very weak portland concrete mix as opposed to clay sand concrete for thermal mass

For the heat battery,, does anyone have any ideas as to why I shouldn't use a very weak portland sand concrete as opposed to clay/sand? With winter around the corner, I just don't have the time or resources to track down and transport a supply of clay, where it would seem a several bags of portland mixed with the masonry sand would accomplish much the same thing. (by very weak, I mean weak enpigh that you could almost break it apaprt with your hands once set.) Thanks.

obviously, I will have quite a bit of gravel in it as well.

(Paul you posted a link in response to a chemical battery by mistake?)

The water in Portland cement is driven out by heat. This destroys the structural integrity.

David; concrete is destroyed over 400 degree F, your mass will not get that hot except close to your transition area. You really need clay/ cob in that area. after you are just a few feet away the temps will fall below the danger level , and you can switch to concrete if you want. Have you considered buying bags of fire clay ? Locally i can get a 50 # bag for 14.00 $ I have heard of it being as low as 10.00 a bag if you buy in quantity. Some building supply's will not know what fire clay is ... just try another supplier. I went to three before they not only knew what fire clay was, there was a pallet of it in the back room !

Mortar Clay (Fire Clay) can usually be sourced at the same location as Portland cement. Here in Tenn., Lowe's carries it for around $8 per 50lb bag. Mix it with sand and gravel, or sand and quarry fines / crusher-run, for your thermal mass / thermal bench fill.

Addendum --> http://www.lowes.com/pd_10367-13040-10367_0__?productId=3110273

If you have pipes which can resist heat, like stainless, at the transtion area. You can go with concrete with no prob.

What we call cleanlyness mix over here. 150kg of cement per cubic metre of concrete. That's enough. No need for 350kg, slab mix.

Thomas Rubino, Concrete start to fail at 400C°, not farenheit. Might spall a little at 200C° already.

Thank you Max, my mistake, I always seem to mix up F & C temps