You (Willen) will not get what you expect to get simply by pouring molten aluminum into a concrete mold.
People have mentioned being sure the mold is dry. Dry at room temperature is not "dry" at 630°. Aside from moisture, you have to consider air. Air in the pores of the mold will also expand greatly. That can cause splashing, but it will also cause defects in the cast ingot. Also, as concrete cures by reaction with CO2 (e.g, Ca(OH)2 + CO2 ==> CaCO2 +H2O ;although, the chemistry of concrete is much more complex), you will need to be aware that other gases may also be produced. That is one reason some fine castings are done in a vacuum.
With sand casting, you need vents and such and will get a rough casting, unless you are expert. With more rigid molds, such as concrete and plaster, you can get finer detail, but you will also need to design the mold differently. You need vents. Dentists and others use a centrifuge to force the molten metal into the mold. I suggest that you look up and read a description of "lost wax" casting, as based on what you have said, that sounds like what you are trying to do.
Finally, have you considered machining rather than casting? However you cast, unless it is a very well done lost wax casting or a boat anchor, will require machining before use.
John