INTRODUCTION: Building materials like mortars are systems with a complex composition. Besides the anorganic binder cement, scores of organic additives give decisive properties to the products. Since ettringite is one of the hydrate phases during cement hydration, XRD investigations on the crystallography of the formed ettringite in cement systems with and without organic additives of commercial quality were carried out and the results were compared.

EXPERIMENTAL METHODS: The XRD investigations were carried out using a Siemens D5000 diffractometer assembled with a SOLX solid state detector from Bruker AXS. A special sample holder [1] with Peltier element allows isothermal conditions at 23 °C +/- 0,2 °C during measurement. Rietveld refinement was performed using the Software TOPAS V3.0 (fundamental parameters approach) and the structural models (ICSD) of all detected phases. For the refinement of ettringite a new published structure was employed [2]. Refined parameters were scale factor, sample displacement, background as Chebychev polynomial of 5th grade, crystallite size and lattice parameters of ettringite and portlandite. The lattice parameters of the cement phases were refined during quantification of the dry cement and kept fixed during refinement of the cement pastes. Furthermore the synthesis of pure ettringite using a suitable precipitation method in presence and absence of the polymer was performed and the lattice parameters were determined.

RESULTS: The investigations prove that the presence of the organic polymer shows influences on the lattice parameters of the ettringite formed during the hydration of the cement. Since there was no difference detected in the lattice parameters of the precipitated ettringites, a possible integration of the polymer in the structure of the ettringites can be excluded. The differences in the lattice parameters of the ettringites have to be caused by the offer of ions in the pore solution in the cement pastes influenced by the presence of the polymer. In hydrating cement systems on the one hand the anions CO₃^2- and OH^- and on the other hand the cations Fe³⁺ and Si⁴⁺ may play the major role for integration in the structure of the ettringite instead of SO₄^2- and Al³⁺ respectively and therefore influence the lattice parameters. Consequently the organic polymer may influence the availability of these different ions for the ettringite formation.

REFERENCES:

[1] Hesse, C., Degenkolb, M., Gäberlein, P., Götz-Neunheoffer, F., Neubauer, J., Schwarz, V. (2008). Untersuchungen zum Einfluss von Temperatur und w/z-Wert auf die frühe Hydratation von Weißzement, Cement International (accepted)

[2] Goetz-Neunhoeffer, F., Neubauer, J. (2006). Refined ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂*26H₂O) structure for quantitative X-ray diffraction analysis; Powder Diffraction 21, pp. 4-11

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