Many refractories must be fired before they are installed.

Most of them contain water when they are consolidated by pressing, ramming, vibration casting etc. This water is removed as steam at 100° you can start the drying/firing process at 200° c because of the latent heat of vaporization of water. It takes a lot of heat.      Joules per gram to boil water at 100C.  Only once it is dry will the surface temperature pass 200 degrees.

Then at 400° c the water of crystallization of calcium aluminate hydrates dissociates and comes off as steam. Steam pressure rises with temperature and at 400° it has very high pressure. When the steam pressure exceeds the tensile strength of the refractory the refractory explodes. If the tensile strength of the refractory is high, then the explosion is very violent.

At 300 to 600 degrees Organics burn off. They can dissociate without burning, but they cannot burn faster than oxygen can diffuse into the material. This is slow because carbon dioxide is diffusing out of the material at the same time. 

From around 500 degrees some soft start to melt then alkalis and chlorides start melting. 

At 700 to 800° c carbonates decompose. limestone, dolomite, and magnesite give off carbon dioxide. 

From around 900° sintering becomes significant. *

At 600 degrees chemically combined water is released from clouds and other phyllosilicates. *

With the high speed of vibration, atoms start to move around in crystals. Crystals change shape to reach lower energy levels. Wetting is in progress and surface tension plays a big role in putting particles together resulting in shrinkage. 

But 1200 degrees there is a lot of liquid phase present. The gas phase is being pushed out through the pores as the material contracts. At some point the porosity becomes discontinuous and they separated pores expand due to gas pressure.