Synthesis of Calcium Silicate Hydrate in Highly Alkaline System
Synthesis of calcium silicate hydrate (C‐S‐H) was conducted over the range of 50°C–90°C and C/S ratio of 0.86–2.14 in the highly alkaline Na2O–CaO–SiO2–H2O system for silicon utilization in high alumina fly ash. Structural change in C‐S‐H formed in the highly alkaline system was investigated using XRD and 29Si MAS NMR spectra. X‐ray photoelectron spectroscopy was used to confirm the amount of sodium ions in C‐S‐H. Conversion of Si may reach 99% under optimum conditions. A higher degree of polymerization of silicate was obtained at lower temperature and C/S ratio. Na+ was confirmed to exist as Na–OSi and Na–OH. The amount of Na+ is the least at C/S ratio of 1.43, which conform to the prediction of topological constraint theory. High Ca/Si ratio leads to the increasing in Na+ combined in the interlayer. Increasing in the Na+ concentration in the system also increases the amount of Na+ combined in the interlayer and reduces the polymerization. Ion exchange was proven to be an effective way to remove Na+ combined in the interlayer of C‐S‐H.
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