I am sitting here thinking of this idea of bricks... What other materials could conceivably be used to make infill bricks

What I mean here by an infill brick is a brick or block of some sort that has a relatively high insulating value that is intended to be used in the space between framing members, and does not necessarily have any structural strength of its own -not something that has to be able to support a roof structure in other words.

Perhaps a bit of historical reflection would be in order here...
In the Middle Ages, Europeans devised a number of methods of infilling their timber structures. Just about everybody came up with wattle and daub.


Bricks were also used later on, particularly those bricks that were of inferior quality and so w=could just be tucked away somewhere inside a wall and covered up.


Since the days of the first Viking settlements, Icelanders built timber framed houses that were infilled and enclosed with huge turf walls several feet thick


Medieval Scandinavians built Stavverk buildings, such as the great Stave Churches that one could say employed a wood infill, as the boarding on these buildings are placed between the framing timbers, not on the outside.


Central Europeans used at times a similar system


This occupies a strange gray area somewhere between infilled timber frames, boarded frames, and log building.

Other techniques used in the past include Stone infill, mud bricks, and so forth.

In the 1960's and 70's, Germans were faced with the prospect of repairing a large number of half timber buildings that had been damaged during the war, and patched with temporary and inferior methods afterward. This sparked a revival in traditional methods, and also great innovation. It is from this that the light clay infill methods developed.

Now I am thinking of straw and wood chip blocks in terms of blocks of rigid insulating material stacked between timbers. Thinking on those terms, I wonder what other materials might be usable in a similar sense, particular natural materials or waste products.

So I have thought of a few materials that might possibly be made into bricks:
Cellulose
Charcoal
bast fibers
Cambium, or inner bark, of certain trees
sawdust, as opposed to woodchips
tire shreds

All of these would be used in conjunction with some type of binding agent that would also inhibit burning, and most would also require something that would protect against insects. Things like clay, lime, or cement (remember, there are types of cement other than portland)

Of these, I know a bit about the thermal properties of charcoal.
Charcoal is produced by forcing volatile chemicals out of wood, leaving behind almost pure carbon. In the process of forcing out the chemicals and leaving behind carbon, a lot of 'extra' space is left. The piece of coal is about 60 to 75% the volume of the piece of wood it once was, but only 20-30% the weight. That means it is now mostly air. You can tell this when you pick up a chunk of charcoal because it is very light relative to its size.
To make charcoal you have to heat wood up to at least 500 degrees fahrenheit, and the finished coal can retain this temperature for hours or even days after it is removed from the kiln. In addition to this, you can be holding a piece of coal in you bare hand while it is flaming red hot just a few inches away and not be the least bit uncomfortable.
It does tend to put off a very fine dust, and it is also really flammable. But I think that enclosed in clay both of these problems could be dealt with (straw for example can have both of the same problems. If you don't believe me, try working in the loft of a barn where straw has been stored. It's very dusty) I have seen already where a trailer load of straw from the field spontaneously erupted into flames because of the heat produced by the stacked bails as they dried. I have also seen barns burn down for the same reason (and I have seen grain bins explode when being loaded with soy beans when the bean dust ignited from friction or static electricity)