I think I can guess why wet sand can hold its shape and dry sand cannot. Polar water sticks to polar sand and acts like a weak glue. If the sand were hydrophobic and non-polar, I’d guess the sand wouldn’t hold its shape as well. The trouble is, if I make a sand castle from wet sand, the sand will dry and still hold its shape. But the glue is gone. How?
Nice question. Umm… maybe the water helps it pack better? The gluing action brings particles into a tighter configurion.
I have a suspicion that there might actually be something chemical going on. Sandstone, obviously, isn’t just packed sand. Might be a similar thing going on.
Also, have you tried comparing freshwater sand castles to seawater ones? The salt would definately be a good cement.
I like the salt idea. As the water would dry it would percipitate out filling in at the contact points (since thats where the water would evaporate last.)
In fact, even with distilled water, stuff will disolve from the sand itself. I see a silica sand castle going into the hot oven at work in the very near future.
I wonder if water would make all that much difference if the sand was nothing but one of the silicon oxides or feldspar without any clay at all? I’ll be going up by the sand dunes at Olancha in the near future. Maybe I can get some of the sand from the very top of a dune and see if water helps. Such sand has very little clay dust because most of that has been blown away. I believe it is virtually all sand and is sorted into fairly uniform grains by the winnowing action of the wind.
Google sandcastle science, here’s a couple of links from the first page that look decent:
http://www.abc.net.au/science/news/enviro/EnviroRepublish_1470831.htm
http://www.firstscience.com/site/articles/sandcastle.asp