Depending on their Si concentration, new steel grades, so- called TRIP steels, have a significantly higher proportion of aluminium compared to conventional aluminium killed steels. The TRIP steels are considered very reactive. This leads to significant changes in the thermo physical properties of the slags, such as the viscosity. So therefore new, New casting powders have to be developed which react less with the TRIP steels. These are called calcium aluminate slags. In this work, a main focus was on the rheological properties and crystallization of new mould fluxes. For more than 100 different mould flux compositions the liquidus temperatures and viscosity at 1300 °C were simulated using the FactSage software program. Based on the results, 13 of these compositions were selected for quenching. These were submitted, melted and potted on a steel plate. The slag viscosity was characterised optically. All slags were ground up and mineralogically analyzed. Nine of these slags were additionally investigated in the simultaneous thermal analysis (STA). The three selected main compositions 66, 68 and 70 have a C/A- ratio between 0.96 and 0.91 at a B2O3- content of 4.5 - 6.5 wt%. They also have a Li2O- content of 6 wt%, a Na2O- content of 8 wt% and have been tested in the main experiments. These tests were the Inclined Plane Test (IPT), the Furnace Crystallization Test (FCT) and measurements using a high temperature rheometer. The chemical compositions of the quenched slags (66 - 70) are very close to each other and so the detected phase compositions are very similar. In all three slags were found a γ-lithium aluminum oxide, takedaite, calcium oxide, tricalcium aluminate and a sodium aluminum silicate. Composition 68 also had a calcium aluminum fluoride oxide added as an additional phase. On the one hand the liquidustemperatures, which were found in the STA, were 1173 °C till 1194 °C and on the other hand the liquidustemperatures, detected at the FCT, were between 1124 °C to 1154 °C, for the compositions 66 - 70. The measured viscosity at 1300 °C were 3.7 poise (composition 66), 1.3 poise (composition 68) and 5.2 poise (composition 70).