In the sharpening video I got, Mr. Carter showed that it is possible to sharpen a knife on a chunk of concrete and a piece of cardboard, to show that in a pinch, those two will work fine to sharpen a knife. Concrete was used for the first five steps, and the last two he used cardboard. Why would cardboard sharpen a knife?
Have you ever noticed how a knife seems to dull quickly when cutting cardboard? This is because the cardboard manufacturer’s use clay to fill in the spaces between the fibers. Whatever can dull a knife can also sharpen it. Pulling the blade backwards along the cardboard will sharpen it very much (though not as much as a special stone), enough to shave hair!