Cold sintering of geopolymer powders

Geopolymers (GP) represent a promising class of inorganic materials with diverse applications due to their properties, including high temperature resistance and strong interfacial bonding ability. They are produced through alkali activation of aluminosilicate sources, such as metakaolin or fly ashes. Despite their attractive characteristics, conventional casting methods for GP production often result in prolonged curing times and inferior mechanical properties to OPC or other benchmark materials. In this study, we investigated the feasibility of rapidly densifying GP matrices using cold sintering technology (CSP), a novel approach previously employed in ceramic systems. Through CSP, it was possible to obtain a dense body starting from GP sodium-based powder with optimal moisture content (10% wt.) under mild isostatic pressure (70 MPa) and moderate temperature (150°C) conditions, with a short duration process (10 min). The resulting products exhibited chemical stability (high resistance to boiling test), high density (> 90% theoretical density) and good mechanical properties (flexural strength equal to 30 MPa and compressive strength over 200 MPa) without requiring additional thermal treatments. SEM, EDS and NMR studies indicated that the predominant densification mechanism was likely to be homogeneous dissolutions and precipitation of the material, consistent with pressure solution creep. Dilatometric tests were performed to track the densification process in real-time and to determine the activation energy, which revealed an exceptionally low value for the system (21.7 kJ/mol). Our results demonstrate the potential of CSP as a rapid and efficient method for producing high-quality GP-based components, paving the way for their broader application in various fields.