Characterization of calcium-silicate-hydrate and calcium-alumino-silicate-hydrate

Abstract

The primary objectives of this thesis were to synthesize calcium silicate hydrate (C-S-H) and calcium aluminosilicate hydrate (C-A-S-H) and characterize their structures with varying Ca/Si ratios and aluminum content. As a secondary objective, carbonation behavior in synthesized C-S-H was studied. C-S-H was synthesized in the lab using two different methods: the double decomposition of calcium nitrate and sodium silicate solutions, and the direct reaction between calcium oxide and fumed silica in water. In C-A-S-H the source of aluminum was sodium aluminate or aluminum nitrate. A variety of phases were present depending on the degree of carbonation of the C-S-H or C-A-S-H. C-S-H, C-A-S-H, alumina/silica gel, calcite, aragonite, and vaterite were all observed throughout the course of study. X-ray diffraction (XRD) was the technique used to identify these phases. XRD was also used to explore the changes in crystal structure with varying Ca/Si ratios and aluminum content. No significant changes to the crystal structure were found. The effect of Ca/Si ratio, aluminum content, and synthesis method were studied on the molecular scale using 29Si and 27Al nuclear magnetic resonance spectroscopy (NMR). An increasing Ca/Si ratio has the effect of a shorter alumino-silicate chain length and an increase in aluminum content increases the alumino-silicate chain length. Aluminum substitution occurred primarily in the bridging tetrahedron position, but also in the pairing tetrahedron position. Aluminum uptake was also examined with respect with Ca/Si ratio and was found to decrease with increasing Ca/Si ratio. The changes in chemical environment of the iii aluminum ion with varying Ca/(Si+Al) ratios was also observed by 27Al NMR. It was found that as the Ca/(Si+Al) ratio increased, the amount of chemical shielding of the 27Al nuclei decreased . Carbonation of C-S-H was also examined and it was found that the double decomposition synthesis method allows for more rapid carbonation of the CS- H. The source of increased carbonation rate is the high pH of the solutions.