Application of 19F NMR relaxometry to the determination of porosity and pore size distribution in hydrated cements and other porous materials

The potential to determine the porosity of cements and other porous materials by employing 19F NMR relaxometry was explored for samples of hydrated Portland cement, white cement and calcium aluminate, filled with Freon 11. The dependence of 19F signal amplitudes on the content of Freon in samples with completely filled pores was linear with a small (< 6%) intercept thus allowing a direct determination of total porosity. Additionally it was found that magnetic susceptibility (hence the content of paramagnetic compounds) of the solid can be evaluated from 19F NMR spectra of samples containing exterior liquid. Information concerning pore size distribution can be obtained from the analysis of multiexponential relaxation of 19F nuclei in samples with completely filled pores. Mathematical models employing sets of distributions of relaxation rates have been developed for this purpose. Distributions were assumed to be of a (fixed) square/triangular type allowing for the calculation of moments of any order from the estimated initial value, width of distribution and asymmetry factor. Longitudinal and transverse relaxation were characterised by either single or several continuous distributions of relaxation rates. One of the relaxating phases (assigned to fluid occupying throats connecting the pores) was found to selectively disappear when Freon was evaporated from samples. Another approach is to analyse the dependencies of means and variances of relaxation rates on Freon content. This has been done by employing the two-site relaxation model with a permanent adsorption layer and pore-dependent relaxation enhancement. Simultaneous fitting of dependencies of means and variances of relaxation rates on Freon content yields parameters of pore size distribution and of the dependence of relaxation enhancement on the pore radius. Analysis of experimental data shows agreement between these two approaches.