Q.: When and why does water change its effect on concrete--from harming it, by too high a water-cement ratio, to helping it during curing? What goes on?
A.: In the early life of a concrete mix one role of the water is to provide workability. The higher the water-cement ratio (within limits) the more workable the mix. But also, in the fresh mix, the higher the water-cement ratio the farther apart the new hydrating cement gel filaments or crystals will spread themselves. (They distribute themselves fairly uniformly throughout the water-filled space.) If more water is added at any time before the concrete has set, it increases this water-filled space and so the cement gel (also called cement paste) will be less dense and the concrete of lower quality.
Once the concrete has set, the total volume of the cement paste has been established; and if water is then added it will not increase this cement paste volume in the way it did before. That is, it will not make the paste less dense and the concrete of lower quality.
Even though the concrete has set, the chemical reaction between the cement and water continues. The various cement compounds are still very slowly dissolving from the surfaces of the original cement grains. After dissolving, they react with the water and form more crystals within the same space, making the paste more dense. This means that anything that can be done to keep plenty of water available within the concrete and replace any water that has been used up in the chemical reaction will contribute to the densification of the cement paste, thereby improving the quality of the concrete.
In summary, too much water in the fresh mix will make the cement paste too low in density and weak. But after the concrete has set, the richness or leanness of the paste has been established; then not enough water for curing will rob the cement of its maximum opportunity to hydrate and make the concrete strong.