Stability of Monosulfate in the Presence of Iron

Abstract

Monosulfate (3 CaO ·( Al x Fe 1 x ) 2 O 3 · CaSO 4 ·12 H 2 O ) is an AF m phase that can be formed during the hydration of cement. Fe‐containing monosulfate and ( Al , Fe ) mixed monosulfate were synthesized and characterized. Fe‐monosulfate is composed of a positively charged main layer [ Ca 2 Fe ( OH ) 6 ] + and a negatively charged interlayer [ 1 / 2 SO 4 ·3 H 2 O ] − , crystallizes in the trigonal symmetry, and is isotypic with Al ‐monosulfate. The solubility product at 25°C was determined to be −31.57. The formation of solid solution due to Al ‐ Fe substitution in the main layer structure of monosulfate was observed. Based on the evolution of the unit cell parameters and the thermodynamic investigations, a presence of solid solution from 0.0 to 0.45 Al /( Al + Fe ) ratio and a miscibility gap in the range 0.45 < Al /( Al + Fe ) ratio <0.95 is suggested.