Cause life
Must be somewhere to be found
Instead of concrete jungle
-Bob Marley and the Wailers

Whether you consider it an architectural triumph or a drab eyesore, concrete is everywhere. The world produces about 30 Gt of it every year, making concrete the most-used building material by far. (1) To put that in context, global crop production (2) and oil consumption (3) are 9 Gt/yr and 5 Gt/yr, respectively.

Concrete is also a major contributor to global warming. Producing cement, one of the key ingredients in concrete, is responsible for approximately 8% of annual greenhouse gas emissions. These emissions can’t be mitigated by switching to renewable energy, making them tough to abate. However, the massive scale of concrete production also creates intriguing possibilities for carbon sequestration. We need to remove gigatons of CO2 and concrete is one of the few materials that we manipulate on a gigaton scale, so can we use it to make construction net negative?

But wait, isn’t this a biotech newsletter? What does concrete have to do with biology? To find out, let’s look at how concrete is made.

Concrete consists of aggregates (i.e. rocks) mixed with cement, which binds them together. Cement is produced by mixing limestone (a.k.a calcium carbonate, or CaCO3) with clay silicates and heating them to 2,700 ℉, which yields CO2 and a solid intermediate called clinker in the following reactions (4):

  1. Limestone decomposes into belite and carbon dioxide. 2CaCO3 + SiO2 -> Ca2SiO4 + 2CO2

  2. Calcium carbonate decomposes into lime and carbon dioxide. CaCO3 -> CaO CO2

  3. Belite and lime form alite. Ca2SiO4 + CaO -> C3SiO5

Then the clinker (alite and belite) is mixed with crushed gypsum, fly ash, and other “Supplementary Cementitious Materials” (SCMs) to produce cement. Cement is mixed with sand and gravel to produce concrete.

The concrete is either cast into pre-made bricks at the factory or poured on-site. Finally, freshly-poured concrete hardens through a process called “curing,” where calcium silicates react with water to form calcium hydroxide crystals (5). This process, which requires maintaining the concrete at an optimal level of moisture for multiple weeks, is essential for concrete to reach its full strength.

Thus there are two types of emissions from cement production: those from the energy to produce heat (~40% of emissions), and those released when limestone becomes lime (>=50%) (6). High-heat processes are notoriously hard to electrify, and the chemical emissions aren’t energy-related at all, so this is a situation where solar panels can’t save us. The graphic below illustrates the entire cement production process, from sourcing raw materials to transporting the finished product. As you can see, the heat and chemistry during clinker production account for the vast majority of CO2 emitted.

How can biology make this process more sustainable? There are two main approaches:

  1. Use photosynthetic microbes to make limestone with CO2 from the atmosphere. Then burning it is net-zero, and building with it directly is carbon negative!

  2. Use biologically-fixed carbon (wood, polymers, ash) as SCMs, sequestering carbon.

Biological SCMs could be very diverse, and they’re part of a larger conversation about carbon-negative SCMs. Research is underway to mix clinker with everything from biochar (7) to the lignin-rich byproducts of biofuel production (8). On the other hand, recent research on abiotic CO2 utilization in concrete casts doubt on the true carbon impact after emissions for the extra processes are accounted for (9). This will be a recurring theme in our newsletter: you can’t truly know the impact of some shiny new technology without careful Life Cycle Analysis (LCA), which quantifies emissions for the whole process from “cradle to grave.”

For the rest of this post, we’re going to focus on approach #1 because it’s driven by one powerful biological process: microbially-induced carbonate precipitation (MICP). Below, we’ll describe the science of MICP and the techno-economic challenges of competing with traditional concrete.

Read the rest here