Classification of rock dust for potential market applications

The process of finding suitable alternate materials for construction is nowadays a necessity, as the use of extracted natural resources is unsustainable. This process, however, is not always straightforward and demands research and material investigation. This thesis focuses on clarifying the current market uses of Rock Dust, a side product of rock crushing and investigates its engineering properties, suggesting potential uses for it. The thesis first categorizes rock dust according to standardized soil classification systems and compares its properties to those of natural soils. This is achieved with literature review and extensive laboratory testing, including Proctor, Permeability, Hydrometer, Oedometer and Direct Shear tests. The study suggests that Rock Dust is underused even with evidence of suitability for various applications inside and outside the field of engineering. The consequence of this underuse is the accumulation of rock dust near crushing plants. Rock dust has interesting engineering properties and is free of additives, as it is comprised just by the finer fractions (ranging from #0-6mm) of crushed rock aggregates. Some properties of rock dust indeed hinder its broader use. It has relatively low permeability, its very fine particles spread through the air easily and is frost susceptible. All these unwanted properties are related to the finest fractions (0.002-0.06mm) present in the rock dust. Perhaps a simple sedimentation process could separate finer particles from the sand and gravel- sized grains. In general, rock dust’s grain size distribution is similar to that of well-graded sandy soils, Proctor compaction properties relate it closely to gravels, the permeability coefficient is similar to that of fine or silty sands, oedometer results remind the behaviour of composite soils of different sand mixtures and sand-silt mixtures, while the Direct Shear test results indicate its strength is remarkably high and close to that of gravel, however dependent on its grain shapes. Microscope observations show that the rock dust grains are very edgy. This positively influences its strength properties, however, in the long term, the particles are likely to crush, reducing the material’s strength. Both Proctor compaction and microscope images indicate that crushing takes place with high vertical loading. As a conclusion, Rock Dust can replace sand depending on the replaced percentage in uses ranging from embankments for roads and ground improvement to concrete structures and even agriculture. Further research is suggested to observe how different percentages of replaced rock dust influence triaxial, oedometer, permeability and compressive test results when mixed into other soils or used in concrete