Methods of porous and foamed structure formation in geopolymer-based materials: a comprehensive review.

Porous geopolymer-based materials are increasingly being considered as solutions for thermal insulation, lightweight construction, adsorption, catalysis and filtration. Existing reviews have mainly focused on individual fabrication methods, porous composites or specific areas of application, whilst direct comparisons of the main methods for creating porous structures, and how they influence pore architecture, remain limited. This review compares direct foaming, moulding using reactive emulsion templates, microwave-assisted foaming, the incorporation of lightweight fillers, additive manufacturing, replica processing and the use of sacrificial templates within a unified processing-structure-properties framework. Particular attention is paid to the interaction between pore formation, pore architecture, fresh-state rheology and geopolymerisation kinetics, which determines pore retention, morphology, connectivity and homogeneity. Analysis shows that direct foaming is the most practical and scalable method for producing low-density thermal insulation materials, whilst lightweight fillers provide an optimal balance between strength and density. Reactive emulsion templating, replica processing and the use of sacrificial templates are better suited to creating the interconnected pore networks required for adsorption, catalysis and filtration. Additive manufacturing offers the greatest control over architecture but remains limited by processing complexity, production speed and costs, whilst microwave-assisted foaming enables rapid processing but faces scaling challenges. The main contribution of this review lies in the development of a practice-oriented framework for selecting technological methods for forming porous structures depending on the desired pore architecture, as this is a determining parameter on which the material's performance characteristics and scope of application depend.