precipitates calcite

The dominant route uses urease-positive bacteria, usually Sporosarcina pasteurii, fed urea and a calcium source.

Urease hydrolyses urea to ammonia and carbamate, which decomposes further, and the net result is ammonium plus carbonate. The ammonium is the operative part: it makes the surrounding solution alkaline. As pH rises, the carbonate equilibrium shifts from bicarbonate toward carbonate ion, and once the product of carbonate and calcium activity exceeds the solubility limit, calcium carbonate precipitates.

So the bacterium does not manufacture limestone. It manufactures the pH at which limestone becomes insoluble — which is a more useful way to hold the mechanism, because it explains what can go wrong.

The cells do one more thing. A bacterial cell wall is negatively charged, so it attracts Ca²⁺ from solution and concentrates it at the surface. The cell therefore acts as a nucleation site, and precipitation begins on and around the bacteria rather than randomly. That is why the calcite forms bridges at particle contacts — where cells accumulate — instead of filling the pore space uniformly, and it is the same efficiency argument as a foundry binder: strength comes from bridges at contact points, not from bulk filling.

Self-healing concrete: the dormancy problem

Embedding this in concrete means keeping an organism alive and inactive inside one of the most hostile matrices in construction. Fresh concrete pore solution sits near pH 13, and the material is dry and mechanically violent during mixing.

The organisms used are therefore spore-formers, since endospores tolerate desiccation and extreme pH for years. Even so, unprotected spores are largely crushed or inactivated during mixing and hydration, so they are encapsulated — in expanded clay particles, in hydrogels or in microcapsules — together with a calcium-source nutrient, commonly calcium lactate.

The trigger is the crack itself. A crack admits water and oxygen, breaches the capsules along its path, and the spores germinate and precipitate calcite until the crack is sealed and the supply of water is cut off. The system is self-limiting, and it is dormant until damage occurs.

The honest limits are dimensional and finite. Healing works on cracks up to roughly a millimetre, because wider cracks need more calcite than the local capsules can supply. Encapsulation reduces the concrete’s strength slightly, since the capsules are weak inclusions. And each capsule works once.

The byproduct that constrains everything

Ureolytic MICP produces ammonium in stoichiometric quantity — roughly two moles of nitrogen for every mole of carbonate. That is not a trace impurity; it is half the reaction.

The consequences are real and are the main obstacle to scale. Ammonia is released as a gas and is an irritant and an air-quality problem in enclosed work. The remaining ammonium oxidises to nitrate, which leaches into groundwater. And nitrogen compounds can promote corrosion of embedded steel reinforcement.

Non-ureolytic routes avoid it — precipitation driven by carbonic anhydrase, by photosynthetic CO₂ uptake, or by oxidising organic acids such as calcium lactate to give carbonate directly. These are cleaner and generally slower, and where bio-bricks are produced by biocementing sand at ambient temperature the choice of route is largely a choice about what happens to the nitrogen.