Stability of Synthetic Calcium Silicate Hydrate Gels in Presence of Alkalis, Aluminum, and Soluble Silica

Abstract

A study is described of the stability (chemical composition and nanostructural change) of a synthetic calcium silicate hydrate gel in the presence of Ca(OH)2, the two main products of portland cement hydration, hours after alkalis, aluminum, and soluble silica were added to the mix. The sol-gel procedure was used to synthesize the gels. Calcium nitrate and sodium silicate solutions were used as the sources of calcium and silicon, respectively. The samples were prepared to a target Ca-Si ratio of 1.9. A 10-M NaOH solution was added to hold the pH at values greater than 13. All processes were conducted in a nitrogen atmosphere. The samples were characterized by using X-ray diffraction, Fourier transform infrared spectroscopy, nuclear magnetic resonance, transmission electron microscopy, and energy dispersive X-ray. The findings showed that addition of a high sodium hydroxide content led to silicate polymerization. The simultaneous addition of alkalis and aluminum induced a rise in the degree of silicate condensation in the gel [Q2(nAl) and possible cross-linking, Q3, via bridging tetrahedral aluminate], along with the formation of a calcium aluminosilicate hydrate similar to strätlingite. Finally, the simultaneous addition of alkalis, aluminum, and soluble silica favored the precipitation of an alkaline aluminosilicate hydrate gel with calcium replacing a high percentage of sodium and a composition clearly different from that of the other components.