Hydration of MgO-based cement and its mixtures with Portland cement: kinetics, state of water, and binder phase structure by NMR spectroscopy and relaxometry

In the last years, MgO-based cements have been proposed both as potential low-CO2 alternatives to traditional Portland cement, the dominant form of cement used worldwide, and as low-pH cements for the immobilization of nuclear or metal-containing waste. The binder phase of MgO-based cement is magnesium silicate hydrate (M-S-H), the amorphous phase that forms from the reaction of MgO with a source of silica and water. Analogously, C-S-H (calcium silicate hydrate) arises from the hydration of alite and belite present in Portland cement. Although a significant quantity of literature exists concerning the structure and nature of the M-S-H gel, 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, notwithstanding 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, novel MgO-based cements obtained by hydration of a 1:1 molar mixture (MgO/SiO2) of MgO and fumed silica io la parentesi la rimetterei qui, and of mixed-formulations containing different amounts of MgO/SiO2 and Portland cement were developed. The hydration process of these systems was monitored by 1H NMR relaxometry by exploiting measurements of both T2 (analysis of signals acquired at 21 MHz by solid echo and CPMG pulse sequences) and T1 (NMRD curves acquired by FFC) of water protons in cement pastes as a function of time. The collected data allowed information on the hydration kinetics and on the state of water and the evolution of the solid matrix during hydration to be obtained. Moreover, the structural properties of the binder phases obtained by hydration at different times were investigated in detail by means of multi-nuclear high-resolution solid-state NMR spectroscopy. In particular, 29Si MAS experiments allowed the different silicon sites, Q1, Q2 and Q3, characterized by different connectivity to -OSi, -OH, -OMg or -OCa groups, to be identified and quantified. In the case of mixed systems, it was also possible to distinguish between M-S-H and C-S-H domains, and to study their properties and relative amounts as a function of composition and hydration time. Spectroscopic data were discussed with the support of results from complementary techniques, such as X-Ray Diffraction, thermogravimetry, IR spectroscopy, Scanning Electron Microscopy, and Differential Scanning Calorimetry. This work was financially supported by MIUR (FIR2013 Project RBFR132WSM).