ECOSUSTAINABLE MgO-BASED CEMENTS: UNRAVELLING THE ROLE OF PHOSPHATE ADDITIVES BY MEANS OF SOLID STATE NMR SPECTROSCOPY

MgO-based cements are receiving great attention both because their production could in principle involve negligible or "negative" CO2 emissions and especially because they are particularly promising for the containment of radioactive waste. The important binder phase in cements obtained from hydration of MgO and silica is the completely amorphous magnesium silicate hydrate (M-S-H), the structure of which has been recently deeply investigated and characterized [1]. In order to pave the way to extensive and optimized applications of these promising cements, many aspects, both fundamental and practical, need to be investigated and understood. So far Solid State NMR (SSNMR) and Relaxometry techniques have given a crucial contribution to the comprehension of these materials at the molecular and nanometric scale, providing a detailed picture of the structure of the silicate binder phase, of the dynamic properties of water included in the pores and on the pore features, also clarifying their time evolution [2,3]. In the practice of cement preparation, additives are always necessary to achieve different goals, such as: accelerate or retard setting or hardening, decrease the amount of water necessary to obtain a given degree of workability, improve the mechanical performances, entrain air, etc. In the case of MgO/silica cements, sodium hexametaphosphate (HMP) nowadays is the only additive used as plasticizer and its mechanism of action is still not understood. In this work we have carried out a wide-ranging investigation on the effects of three different phosphate salts (HMP, sodium trimetaphosphate, TMP, and sodium orthophosphate, OP) on MgO/silica cements by combining fluidity tests, calorimetric measurements, XRD and SSNMR experiments. In particular, 29Si and 31P SSNMR experiments clarified in detail and quantitatively the effects of the different additives on the formation kinetics and structure of M-S-H, highlighting at the same time the chemical modifications underwent by the additives and the interactions with the silicate phase. Combining these results with those of the other techniques it has been possible to propose a rationale behind the macroscopic effects observed for the different additives. References [1] M. Tonelli, F. Martini, L. Calucci, E. Fratini, M. Geppi, F. Ridi, S. Borsacchi, P. Baglioni. Dalton Trans. 45, 3294-3304 (2016) [2] F. Martini, S. Borsacchi, M. Geppi, M. Tonelli, F. Ridi, L. Calucci. Microporous Mesoporous Mater . 269, 26-30 (2018) [3] F. Martini, S. Borsacchi, M. Geppi, C. Forte, L. Calucci. J. Phys. Chem. C 121, 26851-26859 (2017)