Surface Relaxivity of Cement Hydrates

Numerous aspects of the physical chemistry of colloidal systems are conditioned by the solid liquid interface, and this is also the case for hydrated cement systems. Estimating the surface area is thus essential for studying the kinetics of cement hydration and admixture adsorption. Proton nuclear magnetic resonance (NMR) relaxation techniques have already proven useful for this objective, but, for hydrating cements at early ages, it is necessary to know the surface relaxivities of all of the individual phases present to correctly interpret the relaxation data. This paper reports the results of a comparison of NMR relaxometry and Brunauer-Emmett-Teller gas adsorption measurements on various synthetic cement hydrates with the aim of evaluating their individual surface relaxivities. We observe that all of the tested phases (anhydrous and hydrated) except for ettringite have surface relaxivities typical of common natural rocks. We also find that the amount of paramagnetic species as measured by electron spin resonance is not a first-order parameter in determining surface relaxivity. The strong relaxivity of ettringite can be explained by the influence of the dimensionality of diffusive exploration on the relaxation rate. The structure of ettringite essentially provides only ID access to the paramagnetic species, so the water protons only explore a reduced volume and stay in close proximity to paramagnetic ions, maintaining the dipolar pair correlation for a much longer time compared with that observed for the 2D diffusion, which is typical of the other cement hydrates studied.