Formation mechanism of calcium aluminate compounds based on high-temperature solid-state reaction

• CaO gradually diffuses into Al2O3 distribution during solid-state reactions.
• Ca3Al2O6 and Ca5Al6O14 are formed at the initial reaction stage.
• The various calcium aluminates formed present as a layered distribution, and Al2O3 is in the core.
• The formula of mayenite transforms from Ca11.3Al12O32.3 to Ca12Al12O33 with the increasing sintering duration.

Calcium aluminate compounds were synthesized through the sintering process within the temperature ranges of 1100–1400 °C. The obtained compounds were characterized using XRD, FT-IR, SEM, EDS and DSC methods. The results show that the calcium aluminate compounds include Ca3Al2O6, Ca5Al6O14, Ca12Al14O33, CaAl2O4, CaAl4O7 and CaAl12O19 in the sintered products with different molar ratios of CaO to Al2O3. The calcium aluminates present as a layered distribution. The outer layer calcium aluminate contains more CaO, and Al2O3 is in the core. CaO gradually diffuses into the distribution of Al2O3 during the solidstate reactions, and Ca3Al2O6 and Ca5Al6O14 are formed in the initial reaction stage. The variation in the molar ratios of CaO to Al2O3 in the mixtures has no effect on the phase constitutions of calcium aluminates in the initial stage of the reactions. All the calcium aluminates play roles as the intermediate phases to the final equilibrium phases and the molar ratios of equilibrium phases are similar with those of the sintered products. As the sintering process proceeds, the unit cell volumes of Ca3Al2O6, CaAl2O4 and mayenite decrease, and the chemical formula of mayenite transforms from Ca11.3Al14O32.3 to Ca12Al14O33.