A local granite countertop business is unloading broken granite pieces for next to nothing. It's polished on one side, most have a finished/straight edge. It's actual granite, not manmade corian type material. I already picked up a load and plan to use some for the bench/exhaust thermal mass portion of an RMH.
Dale here: I'm not claiming any particular rocket stove expertise, but I can assure you that granite is unsuitable as a replacement for firebrick.
I have cracked granite into thousands of little pieces just by using it as a campfire surround and I've seen it completely crumble after a barn fire. It would do the same thing in the extreme heat of a rocket stove.
Firebrick are manufactured by breaking up fired clay into thousands of little pieces and then compressing these pieces together with fresh clay to make a brick. When this new brick heats and cools it acts like thousands of little bricks so that it doesn't fracture as one unit. Granite on the other hand will act as a single unit and it will break up. Also fired clay can handle higher temperatures than granite can.
If you go into craigslist or any of those other resale sites there are probably lots of free firebricks from demolished chimneys. I give away several tons of them every year.
Location: New Mexico high desert Zone 7a, alkaline soils. 9" average annual rainfall.
Using granite inside the RMH will result is cracking and eventual disintegration of granite. The issue is thermal expansion. Firebrick contains alumina which allows the heat to move through the brick quickly. Granite will move heat slowly: one side hot, the other cold, the result is a thermal fracture.
Used on the outside, it offers a decorative effect as well as mass for storing heat.
Granite weathers well. Would make a good walkway.
Granite is waterproof. How about a backsplash in the kitchen/
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