concrete doesn’t dry out. Quite the opposite, water is an ingredient of concrete. When concrete sets, it is reacting with the water, initiating a chain of chemical reactions to form a complex microstructure deep within the material, so that this material, despite having a lot of water locked up inside it, is not just dry but waterproof. The setting of concrete is, at its heart, an ingenious piece of chemistry, which has powdered rock as its active ingredient. Not every type of rock will work. If you want to make your own concrete you need some calcium carbonate, which is the main constituent
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As this reaction is taking place, the gel that forms inside cement is constantly changing as the solid internal skeleton grows and further chemical reactions take place. That skeleton is comprised of "fibrils that grow and meet, they mesh together, forming bonds and locking in more and more of the water until the whole mass transforms from a gel to a solid rock. These fibrils will bond not only to each other but also to other rocks and stones, and this is how cement turns into concrete."
