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Bentonites, smectite containing clays, are used in the construction industry due to their ability to form stable colloidal dispersions. Bentonite dispersions may be in the state of a sol (stable colloidal dispersion), may be coagulated (destabilised by salts), flocculated (destabilised by polymers) or thickened in the state of a gel (Abend & Lagaly 2000).
Dispersion stability is affected by many parameters. Beside the smectite content there is a strong influence of the electrolyte concentration in the clay-water system. Lagaly (1989) has demonstrated the effect of pH on the flow behaviour (yield value) of sodium bentonite dispersions. The effect of pH is attributed to the pH variable charge of clay minerals edges. The well-known card-house structure only forms at pH below the point of zero charge. Positively charged edges are attracted by the permanently negatively charged faces. Similarly, highly charged anions (e.g. orthophosphate) invert the charge of the edges and lead to the collapse of the card-house structure. Ionic strength strongly influences the rheological behaviour of bentonite dispersions and often shows complex dependencies. The effect of ionic strength is shown together with solid content in a phase diagram with sol-gel transitions for sodium bentonite (Abend & Lagaly 2000). Additionally, it is generally assumed that flow behaviour of bentonite dispersions is controlled by the sodium / calcium ratio (Lagaly et al. 1997). It is known e.g. from geotechnical applications, that age of dispersion, state of dispersion influenced by swelling time, and temperature also strongly influence the dispersion stability.
In combination with cements, e.g. in cement grouts, bentonite dispersions are used to improve dispersion stability, flow properties and penetration depth of the grout paste. We have investigated part of a phase diagram relevant to bentonite dispersions in cementitious systems, which are characterised by high values of pH and calcium concentration. Solid content was held constant (10 % w/w), as well as high constant pH (~12.5) and calcium concentration (~22 mM). Total sodium concentration, thus ionic strength, was varied between 0 and ~2500 mM. As a result of cation exchange, the adsorbed amount of sodium increases with sodium concentration. Similar to Abend & Lagaly (2000), we observed a sharp phase transition at between 100 and 400 mM total sodium concentration, which they have attributed to the transition of a repulsive into an attractive gel. The effect of adsorbed amount of sodium is strongly superimposed by the effect of ionic strength.
G. Lagaly (1989), Principles of flow of kaolin and bentonite dispersions. Applied Clay Science. 4: p. 105-123.
G. Lagaly, Schulz, O. and Zimehl, R. (1997): Dispersionen und Emulsionen. Darmstadt: Steinkopff.
Abend, S. and G. Lagaly (2000), Sol-gel transitions of sodium montmorillonite dispersions. Applied Clay Science. 16(3-4): p. 201-227.
Authors:Müller, Christian and Plötze, Michael
Index Terms:Bentonite; dispersion; rheology; Clay; ClayGroup
Further Information:Date published: 05.02.2004