The effect of calcined mayenite on the hydration of ordinary Portland cement

The synthesis of mayenite with differences in mineral composition and its influence on the properties of Ordinary Portland cement samples were examined. The synthesis of mayenite precursor was carried out in hydrothermal condition (CaO/Al2O3 = 2.8, w/s = 10) within 1-72 h at 130 degrees C. It was observed that synthetic precursor which was formed after 4 h remained stable only till 275 degrees C, and, at a higher temperature (350 degrees C), it fully decomposed to mayenite. In addition, when the temperature increases, the diffraction peaks typical to mayenite increases and changes mineral composition. It was found that the highest amount of mayenite additive (7.5 wt%) with differences in mineral composition has a positive influence on OPC hydration. It was observed that the mineral composition of additives has in-fluence on the OPC hydration mechanism: at the first stage of hydration, the calcination temperature exerts negative influence on the formation of ettringite; at the second hydration stage, the reaction of gypsum with the additive calcined at 900 degrees C is more accelerated. The mechanism of hydration (21-60 min) depends mostly on the calcination temperature of the additive. At this stage, ettringite in the systems OPC-mayenite (synthesized at 350 degrees C) and OPC -mayenite (synthesized at 900 degrees C) is partially decomposed into monosulfoaluminate and gypsum. Meanwhile, the new exothermic reaction between mayenite and ettringite in the OPC-mayenite (synthesized at 550 degrees C) system promoted the formation of monosulfoaluminate and hydrated calcium aluminate compounds.