The fill material could be a variety of material, The key is what the CMU is manufactured with. If the 6" CMU is manufactured using sand and/or limestone aggregate you cannot reach a 4 hr equivalent rating. What you fill the cores with takes on the rating as the CMU. Full lightweight CMU with cores filled will meet a 4 hr requirement. A proper blend of sand and lightweight with filled cores may meet 4 hrs. The blend will have to be verified by an independent lab. Fill material can be practically anything that is fireproof itself. Perlite, sand, expanded clay aggregate and masonry cell fill (course or fine grout) are possibilities.  

The Masonry Society has an excellent book on Fire Resistance of Masonry. Chapter 7 Section 722 covers calculated fire resistance in the IBC. I am attaching a link to the NCMA TEK that covers the same subject. 

FIRE RESISTANCE RATINGS OF CONCRETE MASONRY ASSEMBLIES - NCMA 

Thank you for contacting the MAF.  Our goal is to make sure that when masonry is designed and installed correctly the public has the best wall system available.
Be safe,
Jerry M Painter, FASTM
352-494-8955

​The 12" truss HJR will NOT meet the requirements of TMS 402/602-16 for cavity wall construction. Your wall system will have a total width of 13.5". The actual hjr width is 10". The wire/ rod in the veneer will only have 1 1/4" coverage which is not enough. The code also requires a rod in each face of the CMU. there are several options for anchoring the veneer, If you need seismic protection then you may need the rod in the veneer. I suspect not, The hjr  with a third rod is seriously labor intensive. The bricklayers have to lay brick under the outside rod without raising it. If they do bend it up slightly they will probably not get it to lay flat on the veneer and must take undue care to get the brick above to lay to the line as well as keeping the veneer below from being displaced. The best cavity wall reinforcing and anchoring system is one that utilizes what we call a hook and eye system. It has 8" HJR for the CMU with a double eye welded to it and separate hooks that  will connect to it and lay into the veneer horizontal joint. This is the best system for production and allows for vertical and horizontal movement of the veneer. Ladder type horizontal joint reinforcing is recommended in reinforced concrete masonry.
Be safe.
Jerry Painter, FASTM
[email protected]