Dialog game tools are text chat applications which aim to structure and promote students' collaborative learning by having them select a label and sentence-opener for each message they type to their learning partner. In this experiment, we compared students’ learning from discussions via a dialog game tool to their learning via a standard freechat application. Students discussed topic questions with a learning partner. They then individually completed a multiple choice test, for assessing knowledge-gain, and a short-answer test, to assess readiness for knowledge-building. Results suggest that dialog games applications lead to increased readiness for knowledge-building, in the form of integrating distinct pieces of learned knowledge, than freechat applications. Follow-up analyses suggest that the degree of concept overlap between students' dialog messages and topic keywords, as measured by a "semantic fingerprint" system, is a potentially useful metric for predicting students' knowledge building. Implications and potential applications of our findings are discussed.