MgO/SiO2 and Portland cement formulations: mutual effects on reaction kinetics and hydration products

Cement is the binder at the base of one of the most important building material: the concrete. Nowadays, formulations based on reactive periclase (MgO) [1,2] constitute a promising emerging technology toward eco-sustainable binders, with reduced CO2 emissions as compared to the traditional CaO-based cements. [3,4] In the presence of water, MgO/silicate 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 Portland cements, but with inferior mechanical properties. The aim of the present study was the investigation of the hydration reaction and of the microstructure development of pastes prepared by mixing Portland cement and MgO/SiO2 blend. Different percentages of CaO- and MgO-based binders were evaluated to explore the possibility of preparing pastes that could in principle combine the optimal mechanical properties of calcium based cement with the ecological features of MgO-based cement. The hydration kinetics of the mixtures were investigated by means of Differential Scanning Calorimetry. Concurrently, the pastes were characterized by means of a multi-scale approach and the formation of hydrated phases was studied by Thermogravimetric Analysis, Fourier Transform IR spectroscopy and X-ray Diffraction. The structural properties of all samples were investigated by solid state 29Si-NMR. Finally, the morphology of the pastes has been studied by means of Scanning Electron Microscopy. The results showed that in the mixed pastes both C-S-H and M-S-H binder gel phases form, but, as evidenced in Figure 1, their domains remain segregate in different regions. The results evidenced that the kinetics of hydration in the mixed samples is dominated by MgO/SiO2 hydration in the first period, while the diffusional stage is mostly dependent on the Portland cement presence. Moreover, mutual effects on the structure of the hydration products occur in the mixed pastes. [1] Lothenbach, B., et al., Magnesium and calcium silicate hydrates, Cement and Concrete Research, 2015. 77: p 60-68 [2] Chiang, W.-S., et al., Multiscale structure of calcium and magnesium silicate hydrate gels, Journal of Material Chemistry A, 2014. 2: p 12991-12998 [3] Ellis M. Gartner and Donald E. Macphee. 2011. A physico-chemical basis for novel cementitious binders. Cement and Concrete Research 41: 736-749. [4] Stuart M Evans and Nikolas Vlasopoulos. 2010. Novacem: Carbon Negative Cement and the Green Cement Bond.