Isn't there some contradiction between the concept of vacuum dewatering and the need for keeping the curing water in the concrete?
There is not really a contradiction, but the question does point up the need for promptness in curing vacuum-dewatered concrete to conserve all the water possible. The water in a concrete mix is there for two reasons: (1) to provide workability in the mix so that it is possible to get the concrete in place with a minimum of difficulty, and (2) to react chemically with the cement compounds and harden them. The first is called water of convenience; the second is curing water. In vacuum dewatering as much of the water of convenience is removed as possible within a reasonable period of time. The water that is left is more than the amount needed for curing. The fact that it is possible to immediately take water of convenience out of concrete by dewatering might seem to imply that the water could be removed just as well in another way. Why not simply leave the concrete exposed to the air for a considerable time before beginning to cure it, thus letting the water of convenience leave? Unfortunately, the water doesn't know which molecules should be considered water of convenience and which ones curing water. Consequently, some of both will leave, particularly from the surface where curing water is especially needed and where a shortage of curing water will quickly develop. The virtue of vacuum dewatering is that as it removes water the plastic mix is compacted by atmospheric pressure so that the cement particles remain in intimate contact with an adequate amount of water for curing.