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)O5, have been prepared. T1, T3 and M3 forms have been identified by X-ray powder diffraction, but pure M1 form was not stabilized. The crystal structure of a sample nominally Ca2.96Mg0.03Al0.01(Si0.99Al0.01)O5 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 T3 form with a triclinic cell, space group P 1̄1­, of dimensions a = 11.6389(2) Å, b = 14.1716(3) Å, c = 13.6434(3) Å, α = 104.982(2)°, β = 94.622(1)°, γ = 90.107(2)° and V/Z = 120.346(6) Å3. Laboratory and commercial clinkers were studied by synchrotron X-ray powder diffraction and the Rietveld method. The reported T3 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 Å3 should contain up to ~ 1.0 wt.% of MgO, being T3 type. If refined C3S V/Z is smaller than ~ 119.8 Å3 the clinker may contain more than ~ 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.