50 points. What makes sand hard?

The result of long-term biological, chemical and physical actions of nature.

Biological effects, such as earthworms and plants, change the structure, texture and nutrients of soil;

Chemical action, because soil or sand contains different chemical elements, such as calcium, magnesium, aluminum, silicon, oxygen, carbon, potassium, sodium, phosphorus and so on. Curing by chemical reaction;

Physical actions such as solar radiation and crustal movement can directly provide energy for them on the one hand, and also provide energy for chemical reactions on the other.