The effect of natural dolomite admixtures on calcium zirconate–periclase materials microstructure evolution

The reaction sintering mechanism of dolomite–zirconia mixtures was investigated using fine grounded dolomite raw material and zirconium powder. The used dolomite raw materials differed by the content of impurities (SiO2, Al2O3 and Fe2O3 oxides). The microstructure evolution of MgO–CaZrO3 and CaZrO3 sintered materials was presented as a temperature function. One- and two-step firing processes of calcium raw materials powder mixed with chemically pure zirconium oxide were applied. The kinetics of reaction of CaZrO3 synthesis was estimated by determining the “free” calcium oxide by chemical and XRD analysis. The densification process was evaluated by firing shrinkage, apparent density, pore diameter and pore size distribution measurements. The microstructure of sintered materials was observed by SEM. It was observed that CaZrO3 synthesis was definitely finished at temperature of 1500 °C in the both applied ways of the synthesis (one- or two-step process). The only phase present in the model material synthesized from chemically pure reagents (CaCO3 and ZrO2) after firing at temperature of 1500 °C was calcium zirconate. In the materials synthesized from natural dolomites and ZrO2 two main phases were present—calcium zirconate and periclase. During firing of CaZrO3–MgO materials at lower temperatures the presence of transient phases was detected (mainly ferrites and calcium aluminates, 4CaO·Al2O3·Fe2O3 or 2CaO·Fe2O3). These phases disappeared at higher temperatures. This is probably related to the dissolution of impurities in the main phases of CaZrO3–MgO. The material obtained from the mixture of zirconium oxide and natural dolomite with the high impurities content has the highest densification level (∼95% theoretical density of CaZrO3–MgO) at 1500 and 1600 °C.