Effects of superplasticizer, anti-sedimentation agent, and defoaming agent on the fluidity, pore structure, and strength of landfilled low-calcium fly ash-based geopolymers

• Admixtures were applied to improve the workability of high-strength geopolymers. • Superplasticizer and antifoaming agent increase fluidity diameter effectively. • Open porosity and macropore content were reduced. • The combination with the defoaming agent had the opposite effect. • Strength loss was linked to SP degradation, lower gel density, and weak interfaces. One challenge of alkali-activated fly ash materials is their high viscosity and slow strength development, which contributes to the landfilling of large fly ash quantities, including in Poland. This study examines the effects of a superplasticizer, an anti-sedimentation agent, and a defoaming agent, used individually at various dosages and in combination, to improve geopolymer workability while preserving mechanical performance. Their influence was evaluated using TG-DTA, FTIR, XRD, MIP, nitrogen adsorption, SEM-EDX, and compressive strength testing. The findings emphasize the need for careful selection and compatibility of admixtures and offer new insights for practical application. Admixture addition increased workability (fluidity diameter 153–158 mm vs. 150 mm for the reference) and reduced open porosity detectable by gas adsorption, except in the formulation containing all three admixtures (145 mm). These benefits, however, were accompanied by decreases in compressive strength. The reference geopolymer reached 50 MPa at 7 days and 77 MPa at 28 days. Superplasticizer dosages of 0.05 wt.% and 0.40 wt.% resulted in strengths of 56 MPa and 74 MPa, and 51 MPa and 66 MPa at 7 and 28 days, respectively. The anti-sedimentation agent (0.10 wt.%) and the combined admixture system (1 wt.% defoamer + 0.10 wt.% anti-sedimentation agent + 0.05 wt.% superplasticizer) further decreased strengths to 35 MPa and 47 MPa, and 27 MPa and 41 MPa at 7 and 28 days. Depending on the formulation, strength loss was attributed to superplasticizer degradation, reduced N-A-S-H gel polymerization, and weaker interfacial bonding due to mullite particles embedded in the gel.