Influence of steel slag powder on the early expansion behavior of geopolymers

Volume stability is the key to limiting the application of high dosages of steel slag (SS) in geopolymers (GPs). In this paper, the effects of mixing time, particle size and dosage of SS powder, modulus and Na 2 O content of activators, and immersion treatment on the early expansion behavior of SS-based GPs were studied. The leaching behavior of free-oxides from SS was explored. Moreover, the mechanism of SS-induced early expansion of GPs was analyzed based on mineral composition and micromorphology of hydration products. The results reveal that the rapid expansion of SS-based GPs mainly occurs in the first 2.5–6 h, followed by slow shrinkage. Increasing the particle size of SS powder and the modulus and Na 2 O content of activators, decreasing the dosage of SS powder and the mixing time, and immersion treatment can reduce the early expansion of SS-based GPs. Free-CaO in SS will leach out and participate in the hydration reaction, resulting in amorphous gel phases (C-A-S-H, C-S-H, and N-A-S-H) and Ca(OH) 2 in the hydration products. Indicating that the free-CaO from SS is the key to the early expansion of GPs. The substitution of ground granulated blast furnace slag for SS can reduce the leaching amount of free-CaO and consume Ca(OH) 2 , resulting in a reduction in the early and long-term expansion of GPs. Leaching and hydration of free-CaO from steel slag can induce the early expansion of GPs.