Crystal structure of low magnesium-content alite: Application to Rietveld quantitative phase analysis

Five members of tricalcium silicate solid solution, Ca3-x-yMgxAly(Si1-yAly)O-5, have been prepared. T-1. T-3 and M-3 forms have been identified by X-ray powder diffraction, but pure M-1 form was not stabilized. The crystal structure of a sample nominally Ca2.96Mg0.03Al0.01(Si0.99Al0.01)O-5 has been studied by a joint Rietveld refinement using strictly monochromatic laboratory X-ray and neutron powder diffraction data, with soft constraints of interatomic distances. The crystal structure of this alite is a T-3 form with a triclinic cell, space group P (1) over bar, of dimensions a=11.6389(2) angstrom, b=14.1716(3) angstrom, c=13.6434(3) angstrom, alpha=104.982(2)degrees, beta=94.622(1)degrees, gamma=90.107(2)degrees and V/Z=120.346(6) angstrom(3). Laboratory and commercial clinkers were studied by synchrotron X-ray powder diffraction and the Rietveld method. The reported T-3 structure for alite fits properly a variety of laboratory Portland clinkers with low magnesium contents. The alite refined volume(s) (VIZ) is useful to predict the magnesium oxide content of a clinker and the alite-type. Thus, a refined V/Z value between 121.0 and 120.3 angstrom(3) should contain up to similar to 1.0 wt.% of MgO, being T-3 type. if refined C3S V/Z is smaller than similar to 119.8 angstrom(3) the clinker may contain more than similar to 2.1 wt.% of MgO with alite as M3. For intermediate magnesium (and sulfur) contents, alites phase coexistence may be detected by using strictly monochromatic laboratory or synchrotron X-ray powder diffraction. However, the application of these results to commercial materials has to be taken cautiously due to the influence of other foreign ions in volume and alite-type. (c) 2008 Elsevier Ltd. All rights reserved.