Nanopore Structure Buildup during Endodontic Cement Hydration Studied by Time-Domain Nuclear Magnetic Resonance of Lower and Higher Mobility 1H

Time-domain nuclear magnetic resonance (TD-NMR) of 1H nuclei has been used to monitor and model changes of endodontic cement pastes during hydration, from the initial reaction period up to hours and days. The 1H in the samples are divided into two major spin groups by fitting each free induction decay, acquired after the second pulse of an inversion recovery (I-R) pulse sequence with variable interpulse delay, by the sum of a quasi-Gaussian (signal from low mobility nuclei) and an exponential (from higher mobility nuclei). The extrapolations to zero time of the signals from the two spin groups give two sets of I-R data that have been analyzed to give quasi-continuous T1 distributions. After about a day, two clearly solid components appear. From a day to a few days, three liquid populations are identified, one of them mainly in the low-mobility spin group, which later merge, giving a single T1 or T2 peak. The rapid onset of the solid components, at the cost of the liquid, and the rapid changes of the relaxation time distributions of all components are clear indicators of the amount and kinetics of reaction products formation (C-S-H gel and Portlandite) and of the C-S-H micronanoporous structure buildup and evolution. At 30 days of hydration, the very short T1 and T2 liquid component (T1 = 200 μs and T2 = 50 μs) can be assigned to C-S-H intralayer water (thickness of the order of fractions of a nanometer) and the remaining liquid signal to interlayer water (thickness of the order of 1 nm). Comparisons are made among a widely used commercial endodontic cement paste and two more recent commercial pastes, with additive compounds to make the hydration process faster and to increase the workability. Parameters can be extracted from the data to characterize the different kinetics and nanostructure of the pore space formed up to 30 days. The parameters are in agreement with the expected effects of the additives, so the parameters can be used to optimize the formulation of new pastes, in order to improve their therapeutic performance.