A host of mining practices produce fine-grained mineral sludge, such as mature fine tailing from oil-sand extraction. These sludges are hard to dewater because of their high clay content and their low hydraulic conductivity. This paper describes pressure filtration tests in which eight different nonwoven geotextiles (GTXs) were used to filter high-clay-content sludges. The sludges were formulated by mixing kaolinite and silt to obtain eight different grain-size distributions (GSDs) with d(50) ranging from 3 to 55 mu m and clay content ranging from 0% to 38% (d < 2 mu m). The sludges created for this research had the GSD and variability of real fine tailings. The results indicate that sludges can be filtered with nonwoven GTXs selected on the basis of their pore-opening size. Furthermore, calendered GTXs are more efficient in retaining particles than needle punched GTXs. In addition, only during the early stage of filtration do GTXs really influence filtering because it is at this stage that, by retaining coarser particles, they allow progressive formation of filter cake. Upon forming, the filter cake becomes the major contributor to particle retention. Finally, the sludge composition is found to be the most significant factor affecting the final state of dewatering. (C) 2014 Elsevier Ltd. All rights reserved.