Determination of slag content in blended cement by XRD/Rietveld method without adding internal standard

Blast-furnace slag content in blended cement is a significant parameter affecting both cement properties and concrete performance and durability. The XRD/Rietveld method has been applied for rapid quantifying slag content in blended cement and is expected to be applicable to in-plant processing and quality control. In many cases, however, the amount of amorphous phase is determined indirectly by adding known quantities of alumina (Al2O3) to the cement sample as an internal standard. In this case, a considerable discrepancy exists between measured and actual values because of different grain size distributions or X-ray absorption of cement minerals and the internal standard. Even if the internal standard used is constant, slight differences in fineness or mineral composition of base cements (clinker) due to the variations of production conditions and production date create unacceptable levels of these discrepancies. However, the recently developed Rietveld software enables quantifying the phase with no known crystal structure or amorphous phase in the calculation. We experimentally verified the accuracy and applicability of this technique for quantifying the slag contained in cement. In the experiment, we used many base cements that had different fineness and chemical and mineral compositions. We then used slags from different production areas to make blended cement in a laboratory. The slag diffraction pattern is modeled by measuring one pure slag sample and some fixed parameters by Pawley refinement. The results demonstrate that slag content and cement mineral content can be measured simultaneously with high accuracy, independently of differences in base cement properties and admixed slag brand. The accuracy of this method is comparable to that of the conventional selective dissolution method. This technique is also more reasonable and applicable in an automated plant process because there is no need for calibration or adding a standard, and thus contributes to quality improvement in the blended cement manufacturing process.
Keywords: slag content, XRD, Rietveld method, internal standard, online measurement.
Author: M. Yamashita, H. Tanaka

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