Special Mortars for Fireplace Repairs

Refractory Mortar

• For repairs to the firebox, the internal chamber where the fire burns, a special type of mortar withstands extremely hot conditions and temperature cycles. Known as refractory mortar, it uses binders like calcium aluminate or sodium silicate, which tolerate high temperatures, instead of Portland cement. Refractory mortar is available in hydraulic-setting and air-drying, or premixed, forms. Hydraulic-setting refractory mortar is acid-resistant and water-insoluble once cured, making it suitable for indoor and outdoor applications. Premixed refractory mortar, which dries quickly and is water-soluble, can be washed away with water, making it easy to clean up but only suitable for indoor use.

Standard Mortars

• Standard mortars are made using Portland cement, hydrated lime and sand combined with water to achieve the proper consistency. Depending on the nature of the repair, standard mortar may be adequate or become modified by adding a small amount of admixture to improve certain properties. Type N mortar is the best all-around mortar and recommended for most projects. Type O mortar, also known as pointing mortar, has characteristics that make it the most ideal general mortar for repair in non-load-bearing, above-grade situations. Type K mortar, used in historic preservation circumstances where load-bearing strength is unimportant, has a porous nature that helps prolong the integrity of older bricks.

Admixtures

• A number of ingredients can be added to mortar to create specialy mortars for certain circumstances. Many repair mortars are created using a polymer additive like polyvinyl acetate, polyvinylidene dichloride and acrylics or modified acrylics that act as bonding or water-reducing agents, increase strength and limit permeability. Accelerators can be added to mortar to shorten its setting time, a quality especially useful in cold weather. Retarders are added to extend mortar working time and slow cement hydration in hot weather. Color pigments are also commonly added to repair mortars to allow replacement mortar to match existing mortar.

Performing Mortar Repairs

• To perform mortar repairs, you may need to clean the fireplace interior to identify all faulty joints. Use a fireplace cleaner as directed. Remove damaged mortar or bricks with a mortar-raking tool, chisel, hammer and stiff-bristled brush. Dampen the surface of bricks that bond to mortar. Prepare the mortar and apply mortar to the joints in 1/4-inch layers, letting the mortar cure for about 30 minutes between layers. Once the brick surface is reached, tool the mortar so it matches existing mortar. Clean off the face of the bricks with a stiff-bristled brush or damp sponge.