When cement is mixed with water, it begins to undergo a chemical reaction called hydration. This reaction causes the cement to harden and set, eventually becoming rock-like in consistency. Once this process has begun, it cannot be reversed.
However, there are ways to soften hardened cement. One method is to soak the hardened cement in water for several days. This will rehydrate the cement and make it softer and more pliable. Another method is to grind the hardened cement into a powder using a tool like a hammer drill or angle grinder fitted with a diamond blade.