POTENTIAL VALUE OF TRACE ELEMENTS IN THE SOURCING OF CEMENTITIOUS MATERIALS

摘要

Modern concrete production methods result in cementing phases with tightly controlled bulk chemistries, however, aggregates are selected mainly on the basis of regional availability. Since a wide variety of different minerals are used as aggregate, petrographic microscopy is the primary method used during the examination of concrete trace evidence in forensic contexts, whether in criminal cases, where concrete seems to be a material of choice for concealing evidence, or in civil cases regarding structural failures or disputes over workmanship. Petrographic microscopy is semi-quantitative and generally limited to determining whether the mineralogical classifications of aggregates are consistent between a sample and a potential source. Such analyses depend heavily on the experience of the investigator, and generally require relatively large samples and time- and resource- intensive sample preparation processes. Although petrographic microscopy is a reliable tool for excluding evidence if the aggregate classifications are dissimilar, new， operator-independent, qualitative methods are needed for establishing positive relationships. The cementing phase, rather than the aggregates, may be of use to the forensic investigator. A wide variety of trace elements (those measured in the ppm range) can enter ordinary portland cement through a wide variety of paths: As impurities in raw or supplementary materials; through variations in plant processes or equipment; through the reuse of materials such as cement kiln dust; and through the use of alternative fuels. While the exact content of trace elements in the cementing phase of a concrete system can be changed based on mix design, it is hypothesized that the ratios of these elements one to another will serve as a unique 'fingerprint' that can be used to identify cementitious trace evidence. Samples of several ordinary portland cements, secured from geographically diverse facilities, were prepared through acid dissolution and investigated using Total X-Ray Fluorescence (TXRF). 23 trace elements were quantified in each sample, however, analyses indicate that only eight of these elements show the promise of being usable as a characteristic 'fingerprint'. Two graphical interpretation methods (bar charts and ratio scatter plots), each display unique merits and flaws. This initial report provides encouraging data, however, significant future research is necessary to establish the usefulness of trace element ratios to forensic investigators.