Scientists Propose Feeding Concrete To Plants, Trapping More C02
by John Laumer, Philadelphia 
on 04. 2.08
UK researchers are amending soils with powdered calcium-silicate (concrete dust) to determine if a carbon fix-boost hypothesis is correct: that crops will be induced to bind extra carbon dioxide, reacting it with calcium taken from the concrete dust (in the soil matrix).
This reaction, whether directed by, or simply mediated by plants and/or soil organisms, would sequester more atmospheric carbon than is possible by production of plant tissue. Good for the climate.
Assuming their work validates plenty of extra sequestration, we think it makes complete sense if the source of calcium-silicate is demolition debris or "rubble". But, it would be idiocy to use commercial concrete that had not gone through a building-product life cycle.
This work conjures up images of abandoned concrete and plastered buildings (as pictured), overgrown by plants busily sequestering C02. Who knew?
Pehaps TreeHugger Lloyd will learn to love concrete in its afterlife...at least a little bit. See: "Sustainable Cement is Like Vegetarian Meatballs"
The concept underlying the initiative exploits the fact that plants, crops and trees naturally absorb atmospheric carbon dioxide (CO2) during photosynthesis and then pump surplus carbon through their roots into the earth around them. In most soils, much of this carbon can escape back to the atmosphere or enters groundwater.But in soils containing calcium-bearing silicates (natural or man-made), the team believe the carbon that oozes out of a plant's roots may react with the calcium to form the harmless mineral calcium carbonate. The carbon then stays securely locked in the calcium carbonate, which simply remains in the soil, close to the plant's roots, in the form of a coating on pebbles or as grains.
Here's the money quote::
"The process we're exploring might be able to contribute around 5-10% of the UK's carbon reduction targets in the future," says Professor Manning. "We could potentially see applications in 2-3 years, including a number of 'quick wins' in the land restoration sector."
So, who gets the offset credit? The original concrete supplier? Building owner? Farmers?
Via::TerraDaily, "Specially-Designed Soils Could Help Combat Climate Change" Image credit::Singapore Haunted Houses, building overgrown with plants
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This is great for buildings in ruin I guess, but already constructed buildings would be torn apart by the trees.
Crushed concrete soil ammendments will perform much like a lime fertilizer. Plants that like sweeter soils will grow more prolifically in a high calcium matrix.
This may be a good use for some waste concrete. Other uses for old concrete can be as aggregate for new concrete, roadway base, and fills. In places, I've seen old concrete sidewalks and driveways sawn up and stacked to form walls and retaining walls. There's a question of how to best reuse all of the resource and embodied energy. As mentioned, it takes a great deal of energy to produce cement to begin with, so you want to take care of it for the long haul.
Calcium silicate doesn't make soil significantly sweeter (less acid); that is done with lime.
What this scheme does is recover some of the carbon dioxide that is released when calcium carbonate is roasted to give calcium oxide (for making concrete) and carbon dioxide. It does require the concrete to be pulverized, which takes a good bid of energy. Vegetation growing on a building isn't going to trap significant carbon dioxide.
Decades ago there was an idea to feed concrete dust to cows as a source of calcium; I wonder whatever happened to that.
Back up. "Trapping more CO2"? Once the cement is produced, the CO2 has already been released. It doesn't matter if the concrete stays as part of a building, or gets buried, or crushed into soil or anything else. The only ways to reduce the CO2 footprint of concrete is to use less or try to capture at the production facility...
==== authors' response follows ====
You argument holds if the debris is landfilled or used for fill; but this work indicates that if crushed and used as soil amendment, some of the environmental burden of manufacture will be overcome, courtesy of plants.
Haha poor Lloyd...
Although I wonder if any benefits in captured carbon will overcome the GHGs released from the energy required to grind up the old concrete...