Solid state 29Si and 27Al magic angle spinning NMR studies of mechanochemically modified cement

Abstract: Solid state 29Si and 27Al magic angle spinning (MAS) NMR spectroscopy has been used to investigate how mechanochemical modification affects the hydration kinetics, as well as the structural and chemical properties, in cement blends. The modification comprises intensive treatment of rapid hardening Portland cement with condensed silica fume additive in a vibrating grinding mill. 29Si MAS NMR was utilised to follow the hydration reactions of calcium silicates at five different ages and with two different water/binder ratios. The hydration rate was higher in the cement pastes formed by the treated blends, especially at an early age, and more pronounced at high water/binder ratios. Variations in relative intensities of 29Si resonances suggest different structures of calcium silicate hydrate nets in the pastes. This can be correlated with enhanced compressive strength in cement pastes produced by modified blends. 27Al MAS NMR was used to characterise the coordination state of aluminium in anhydrous and hydrous aluminate and ferrite phases. The hydrated cement blends showed 27Al resonances characteristic of four-, five-, and six-coordinated aluminium. 1H-27Al CPMAS experiments revealed that only the octahedral phases are located in a proton-rich environment. The signal in the Al(IV) range is due to incorporation of aluminium in the hydrated silicate phases, and is most likely an intermediate formed prior to the octahedral hydration products. A higher Al(IV)/Al(VI) ratio in cement pastes formed by mechanochemically treated blends may contribute to increased early strength development.