Study on mix ratio optimization, microstructure evolution, and economic benefits of iron tailings-fly ash-limestone powder ternary geopolymer

Iron ore tailings (IOT) can be synergistically combined with other solid wastes under alkaline conditions to form eco-friendly cementitious materials. This study synthesized a ternary geopolymer using sodium silicate (Na 2 SiO 3 ) and sodium hydroxide (NaOH) as a composite alkali activator, along with IOT, fly ash (FA), and limestone powder (LSP) as precursors. We systematically examined the effects of activator modulus ( M ), alkali-to-precursor ratio ( A / P ), water-to-binder ratio ( W / B ), and FA replacement ratio ( ω ) on mechanical performance through orthogonal design and single-factor testing. Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analysis were employed to characterize the microstructure, pore features, and alkali-activation mechanism. A cost comparison between the developed geopolymer and ordinary Portland cement (OPC) was also performed. Orthogonal test results revealed the following order of factor significance on strength: M > ω > A / P > W / B , with optimal levels of M = 1.2, A / P = 0.38, W / B = 0.26, and ω = 30 %. In single-factor tests, strength increased nonlinearly with extended curing time and reduced alkali modulus. A low-modulus activator ( M = 1.0) created a highly alkaline environment that accelerated the dissolution of reactive Si-Al components from FA/IOT and calcium from LSP. Interactions among these dissolved species promoted C-(A)-S-H gel formation, leading to a denser matrix. Compared with OPC, the geopolymer offered clear economic advantages in non-structural applications such as road subbase layers.