The chemical process that forms concrete is as follows:

Limestone (CaCO3, plus imputities) is crushed and baked in large ovens and "calcined" - CO2 is driven off and CaO results - "Portland Cement"

This is stored until needed, then mixed with aggregate (sand and gravel) plus water.

Water reacts rapidly and exothermically with CaO to form CaOH, a weak binder. In a few hours, the raw or "green" concrete becomes dimensionally stable, but is not yet load-bearing. In hot climates, the heat produced by this reaction can fracture and split the concrete. This is one reason why concrete pours are limited to modest thicknesses at one time.

Over a period of days to weeks (and extending to years, as the Biosphere people learned), atmospheric CO2 reacts with CaOH to form CaCO3, plus H2O. The H2O is ultimately lost from the concrete. Keeping the concrete moist promotes this "curing" process which develops the full load-bearing strength of the concrete.

Application of high pressure CO2 gas, or supercritical or liquid CO2 speeds the curing.

Well-aged cement has little intrinsic water content, other than hydrated minerals associated with the impurities in the original cement and the aggregate, so Gary is actually wrong here. There is no more water in sidewalk, than in sandstone, and much less than in clays, zeolites, and other hydrated minerals.