The bricks surrounding my fireplace have cracks between them. It's as if the thermal expansion/contraction has eroded the mortor between the bricks. I have had some serious fires in there and haven't had a problem but I am worried that the lack of grout may be a fire hazard. Is there a grout or mortor than I can force into the cracks?
Sure, you can re-cement those. You'll want to dig them out as much as you can, rinse them out with water, let dry, spray with water to wet them & then force in the new cement.
You can put a batch in a Zip-Lock type bag & cut off a tiny bottom corner of the bag to squirt the mortar into the joints. Below, is what the Home Improvement stores offer. The Bucket is good for up to 3000°F & tubes are good for up to 2000°F.
Wood breaks down at 500° F and vapors containing 50% to 60% of the heat value are released by 1100° F. After the gases burn off and moisture is evaporated, charcoal remains and burns at a temperature exceeding 1100° F.
The stove gasket sealant in the picture has a maximum operating temperature of 900 °F. I believe this would work because of my experience with a similar problem. I had an emergency one day and needed to fix several broken firebricks. I used the red kind of high temperature gasket sealant, for engines. This was two yeas ago and this has been holding up very well. The red gasket sealant generally has a maximum operating temperature of 600 - 650 °F. After a bit of research, I see that the copper kind (of RTV sealant) seems to have a sometimes higher or lower maxiumum. I found one kind of copper sealant with a maximum of 700 °F.
But my advice would be to try the 900 °F sealant, specifically for stoves. You will need to clean the surface as well as possible (maybe use a spray bottle to rinse the area with water) before applying it.
