Stability of Monosulfate in the Presence of Iron

Monosulfate (3CaO center dot(AlxFe1x)2O3 center dot CaSO4 center dot 12H2O) is an AFm 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 [Ca2Fe(OH)6]+ and a negatively charged interlayer [1/2SO4 center dot 3H2O]-, crystallizes in the trigonal R (3) over bar symmetry, and is isotypic with Al-monosulfate. The solubility product at 25 degrees 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 similar to+similar to Fe) ratio and a miscibility gap in the range 0.45 similar to