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Concrete is a mixture of cement, water, and aggregates that gradually hardens and gains strength over time through a process called hydration. The hydration process involves the chemical reaction between cement and water, which creates a solid matrix within the concrete. The rate of this reaction is influenced by various factors such as temperature, moisture, and the type of cement used. Generally, it takes around 28 days for concrete to reach its maximum strength and hardness. Therefore, the correct answer is 28 days.