Abstract: Alkali-activated class F fly-ash-based polymer materials (FA-GEO) by NaOH solution has not ideally solidified at room temperature and atmospheric pressure. In this research, the properties of fly-ash-based geopolymer materials are improved by soda residue. The early mechanical properties, microstructure and thermal stability of the modified materials are researched through flexural strength test, compressive strength test, XRD, SEM-EDS, FTIR and TG-DSC test. Results show that the early mechanical properties and microstructure of FA-GEO are significantly improved by soda residue. When soda residue content is 27%, the 60 d compressive strength of mortar increases by 1.4 times. Soda residue reduces the crystallinity of zeolite crystals of FA-GEO. Soluble Ca²⁺ makes the modified gels tend to form a more cohesive calcium-containing zeolite-like phase geopolymer which is particle-shaped. Soda residue promotes the polymerization of Si-O-Al chain and the absorption of Ca²⁺ and Na⁺. The research result could be regarded as a new idea in optimizing FA-GEO.

Key words: soda residue, modification, fly-ash-based geopolymer, mechanical strength, microstructure