title: Probing microstructure evolution during the hardening of gypsum by proton NMR relaxometry.
abstract: We report a comprehensive proton NMR relaxation study of the water confined in the evolving porous structure of hardened gypsum prepared with different water-to-plaster ratios (w/p) and increasing additions of crushed gypsum. This study gives some new information on the microstructure, the water distribution, and the hydration kinetics without any drying or perturbing preparation. The bi-exponential transverse magnetization decay reveals the existence of two water populations in slow exchange. However, the different behaviors of these populations during saturation and desaturation experiments show evidence of a fast exchange of each population with the surface. Two modes of organization of the microstructure of this material are identified through an original model of exchange as a function of the water-to-plaster ratio (0.4 < or = w/p < or = 0.6 and 0.7 < or = w/p < or = 1). A clear gap is shown in the exchange rate value above w/p = 0.6 that could be representative of a percolation threshold. Both the method and the theory presented can be applied more widely to other porous media with reactive surface areas.
authors.affiliation: Laboratoire de Physique de la Matière Condensée, UMR 7643 du CNRS, Ecole Polytechnique, 91128 Palaiseau Cedex, France.
authors.forename: Hamouda
authors.initials: H
authors.lastname: Jaffel
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Korb
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Ndobo-Epoy
authors.forename: Vincent
authors.initials: V
authors.lastname: Morin
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Guicquero