When cement, sand, stone and water are combined, a chemical process called “hydration” occurs in the cement particles. The cement particles develop finger-like extensions which become interlocked with other nearby cement particles, much like Velcro balls sticking together. These hydrating particles also bond to the sand and stone, increasing strength. The amount of water and cement in the mix determines how close these cement particles are, which is a good indicator of the strength (water/cement ratio). As these components bond together, moisture is pushed out, leaving hard concrete in its place.