Acabar de is a periphrasis, in Spanish perífrasis verbal. A periphrasis is formed by combining two verbs. When this occurs, the first verb is conjugated, while the second verb remains in the infinitive form, many useful expressions are formed like this, that you have probably already seen:
Voy a comprar una casa (I am going to buy a house).
Tengo que hacer la maleta (I have to pack the suitcase).