Truth is that many masons are not really great at making fireplaces or fireboxes and fail to use refractory mortar. They confuse the refractory mortar with fireclay mortar or worse yet ordinary Portland cement with maybe a bit of fireclay added to make “fireclay mortar” themselves.
That is actually not terribly surprising as many are not familiar with the proper product and use. However for some time NFPA-211, which is an American reference, has clearly shown the requirement for refractory mortar for the construction of fireboxes, smoke chambers, and flue linings. The real problem with using “fireclay mortar” is that Portland cement simply can not withstand the heat; oh it can manage the increase in temperatures fairly well but it is the cooling process that causes it to fail and over time it will begin to fall apart – it essentially looses its strength and wide joints make it even worse. However, proper refractory mortar is made of high temperature cements and aggregates which do not expand and contract with temperature changes, this lack of movement prevents the mortar from failing and falling apart.
And to make things even worse, proper refractory mortar not only performs better but it also looks better, and is easier to apply! There are two types of refractory mortar conforming which are permitted to be used and each does the job well. The first is a dry mix, hydraulic-setting mortar that sets up, or cures just like Portland cement based mortars, and I have and use this at times. There is however another, premixed mortar is ready to use and is much like a dry wall mud in consistency and can even obtained in caulking tubes. The first choice, the hydraulic-setting refractory mortar often the best all-around solution as it is able to be used to lay the firebrick, smoke chamber, lay clay tiles for liners, etc. and can be mixed to match virtually any color of mortar. Once dry and cured the hydraulic refractory mortar is water insoluble and acid resistant. The premixed mortar on the other hand will dissolve in water even after it has set and dried, and is therefore not a great solution for all uses and it can be more difficult to store long term than the hydraulic refractory mortar which comes in pails.