# Bio-cementation and MICP materials

How ureolytic bacteria bind sand with calcite without a kiln, why the carbon in the stone is mostly re-stored industrial CO2 rather than removal, and why binder dose and the ammonium byproduct cap the scale.

Bacteria can grow a cement from calcium carbonate at room temperature — but the carbonate usually arrives as urea, and every mole of binder drags two moles of ammonium behind it.

Source: https://en.bioecon.ru/docs/bioenergy-climate/cdr-carbon-management/bio-cementation-micp-materials/
Updated: 2026-09-07



Portland cement is made by calcining limestone — CaCO3 to CaO plus CO2 at clinker temperatures near 1,450 °C — and the CO2 comes as much from the rock as from the fuel, which is why cement stands for roughly 8 percent of global emissions. Microbially induced carbonate precipitation (MICP) proposes to skip calcination entirely: bind sand with calcium carbonate precipitated at ambient temperature.

## The chemistry that binds without heat

Ureolytic bacteria such as Sporosarcina pasteurii carry urease, an enzyme that hydrolyses urea: CO(NH2)2 + 2 H2O → 2 NH4+ + CO3²−. In a calcium-bearing solution the carbonate quickly overshoots calcite solubility and precipitates as CaCO3 — preferentially on the cells themselves, whose negatively charged walls act as nucleation sites. Sand grains with cells and reagents between them acquire crystal bridges at the points of contact; enough bridges, and sand is stone. The mechanism is genuinely low-temperature: no mineral is calcined and no process CO2 is generated. The binder is grown, not fired.

## Whose carbon the stone carries

Here is the honest accounting question. The carbonate in urea-driven MICP comes from the urea molecule, and industrial urea is made by combining ammonia with CO2 — in practice concentrated process CO2, often captured from ammonia plants. Urea-driven MICP therefore does not take carbon out of the air: it takes CO2 that was already captive, lends it to fertiliser chemistry and re-stores it as mineral. Against a Portland baseline that is valuable — every tonne of stone grown without clinker avoids both fuel and calcination emissions — but it is avoided emissions, not removal. A removal-style claim belongs only to routes where the carbonate forms from atmospheric or biogenic CO2, such as accelerated carbonation of aggregate with captured CO2, and even then only for as long as the stone stands.

## What limits the scale

CaCO3 bridges form at grain contacts, so strength is bought with binder dose — cycles of urea and calcium solution until the contacts fill — and dose is where the route pays. Every mole of urea hydrolysed releases two moles of ammonium, which leaves with the process liquor and needs treatment or disposal, and the calcium salt is itself an industrial input. The practical consequences follow: MICP strengths and setting times of days suit tiles, pavers, ground stabilisation and self-healing — encapsulated spores that germinate in crack water and precipitate calcite to seal it — rather than structural frames. Delivering reagents uniformly deep into a sand mass runs into permeability, and growing sterile bacterial culture by the cubic metre, or supplying instead enough stable enzyme, is a supply chain in its own right. The limiting quantity is the binder dose — moles of CaCO3 per unit volume needed for strength — because each one drags its ammonium byproduct and its calcium supply chain along.