The Preparation And Properties Of Fluorine-doped Calcium Silicate Hydrate

Synthesized calcium silicate hydrate is a kind of functional inorganic non-metallic materials prepared under certain conditions. Calcium silicate hydrate which prepared through the synthetic methods are widely used, but due to the structural features of CSH itself, it has poor erosion resistance, heat resistance and wet fastness, which will affect its functional performance, shorten its service life, and limit its application. In order to improve the performance and application of calcium silicate hydrate, this paper has been introduced the stable fluorine ions into calcium silicate hydrate and study the modification methods and mechanism of F ions doping, so as to improve the comprehensive performance of calcium silicate hydrate and extend its application field, then it will meet the demand of practical engineering. At the same time, the national strategy for the energy conservation and emissions reduction and low carbon production can be come ture.Prepare calcium silicate hydrates?CSH? and fluorine-doped calcium silicate hydrates?F-CSH? under different conditions of hydrothermal synthesis. In order to provide the technical support to promote the comprehensive performance of F-CSH and extend its application range, the structure and properties of CSH and F-CSH have been studied, and the modification mechanism of F-CSH is explored. To fully understand the viriation of calcium silicate hydrates before and after fluorine doping, this study take white carbon black as siliceous material and calcium oxide as calcareous material. Introduce fluorine element in the form of sodium fluoride. By changing the technological parameters of each sample, preparing different structure and morphology of CSH and F-CSH under different conditions of hydrothermal synthesis. Samples have been characterized through XRD, SEM, FTIR, NMR and N₂ adsorption and desorption analysis, and the modification mechanism of F-CSH is analyzed. In order to further understand the viriation of the calcium silicate hydrates performance after fluorine doping, the moisture adsorption and carbonation resistance of samples have been tested. And the cause of viriation of moisture adsorption and carbonation resistance before and after fluorine doping have also been analyzed. The main conclusions are as follows:?1? According to XRD and SEM analysis, the hydration products of CSH mainly include flocculated CSH gel, flake-like 14? tobermorite. Due to the sodium ions doping, F-CSH can generate the phase of small plate crystal structure of gyrolite more easily, at the same time, the generation of 14? tobermorite reduced. Due to the flourine ions doping, the produced CaF₂ wraps the surface of F-CSH, which will makes the crystal more close-grained and granular hydration products distribution more uniform.?2? FTIR results indicate that the silica tetrahedron synthesized under 200°C mainly contains structure of Q2, and includes little Q1; the vibration type of Si-O bond of the synthesized sample under 120°C only finds the structure of Q2, and the crystalline degree is relatively low. Moreover, F-CSH prepared under the condition of of 120 °C contains more CaF₂.?3? It can be known from analysis of 29 Si MAS NMR spectrum and 19 F MAS NMR spectrum that F atom in the products exists mainly in two forms. Firstly, it combines with Ca ions to generate CaF₂ that wraps the surface of CSH. Secondly, it will enters into the space between layers of CSH to combine with Ca.?4? N₂ adsorption and desorption analysis results indicate that synthesized CSH sample under the condition of 120°C, 6h and calcium silicon ratio 0.65 and the corresponding F-CSH sample with calcium fluoride ratio 0.5 are both mesoporous material. It can be found the type of adsorption isotherm of the two is both IV type?classified based on IUPAC?. The specific surface area and pore volume of F-CSH sample is almost a quarter of that of CSH sample, and the corresponding average pore size also drops.?5? Due to the lower specific surface area and correspondingly fewer numbers of hydrophilic group, F-CSH has better wet resistance enhancement than that of CSH. Moreover, more CaF₂ content and lower specific surface area hindered the diffusion of CO₂, at the same time H₂O molecules in F-CSH which can be dissolved CO₂ and the number of detached Ca2+ are both reducted, makes its carbonation resistance increased. In general, the overall performance of F-CSH is greatly improved.