Can grass clippings be used as mulch ? Not composted clippings. Just rake up and put around base of trees. If so, how ? If not, why not ?

No. Grass is too compact, so will just mold and rot instead of mulching and slowly decomposing. Mulch has to be loose enough for air to get around it, and is best if it is dried too. Grass clipping are better left right where they are when mowed, to work back into the soil. If raked, they need to be mixed well with leaves and other looser material.

The only way I have been able to really compost grass is with a tumbler type composter. Drum gets half filled with grass, dirt and some bacteria and then gets turned every week. Not worth the trouble in my book..