Tiles made of slate have been used for centuries to cover roofs for their aesthetic and technical properties. The “Geo Slate” project was developed as part of the restoration work of the Dutch Schaesberg Castle. The roof of this castle was to be covered with a viable alternative to traditional slate: firstly, because almost all slate quarries in Europe have been closed in the past century, which made local sourcing impossible; and secondly, because the production process of slate is highly inefficient and not sustainable, with only three per cent of the material extracted from quarries being sold on the market as slate. The material for Geo Slate comes from the residual flows of slate extraction and other mineral raw materials from Dutch soil. This powder-like material can only be mixed and moulded with geopolymers as binders and is usable after a drying time of 24 hours. Alternatively, a powder-based 3D printer can be used to obtain individually shaped tiles, as in the case of the castle restoration project. After the printing process, the tiles only need to be placed in water and then dried in the air. After processing, Geo Slate possesses properties that are comparable to natural slate.