Monitoring the hydration of MgO- and CaO-based cements by NMR and calorimetric techniques

In recent years MgO-based cements have attracted an increasing interest in the field of building materials research. First, they have been proposed as an "ecofriendly" alternative to traditional CaO-based cements, with the aim of reducing the footprint associated with the production of Portland cement, responsible for 8 % of global anthropogenic CO2 emissions.1 Moreover, thanks to the lower pH of their pore solution, they are particularly promising for specific applications in the field of radioactive waste disposal.2 The binder properties of MgO-based cements are due to the Magnesium Silicate Hydrate (M-S-H) phase, an amorphous phase formed by the hydration of a source of magnesium (commonly MgO) in the presence of a source of silica. The structure of M-S-H, from the molecular to the micrometric scale, and the hydration mechanisms behind its formation have recently been object of intensive research.3-6 Nevertheless, a full comprehension of these properties and of their correlations with the mechanical properties of these cements is still lacking. In our work we developed new cementitious materials obtained 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. The nature and the molecular structure of the hydration products were investigated on samples lyophilized at different times of hydration by means of 29Si high-resolution solid-state NMR (SSNMR) experiments, combined with the results from X-Ray Diffraction (XRD) and thermogravimetric analysis (TGA).3 Moreover, the status of water in cement pastes during hydration was monitored by 1H NMR relaxometry and calorimetric techniques. This approach provided information on the hydration kinetics and on the evolution of the porous structure of cement,4 which is strictly related to the mechanical strength and durability of the final material. This work was financially supported by MIUR (FIR2013 Project RBFR132WSM). References (1) N. Vlasopoulos, Process for producing cement binder compositions containing magnesium, WO/2012/028471, 2012. (2) Walling, S. A.; Provis, J. L. Chem. Rev. 2016, 116, 4170. (3) Tonelli, M.; Martini, F.; Calucci, L.; Fratini, E.; Geppi, M.; Ridi, F.; Borsacchi, S.; Baglioni, P. Dalton Trans. 2016, 45, 3294. (4) Martini, F.; Calucci, L.; Geppi, M.; Tonelli, M.; Ridi, F.; Borsacchi, S. Micropor. Mesopor. Mater. 2018, 268, 26. (5) Nied, D.; Enemark-Rasmussen, K.; L'Hopital, E.; Skibsted, J.; Lothenbach, B. Cem. Concr. Res., 2016, 79, 323. (6) Walling, S.A.; Kinoshita, H.; Bernal, S.A.; Collier, N.C.; Provis, J.L. Dalton Trans. 2015, 44, 8126.