INNOVATIVE ECO-COMPATIBLE MgO-BASED CEMENTS: A SOLID-STATE NMR AND RELAXOMETRY STUDY

The development of eco-compatible cements represents a relevant issue in the field of building material research. To this aim, cement based on reactive periclase (MgO), which in the presence of a silica source forms a binder phase, M-S-H (Magnesium Silicate Hydrate), is one of the most promising emerging technologies, allowing to greatly reduce CO2 emissions involved in the production of the traditional CaO-based cements. Although the increasing research interest in MgO-based cements, a thorough investigation of their properties, such as hydration kinetics, the nature of the hydrated products, the structural and morphological properties of the obtained phases, is still lacking. Nevertheless, this understanding is fundamental to achieve the industrial breakout of these systems, whose production is today limited to pilot plants. In this work, a detailed and systematic study of MgO-based cement formulations was carried out by means of multinuclear solid-state NMR (SSNMR) and relaxometry. Both these techniques already proved to be very powerful for the investigation of the structural and morphological properties and hydration kinetics of the traditional CaO-based cements.1-3 In this study, 29Si and 1H one- and two-dimensional experiments were performed on pastes freeze dried after up to 28 days curing, in order to characterize the structural properties of the M-S-H binder phase, still almost completely unkonwn, also in dependence on the hydration time. In particular, 29Si MAS spectra allowed to identify and quantify the different silicon sites in M-S-H, characterized by different connectivity to -OSi, -OH and -OMg groups. Moreover, in order to shed light on the hydration process and the morphological evolution of these systems, the status of water in pastes at different hydration times was investigated by analyzing 1H T1 relaxation times measured by FFC NMR relaxometry and 1H T2 relaxation times determined by low resolution experiments at 20 MHz. The obtained results were combined with complementary data obtained by DSC, TGA, XRD and SEM measurements. This work was financially supported by MIUR (FIR2013 Project RBFR132WSM). (1)Rawal, A.; Smith, B. J.; Athens, G. L.; Edwards, C. L.; Roberts, L.; Gupta, V.; Chmelka, B. F. Molecular Silicate and Aluminate Species in Anhydrous and Hydrated Cements. J. Am. Chem. Soc. 2010, 132, 7321-7337. (2)Barberon, F.; Korb, J. P.; Petit, D.; Morin, V.; Bermejo, E. Probing the Surface Area of a Cement-Based Material by Nuclear Magnetic Relaxation Dispersion. Phys. Rev. Lett. 2003, 90, 116103-1-116103-4. (3)Bortolotti, V.; Brown, R.; Fantazzini, P. Evolution of a Short-T2 Liquid-Like 1H Signal During the Hydration of White Portland Cement. Micropor Mesopor Mat 2013, 178, 108-112.