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

Abstract Five members of tricalcium silicate solid solution, Ca 3 −  x  −  y Mg x Al y (Si 1 −  y Al y )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 Ca 2.96 Mg 0.03 Al 0.01 (Si 0.99 Al 0.01 )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 11­, of dimensions a  = 11.6389(2) A, b  = 14.1716(3) A, c  = 13.6434(3) A, α  = 104.982(2)°, β  = 94.622(1)°, γ  = 90.107(2)° and V / Z  = 120.346(6) A 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) ( V / Z ) 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 A 3 should contain up to ~ 1.0 wt.% of MgO, being T 3 type. If refined C 3 S V/Z is smaller than ~ 119.8 A 3 the clinker may contain more than ~ 2.1 wt.% of MgO with alite as M 3 . 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.