I have been pondering the best way to achieve high thermal density in a thermal mass bench without the expense of using blocks of cut granite, marble, or soapstone. It occurred to me that using crushed granite of varying sizes as an aggregate would allow me to cast a solid dimensional block using concrete as a binder.
I did some looking and found that #10 and #57 granite (fine screenings and 1/4" to 1/2 inch pieces respectively), would likely permit a block to be cast that is 90 to 95 percent granite. That's as close to granite as it gets. While I'm presently tied up with my Recycled Natural Gas Water Heater to SolarWater Heater build, I'll likely not be getting around to testing this idea for some time, though no doubt I'll be looking up cheap and efficient ways to cast them.
If anyone has done this, or is willing to give it a go, I'd love to hear about how it performs.