You need a binder. Many plants and trees can provide gum to accomplish this. It is best to use a gum that will harden over time and not remain elastic. One way to do this is by the removal of moisture from the gum. This could be accomplished through evaporation in sunlight. Or it may involve another vegetable ingredient, an emulsifier, to the recipe to cause an elastic gum to harden sufficiently. A protein would probably work. No water needed in this mixture.
Another binder is lime. Lime deposits are very common especially inland from sandy shores and on alluvial plains. This involves water. Fresh water is much better to use than salt water, because the salt crystals interfere with the integrity of the mixture during the hardening process. So, the sea water would have to be distilled in order to separate the salt from the water. That is a simpe process. .
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