Fresh cement paste releases calcium hydroxide, which is why concrete washout water is strongly alkaline, often above pH 12, and carries dissolved calcium. Sodium polyacrylate swells by drawing water into a network of charged carboxylate groups. Divalent calcium ions bridge those groups and pull the network tight, so a general-purpose grade can hold far less here than its clean-water rating suggests.
Grout admixtures such as plasticisers, retarders and accelerators, and the bentonite used in some grout mixes, add further variables. The slurry also keeps stiffening on its own as the cement hydrates, which changes how much water is still free to capture by the time treatment happens.