College Park, Maryland      June 6 - 10 , 2004

MP4: Neutron diffraction investigation of the structural deviations of components in cement clinker

V. K. Peterson (NIST Center for Neutron Research; University of Maryland, College Park)

The deviations in structural models of tricalcium silicate phases in a Rietveld model of cement clinker after atomic position refinement using neutron powder diffraction data were investigated. Results for phase quantification before, and after, atomic position refinement are reported, and alterations to Bragg-R factors of phases are recorded. In this neutron study, relatively minor adjustments to some tricalcium silicate structural models resulted in decreased R-Bragg factors of those and other phases.

Tricalcium silicate is the major phase in cement clinker, and is crystallographically complex. Tricalcium silicate exhibits polymorphism, and it is usual for several polymorphs to co-exist as a solid solution in cement clinker. At least seven polymorphs exist, and three have structural solutions that are commonly used in the Rietveld analysis of clinkers. Full-profile Rietveld methods have the ability to define the tricalcium silicate form. It is usual to include a polymorph of tricalcium silicate in each of the three crystal systems encountered when beginning a refinement and then exclude the crystal system(s) not found. Care must be taken so as not to enable one crystal system to alter itself to compensate for others not being modelled.

The crystallography of tricalcium silicate in further complicated as both triclinic and monoclinic forms of tricalcium silicate found in cement clinker are structurally modulated. The structural models used in the Rietveld analysis of cement clinker are averages or approximations only to these true, modulated structures. Additionally, only one triclinic and one monoclinic structural model exist to represent the three possible forms in each of these crystal systems.

Rietveld refinement is commonly used as a method of phase quantification of cement. Given that the structural models for tricalcium silicate may not be accurate or appropriate descriptions of the tricalcium silicate forms in cement clinker, the effect of refining these structures using data of real cement samples is of interest. Thus, any deviations of the "real" structures from the "model" structures can be investigated.

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