When water is added to cement, the water reacts with the surface of the cement grains to make a jelly-like material which holds the grains together.
Three or four hours later the gel starts throwing out tendrils, radiating from each grain of cement. The tendrils, from neighbouring grains intertwine, trapping any sand or gravel mixed with the cement.
Over a period of days or even weeks the tendrils harden – locking the grains together in a way that makes cement almost impossible to crush. However, the tendrils are quite easy to pull apart, and because of this weakness concrete is usually reinforced with steel bars.
Picture Credit : Google
