Gypsum-based materials for exterior applications using lime and recycled pozzolanic additions

Gypsum plaster could replace more energy-intensive building materials, especially considering the versatility of traditional gypsum used for exterior/structural applications since ancient times. However, low weathering resistance limits the exterior use of modern industrial gypsum and suitable additives are required to design optimized plasters and renders. Here, ternary plasters containing 50-95 wt% gypsum and varying amounts of lime and a commercial recycled pozzolanic additive (Metapor (R)) (R)) are compared with pure gypsum plaster to verify whether pozzolanic reaction products are responsible for the often praised superior performance of traditional gypsum. A detailed mineralogical, physico-mechanical, and hygric characterization shows that bassanite hydration is delayed and calcium carbonate polymorphs are stabilized in the presence of additives, while porosity and pore size distribution can be controlled by varying the additive content. Compressive and flexural strength reveal a negative correlation with additive content and a high additive content is required to improve weathering. However, a 25-40 % reduction in embodied energy can be achieved by replacing commonly used metakaolin or Portland cement with Metapor (R). (R). Our findings contribute to a better understanding of underlying reaction processes in gypsum-based materials and facilitate a wider use of modified gypsum formulations in sustainable construction and rehabilitation/conservation interventions.