Objection 1. It would seem that all will not rise again from ashes. For Christ's resurrection is the exemplar of ours. Yet His resurrection was not from ashes, for His flesh saw not corruption according to Psalm 15:10; Acts 2:27-31. Therefore neither will all rise again from ashes.
Objection 2. Further, the human body is not always burned. Yet a thing cannot be reduced to ashes unless it be burned. Therefore not all will rise again from ashes.
Objection 3. Further, the body of a dead man is not reduced to ashes immediately after death. But some will rise again at once after death, according to the text (Sent. iv, D, 43), namely those who will be found living. Therefore all will not rise again from ashes.
Objection 4. Further, the term "wherefrom" corresponds to the term "whereto." Now the term "whereto" of the resurrection is not the same in the good as in the wicked: "We shall all indeed rise again, but we shall not all be changed" (1 Corinthians 15:51). Therefore the term "wherefrom" is not the same. And thus, if the wicked rise again from ashes, the good will not rise again from ashes.
On the contrary, Haymo says (on Romans 5:10, "For if when we were enemies"): "All who are born in original sin lie under the sentence: Earth thou art and into earth shalt thou go." Now all who shall rise again at the general resurrection were born in original sin, either at their birth within the womb or at least at their birth from the womb. Therefore all will rise again from ashes.
Further, there are many things in the human body that do not truly belong to human nature. But all these will be removed. Therefore all bodies must needs be reduced to ashes.
I answer that, The same reasons by which we have shown (1) that all rise again from death prove also that at the general resurrection all will rise again from ashes, unless the contrary, such as the hastening of their resurrection, be vouchsafed to certain persons by a special privilege of grace. For just as holy writ foretells the resurrection, so does it foretell the reformation of bodies (Philippians 3:21). And thus it follows that even as all die that the bodies of all may be able truly to rise again, so will the bodies of all perish that they may be able to be reformed. For just as death was inflicted by Divine justice as a punishment on man, so was the decay of the body, as appears from Genesis 3:19, "Earth thou art and into earth shalt thou go [Vulgate: 'Dust thou art and into dust thou shalt return']."
Moreover the order of nature requires the dissolution not only of the union of soul and body, but also of the mingling of the elements: even as vinegar cannot be brought back to the quality of wine unless it first be dissolved into the prejacent matter: for the mingling of the elements is both caused and preserved by the movement of the heaven, and when this ceases all mixed bodies will be dissolved into pure elements.
Reply to Objection 1. Christ's resurrection is the exemplar of ours as to the term "whereto," but not as to the term "wherefrom."
Reply to Objection 2. By ashes we mean all the remains that are left after the dissolution of the body--for two reasons. First, because it was the common custom in olden times to burn the bodies of the dead, and to keep the ashes, whence it became customary to speak of the remains of a human body as ashes. Secondly, on account of the cause of dissolution, which is the flame of the fomes [Cf. I-II, 82, 3] whereby the human body is radically infected. Hence, in order to be cleansed of this infection the human body must needs be dissolved into its primary components: and when a thing is destroyed by fire it is said to be reduced to ashes. wherefore the name of ashes is given to those things into which the human body is dissolved.
Reply to Objection 3. The fire that will cleanse the face of the earth will be able to reduce suddenly to ashes the bodies of those that will be found living, even as it will dissolve other mixed bodies into their prejacent matter.
Reply to Objection 4. Movement does not take its species from its term "wherefrom" but from its term "whereto." Hence the resurrection of the saints which will be glorious must needs differ from the resurrection of the wicked which will not be glorious, in respect of the term "whereto," and not in respect of the term "wherefrom." And it often happens that the term "whereto" is not the same, whereas the term "wherefrom" is the same--for instance, a thing may be moved from blackness to whiteness and to pallor.
Source: Summa Theologica (New Advent)