Dipotassium hydrogen phosphate activated Al-rich steel slag: The role of layered double hydroxides and aluminum hydrate gel

Due to absence of C-S-H gel, chemically activated mixed Al-rich steel slag often shows low early-age strength. This work proposes a novel method to accelerate the hydration of Al-rich steel slag by dipotassium hydrogen phosphate (DHP) activation. Results reveal that DHP significantly promotes the formation of layered double hydroxides (LDHs) by increasing pH of the pore solution and supplying phosphate ions as the interlayer anion. Ca-Al-LDHs competes with the ${\mathrm{Ca}}^{2+}$${\mathrm{Ca}}^{2+}$ used to form metastable C₂AH_8, thereby releasing additional $\mathrm{Al}{\left(\mathrm{OH}\right)}_4^{-}$$\mathrm{Al}{\left(\mathrm{OH}\right)}_4^{-}$, which is further accompanied by the increased formation of AH₃ gel. Consequently, DHP-activated steel slag develops a denser microstructure dominated by highly-crystalized LDHs and amorphous AH₃ gel and achieving impressive compressive strength (40 MPa at 28 days). Overall, this study provides a unique activation method for enhancing the early hydraulic activity of steel slag by in-situ formation of LDHs and AH₃ gel, contributing to the unique way to utilize steel-making byproducts.