Pop-outs are esthetical and structural unwanted holes that appear on the concrete by flied off surface material. The ejection of concrete pieces is caused by the pressure that is built up around an aggregate as a result of physical or chemical reactions. Because of approaching shortness of gravel in Flanders (north of Belgium), alternatives are needed. One of the possibilities is the use of secondary granulates, i.e. concrete residues coming from different industries, in concrete. Experience learned that some of those granulates can cause pop-outs. Earlier research showed that the damage often is due to chemical reactions between the aggregates and the cement paste. In many cases pop-outs are associated with metallic particles or with the hydration of Ca- en Mg-oxides. However, a systematic study has never been performed. This kind of research is required in order to be able to estimate the risks when using stony residues in concrete. Once the causes identified, directives can be stated to minimize the risks of pop-outs. VITO (Flemish Institute for Technological Research) and Buildwise are working together on the research of the nature of the damaging materials and the mechanisms of pop-outs. This project is subsidized by the 'Research committee', established to financially support projects in order to find alternatives for gravel. By petrography and electron microscopy, three kinds of secondary granulates are being analyzed. In the next step, the aggregates will be used in concrete and these samples will undergo some durability tests to generate pop-outs. The created holes will be examined by (electron) microscopy to find out more about the triggers.