An experimental investigation into the micromechanics of non-active clays

Abstract

The paper presents an experimental investigation into the micro-mechanisms controlling the behaviour of non-active clays. Clay microstructural behaviour was investigated via Mercury Intrusion Porosimetry accompanied by Scanning Electron Microscope images. To gain insight into the mechanisms underlying reversible and non-reversible compression, samples for MIP testing were taken along both normal compression and unloading-reloading lines. To investigate the nature of inter-particle forces, the response of clay samples prepared with deionised water (characterised by acidic pH) was compared with clay samples prepared with alkaline water. A high pH 'deactivate' the edge-to-face contacts that are indeed active in the clay prepared with deionised (acidic) water. The pore-size distribution data clearly highlighted that the smaller pores are associated with particles in non-contact configuration, i.e. only interacting via the overlap of the repulsive electrical field generated by the negatively charged faces. On the other hand, larger pores are associated with contact configuration, generated by the attraction between the positively charged edge and the negatively charged face of the clay particle. The pore-size distribution data also allowed inferring that reversible behaviour is mainly associated with the reversible overlap of the repulsive electrical field in contact configuration whereas the plastic response appears to be associated, at the micro-scale, with the loss of edge-to-face contacts. Finally, an embryonic 1-D discrete element model was developed to show the potential of the micromechanical conceptual model to be implemented into a DEM model

Microstructural-based approach to the interpretation of clays and transitional soils behaviour

“…Thus, within an alkaline solution the resulting in a net negative charge on the edge's surface. Pedrotti (2016) reported that it is common practice to use potassium hydroxide KOH or similar to raise the pH of the solution in order to assess the edge charges. Sodium hexametaphosphate, (NaPO 3 ) 6 , is also known to Thus, when kaolin is mixed with distilled water (pH7), the pH of the mixture is found to be somewhere below 5.…”

“…On the contrary, as mentioned before, when kaolin is mixed with very alkaline solutions (e.g., KOH) its particle edges are charged negatively and pH > 6. Figure 2.7 -Surface charges on Kaolin particles (after Pedrotti, 2016)…”

“…Consequently, particle distance is controlled mainly by Coulombian interactions. Pedrotti (2016) reported that particles are considered to interact in two different ways when kaolin is in an acid suspension (distilled water). The first is Face-to-Face (FF), where Distilled water…”

“…(a) (b) Figure 2.13 -(a) MIP data, bi-modal and mono-modal fitting functions -experimental data from reconstituted Kaolin sample (Pedrotti, 2016) and (b) Frequency PSDs generated with various interval sizes for a reconstituted Kaolin sample -experimental data from Pedrotti (2016)…”

“…The materials used in this research were the two commercially available clays mined from Cornwall, England, and supplied by Imerys Minerals: Kaolin Clay and Ball Clay, the latter being a sub-product of the Kaolin Clay (Figure 3 Table 3.1 shows the three tests used to characterize the materials used in this research and the respective standards followed, while Table 3.2 presents the most relevant properties of the soils tested. The specific gravities of the three materials were obtained from previous studies conducted with samples of Kaolin Clay (Tarantino & De Col, 2008;Pedrotti, 2016), Ball Clay (Imerys Minerals, 2015) and Brasilia Soil (Borges, 2014). The pH9 water content was determined by this research using the trial and error approach, taking into account the experience provided by Pedrotti (2016) in samples of Kaolin Clay.…”