All concrete gets stronger over time! https://www.concrete.org/tools/frequentlyaskedquestions.aspx... edit: [adding more detail, removing all caps] Contracts will often specify a 7day or 28day strength because the concrete will keep getting stronger over time. Separate cylinders are poured for independent testing of the strength

Here’s a good textbook on concrete: Concrete: Microstructure, Properties, and Materials Textbook by Paulo J. M. Monteiro and Povindar Kumar Mehta