Stacie; An option for you might be to enclose your mass with regular clay bricks. This allows you to easily mix any clay / dirt / and sand into a cob like mud . Use as many large stones or
concrete chunks as you can and fill in the spaces between with your mud/cob mix. The stones are superior to cob at holding heat and your mud is simply filling in the air gaps. Sandy clay mud can often be found on a forest service road or next to a running creek . Contractors excavating a foundation routinely run into a clay layer. They would happily give you as much as you wanted! If you can find a substitute mud , your fireclay/ sand can be used to glue your bricks and around your core , instead of in your mass.