The influence of the cationic surfactant dodecyltrimethylammonium bromide (DTABr) on the electrokinetic and theological properties of a soda-activated bentonite was investigated. The electrokinetic and theological measurements of the surfactant-bentonite (2 wt.%) system were performed at three pH values of 4,8 and 12. The electrokinetic measurement results show that the DTABr was adsorbed on the bentonite mostly at pH 4 and 8, whereas X-ray diffraction (XRD) studies revealed that the DTABr surfactant enters the interlamellar space of the clay mineral at pH 8 and 12. Rheological data showed that the flow behavior changed with the surfactant concentration. The basal d((001))-spacings of DTABr-treated bentonite samples revealed expandability, from 14.8 to 16.8 angstrom. XRD results indicate a monolayer arrangement of the DTABr molecules parallel to the silicate layer. (c) 2007 Elsevier B.V. All rights reserved.