I think the target is a 60% reduction. If we can replace electric generation, half of concrete, and transportation that would account for about 49%. The rest is agriculture and methane which are their own headaches.
Concrete is probably one of the last things on our CO2 emission hit-list. Kind of an extra credit compared to power generation and transportation.
https://www.c2es.org/content/international-emissions/#:~:tex....
So I have this conversation with coworkers all the time. Thinking in fractions of the status quo is a logic trap, and most of us fall into it.
As Gretzky famously said, "skate to where the puck is going to be."
We need to halve our CO2 emissions. That means concrete is 16% of the target. That is way too large of a fraction to ignore. We will not get where we are going if we don't look at this, and hard. 1/6th of our target is practically a call to action. 8% is more likely to invite foot dragging.
If you lose your job and have to change careers, it doesn't matter if your fancy car is 12,15,18% of your current monthly expenses. What matters is that it's now 40% of what you have available to spend, and you simply cannot be spending that much on a car. Getting rid of the car is more important that weening yourself off of a daily Starbucks run, steaks, or starting to use coupons.
We cannot be spending 1/6th of all carbon output on concrete. Especially if we're going to make suggestions that involve huge increases on concrete usage.
I wish as a species we could all wrap our heads around this. It gets really old explaining to people that they need to look at the thing that is taking 5% of run time because it's "only 5%" when rationally it should be using .25% of resources and thus it's taking 20x as long as it has any right to be. They won't look at it until overall runtime is down 25% and now we are creeping up on double digits. It's a delaying tactic, and we can't wait for other people to do their part before we start on ours. These are not ordered lists.
Concrete isn't negligible, however the unavoidable 4% of emissions for standard concrete isn't enough of a deterrent to require us to curb its use in favor of other methods immediately. Especially considering it's utility. With enough carbon free energy we could offset that 4% very easily. In the same way we can create carbon neutral fuels with sufficient cheap energy.
Some cement plants are reducing their emissions by including fly ash into their mix, and as coal plants wind down across the world, I don't know what other waste streams there are to backfill that, or if in the near term removing coal from our energy mix actually amplifies the percentage of our overall carbon footprint represented by concrete.