Fracture properties and response surface methodology model of alkali-slag concrete under freeze–thaw cycles

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Alkali-slag concrete(ASC) is prepared with Na2SiO3 and NaOH composite activator. Fracture properties of ASC under freeze-thaw condition are studied using response surface methodology(RSM), Design Expert software and Box-Benhnken design(BBD). The RSM model of fracture toughness is built, thus the influence law of sol ratio, slag content, age on fracture toughness and the relationship between initiation fracture toughness and unstable fracture toughness is established. The influence law of freeze-thaw, sol ratio and slag content under freeze-thaw on unstable fracture toughness is built. Then the impact mechanism of the two factors on unstable fracture toughness is analyzed. Results show that the RSM model fits well and can be used to analyze and predict ASC fracture toughness. The rank of three factors’ significance for unstable fracture toughness is slag content>age>sol ratio, and the most significant interaction is between slag content and age. Initiation fracture toughness is about 50%~70% of unstable fracture toughness, crack initiation should be the warning indicator of ASC structure. Fracture toughness decreases with the increasing of freeze-thaw times, its declining rate and degree increase with sol ratio and slag content. With sol ratio increasing, more intense is the alkali slag reaction, reducing internal density and hindering depolymerization. With more cementitious material content, effective water-cement ratio is relatively decreased, subsequent hydration is limited, some slag powder doesn’t participate in the reaction, reducing ASC toughness.

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