Sequestering carbon from the air is only a small element in this particular trial (and to be honest I suspect that's a journalistic error).

The bulk of carbon emissions in the manufacture of concrete come from the processes which occur during the creation of cement. There's two elements to this:

• Current methods of creating cement involve heating limestone and clay to around 1400 degrees Celsius. Its hard to hit these temperatures without large inputs of fossil fuels.
• The chemical process which occurs within the limestone during this baking releases A LOT of CO2. (This is the most carbon intensive bit).

So to achieve lower carbon intensity, we need new methods of producing concrete that use significantly less cement (which is how this trial aims to achieve the bulk of its CO2 reductions); and/or we can capture some of the CO2 from the limestone and embed the carbon back into the concrete.

Pulling it from the air does seem incredibly inefficient when the concentrations will be much higher in the exhaust and waste gasses than the atmosphere. This is where I think the journalistic error occurs. The article says they'll use the AirCapture technology - but I suspect they'll use that technology within a closed system - not just pulling it from the nearby atmosphere.

And yeah - it still won't be carbon neutral, but a significant reduction in a very carbon intensive process is still better than not doing it.