These polyanions have carboxylate groups whereby cations bind them, especially Ca2+ in this early phase, as it is the most readily available ion, crosslinking into calcium polyacrylate chains that begin to form a gel matrix, resulting in the initial hard set, within five minutes.
These polyanions with carboxylate groups are capable of binding cations, especially Ca2+, which forms crosslinks in the calcium polyacrylate chains, leading to the formation of a gel matrix that hardens quickly within five minutes.