Improvement of the properties of gypsum-based composites with recycled isostatic graphite powder from the milling production of molds for Electrical Discharge Machining (EDM) used as a new filler

• Graphite powder from the milling of isostatic graphite molds for EDM, EDM graphite, can be used as fillers in gypsum pastes.
• EDM graphite is inert in gypsum pastes.
• EDM graphite as filler in gypsum-based products, increases the mechanical strength and waterproofing.
• The gypsum composites with graphite filler present different possible uses in the gypsum precast industry, especially to improve the resistance of the core of dry-walls and panels.

The study here exposed was developed in order to contribute to the recycling and re-valorization of isostatic graphite powder as filler in gypsum pastes. Graphite powder is a waste product obtained from isostatic graphite blocks ground for mold production. The design of these molds is previously processed by Computer-Aided Manufacturing (CAM) and CAD/CAM software applications that uses a tool path for three-dimensional modeling to assist computer-numerically controlled (CNC).In order to contribute to the knowledge of this material as filler, this paper describes and assesses the microstructure of gypsums with graphite, through the use of a Scanning Electron Microscope and an Energy Dispersive X-ray Spectroscopy (SEM/EDX). Furthermore, the density, water absorption, and mechanical properties of gypsum pastes with addition of graphite filler have been analyzed.In order to deepen on the knowledge of the influence in the behavior of gypsum commonly used in building, several series of gypsum pastes with 0–5–10–15–20–25% of graphite filler by weight replacement of gypsum, and three different water/gypsum ratios – one of them with plasticizer – have been tested. It was observed that isostatic graphite powder from milled molds for Electrical Discharge Machining (EDM) significantly increases the mechanical properties of the gypsum pastes, more than other conventional fillers. This has been achieved by the great compatibility of the gypsum microstructure and the graphite micro-grains, which fill the microstructure of the pastes and increase the density of the hardened paste. The progressive increase of the amount of graphite influences the pastes properties, increasing their density and their mechanical resistances and reducing their porosity.