Difference between scale and sludge in boilers?

In boiler when water is heated, if the water is hard then would contain salts. So on heating the water the water particles are evaporated by the time and the saturated salts are left behind. Now its the part of the boiler, if the inner walls are made up of non stick material or such then the salts which is left as a precipitate will not stick on the same and hence make the water thicker and thicker as more and more water will evaporate which would result in a very thick liquid substance having a lot of salt rather than water, this is known as SLUDGE. On the other hand if the inner walls of the boiler is not made up of such non sticking material then the salts would stick on them resulting a formation of a thick layer of salts on the walls, this is known as SCALE.