In the last decade, research efforts on the investigation of MgO/SiO2 cements have significantly grown because they are one of the most promising materials for the encapsulation of radioactive waste.(1-4) In the presence of water, MgO/SiO2 mixtures hydrate and form a binder phase, M-S-H (magnesium silicate hydrate), a colloidal gel analogue to calcium silicate hydrate (C-S-H), present in traditional cements. Even if research interest in MgO-based cement is growing, only one additive has been used so far with this kind of cement, while in CaO-based cement many additives are commonly used to modulate the performances to specific applications. It has been recently reported that sodium hexametaphosphate in MgO-based cements increases the fluidity of the pastes, which is essential for practical applications, but its action mechanism is still unknown.(5-7) In this study we investigated the mechanism of interaction of different phosphate-based salts with MgO-based cements, in order to understand the structural changes induced by phosphate presence on the hydration reaction and on the M-S-H structure. We evaluated the effect of adding sodium orthophosphate (OP), sodium trimetaphosphate (TMP) and sodium hexametaphosphate (HMP), in comparison with a sample without additive. The effect of the investigated additives on M-S-H has been studied with a multi-technique approach: the kinetics of hydration of the studied pastes were monitored by means of differential scanning calorimetry, the hydrated phases formed during the reaction have been characterized by thermogravimetric analysis, infrared spectroscopy, X-ray diffraction and solid state NMR. The morphology and topography of the pastes have been studied by scanning electron microscopy and atomic force microscopy. Moreover, we performed a characterization of the mechanical properties by statistical nanoindentation and Vickers micro-indentation. The results evidenced that phosphate salts influence the hydration reaction, which is more efficient especially in presence of sodium ortophosphate that enhances M-S-H precipitation even more than the currently used additive sodium hexametaphosphate. References: 1. M. Schneider, et al., Cem. Concr. Res., 2011, 41, 642. 2. E. M. Gartner and D. E. Macphee, Cem. Concr. Res., 2011, 41, 736. 3. S. A. Walling et al., Dalton Trans., 2015, 44, 8126. 4. F. Jin and A. Al-Tabbaa, Cem. Concr. Compos., 2014, 52, 27. 5. Z. Li et al., Constr. Build. Mater., 2014, 61, 252-259. 6. T. Zhang et al., Cem. Concr. Res., 2014, 65, 8-14. 7. Y. Jia et al., Cem. Concr. Res., 2016, 89, 63-71. Acknowledgements: CSGI and FIR2013 (Project RBFR132WSM) for financial support.

Interaction of phosphate-based additives with MgO/SiO2 cements

L Calucci;M Geppi;S Borsacchi;
2017

Abstract

In the last decade, research efforts on the investigation of MgO/SiO2 cements have significantly grown because they are one of the most promising materials for the encapsulation of radioactive waste.(1-4) In the presence of water, MgO/SiO2 mixtures hydrate and form a binder phase, M-S-H (magnesium silicate hydrate), a colloidal gel analogue to calcium silicate hydrate (C-S-H), present in traditional cements. Even if research interest in MgO-based cement is growing, only one additive has been used so far with this kind of cement, while in CaO-based cement many additives are commonly used to modulate the performances to specific applications. It has been recently reported that sodium hexametaphosphate in MgO-based cements increases the fluidity of the pastes, which is essential for practical applications, but its action mechanism is still unknown.(5-7) In this study we investigated the mechanism of interaction of different phosphate-based salts with MgO-based cements, in order to understand the structural changes induced by phosphate presence on the hydration reaction and on the M-S-H structure. We evaluated the effect of adding sodium orthophosphate (OP), sodium trimetaphosphate (TMP) and sodium hexametaphosphate (HMP), in comparison with a sample without additive. The effect of the investigated additives on M-S-H has been studied with a multi-technique approach: the kinetics of hydration of the studied pastes were monitored by means of differential scanning calorimetry, the hydrated phases formed during the reaction have been characterized by thermogravimetric analysis, infrared spectroscopy, X-ray diffraction and solid state NMR. The morphology and topography of the pastes have been studied by scanning electron microscopy and atomic force microscopy. Moreover, we performed a characterization of the mechanical properties by statistical nanoindentation and Vickers micro-indentation. The results evidenced that phosphate salts influence the hydration reaction, which is more efficient especially in presence of sodium ortophosphate that enhances M-S-H precipitation even more than the currently used additive sodium hexametaphosphate. References: 1. M. Schneider, et al., Cem. Concr. Res., 2011, 41, 642. 2. E. M. Gartner and D. E. Macphee, Cem. Concr. Res., 2011, 41, 736. 3. S. A. Walling et al., Dalton Trans., 2015, 44, 8126. 4. F. Jin and A. Al-Tabbaa, Cem. Concr. Compos., 2014, 52, 27. 5. Z. Li et al., Constr. Build. Mater., 2014, 61, 252-259. 6. T. Zhang et al., Cem. Concr. Res., 2014, 65, 8-14. 7. Y. Jia et al., Cem. Concr. Res., 2016, 89, 63-71. Acknowledgements: CSGI and FIR2013 (Project RBFR132WSM) for financial support.
2017
Istituto di Chimica dei Composti OrganoMetallici - ICCOM -
cement
MgO
ecosustainable
nuclear waste
additives
phosphate
nmr
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.14243/342015
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