Microanalysis and Optimization of Sulfate Corrosion Resistance of Flowable Concrete Using Response Surface Methodology.

In this study, the Box-Behnken response surface method was used to investigate the effects of coarse aggregate gradation (percentage of 5-10 mm stone), water-reducing agent mixture, and aggregate-to-cementitious material ratio on the workability, compressive strength, and sulfate corrosion resistance of flowable concrete. These three factors served as the independent variables. A response surface regression model was constructed using data from 17 tests to analyze the variations in slump flow, J-ring extensibility, 28-day compressive strengths, compressive strength retention ratio under sulfate attack, and relative mass loss ratio. The results showed that coarse aggregate gradation (A) had the most significant effect on each performance index, followed by the aggregate-to-cementitious material ratio (C); the water-reducing agent mixture (B) had the least impact. The optimized proportions were determined as follows: 71.149% for the 5-10 mm stones, 1.695% for the water-reducing agent mixture, and an aggregate-to-cementitious material ratio of 2.749. Microanalysis revealed that during the initial stage of sulfate corrosion, ettringite (AFt) and gypsum were generated to fill the pore spaces and improve densification, resulting in an increase in quality and strength; however, prolonged corrosion led to expansion damage and accelerated deterioration of the concrete. Scanning electron microscope (SEM) tests further verified the correlation between the macroscopic and microstructure properties.