Low-temperature thermal activation of diabase tailings-based geopolymers: orthogonal design and structural mechanisms

Diabase tailings–based geopolymers (DTG) were investigated through stepwise low-temperature activation at 100 °C, 200 °C, and 300 °C to assess their potential as sustainable, high toughness binders. However, the intrinsically low reactivity of diabase tailings, combined with the high energy demand required for conventional thermal activation, has posed a major barrier to their efficient large-scale utilization in geopolymer systems. A three-factor, four-level orthogonal design was implemented to systematically evaluate the effects of activation temperature, alkali modulus (Ms = 1.0, 1.2, 1.4, and 1.6), and Si/Al molar ratio (2.6, 2.8, 3.0, and 3.2) on the mechanical performance and structural evolution of DTG. XRD, FTIR, and SEM analyses revealed a progressive transformation sequence involving dehydration, hydroxyl exposure, gradual dehydroxylation, and partial amorphization, which collectively enhanced the availability of reactive Si and Al species. Statistical analysis (ANOVA) indicated that activation temperature, alkali modulus, and Si/Al ratio contributed 56.2%, 33.6%, and 10.2% to strength development, respectively. The optimal combination—300 °C, Ms = 1.2, and Si/Al = 2.6—produced 7-day compressive and flexural strengths of 42.9 MPa and 12.8 MPa, with a flexural-to-compressive ratio of 0.30, reflecting enhanced toughness and a well bonded gel framework. These findings demonstrate that controlled stepwise activation below 300 °C significantly improves the reactivity of diabase tailings while minimizing energy demand, providing a practical strategy for fabricating high toughness, low-carbon geopolymer binders for sustainable construction.