Place a drop cloth under the gaps in the fireplace to protect the floor from any mortar that may drip while you work.
Combine 3 parts masonry sand with 1 part Portland cement in a bucket. Stir the dry ingredients with a paddle mixer until well-blended.
Add water and continue to mix the mortar until it has a mud-like consistency.
Scoop wet mortar out of the bucket with a trowel and fill the gaps in the fireplace with the mortar. Scrape away excess mortar and smooth with the trowel. Wipe any mortar off the brick with a damp rag. Let the mortar dry for 24 hours.
Spray the newly patched mortar with mortar and grout sealer. Let the sealer dry for at least 48 hours before using the fireplace.