Continuous monitoring of the zinc-phosphate acid-base cement setting reaction by proton nuclear magnetic relaxation

Proton nuclear magnetic relaxation is a well-established technique for cont inuous and non destructive monitoring of hydration of conventional Portland building cements. Here, we demonstrate the feasibility of nuclear magnetic resonance (NMR) monitoring of the setting reaction of zinc-phosphate acid- base dental cements, which harden in minutes as compared to days, as in the case of Portland cements. We compare the setting of cement powder (mainly, zinc oxide) prepared with clinically used aluminum-modified orthophosphori c acid solution with the setting of a model system where cement powder is m ixed with pure orthophosphoric acid solution. In contrast to previously pub lished NMR studies of setting Portland cements, where a decrease of spin-la ttice relaxation time is attributed to enhanced relaxation at the growing i nternal surface, spin-lattice relaxation time T-1 increases during the set of clinically used zinc-phosphate cement. Comparison of these results with a detailed study of diffusion, viscosity, and magnetic-field dispersion of T-1 in pure and aluminum-modified orthophosphoric acid demonstrates that th e increase of T-1 in the setting cement is connected with the increase of m olecular mobility in the residual phosphoric acid solution. Although not ta ken into account so far, such effects may also significantly influence the relaxation times in setting Portland cements, particularly when admixtures with an effect on water viscosity are used. (C) 2001 American Institute of Physics.