You topic is akin to powder metallurgy, even though your source material isn't a metal. Yes, you can sinter NaCl together, I have done it. And if you melt it completely, and let it cool slowly, you'll get a single crystal. If you cool quickly, you'll get a mass of smaller crystals stuck together. NaCl crystals aren't particularly strong, and sintered together, or as a mass of smaller crystals, even less so. Some sintered materials can be pretty tough -- for example, blocks of PTFE are made by powder metalurgy, then machined into small Teflon parts, such as burette stopcocks. But I'm not expecting dimensional strength from sintered powders.