Natural fine-grained suspensions usually exhibit a complex rheological fingerprint – in particular a two-step yielding phenomenon – due to the presence of mineral clay particles and organic matter (often found in a flocculated state). These rheological properties may vary considerably from one location to another due to the differences in mud composition (specifically in organic matter content). In this study, the origin of this two-step yielding behaviour for natural suspensions is discussed with the help of different experimental techniques including rheology, particle sizing, rheo-optics, and video microscopy. The samples were collected from different locations in the Port of Hamburg, Germany. A rheological analysis of the samples was performed with amplitude sweep, frequency sweep, stress ramp-up and structural recovery tests. The shear-induced structural changes of mud samples was studied by using a parallel plate shearing device with a microscope. Mineral clay-organic matter flocs were studied using video microscopy to obtain the floc size, floc density and settling velocity of flocs. Higher values of rheological properties such as cross-over stress, yield stress, and moduli were observed for samples having higher organic matter content. These samples also produced the largest floc sizes. The rheo-optical analysis showed the formation of cylinder-like structures in fine-grained suspensions upon shearing action, which reflect the origin of two-step yielding behaviour in mud samples, observed in stress ramp-up and amplitude sweep tests.@en

In the mining industry, tailings are conventionally stored in artificial ponds that are susceptible to ruptures, as occurred in Southeast Brazil in November 2015. The Fundão failure dumped approximately 40 million m3 of very fine iron-rich tailing material into the Doce River watershed, where part reached the adjacent continental shelf. Part of the spilled material remained suspended in the water column after reaching the ocean. Zeta potential measurements (pH and salt dependence) were conducted to verify the flocculation tendency of suspensions. Surface sediment samples were collected in areas representative of key stages of tailings transport through the river and subsequent deposit on the continental shelf. A surface sediment sample from the continental shelf prior to the failure was used as a control. Previously published data suggested an influence from goethite on the flocculation of samples containing iron ore tailing sludge. Here we show that although a flocculation tendency was observed under influence of salts, the presence and quantity of iron oxyhydroxide were determinant in the flocculation pattern of contaminated sedimentary samples. Only sediment from the continental shelf before the mining dam breach exhibited classical zeta potential behavior dependence on pH and salinity expected in natural sediments containing clay minerals. Sediments contaminated with the iron ore tailing sludge displayed a clear influence of goethite on their zeta potential.@en

A modified version of the commercially available RheOptiCAD® was developed to visualize the microscopic structural changes occurring in concentrated suspensions, such as the break-up of flocs in clay suspensions, under shearing action. This is made possible by replacing the inverted microscope used in the traditional RheOptiCAD® set-up by an upright modular microscope equipped with a CMOS camera and epi-illumination. Our device retains the following features of the previous version of RheOptiCAD®: [i] uniaxial translational motion of two parallel plates, [ii] three modes of shear straining, [iii] controlled thermal environment, and [iv] vacuum joining of microscopy glass slides. The validation of the new design was done using a model system of un-flocculated and flocculated kaolin suspensions and concentrated natural mud suspension. The results showed that the constructed device is a promising tool for studying, from fundamental and industrial perspectives, the microstructural behaviour of complex suspended systems under controlled thermal and mechanical conditions.

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