Why fireclay mortar can develop cracks


Fireclay mortar binds off in a different way than conventional cement mortar. After drying and evaporation of the water, the setting process is not yet complete. In order to avoid bursting and cracks, the grill or kiln masonry must be carried out with the first firing with targeted temperature increases.

Ceramic bonding

The setting of fireclay mortar is ceramic and differs from the hydraulic setting of other mortars. Care should be taken during selection not to use a hydraulically setting mortar.

In order to prevent crack formation later, wet-on-wet work is an important basis when processing fireclay mortar. The surfaces of the firebrick stones should be heavily watered to help the adhesion of the two materials.

Ready-mixed mortars are more reliable

In general, fireclay mortar can be mixed by itself. Since this process leads to additional possible sources of error, it is advisable to use a slightly more expensive ready-made fireclay mortar.

Premixed mortar must not be stretched, thinned or extended. The addition of chamotte and the procedure called "leaning" change the physical reactions during setting. That does not hold the fireclay mortar or cracks, is as common as typical damage.

Drying and "burn-in" crucial for cracking

In addition to the drying time of up to two weeks, initial heating of masonry and mortar in slow and careful steps is helpful in preventing cracking. It is true that per hundred degrees of temperature increase from lower to increasing heating rate is changed. The general rule:

  • Starting speed ten degrees Celsius per hour
  • per hundredth jump five percent speed increase
  • Maximum hourly temperature increase thirty degrees

If existing cracks are filled with mortar, generous soaking and watering the environment is also helpful. After the repair work, the grill or oven should be heated similar to the first firing.

Only after complete cooling is the masonry of the grill or oven ready for the first functional heating. With premature and too fast action cracks are a sure consequence. During drying time and initial firing extreme external influences such as precipitation or temperatures below ten degrees and high temperature fluctuations should be excluded.

Tips & Tricks

If you discover fine cracks in fireclay or fireclay bricks, that's not unusual. For crumbling interior walls with larger holes, you should commission a professional stove builder with the repair indoors and thoroughly rehabilitate the exterior.

Product Image: Pablo Rogat / Shutterstock


Video Board: How To Fix Cracked Mortar or Concrete in a Fireplace with Mor-flexx