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The reason why is you are wrong.

Portland cement, when hardened, does not absorb any significant amount of CO2. It does not form limestone. It mostly forms CSH [0] which just takes water, silicon dioxide, and quicklime. The last is sourced from limestone but does not turn back into limestone.

[0] https://en.wikipedia.org/wiki/Calcium_silicate_hydrate

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Across the world, what percentage is Portland cement vs the other CO2 absorbing kind? I googled around, but couldn't find the answer.

Per TFA, 98% of concrete is made from Portland cement.

Nearly all, much to the frustration of heritage building owners everywhere. I just spent a few weeks manually chiselling off a godawful cement render from a wall to restore a naturally-hydraulic-lime based one. The cement crap is rock-hard and doesn't have any moisture permeability, so water gets in and just dissolves your building (or more importantly, the old lime-based mortar holding up the dozens of tonnes of stone that is your home)

And basically all the other forms of cement in use starting with the Romans.

https://rootsofprogress.org/cement-redux

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