Neutron scattering studies of hydrating cement pastes

The progress of the hydration reactions of tricalcium silicate (C₃S) has been followed using quasielastic neutron scattering (QNS) at an energy resolution of 60 μeV and a momentum transfer of q = 1 Å^-1. The degree of reaction in the hydrating cement paste is inferred from the fraction of water that is chemically bound to the cement reaction products and the known stoichiometry of C₃S hydration. Three different water-to-cement ratios were studied in this experiment: W/C = 0.7, 0.5 and 0.3. The results of an Avrami-model analysis of the first 15 h of the reaction are consistent with three types of C₃S-H₂O reaction product morphology and growth mechanisms: (i) a plate-type product phase from either phase boundary growth with no nucleation; (ii) diffusion-limited growth with constant nucleation; or (iii) a needle-type product phase with phase boundary growth and constant nucleation. Analysis of the later-time diffusion-limited portion of the reaction provides apparent diffusion constants for the migration of water through the C₃S hydration products. These data indicate that the diffusion constants vary approximately exponentially over the range of water-to-cement values studied,