Abstract In this study, fly ash and metakaolin are examined as replacements in cementitious materials to reduce environmental pollution. An orthogonal test was conducted with nine groups of geopolymer mortars prepared by varying the ratio of the cementitious materials and the amount of composite alkali activator (NaOH-Na2SiO3). The optimal ratio of the geopolymer mortar and its feasibility as a non-vibrated pipe trench backfill mortar were examined using fluidity and mechanical property tests. Combined with the microscopic tests, the pore structure and hydrate phase of the geopolymer mortar were analysed. The results showed that the geopolymer mortar with 30% fly ash (F30), 10% metakaolin (M10), and 4% alkali activator (A4) had the best performance and achieved compressive and flexural strengths of 17.2 MPa and 5.55 MPa, respectively, with a fluidity of 157 mm; these results meet the requirements for backfill materials. In addition, according to the strength comparison of the different groups, metakaolin had a better effect on the mechanical properties of the material than fly ash. Microscopic analysis revealed that the hydratable substances such as potassium feldspar and montmorillonite decomposed under the action of OH− and formed C-(A)-S-H and N-A-S-H gels, along with potassium A-type zeolite crystals; these substances improved the pore structure and density of the mortar. This study provides valuable insights into the potential use of alkali-activated geopolymer mortars as vibration-free backfill materials for pipeline trenches.