lord of the rings - Did Tolkien ever specify that Orodruin was a volcano?

No, Tolkien never uses the word "volcano" to describe Orodruin.

The closest he comes is "Mountain of Fire", an epithet (note the capitalization) that gets frequently applied to it. In Fellowship, for example (emphasis mine):

Then all listened while Elrond in his clear voice spoke of Sauron and the Rings of Power, and their forging in the Second Age of the world long ago. A part of his tale was known to some there, but the full tale to none, and many eyes were turned to Elrond in fear and wonder as he told of the Elven-smiths of Eregion and their friendship with Moria, and their eagerness for knowledge, by which Sauron ensnared them. For in that time he was not yet evil to behold, and they received his aid and grew mighty in craft, whereas he learned all their secrets, and betrayed them, and forged secretly in the Mountain of Fire the One Ring to be their master. But Celebrimbor was aware of him, and hid the Three which he had made; and there was war, and the land was laid waste, and the gate of Moria was shut.

^{_{Fellowship of the Ring Book II Chapter 2: "The Council of Elrond"}}

In The Silmarillion is is also referred to as a "fiery mountain":

There above the valley of Gorgoroth was built his fortress vast and strong, Barad-dûr, the Dark Tower; and there was a fiery mountain in that land that the Elves named Orodruin.

^{_{The Silmarillion V Of the Rings of Power and the Third Age}}

And in a draft of what would become chapter 2 of Fellowship, Orodruin is referred to as a "fire-mountain":

I can think of only one way: one would have to find the Cracks of Doom in the depths of Orodruin, the Fire-Mountain, and case the Ring in there, if he really wished to destroy it, or put it beyond all reach until the End.

^{_{History of Middle-earth VII The Treason of Isengard Chapter II "The Fourth Phase (1): From Hobbiton to Bree"}}

"Mountain of Fire" is actually a fair translation of "Orodruin", in fact; The Silmarillion "Index of Names" translates it as "Mountain of Blazing Fire", and from the Appendix we can see that it's comprised of the words orod, "mountain", and ruin, "red flame".

The only time I can find where Orodruin is referred to as a "volcano" is in one of Christopher Tolkien's notes:

No doubt because Gil-galad had by then discovered that Sauron was busy in Eregion, but had secretly begun the making of a stronghold in Mordor (Maybe already an Elvish name for that region, because of its volcano Orodruin and its eruptions - which were not made by Sauron but were a relic of the devastating works of Melkor in the long First Age)

^{_{History of Middle-earth XII The Peoples of Middle-earth Part 2 Late Writings Chapter 13 "Last Writings" Note 14}}

However, there is one interesting almost-connection. One of the few uses of the word "volcano" by Tolkien occurs in the Notion Club Papers, an abandoned novel which revolves around contemporary Oxford scholars discussing dreams of Númenor:

'Still that seems to be where you got your Volcano and Tree from. But you've given them a twist that's not in your source. You've put them in a different order, I think, making the Tree further west; and your Volcano is not a hell-smithy, but apparently a last peak of some Atlantis.

^{_{History of Middle-earth IX Sauron Defeated Part Two: "The Notion Club Papers" The Notion Club Papers Part Two}}

In-context, this description is quite clearly of the Meneltarma, which was believed to remain above water after the drowning of Númenor¹.

What I found interesting was a note by Christopher Tolkien immediately following the paragraph, where he notes:

The passage Lowdham refers to is 33-52 [of the poem they are discussing], where when 'the smoking cloud asunder broke' they 'saw that Tower of Doom': in the earliest text of the poem the mariners 'looked upon Mount Doom'.

^{_{History of Middle-earth IX Sauron Defeated Part Two: "The Notion Club Papers" The Notion Club Papers Part Two. Note 81}}

While there's no context where it makes sense for this mountain to be Orodruin, I believe this writing predates Tolkien's work on Lord of the Rings, so we can see where he was getting the idea.

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¹ From Akallabêth:

Among the Exiles many believed that the summit of the Meneltarma, the Pillar of Heaven, was not drowned for ever, but rose again above the waves, a lonely island lost in the great waters

^{_{The Silmarillion IV Akallabêth}}