engleski

X-ray Rietveld analysis of the calcium-aluminate-cement clinker

Calcium aluminate cements were developed in France at the beginning of the 20-th century. They are usually called bauxite cements, melted cements or La Farge cements. Originally thay had to improve the durability of concrete in sulphate-bearing environments. Calcium-aluminate-cement clinker is produced by melting of bauxite (containing iron) and limestone at temperature of 1500-1600 ○C. The melt is homogenized and slowly cooled down to RT during 24 hours. After cooling, the material is ground to a fine powder (particle size ~20 μm) in a ball mill. Important components of the calcium-aluminate-cement clinker are CaO·Al2O3 (designated CA) and 12CaO·7Al2O3 (designated C12A7). Iron is situated in C-A-F solid solutions of variable compositions, from 4CaO·Al2O3·Fe2O3 (designated C4AF) to 6CaO·Al2O3·2Fe2O3 (designated C6AF2), and also can form oxide FeO (wuestite). The following phases are present in lower amounts: CaO·2Al2O3 (designated CA2), -2CaO·SiO2 (designated -C2S), 2CaO·Al2O3·SiO2 (designated C2AS) and CaO·TiO2 (designated CT). CA is the most important component of calcium-aluminate-cement. Hydration of CA is an exothermic process. This makes a concrete production by using the calcium-aluminate-cement possible even at very low temperatures. Also, the strength development in such concrete is very fast. Samples of calcium-aluminate-cement clinker have been investigated in order to get better understanding of crystallization process of the present mineral phases in this system. Three representative samples of calcium-aluminate-cement clinker from the cement plant Istra Cement International were selected. Elemental chemical composition of the samples was determined by means of XRF (X-ray fluorescence) spectroscopy, and then the samples were examined by X-ray powder diffraction at RT. Unit-cell parameters of the identified mineral phases in samples were determined using Si powder as an internal standard, and refined by the whole-powder-pattern decomposition method [WPPF program, H. Toraya, J. Appl. Cryst. 19 (1986) 440]. Rietveld method was performed to refine the crystal structures of the present mineral phases in samples [Program X'Pert Plus–1999, Koninklijke Philips Electronics N.V., Philips analytical, Almelo, The Netherlands]. Starting structure models were taken from the ISCD database [Inorganic Crystal Structure Database, Fachinformationszentrum Karlsruhe, Germany] and slightly modified in accordance with the chemical composition of samples and the refined unit-cell parameters of present phases. Rietveld refinement for the samples converged with Rwp being in range from 0.088 to 0.095. The results showed that the main components of the investigated samples are CA (in range 58-61 wt%), C4AF (in range 35-36 wt%) and FeO (in range 3-6 wt%). C12A7 is present in a small amount of about 2 wt%, while the amount of nonidentified phases was less than 3 wt%. The distinct mineral phases containing Si and/or Ti were not noticed, so it is likely that small amounts of Si and Ti are incorporated into dominant phases.

Aluminate cement clinker; X-ray diffraction; Rietveld refinement

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