It is a gravel head or pebble with larger particle size in sand, which is called gravel. There is almost no difference between sandy pebble and gravel. Sandstone is a general term for sand and stone. Gravel is made of larger pebbles, at least one side is rough, and the other side can be shiny, which has poor adhesion to cement slurry. Gravel is an angular stone made of broken rocks, which is generally rough and has good adhesion with cement slurry. Pebbles are pebbles with smooth surface and no edges and corners formed by river water all the year round. The difference between them in engineering is mainly the bond with cement.