Cornice cement is a setting type compound that sets and dries quickly through a chemical reaction.
Joint compound is a powder that has to be mixed with water. At best you can try to thin it out with some kind of liquid (water or something else).
Cornice cement is an actual mix of cement and other ingredients in powder form; in most cases it also contains sand. If you try to use it like a joint compound (by adding water) you might end up with a weak wall full of holes. Cornice cement can be made slightly thinner by adding a small amount of water, but if your goal is to fill in large gaps then you are better off using a joint compound.However, cornice cement is a setting type compound that sets and dries quickly through a chemical reaction and can easily be used for quick fixes, repairing existing cornice and cornice joints with a small broadknife or to install cornice between plasterboard walls and ceilings using a paper tape or a mesh tape.
If you have a tiny hole in your plasterboard wall, a concrete patch might do the trick to save some time and effort. Cement patches are sold as "dry-pack" so there is no need for mixing or preparing. There are two types of patches that can be used: those that come pre-stocked with spackling paste (or something similar) and those that don't. The ones packed with filling material usually work better because the builder has taken more time to indent the middle part of the pad, making it much easier to hold against a plasterboard wall while it sets up.
Self-levelling compounds are liquid and usually come in a small bucket. They are used to level uneven walls or ceilings, but they can also be used to cover up small imperfections. If you want to fill large gaps or deep holes then regular drywall joint compounds are more suitable for you (which is easier for the average homeowner to find).
Mortar mix should only really be used if you are building an actual cement wall. For other uses around the house, it's best not to use mortar mix because it doesn't harden as much as cement so your results won't last long. It can be made thinner by adding sand, however most people opt for ready-mixed joint compound instead.
Yes, you can use a drywall joint compound for a cornice. The main problem with this type of project is that the joints will show up much more than they would on plaster. If you were to join several sheets of drywall together then it might not be as noticeable since there will be large pieces of overlapping layers where the flaw won't be as obvious. If you are using concrete joint mix or something similar then this might not really matter because your overall end-result will look like a solid wall with a texture that matches the rest of your home. You can also do regular patching work if you need to but wetting down the surface is going to make it very difficult for the material to stick properly once it dries out.
When using joint compound as a base coat to fill in cracks and holes using a mesh or paper tape, if the walls are properly prepared then it will leave gaps around the edges: this is normal and your goal is to seal up those edges so they aren't visible anymore. When working with an actual cornice cement as the base coat you can't really add more material on top of what's already there without leaving visible lines. The only exception could be if your existing plasterboard wall is very uneven or there was something else that damaged your cornice (such as active termites).
Once everything is set up properly then the outside texture will blend in much better than any kind of joint compound would. If you happen to use a standard mud mix as your base coat then it might look OK as long as you're able to conceal the edges properly.
No, you shouldn't use a joint compound to patch up a cornice because this will lead to large areas of visible cracks and crumbles over time on the dried plaster. A good quality dried cement paste will do a much better job at binding with your existing plasterboard wall. If there are any gaps or irregularities then the best thing would be to scrape and cut everything back down with some grinding equipment and start from scratch. If you can't remove all of it then try using an aerosol spray adhesive that is compatible with concrete. Most cans come pre-mixed so all you have to do is spray it over the area and then apply your drywall cement patch.
No, cornice cement or cornice adhesive is not the same thing as plaster. Plastering is a much bigger project than just patching up an old cornice and it requires a lot more skills - in fact, if you can't find someone to work with you then this would be a good time to consider hiring a professional contractor or architect to prevent future damage and cracking instead.
Instead, cornice adhesive can be used to bond paper-faced cornices and plaster cornices to plasterboard wall and ceiling, providing a high bond strength and improved setting time. Proper sealing is essential to ensure your home has the best ventilation, according to YourHome.