Quasi-Elastic Neutron Scattering Investigation of the Effect of Water/Cement Ratio on Tricalcium Silicate HydrationQuasi-Elastic Neutron Scattering Investigation of the Effect of Water/Cement Ratio on Tricalcium Silicate Hydration

摘要

The effect of water/cement variation on the hydration of C3S was investigated with Quasi-Elastic Neutron Scattering (QENS). This technique measures nondestructively the amount of water that is bound up in the hydration products as well as the remaining free water. The experimental approach consisted of mixing samples of C3S powder with varying amounts of water and then performing QENS measurements on them continuously for 48 hours on the Fermi Chopper instrument at the NIST Center for Neutron Research. The results were plotted as the boundwater fraction, β(t), versus time. The plots all showed a characteristic shape with an initial flat induction period followed by an acceleratory period with nucleation and growth kinetics and finally a transition to diffusion-limited kinetics. The time of the transition was the same for all the w/c ratios. The degree of hydration β(t) at the transition was different for each case, but they could all be collapsed into a single master curve by a simple vertical scaling. The scaling factors showed an excellent linear correlation with the w/c ratio. This result suggested rescaling β(t) data by multiplying by the w/c ratio. The product β(t)· w/c has units of boundwater volume per mass of cement. It was found that at the transition point, this took on a fixed value of 0.25 cm3/g for all the curves. This rules out the pore-filling mechanism which is conventionally given as the explanation for the transition point. Moreover, this value agrees with estimates of the amount of reactive silica formed during the prior induction period as measured by Nuclear Resonance Reaction Analysis. It thus supports the recently proposed theory that during the peak reaction period, the hydration reaction is characterized by boundary nucleation kinetics rather than the volume nucleation kinetics of the conventional Kolmogorov Avrami Mehl Johnson theory.