Abstract

Objective This study aims to investigate the effects of various types，concentrations， and proportions of composite activators on the strength properties of alkali-activated steel slag-based cementitious materials （ASCM）. By analyzing the synergistic effects of composite activators，the study examines their influence on the microstructure， chemical composition， and hydration products of ASCM. It aims to reveal the underlying mechanisms of strength enhancement in ASCM， thereby providing a theoretical foundation and technical support for the efficient preparation of these materials in engineering applications.

Methods A systematic experimental approach was employed using Box-Behnken response surface methodoligy （RSM）. Seventeen experimental groups were designed with silica fume， quicklime， and Na₂SO₄ contents as independent variables. The compressive strength of ASCM specimens at 3， 7， and 28 days was selected as the response variable. A quadratic polynomial regression model was developed to identify the relationship between activator variables and strength performance.To optimize theactivator formulation， model parameters were analyzed using the Numencial optimization function. This enabled the identification ofthe optimal activator proportions. Advanced characterization techniques， including scanning electron microscopy （SEM）， X-ray diffraction （XRD）， and Fourier-transform infrared spectroscopy （FTIR）， were used to analyze hydration products and microstructural morphology， revealing the mechanisms driving strength enhancement.

Results and Discussion The results demonstrated that the interactions between quicklime and silica fume， as well as between Na₂SO₄ and silica fume， significantly influenced ASCM strength.The optimal composite activator composition was identified as 1.1% silica fume， 4% quicklime， and 1.3% Na₂SO₄. Under these conditions， the compressive strength of ASCM reached 27.51 MPa， 36.76 MPa， and 45.24 MPa at 3， 7， and 28 days， respectively. Notably， the relative error between experimental results and model predictions was less than 5%， indicating high accuracy and reliability of the regression model.The interactions among activator components promoted the formation of calcium silicate hydrate （C-S-H） gel sand ettringite （AFt）， which are crucial for improving the strength of cementitious materials. The dense reticulate structure formed by these hydration products improved microstructural integrity of ASCM. Furthermore， silica fume particles， which did not directly participate in the activation reaction， served as a filler， further enhancing packing density of the matrix. This combination of chemical and physical enhancements resulted in a significant improvement in the macroscopic mechanical strength of ASCM.The analysis of hydration products confirmed the formation of a robust microstructure，which contributed to the enhanced strength properties of ASCM. The composite activators worked synergistically to optimize the development of hydration products and address potential microstructural weaknesses. This synergy ensured that ASCM exhibited excellent strength and durability， making it a viable material for a wide range of engineering applications.

Conclusion This study highlights the role of composite activators and silica fume in enhancing ASCM mechanical properties and durability by improving microstructure and packing density， offering valuable insights for optimizing formulations tailored to engineering needs. By utilizing industrial byproducts like steel slag， ASCM serves as a sustainable alternative to Portland cement， reducing environmental impact and promoting waste management. These findings lay a foundation for developing next-generation construction materials， with future research focusing on ASCM's long-term durability and large-scale application performance.

Keywords： response surface methodology； steel slag； alkaliactivation；cementitious material； microscopic structure