I'm looking to design a small calciner that heat gypsum at a constant temperature of 350Celsius. The kiln is made off mild steel and I'm looking for a good insulator to fill the housing that keeps the heat onto the rotary kiln. The kiln is heated with burners. The kiln have a diameter of 0.5 [m]. I need a material to fill the sides of the housing as can be seen in the image attached. I would appreciate any helps and tips.