The voids themselves are vacuum, just empty space with no support. When I mention compression stregnth that's the ultimate strength of the material before it breaks. As the density of this stuff goes up the compressive strength goes up also.(all other things being equal) The modulus of this stuff is all the same, regardless of density. That means the strain in these ceramics is always tiny with a huge stress. The modulus is the deformation/applied stress. That's why this high void material seems like a dinner plate material. If you applied successive weights to the high and low density mat'ls, you'd find the low density stuff being crushed with a lower applied weight than the high density stuff. It has a lower bulk ultimate compressive strength.
I believe the low density does in fact reduce the strength.
Part of my carreer was also spent in the steel business, so I am familiar with hardness and moh scales and such. I suppose what I am feeling is a gut reaction based on work experiances and knowledge of materials. I am familiar with heat and impact studies and all of it tells me that there is some other factor that is damaging these tiles.
I just don't know what it is, which leads me to believe it may be a event that is not consistant, not measurable and not avoidable since we do not know the source. It may be a one time thing with no way of anticipating.
This is why I came up with the space debris theory and or micro meteorites.