Of Sand Sculpture and Hairspray
Here resides a more technical explanation of why our sand sculptures hold together... for those who really need to 'geek out'.
First, I would like to finally end a few seemingly un-killable myths. Wherever we go we invariably hear someone in the crowd say that the "Big Secret" is: sugar water, hairspray, unicorn tears or Mountain Dew. Other than attracting every ant within five miles and throwing piles of cash out the window these magic additives would accomplish little, if anything.
Even within the realm of top-flight sand sculptors I've heard claims that an attractive molecular charge between water and sand grains, surface tension, etc. are the one true binding force behind what we do. Baloney.
Back to Why Sand Matters
Friction is also why certain sands are better than others. Finer sands will have naturally smaller pore spaces and angular grains are most likely to tightly interlock and cross-link. Rounded grains will always have larger pore spaces between grains no matter how well compacted, and a naturally smooth surface further reduces friction. Beyond being merely rounded as the individual grains become more spherically shaped the grains also become incapable of cross-linking. Try to imagine stacking a pile of bowling balls. The advantages of a sand that compacts well are easy to see and is why starting with a good sculpture sand always makes for a more enjoyable day.
In nature sandstone is formed in a similar fashion but under extreme pressures. The pore spaces become very small and can be reduced further when small amounts of 'matrix' (silt or clay) are present. Eventually minerals suspended in water crystalize cementing compacted sand into stone.