What reaction happens when you mix sand and water?

When we mix sand and water, no reaction takes place. The sand simply settles down at the bottom of the water container. This is because sand is heavier than water and hence cannot float in water. The sand can be separated from the water by simple filtration and can be obtained in its original condition after drying. Since no chemical reaction takes place, we can term this reaction as a physical reaction (as gravity and density are responsible for this reaction). The separation of sand from water is called gravity separation. 

In comparison, if we mix something like salt or sugar in water, they will react and make the water salty or sweet. Sand causes no such changes in the quality of water.

Hope this helps.