Thermal Mass material

I am working on plans to build a RMH in my cottage.

The cottage is approx. 130 yrs old, made of limestone, about 1000 sq. ft. As part of renovations I'm planning to remove the rotted wood flooring and pour a concrete floor. I'm also going to add inner walls with insulation.

Would it be reasonable to make the RMH out of concrete as well? Cob is not an option, since I don't have access to any adequate supply of clay. My two only real options are concrete/cement or fill dirt.

Any advice? Suggestions?

Chris,

Please try to add insulation to the outside of the thermal mass, otherwise you will lose its ability to work as the buffer of heat and cold.

If your dirt has sufficient plasticity when wet (contains enough clay) you could use it as your cob. Sometimes adding a very small amount of clay dramatically changes the properties of the mix. Year ago I have plastered three rooms of my house with a plaster made by grinding remnants of the CEB bricks that I used for building the house, 5-10% of cement to reflect the mix used to manufacture the blocks and some rice straw. It turned out to be a very bad mix that looked bad, was soft, dusting and did not want to get lime washed. I decided to chip it off and since the plaster was such a low quality it was quite easy. I have reground it and mixed with the same amount of cement and ONLY 5% of clay. It turned out to be smooth when plastering, hard after drying and after lime washing it gave a gorgeous looking wall.
I would start with determining the clay contents (doing a jar test: mix dirt with water, shake vigorously for few minutes and leave for a few weeks and then measure with the ruler the ratios of sand, silt, clay). Then you could just buy 1 bag of clay in a ceramic supply store - locally or online and it could be enough to amend your dirt to make good cob.

Thank you! I'll test the soil and see what it needs. And I'll definitely put insulation in by the wall. I was planning to put up the insulation and wall board, and then brick between that and the main mass.

Normal concrete might crack and fail near the bell and around the fire box where it’s hottest. However it should be ok on the bench portion. Maybe there’s a trick or product to make concrete more heat resistant.