Synthesis of the U phase (4CaO . 0.9Al 2O 3 . 1.1SO 3 . 0.5Na 2O . 16H 2O)

A mineral called “U phase” was recently identified in cement-based systems at high alkali concentration. This sodium-substituted AFm phase with chemical composition 4CaO.0.9Al 2O 3.1.1SO 3.0.5Na 2O.16H 2O can induce deleterious effects through its secondary formation and its transformation into ettringite. In order to go further in its properties, this paper reports some methods of the pure U phase synthesis. Ten compositions were chosen and tested. It was found that the reaction between Na 2SO 4(0.5mole) + Ca(OH) 2 (0.5mole) and a fresh sodium aluminate solution (dissolution of 0.41~0.82 mole metallic aluminum fine powder in 1 liter 1N NaOH solution) may be considered as the most efficient way for synthesizing the U phase.