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Comment by grenoire | Hacker News Reader

Just to get a frame of reference: How much CO2? I'd assume that building once, and then not having to replace it for hundreds of years is the long con and more energy-efficient, if that's the metric we're going by.

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Cement production accounts for ~7-8% of all global CO2 emissions. Wikipedia cites sources which claim ~900kg of CO2 is produced for every ton of cement. It's possible that frequent rebuilding could be more emission-efficient.

This entire debate is silly without numbers, but even worse it's missing the point - 38% of UK CO2 is from heating. Most of CO2 is produced to heat and cool building while the stand, not to build them.

Even tiniest improvements in quality of building, it's heating system and insulation will result in dramatic changes. This favours something well built, well insulated, with a heatpump, high thermal mass, and most importantly with heat recovery ventilation. Not hastily constructed crap that goes to shit in 30 years.

IMO the bigger question are: why does cement produce CO2? Could we use a different kind that doesn't? Could we capture CO2 from the atmosphere to produce cement, making it a closed loop?

Cement is a binder mostly made up of Ca and Si oxides. It produces CO2 mainly from the heat input required in production, pure and simple.

I have not heard of any carbon capture technologies that are more efficient than trees and oceans, so the best we can do is facilitate a process that generates less CO2 from fuels spent.

In order to produce CO2, the cement has to contain carbon. Where does that carbon come from? Potentially, we could use plants to capture carbon, and put plant matter in the cement. We could also use chemistry and catalysts to skip the plants. It might not be energy efficient, but maybe that doesn't matter. You can use solar panels and run your cement-generation-reactor only during the daytime, as well as put measures in place to cut back on cement usage. This problem is solvable, it just needs creative thinking and an active decision that we are going to solve it.

One of the inputs in cement manufacture is limestone which is Calcium Carbonate. That is ground and heated at high temperatures with clay minerals to produce portland cement.

So CO2 comes from the reaction between limestone and clay. And comes from burning natural gas or coal to generate the heat needed to drive the reaction.

So we use solar instead. We might not even need solar panels, we could potentially use one of those solar thermal reactors with an array of mirrors pointed to a central focal point. They can produce heat up to several hundred degrees C AFAIK.

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