Online quality control of aggregates based on the measurement of magnetic susceptibility
Aggregates are an important part of society, especially in construction and road construction. A major problem in the mining of magmatic aggregates is the changing quality of the material within a deposit. Geological alteration processes can impair the quality characteristics of the stone, such as strength and weathering resistance. The consequences of these processes are not always visible. The unconscious sale of inferior aggregate qualities might lead to the loss of customers. Especially, for the mostly medium-sized extraction companies, this is a consequence, which endangers their existence. The structural stability of aggregates is currently only determined in time-consuming laboratory tests. An economic analysis of representative sample quantities is not possible. This report presents a procedure in which an online analysis of a representative sample quantity can be carried out for each blasting well. The basis of this method is the measurement of the magnetic susceptibility of the drilling dust. Such a measurement can be performed in less than a minute using a low-cost hand-held instrument. The magnetic susceptibility is used as an indicator for alteration processes. This report examines for which magmatic rocks the method can be used and for which rocks the magnetic susceptibility serves as an indicator for alterations. These investigations are based on data from ten aggregate plants, which are composed of structural geology, magnetic susceptibility, petrological properties of the rocks (microscopy and overall rock analysis) and common indicators of structural stability (long-term weathering test, loss of ignition, glycol test). In this paper, the results in basaltic andesite and in phonolithic basalt are discussed. For basalt or andesite quarries the measured values of magnetic susceptibility indicate various populations, which can be linked to faults and discontinuities. The magnetic susceptibility of samples shows an inverse correlation to the loss of ignition of the same samples. This indicates that a higher rock quality relates to a higher magnetic susceptibility. This assumption is supported by the results of long-term weathering tests, ethylene-glycol tests and methylene blue tests. In basalt and andesite, the magnetic susceptibility is inversely proportional to the degree of alteration. In alkali basalts containing nepheline, however, no significant populations of magnetic susceptibility could be determined. Neither could any relation between magnetic susceptibility, loss of ignition and other rock quality parameters be shown. For these rocks the measurement of the magnetic susceptibility is not sufficient to test the rock quality and has to be extended by other quality determining methods, such as the determination of the loss of ignition.