Influence of natural coating type on the frictional and abrasion behaviour of siliciclastic-coated sedimentary sand grains

Sedimentary deposits often display a variety of grains and especially in gap graded materials or soil mixtures, it is expected that the finer particles may develop a coating in the contact region of coarser particles. Development of coatings in the vicinity of the contacts of coarser grains may also be encountered due to increase in depth and pressure which cause breakage of grain asperities forming in this way smaller particles which interact with larger grains and coatings may also influence reservoir quality. As the interactions of grains at their contacts have a significant influence on the mechanical, geophysical, hydraulic and thermal properties of sediments, it is essential to be examined in the laboratory through grain-scale tests to provide insights into their behaviour. In the present study, a new technique was developed in coating sand grains with siliciclastic non-plastic powder and emphasis was placed in the assessment of the mechanical behaviour of the specimens, comparing them with plastic-coated grains. The experiments were performed with grain-scale methods and were further supported with direct shear tests. The results showed that different frictional mechanisms involve in non-plastic coated grain contacts compared with the case of plastic coatings and that the influence of loading history played an important role on the tribological behaviour of the grains. Microcrushing and rearrangement of the siliciclastic non-plastic coating were the major contributing mechanisms, and their influence was found to be dependent on the magnitude of the applied normal load, which was not the case of plastic-coated grains. The results from the study provide fundamental understanding on the role of the type of natural coatings on the interface behaviour of sediments and highlight the important differences at the grain-scale between sand-silt and sand-clay mixtures.