Nano-Silica as Designer Tools for Geopolymer Microstructure Optimization: Effects on Porosity, Interfacial Transition Zone (ITZ), and Mechanical Performance

Nano-silica (nano-SiO2) has emerged as a powerful designer tool for engineering the microstructure of geopolymer composites, enabling precise control over porosity, interfacial transition zone (ITZ) characteristics, and resultant mechanical performance. The main aim of this review is to evaluate the role of nano-silica as a reinforcement and pozzolanic accelerator. The paper delivers a critical literature overview. It is based on a comprehensive critical review of the existing literature and illustrative case studies demonstrating practical applications in geopolymer composites. The article presents the key mechanisms connected with the application of nano-additives, including accelerated geopolymerization kinetics and heterogeneous nucleation on nano-silica surfaces. Comprehensive characterization methods are critically assessed, including SEM/EDS for gel morphology, MIP for porosity profiles, XRD/FTIR for reaction products, micro-CT for 3D void networks, and nanoindentation for ITZ mechanical gradients. The article also shows the main applications span high-performance concretes, 3D-printed geopolymer elements (improved buildability and interlayer adhesion), and durable overlays. The article is a closed presentation of challenges such as long-term stability, alongside future directions. The main findings show that nano-silica offers a pathway to tailored, low-carbon geopolymers with superior microstructure–performance relationships aligned with sustainable construction goals.