The stone floor of my 1930s fireplace was badly repaired many years ago, and the mortar patch is loose and crumbling. A chimney restoration service recommended a material called "Chamber Tech." I googled it and it seems to be a mortar or parging that is primarily used to patch and contour the smoke chamber. Is this stuff durable enough to withstand the weight and scraping of a cast iron grate that gets picked up, put down and moved around during cleaning? I don't think of "parging" as a substitute for rock.