Resource Recovery of Granite-Processing Sludge for Extrusion-Based 3D Printing of Ceramic Products

The valorisation of industrial by-products represents an important opportunity to improve resource efficiency and circularity in manufacturing. This study investigates the potential of granite-processing sludge as a secondary raw material for ceramic products manufactured through extrusion-based additive manufacturing, specifically Slurry Deposition Modelling (SDM). Granite sludge was incorporated into clay-based ceramic formulations at different concentrations, ranging from 10 to 50 wt%. Formulations were optimised through adjustments to water content and additives, followed by assessments of rheological behaviour, pumpability, printability, mechanical strength and water absorption. Experimental results demonstrated that a water content of 25 wt% provided suitable rheological properties for stable extrusion, with Gallenkamp values reaching 29–32°. Increasing granite sludge content improved the mechanical and physical properties of the sintered specimens. Bending strength increased from 6.8 MPa to 43.4 MPa, exceeding the 20 MPa construction performance target for formulations containing at least 30 wt% granite sludge. Water absorption decreased by approximately 44%, from 5.3% to 3.0%, indicating progressive densification during sintering. The formulation containing 50 wt% granite sludge exhibited the highest bending strength and lowest water absorption, making it the preferred composition among those evaluated. Additionally, the SDM approach reduced machining waste from approximately 45% to 6%. These findings highlight the feasibility of incorporating granite-processing waste into printable ceramic materials, contributing to industrial waste valorisation, resource efficiency and circular manufacturing. Future investigations will focus on reducing firing temperatures and developing firing-free formulations to further improve process sustainability.