Free mica in crushed rock aggregates
Abstract: Free mica particles in crushed rock aggregates for constructional purposes affect the quality of the end product. It is also known that mica-bearing rocks may cause damages of the road constructions, especially in regions with a temperate climate. The negative influence arises in both bounded and unbounded layers. The deterioration of the construction is related to the ability of mica particles to be released during crushing and to concentrate in the aggregate fine fractions. As the free mica particles absorb and hold liquid such as bitumen and water, the unbounded base course becomes susceptible to frost weathering and the mechanical properties of the asphalt mixture are negatively influenced. The water absorption ability is in turn caused by an increased specific surface area of the fine fraction depending on the grain shape of mica particles and their intra crystalline cavities. The aim of the present study was to investigate, emphasise, and contribute to an understanding of the performance of free mica particles in the fine fractions of crushed rock aggregates. The free mica problems are well- known, but sparsely taken into account in projecting and prospecting activities. The mineralogical composition (i.e. the quality) of the rock material is often a subordinated task in early stages in planning for roads and railways. Connected with establishment of quarries the estimation of rock materials quality is generally based on petrographic and mechanical analyses of drill-cores.The study presented consists of: ' The investigation of the ability of free mica particles to enrich in fine fractions of granitoid crushed rock aggregates. The comparison study to examine a possible correlation between the content of mica in original rock materials, the content of free mica particles in rock aggregate products, and the content of free mica particles in drill cuttings. ' The development of a method to estimate free mica particles in aggregate fine fractions using image analysis of grain mounts. ' The development and practical applicability of a method to estimate rock aggregates quality using analyses of drill cuttings. The study reveals that mica-bearing rocks exposed to mechanical impact release free mica particles that concentrate in the fine fractions of crushed rock aggregates. The original grain size of mica in the host rock is suggested to influence on the process. Coarse-grained rocks seem to separate free mica grains in earlier phases of the crushing process than fine-grained ones. The enrichment is suggested to depend on the textural characteristics of the rock rather than on the mineralogical composition. These aspects must be taken into consideration when the grain size fractions for analyses of free mica content are selected. The statistical evaluation of the new method for estimation of free mica particles points out its certainty and repeatability. Due to the possibility to save the images of the statistical operation, the results of the method are controllable. These factors mentioned make the method satisfactory and statistically acceptable from a scientific point of view. The concept using drill cuttings for estimation of rock aggregates quality is practically tested and evaluated as an effective field method. The further development and calibration should focus on improvement of sampling concerning the content of coarse drill cuttings. The homogeneity/heterogeneity of the bedrock is crucial for the selection of the samples and stresses the necessity of representative sampling.
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