You can use fireclay, refractory cement is commonly used to arch. fireclay has good viscidity, it is handy to use.
consistence Mainly concern about the strength, generally masonry mortar does not consider the degree of fire resistance
use refractory clay, refractory clay is made of refractory clinker clay mixing with cornstarch in a certain proportion, generally refractory cement refers to alumina?cement, which is generally used in castable refractory, producing binder.
