What happens when cement gets wet?

Why is cement such a good bonding agent for rock and sand?

When a cement particle absorbs water it grows hair (like Velcro) on the surface. This has two effects, the first is that the volume of the cement particle grows, thus filling in the open space between the rock and sand particulates. the other effect is that the "hair" that gow upon the cements hydrated surface attched and bind (like Velcro) to the other materials in the mix.

Above: Image of hydrated cement