Bench-scale and industrial expanded polystyrene formworks (EPSFWs) were prepared and tested in terms of shock resistance and compression analyses. The composition of the EPSFWs was varied and the influence of the substitution of calcium carbonate by granite, kaolin or slate on the mechanical properties of the EPSFWs was analyzed. Bench-scale samples were subjected to a previous selection based on the optimal mechanical behavior. Samples containing large grain sand and slate were chosen for the subsequent industrial scale preparation. The real EPSFWs were also tested from the shock resistance and compression standpoints. The results obtained suggest that the samples showing the best performance among all the tested EPSFWs are those in which calcium carbonate was substituted by slate. This fact is of special interest since large amounts of slate dust are obtained in the area of Spanish Extremadura from local mines. Thus, the preparation of formworks represents a good alternative for the valorization of this by-product.