Design of ultra-low water content geopolymers through particle packing optimization: Reaction kinetics and performance

Ultra-low water content geopolymer preparation is constrained by insufficient geopolymerization, restricted ionic transport, and poor specimen consolidation, yet the role of initial particle packing state in coordinating these processes remains unclear. This study investigates how Compressible Packing Model (CPM)-guided particle-size-distribution (PSD) optimization influences the apparent reaction evolution, microstructure, and mechanical performance of a basaltic volcanic ash (BVA) geopolymer under ultra-low water conditions. CPM optimization increased the packing fraction from 65.6% to 78.8% at 25 MPa, producing a dense pressure-formed particle framework. Isothermal calorimetry, two-dimensional correlation spectroscopy (2D-COS) and X-ray photoelectron spectroscopy (XPS) captured an earlier apparent reaction response after optimization; specifically, the Si-O-T-related spectral variation preceded the water-related variation. Correlation analysis between the calorimetric peak time and a transport-related geometric proxy supports a coupled physical-chemical interpretation in which packing optimization is associated with both a shorter model-estimated transport-related timescale and an earlier apparent reaction response. The optimized material achieved 36.1 MPa at a W/S ratio of 3% and retained more than 90% of its pre-exposure strength after short-term vacuum or hot-dry exposure. These findings establish particle packing state as a controllable component of ULWG design and provide a potential strategy for balancing physical densification, localized reaction, and mechanical performance.