When repairing your fireplace, you can avoid the cost of buying a full container of firebrick mortar by mixing your own to use from scratch or to add to the supply you have, as an "extender." There are two recipes for mixing firebrick mortar. One recipe has standard Portland cement as one of its ingredients, while the other calls for the more difficult-to-find calcium aluminate cement. Whichever you use, do not deviate from the prescribed mixture proportions, as it is very important to mix the amounts properly.
Put 10 parts sand in the mixing container
Mix 6 parts fire clay with the sand in the mixing container.
Add 2 parts Portland cement to the sand and fire clay, and then mix them thoroughly.
Add 3 parts lime to the mixture, mix it thoroughly with the garden hoe, and then add water to the mixing container until the mortar has reached the consistency of peanut butter. Apply the mortar to the brick so that no more than 1/4 of an inch of mortar is used between the bricks. This will help prevent cracking of the mortar when it is heated.
Place 10 parts sand into the clean, dry mixing container.
Add 3 parts calcium aluminate cement to the sand and mix it thoroughly, using the garden hoe.
Add 1 1/2 parts fire clay to the mixture, and then fold it into the sand and calcium aluminate. Add water to the mixture and mix it thoroughly until it has reached the consistency of peanut butter. Apply the mortar to the firebrick so that no more than 1/4 of an inch of mortar is used. This thin application is necessary to prevent cracking.