Probing microstructure evolution during the hardening of gypsum by proton NMR relaxometry

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.     

Multi-scale approach continuously relating the microstructure and the macroscopic mechanical properties of plaster pastes during their settings

A new multi-scale experimental approach is proposed to continuously relate the microstructure and the macroscopic mechanical properties of plaster pastes during their settings. (1)H NMR relaxometry is used to follow continuously and not destructively, the degree of hydration and the microstructure evolution during the setting and hardening of plaster paste. Transmission of shear and compressional ultrasonic velocities enable the determination of macroscopic mechanical properties of the material during the setting. On the basis of similar behaviors of Young's modulus and NMR-population of confined water as function of the degree of hydration, we conclude that NMR gives a better understanding of the evolution of the microstructure at the origin of a better control of the macroscopic mechanical properties.     

The mechanism of fluidization of concentrated calcium carbonate slurries by poly(oxyethylene) diphosphonates

Nonionic poly(oxyethylene) polymers having a diphosphonate group at one chain end strongly adsorb onto CaCO₃ particles. The main consequence is a considerable lowering of the viscosity of concentrated slurries. This effect occurs because of the break up and redispersion of aggregates of flocculated CaCO₃ particles by the polymer adsorption. The mechanism of colloidal stabilization is steric, the particles becoming uncharged as the polymer adsorbs at their surface. As a consequence, the colloidal suspensions remain stable and fluid at high volume fractions and at high ionic strengths. On the other hand, because of the strong affinity of these polymers for CaCO₃ surfaces, the larger part of the polymer is adsorbed until the coverage of the particles reaches completion. The easy to handle polymer-to-solid weight ratio can then be used as a formulation parameter. The depletion flocculation by the nonadsorbed polymer is avoided. Received: 12 March 1999 Accepted in revised form: 2 July 1999     

Study of structural and electronic transport properties of Ce-doped LaMnO3

The structural and electronic transport properties of La_1−x Ce _x MnO₃ (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration up to 50% doping. The system La_0.5Ce_0.5MnO₃ also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition.