I am getting a Jotul Oslo installed in a couple weeks. I am in the process of making a hearth pad. I have some 1.5 inch thick flagstone that I am setting in Flexbond thinset. My question is: What should I use to fill the gaps between the stones? They are irregular shapes, so some of the gaps will be up to 1.5 inches. I'm assuming this kind of gap is too large for normal sanded grout. Can I use the Flexbond thinset? The grey color is fine.