In engineering terms, concrete is σκυρόδεμα and cement is τσιμέντο. These are the right words to use. An engineer would never say "κτίριο από τσιμέντο" It would sound very weird. But in everyday language indeed τσιμέντο is also used for concrete, strangely... "κτίριο από μπετόν" would be better and more accurate than "τσιμεντένιο κτίριο" in any case
Similarly in English, people say "cement" when the correct term is "concrete."
True, but nobody (in Britain anyway) would talk about a cement building.
The word Cement refers to any kind of binder that holds other materials together, while Concrete (also known as μπετόν, from the french word) is a mixture of materials combined with any type of cement and water. Technically, Cement is a part of concrete. However, in Greek, we say nothing like "το κτίριο από σκυρόδεμα". We just say "Το τσιμεντένιο κτίριο". I don't think we even have an adjective from σκυρόδεμα to describe it, either. So yeah. You can say that cement and concrete are kind of used as synonyms, even if they are not exact ones. :P