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