There are a few things a little weird about your image:
- First, for a CT image, I'd expect the background to have a lower intensity than everything else since this is void space (unless is was scanned in a liquid of some kind). What do you know about the setup of the system and do you have control over it?
- Second, it appears the histogram has already been modified in some way. I would expect this to be fairly bimodal for an object in the forground.
As far as identifying the cement matrix, here's a rough first pass to get you started:
mask of the concrete part
disk = getnhood(strel('disk',4));
Istd = stdfilt(I,disk);
Mcyl = Istd>2;
C = (conv2(double(I),double(disk),'same'));
levels = multithresh(C,2);
L = imquantize(C,levels);
L = L+1;
L(~Mcyl) = 0;
cmap = [1 1 1;lines(3)];
Lrgb = label2rgb(L,cmap);
hCb = colorbar;