Influences of Different Solid Waste Powders on the Drying Shrinkage Characteristics of Metakaolin-Based Geopolymers

This study investigates the effects of three solid waste powders—fly ash (FA), silica fume (SF), and phosphogypsum (P)—on the drying shrinkage behavior of metakaolin-based geopolymers. To systematically evaluate the performance and underlying mechanisms, a comprehensive experimental program was conducted, including compressive strength and elastic modulus testing, early-age and variable-humidity drying shrinkage monitoring, mercury intrusion porosimetry, and microcalorimetry analysis. Results demonstrate that all three materials effectively reduce drying shrinkage through distinct mechanisms. The incorporation of 30% FA optimized the capillary pore network and densified the matrix, achieving a peak compressive strength of 53.51 MPa and an elastic modulus of 9.23 GPa. SF exhibited a dose-dependent effect; at an optimal content of 7%, it enhanced compressive strength by 28.3% through its nucleation effect and micro-aggregate filling. However, excessive SF (9%) led to pore coarsening and increased shrinkage. Although P incorporation slightly reduced mechanical strength, it decreased cumulative porosity by up to 8% and formed needle-like Wairakite-Ca crystals that provided micro-structural support, resulting in a net shrinkage reduction of up to 137.83 µε. This study provides a scientific basis for designing low-shrinkage, low-carbon geopolymers by tailoring solid waste incorporation to engineer multiscale pore structures.