H-1 NMR study of the local magnetic field gradients in evoluting porous structures. An application to cement gels

When placed into an external magnetic field inhomogeneous porous structures , like rocks, cement gels, ceramics, etc., exhibit strong local magnetic fi eld gradients (LMFG), which are inherently related with the size, shape, an d distribution of their pores. In this paper, we present a method of measur ing LMFG in a liquid-filled porous media, provided that the fast exchange m odel sufficiently describes water motion in the pores, which is based solel y on the measurement of the water H-1 NMR spin lattice relaxation rates and the knowledge of the unrestricted diffusion coefficient. The method is app lied to two hydrating white cement samples with different hydration kinetic s, and the results are correlated with the time evolution of the pore struc ture. It is shown that the measured LMFG have extremely high values, which qualitatively reflect the sharp needlelike morphology of the internal cemen t gel surface. (C) 2000 American Institute of Physics. [S0021-9606(00)70241 -8].