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Cement stable bentonites and cation exchange

Abstract:

Sodium bentonites, sodium montmorillonite containing clays, are used in the construction industry due to their ability to form stable suspensions (e.g. cut-off walls, boreholes). In combination with cements, e.g. in cement grouts, they are used to improve suspension stability, flow properties and penetration depth of the grout paste. In cementitious systems, buffered by calcium hydroxide, the calcium concentration is approx. 22 mM dependent on the alkali concentration. In such systems a stable bentonite suspension may fluidise and eventually break down due to cation exchange of sodium against calcium. Bentonites that form stable suspensions even in contact with cementitious pore fluids are called cement stable bentonites. Freeze-dried samples of three months reacted mixtures of bentonite “Volclay” and calcium hydroxide were investigated for the basal spacing of montmorillonite. It was observed, that this basal spacing was not equivalent to the one of homoionic calcium form (~15 Å). Moreover a clear dependency of the basal spacing on the Na/Ca ratio in solution was found (fig.1). For this experimental setup, the clay is the sodium source and the calcium solution concentration is buffered by the presence of calcium hydroxide. Therefore, the Na/Ca mole ratio in solution is roughly equivalent to the sodium solution and also the clay concentration (Na / Ca mole ratio of 17.9 in fig.1 was therefore extrapolated from a strong correlation between this ratio and the clay concentration). This dependency is best explained by cation exchange. Then, fig.1 displays part of an exchange isotherm with the basal spacing as a function of the adsorbed Na / Ca ratio. For increasing Na / Ca mole ratios in solution, the basal spacing approaches the value of the homoionic sodium montmorillonite (~12.4 Å). In the case of low Na / Ca mole ratios, basal spacings close to that of the calcium form are observed. In order to test this hypothesis, montmorillonites with different adsorbed Na / Ca mole ratios were produced in calcium hydroxide buffered solutions. The dissolved resp. adsorbed Na/Ca ratio is measured by ICP-AES and by an exchangeable cation method, the resulting basal spacing by X-ray diffraction. Additionally, rheological experiments are in progress.

Authors:

Müller, Christian and Kahr, Günter and Plötze, Michael

Index Terms:

cement stable; bentonite; cation exchange; basal spacing; Clay; ClayGroup

Further Information:

Date published: 22.06.2003