Recycling of steel slag as an alkali activator for blast furnace slag: geopolymer preparation and its application in composite cement

The recycling–utilization rate of steel slag in China is less than 30%. Steel slag is rich in CaO and can produce the Ca(OH)₂ after hydration, which are expected to activate the pozzolanic activity of blast furnace slag. This work recycled steel slag as an alkali activator for the activation of blast furnace slag and prepared a steel slag-blast furnace slag-based (SS-BFS-based) geopolymer. The effect of steel slag addition on the hydration mechanism of blast furnace slag was investigated by analyzing the pH value and the hydration product and its yield and morphology. The compressive strength and activity index of the SS-BFS-based geopolymer was investigated. The results show that the pozzolanic activity of blast furnace slag can be activated in the alkaline environment (the digestion of free CaO; the Ca(OH)₂ hydrated from steel slag) provided by steel slag. The silicate–alumina phases dissolve to form the [SiO₄]⁴⁻ and [Al₂O₄]²⁻ and then repolymerize with the Ca²⁺/Mg²⁺ and OH⁻, forming the calcium silicate hydrate, hydrotalcite, and calcium (magnesium) silicate hydrate. Results also show that because steel slag can hydrate itself, the optimal addition of steel slag to blast furnace slag at 3, 7, and 28 days are 20, 40, and 60 wt%, respectively. However, the SS-BFS-based geopolymer (60 wt% steel slag and 40 wt% blast furnace slag) reaches the highest compressive strength (12.41 MPa) at 28 days and its activity index is 82.38%. This work greatly realizes the recycling–utilization of steel slag on a large scale, answering the call of the policy of carbon peak and carbon neutralization.