Curing Concrete and Masonry

[Xabia Jim]

Okay, it's Calcining according to this excerpt....

• Notes on Refractory Curing:
  Refractory material has various materials in it to create tiny air pockets after curing. These air pockets are what actually insulates the furnace. Typically the manufacturer creates these pockets by adding vermiculite to the mix. This is can cause the curing to be deceptive, because the vermiculite can trap and hold water, even though the cement has kicked off and cured. Sorta like a cement sponge!
  
  The refractory should be allowed to cure a few days before disturbing. This is an important time, and the longer you can wait, the better. As with concrete, keep the surface from drying out by covering with wetted cloth or burlap during hot, dry days. Also keep in mind that this material does not set as hard and strong as concrete, due to all the trapped air pockets, different cement, and lack of aggregate.
  
  After the initial curing process, it is good to heat the material using a heat source such as a hot plate for a period of 8 hours or so. This will help to drive off the water in the cement prior to the initial firing.
  
  The initial firing, known as calcining, is critical. During this time the refractory is slowly heated from room temperature to the full operating temperature. This should be done over a long time as well, to allow the steam to escape the refractory. A calcining time of 12 hours is not unreasonable. Increase the heat very slowly at first to prevent the escaping steam from causing cracks, or worse.
  
  After calcining the unit is ready for use

[david s]

Jim, There are two types of refractory, insulating or dense. The dense type does not contain material like vermiculite to open it up. It is designed for the hot face. The problem is that without these spaces the moisture has more difficulty in escaping. I was told buy my supplier, after I noticed some fibres in the material and was concerned about their safety, that these fibres are designed to burn away at low temp and provide tiny pathways for moisture to escape but still leaving a dense material. The temp that ceramic materials become permanent (irreversible change) is 573 C For our ovens we really only get some of the surface to this temp. But the refractory material uses calcium aluminate cement which does a good enough job to make the material hard enough.