For collaborative learning environments, the support of interaction is mainly focused on communication since directly experiencing a situation and learning by observing are mostly inapplicable. We describe communication based on a context-oriented communication model that focuses on the dialogical communication and mediation of context. The model leads to an abstract identification of general communication tasks and the use of learning material in collaborative learning processes. The difference between uploading material and communicative contributions has to be made distinct and different ways of combining both need to be supported. The prototype system "KOLUMBUS" which has been used for an experimental investigation in a university course and in a working group is used to demonstrate such features. The evaluation brought evidence that the concept of annotations to support context-oriented communication in collaborative learning processes is well received.