If your talking about the large circular concrete blades, with diamond tipped teeth, they won't make a decent knife blade. A few years ago I had a local construction outfit who wanted to bring me several, and have me build knives for them out of the used blades......I had it tested and the results came back as 4140 and/or 4340, depending on the manufacturing company.
4140 and 4340 are great for impact type applications like hammers and such, but the .40% carbon content limits the hardenability to the point that a thin cross section (such as a knife blade) is too soft.