While the mixing process is the same for fire clay mortar or refractory mortar, the portions of its materials will vary depending on the type of cement used. The mix for refractory or fire clay mortar is different from typical mortar to compensate for its exposure to high temperatures. Refractory mortar should also not be exposed on the inside part of an oven to minimize its shrinkage.
When using calcium aluminate cement, the recommended ratio for the materials is 10 parts sand, 3 parts calcium aluminate cement and 1.5 fire clay. If Portland cement is preferred, the recommended proportions for the mixture are 10 parts sand, 6 parts fire clay, 2 parts Portland cement and 3 parts lime. Portland cement helps the mixture to set but will be eventually burned off leaving lime, which is a type of calcium, to take its place.
The following shows a method of mixing fire clay mortar.
- Combine the dry materials Mix all the dry materials thoroughly. Continue mixing well into the next step.
- Add water Gradually add water while mixing. The water should be clean and at room temperature. Continue to add water until a peanut-butter-like consistency is achieved.
When used as mortar to hold fire bricks in an oven structure, this fire-resistant mortar formula should be applied to no more than 1/4 of an inch thick. If the mortar is applied too thick, the shrinkage might cause movement in the bricks, which will require repair work that would have been avoidable.