SOLID STATE NMR SPECTROSCOPY AND 1H RELAXOMETRY FOR A MULTI-SCALE INVESTIGATION OF INNOVATIVE MgO-BASED CEMENTS

In the last years, MgO-based cementitious materials have attracted great interest because of the low CO2 emissions associated with their production and especially for applications requiring relatively "low" pH of the cement pore solution, such as nuclear waste containment in deep repository [1]. The binder phase of MgO-based cements is magnesium silicate hydrate (M-S-H), the amorphous phase that forms from the reaction of MgO with a source of silica and water. Although a significant quantity of literature exists concerning the structure and nature of M-S-H [2-4], a full comprehension of properties, such as the hydration kinetics, the nature of the hydrated products and their multi-scale structure and organization, is still lacking. The investigation of these properties, as well as the research for new formulations with improved performances, is fundamental to achieve the industrial breakout of these materials. In this work we have combined Solid State NMR spectroscopy (SSNMR) and 1H relaxometry, already proved to be powerful for the characterization of traditional cements, to obtain a detailed multi-scale description of novel MgO-based cements prepared by hydration of a 1:1 molar mixture of MgO and fumed silica (MgO/SiO2) and of mixed formulations containing different amounts of MgO/SiO2 and Portland cement. A 29Si SSNMR investigation on samples freeze-dried at different hydration times allowed us to obtain quantitative information on the nature and structure of the binder phases at the nanometric level, as well as on their formation kinetics [5, 6]. The analysis of 1H T2 measured at low magnetic field and 1H T1 obtained by means of Fast Field Cycling relaxometry directly on cement pastes during their hydration provided a description of the state of water and of the evolution of the solid phases during the hydration process [7]. This work was financially supported by MIUR (FIR2013 Project RBFR132WSM).