In contradiction to the proposition of Nicoleau and Nonat, it is shown that the rate of hydration of anhydrous tricalcium silicate (C3S) cannot be controlled by the rate of dissolution of the anhydrous phase in typical C3S pastes except at extremely early ages. The strongest evidence for this is the observation that the hydration of C3S in pastes is strongly accelerated by the addition of afwillite seeds, to the extent that it hydrates almost completely by 7 days. It is suggested that the hydration rate at later ages is primarily controlled by the porosity and permeability of the hydration products.