Nano-scale aluminium interaction in synthetic hydrated calcium silicate gel studied by 29Si MAS-NMR

Abstract

This research consists of the fabrication of synthetic gels of hydrated calcium silicate (C-S-H gel) and hydrated calcium aluminate silicates (C-A-S-H gel) in aqueous solution oversaturated in calcium hydroxide. These gels were fabricated using nanomaterials with different specific surface area; two nanosilicas (NS), OX50 and A200 (50 and 200 m²/g respectively) and two nanoaluminas (NA), A65 and A130 (65 and 130 m²/g). Mixtures were carried out maintaining a Ca/Si = 2 ratio and variable Al/Si ratios of 0.1, 0.5 and 1. The effect of aluminium incorporation in the C-S-H gel was studied using the nuclear magnetic resonance technique (²⁹Si MAS-NMR), and the information obtained was further processed using the mathematical deconvolution method. Chemical shift bands were delimited to identify the structures. From the results obtained, modifications of the tetrahedral (Q_n) in the dreierketten structure were observed in the different combinations, as well as the modification of the bridging tetrahedral (Q₂b) due to the presence of aluminium replacing the silica bridging tetrahedron Q₂b(1Al). High Q₄ values were detected in the C-S-H gel with NS OX50 and this could be associated to a double chain formation very similar to a perfect tobermorite. The length of the mean chains (MCL) was very variable in each blend, but some trends were observed as the Al/Si = 1 ratio and the Al/Si = 0.1 ratio maintain or increases the MCL respectively. The results are interesting and concrete case mixtures with NS OX50 show original trends that have not yet been reported in the literature.