> hungarian notation vs meaningful identifiers, what identifiers are more meaningfull than sznMaterial?

Unfortunately, this is the convention. "material" is as helpful as "szMaterial" since to determine the proper type, you look into the function call anyway (where it is either string material or int material).
Please consider not using abbreviations like "mat" if "material" would do as well.

> i did

Please don't see this as a general critique on your work, I just wanted to give a reason why Maikel made such an appeal.

> what else should i write there?

You might want to have a look at the PowerConsumer and PowerGenerator libraries. They are good examples of well documented libraries. You don't have to do this since there are still many libs badly documented but I would be helpful for all other developers.

> Shall i use int mat or string mat as parametre?

That's up to you. I myself would prefer string because it's more likely human readable and people will end up using LiquidInput(Material("Water"), ...) anyway.