I bought a house with a gunite in ground pool with tile around the top edge. Water had gotten under the edge where it meets the pool rim tiles above and freezing had made it all expand out. About one quarter of the tile was gone or popping off. When I started pulling off the tile, chunks of the concrete under the rim of the pool starting coming out and I realized the top of the pool has sandy concrete that was not mixed right. I wanted to fill that in first and then build back level the material that chunked off with the tile so I could apply the thinset and tile. I need something super sticky with the consistency of putty to stick on a vertical wall. Someone said sticky mortar. Someone else said just put the tile thinset in there to fill it up. I want to do it right. Help!
How big are the voids that you are trying to fill? If they are small enough then thinset mortar should work. If larger than that you will probably need to create a form out of bender board and wood to pour with. In this case I would use the highest PSI concrete that you can get off the shelf and use a small aggregate. I am sure that you will get some very technical response to your question with some very detailed information. Many of which will tell me that I am wrong. I can only go by the experience that I had when fixing my own pool. So far it has turned out OK. One big difference is that I do not live where it freezes. Chances are that you will have further problems in the future since it is very difficult to marry two concrete surfaces together and get a real good bond.