Durability Degradation and Fractal Strength Prediction of Bentonite-Slurry/Steel-Slag Foamed Concrete Under Corrosive Wetting–Drying Exposure

Bentonite slurry (BS) and steel slag powder (SS) were co-utilized to develop bentonite-slurry/steel-slag foamed concrete (BS-SSFC). The evolution of compressive strength and the associated deterioration mechanisms were examined after repeated wetting–drying exposure in four environments, namely H2O, H2SO4, NaOH, and Na2SO4, by combining mechanical testing with microstructural observations. The mix-design results indicate that, for the SS-only mixtures, 20% SS replacement produced a relatively high strength, whereas the binary SS-BS system reached its maximum strength at 10% SS and 5% BS; this combination was consequently adopted for the durability experiments. After 20 cycles, the severity of degradation followed Na2SO4 > H2SO4 > NaOH > H2O. XRD and SEM-EDS evidence shows that sulfate ions in the H2SO4 and Na2SO4 solutions favored ettringite-type expansive products, and Na2SO4 further caused salt-crystallization pressure during drying. For NaOH exposure, the main damage was related to reduced stability of cementitious phases together with ion redistribution and localized re-precipitation in a strongly alkaline pore environment. Based on fractal theory, an empirical strength–degradation correlation model was established by using SEM-derived two-dimensional apparent areal porosity as a structural parameter and by linking fractal dimension with the number of cycles. Within the scope of the present experiments, the model captures the empirical link between strength loss and apparent pore-structure deterioration in BS-SSFC; however, its use remains dependent on the image-acquisition procedure, thresholding method, and material system considered. The results provide useful support for using BS-SSFC in aggressive engineering settings such as saline ground and acid-rain regions.