Will the solid formed after dehydration of a disodium hydrogen phosphate + sodium hydroxide solution be hygroscopic at STP?

Given a solution consisting of disodium hydrogen phosphate + sodium hydroxide (approaching a pH of 12), will the solid formed after dehydration of the solution be hygroscopic at STP? Which salts are likely to precipitate? In the event that the resulting solid is hygroscopic, how might the addition of KCl or NaCl salts to the solution, before dehydration, affect the extent to which the dehydrated solid absorbs moisture if exposed to a humid atmosphere?




The solid is made of trisodium phosphate $\ce{Na3PO4}$. Trisodium phosphate has a pH equal to $11.9$ in a $1$% aqueous solution, according to the Merck Index, in article $8736$. If evaporation is made near room temperature, the solid is a dodecahydrate $\ce{Na3PO4·12H2O}$. It is a little hygroscopic. Caking appears after some weeks. It looses $4$ molecules water by heating.


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