Samples of sandstone with and without deposits of silicon oxide stone strengthener as well as samples of historical brick material were analyzed by transverse NMR relaxation and mercury intrusion porosimetry. Relaxation times and relaxation time distributions of the protons from the water saturated samples were measured by low-field NMR using homogenous and inhomogeneous fields. The measurements in inhomogeneous fields were performed with two different NMR-MOUSE® sensors, one with a field gradient of 2 T/m and the other with an average field gradient of about 20 T/m. In the sandstone samples the application of stone strengtheners was shown to result in a confinement of the large pores within the outer layer of a few millimeters depth. Depending on the ferromagnetic contamination of the brick samples, the relaxation time distributions can be affected. The agreement of T2 relaxation time distributions and pore size distributions from mercury intrusion porosimetry was found to be better for the NMR-MOUSE sensors than for the homogeneous field measurements. This is true even for different brick samples, unless the content in ferromagnetic particles is very strong.
S. Sharma, F. Casanova, W. Wache, A. Segre, B. Blümich; Magn. Reson. Imag. 21 (2003) 249-255; DOI: https://doi.org/10.1016/S0730-725X(03)00132-2