Melt Differentiation Induced by Crystallization of Cement Clinker Minerals in a CaO-SiO2-Al2O3-Fe2O3Pseudoquaternary System

Abstract

The phase stability in part of the P 2 O 5 -bearing pseudoquaternary system CaO-SiO 2 -Al 2 O 3 -Fe 2 O 3 has been studied by electron probe microanalysis, optical microscopy, and X-ray powder diffractometry. At 1663-1623 K, the four phases of melt, calcium aluminoferrite solid solution (ferrite), Ca 3 SiO 5 solid solution (C 3 S(ss)(, and α-Ca 2 SiO 4 solid solution [α-C 2 S(ss)] coexisted in equilibrium. On the basis of the chemical variations of these phases as a function of temperature, a melt-differentiation mechanism has been suggested to account for the crystallization behavior of Ca 3 Al 2 O 6 solid solution [C 3 A(ss)]. When the melt with Al 2 O 3 /Fe 2 O 3 = 0.25 was cooled from 1663 K, it underwent differentiation by the crystallization of ferrite, C 3 S(ss) and α-C 2 S(ss). The Al 2 O 3 /Fe 2 O 3 value of the melt steadily increased to 1.74 when cooled to 1613 K. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and the coexisting melt during cooling, the resulting ferrite crystals would show a zonal structure with the Al/(Al+Fe) value steadily increasing from 0.203 to 0.57 from the cores toward the rims. During cooling from 1623 to 1613 K, the C 3 A(ss) would crystallize out of the differentiated melt. The resulting phase compositions at ambient temperature were C 3 S(ss), β-C 2 S(ss), ferrite, and C 3 A(ss). On the other hand, when the melt of Al 2 O 3 /Fe 2 O 3 = 4.0 coexisting with α' H -C 2 S(ss) and C3A(ss) was cooled from 1615 K, the Ca 12 Al 14 O 33 solid solution [(C 12 A 7 (ss)] eventually crystallized out of the melt, with the phase composition of β-C 2 S(ss), ferrite with Al/(Al+Fe) = 0.7, and C 3 A(ss) and C 12 A 7 (ss) at ambient temperature.