Catechism of the Catholic Church §2435 — excerpt
Recourse to a strike is morally legitimate when it cannot be avoided, or at least when it is necessary to obtain a proportionate benefit. It becomes morally unacceptable when accompanied by violence, or when objectives are included that are not directly linked to working conditions or are contrary to the common good.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.