Can we use the carbonate "rocks" from carbon sequestration?

Photo credit: Construction Consulting and Testing

In my previous post, I discussed the launch of a pilot plant by a group of entities with the goal of sequestering CO₂ in manufactured carbonates that would subsequently be used in construction. These uses may include bricks, aggregates, pavers, etc. Of course, even were the CO₂ not used, it's better to have it in an inert mineral material than contributing to radiative forcing in the atmosphere. I suppose that any unsold inventory could simply be dumped. But what about a market?

The claim I mentioned in the previous post was that 50 plants could sequester a gigatonne of CO₂. The veracity of the claim aside, what would this mean? For a start, how much rock is represented by converting a gigatonne of carbon dioxide to calcium carbonate (CO₂ + CaO → CaCO₃₎, or possibly magnesium carbonate (CO₂ + MgO → MgCO₃)?  As an aside, it should be noted that these are EXTREMELY simplistic versions of the actual production reactions. A paper giving technical details is available here. Anyway, the molar weight of CO₂ is 44 grams, the molar weight of CaCO₃ is 100 grams. Thus, converting a gigatonne of carbon dioxide would produce (100/44)*1 billion tonnes, or 2.27 gigatonnes. The same calculation for MgCO₃ yields 1.92 gigatonnes. Let's call it 2 gigatonnes. Of course, it wouldn't make a lot of sense to convert this to cement (!) so what's the demand for bricks and aggregate worldwide?

What is typically thought of when "brick" is mentioned is the iconic red clay brick. These are usually formed by an extrusion process of pulverized clay materials mixed with water. The most common result is a brick of length 4", height 2 1/4", and depth of 4" weighing 2.7 kg. A similar volume of calcium carbonate weighs 3.20 kg, and of magnesium carbonate, 3.49 kg.  A gigatonne is 1000 kilograms, so 2 gigatonnes is 2 trillion (~2*10^{12}~) kg. I'll use 3.3 kg to determine that 606 billion bricks could be manufactured. The best information I was able to find says that around "seven to nine billion" bricks per year are used.

OK, can't use it all in brick, what about aggregate? This site estimates that demand for construction aggregate worldwide is on the order of 26 gigatonnes. Clearly, this is where the manufactured carbonates are best used. And, it would seem, there is sufficient demand. In fact, working backward, 26 gigatonnes of calcium carbonate aggregate would absorb 13 gigatonnes of CO₂, about 40% of our annual emission. And, one would assume, as emissions rise with a growing developing nations economy, so would aggregate demand. If it works, I like it!

There are two further considerations: energetics and economics. After all, if the energy required to manufacture the carbonates is excessive, particularly if it involves fossil fuel energy, there's a problem. And if the cost is too high, it won't matter about demand because it won't be purchased (unless, of course, carbon is taxed or credited in such a way as to balance the price).

I may or may not be able to get a handle on the economics but I should be able to nail down the energetics. I'll do that in my next post.