help! thermal mass not heating up

I recently replaced my traditonal woodstove with a rmh, we had to cut some corners due to a lack of space and material and im afraid it may be very cold this winter.

Issue 1. The exhaust exiting the bottom of the barrel only travels 5-10 feet before entering the chimney. I dont think this is enough length to soak up all the heat

Issue 2. We could not find a cheap or close by source for clay so the thermal mass is a mixture of fire brick, large granite stones. Pieces of concrete blocks and a 2:1 mixture of sand and high strength concrete with peastones mixed throughout. Will these materials be viable for thermal mass?

Here is a picture of what we have put together

The thermal mass does heat up but only within a few inches of the barrel. The thermal mass was all put in over the past few days so perhaps it is still drying and will operate better in a week or so. But the room is still fairly cold unless im hanging out right around the barrel.

I am noob here
What is the temperature coming out of the flue or just out of the thermal mass?

I don't think fire brick is good for thermal mass (maybe I am wrong) but doesn't it have the property to refract the heat?
Like if it was insulating the heat on one side from getting to the other side.

Before cementing/cobing all this up you should have tested it.
I postulate you may need to take out the fire bricks.

The drying process maybe stealing a lot of the heat and part of the problem.
But I suspect the fire bricks are the problem.

Go see any farmer and ask if you can load up your car with rocks that he took out of his field.
They have tons of rocks that they discard along the fields.
I would get as many bowling ball size rocks as you can or smaller that can fit in your car (without damaging the suspension).
Then it is a matter of how many trips will it take to fill the bench.

Please take care to verify what I share as I am a newbie
Hopefully a more experienced RMH builder will confirm or correct me.
I humbly apologize for any errors in my comments.
In any case we will both learn something out of this.

Good Luck

The Firebricks Are Only Along the front that you can see in the picture and insulating the burn chamber they arn't actually throughout the thermal mass.the top of the barrel gets up to 600 F but the mass is only heating up within a couple inches of the barrel and on top of the burn chamber. i will update how well its heating in a few days it should be dry by then.

How long ago did you finish the build?

In the past, the answer to this question has been, "Keep burning it regularly for a couple of weeks before expecting the mass to be hot." You've got to evaporate a lot of water before you can start raising the temperature.

Brandon: Keep Burning ! It takes weeks to dry out a cob mass... not sure how long your mass will take but it will take time. Give us more info on your build , pictures if you have them.