In Flanders (north of Belgium), a shortness of gravel is at hand. Therefore, the “Research committee” was established to financially support projects in order to find alternatives for gravel in all its applications. With this subsidy, VITO (Flemish Institute for Technological Research) and BBRI (Belgian Building Research Institute) worked together on the research of the mechanisms of pop-outs in concrete and of the nature of the damaging secondary granulates, i.e. residues coming from different industries such as bottom ashes, slag, ... . 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. 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. After mineralogical, chemical and micro structural analysis, the selected aggregates were used to make concrete samples. Three tests were developed to which the samples were exposed in order to evoke pop-outs in the concrete. Of the four different tested aggregates, only one lead to pop-outs and this with only one of the developed tests. Examining the core of the created pop-outs with the electron microscope, the hydration of calcium oxides proved to be the cause. Other than pop-outs, the damaged samples showed also an intense fissuring from which the origin could be revealed by SEM-analysis. Several attempts to make thin sections of the concrete samples failed because of the hard slag components and up till now the cause of the fissuring remains a question mark.