if i filled a balloon with air, covered it with cement, waited a few hours, deflated the balloon, pulled the balloon out, could i create a hollow cement sphere?????
Well, yes ... as long as you accept your sphere won't be very spherical. Cement is heavy, and will deform the balloon. A much tougher balloon would resist better, but would be tougher to pull out afterward. If the interior doesn't matter to you as long as it's hollow, try to make a light but strong spherical form. There must be ways to make paper mache spheres; perhaps they can even be bought. And there are many sources for hollow plastic spheres. Naturally, the form remains inside the concrete sphere. You might also consider building forms for making concrete hemispheres, which can just be cemented together later. That will allow you to select your thickness (and even adjust it easily), create a high-quality hemisphere, and fill or adjust the interior as desired. Since concrete is poor in tension, any opposing pressure on the sphere will create an equator where cracks threaten to appear. Hence, when forming the sphere, some sort of embedded layer of mesh (plastic, metal) is a good idea.
My guess would be no. Cement hardens by chemical reaction to water and the curing cycle generates heat. I would wager that a latex balloon would not be able to withstand the heat.
Coat the balloon with hand cream to prevent cement from adhering to it. Might want to fill the balloon with water for better support. Or coat the balloon with a few thin layers of cement instead of one thick layer.
There are two things you mention that will lead you to an answer of No. First, in order to pull out the balloon, you will need a hole and strictly speaking, you will not have a true sphere because of the hole. Overlooking that little problem is a more significant one. A few hours is not long enough. Cement takes a very long time to achieve its maximum strength. When cement is used in construction, it is left to harden for at least 7 days before any weight is placed on it. Ideally, you should wait about 30 days before expecting anything made of cement to sustain a significant load. There is another problem that you did not mention. That is that cement can be very strong in compression but will never be as strong in tension. That is why any cement structure that has to support anything is usually reinforced with steel. Will your cement balloon float? It could, if you size it properly and fill it with helium or hydrogen.
Yes, in fact it has been done, though it's a little more complicated than you describe. The problem is that wet cement is pretty soupy, and it's hard to get it to form a layer around the baloon. So you also need an outer form around the layer of cement. That could be made of cardboard and duct tape on a small scale, and cut away after the cement has hardened. You can pop the balloon and slip it out through the hole. The interior void will not be exactly spherical, of course.
