Exploring properties and hydration mechanisms in clinker-free cement formulated from steel industry solid waste using the extreme vertices method

Abstract

The development of clinker-free cementitious binders (CFCB) using industrial solid waste has attracted widespread attention due to their environmental and cost benefits. This study developed a CFCB using ground blast furnace slag (GBFS), steel slag (SS), and flue gas desulfurization gypsum (FGDG) as raw materials, utilizing an extreme vertex design method. The study systematically assessed the effects of each component on the CFCB's properties, hydration behavior, and microstructure, and based on these findings, further elucidated its hydration mechanism using thermodynamic simulations. Results indicated that FGDG played a critical role in regulating the fluidity of the fresh pastes and the compressive strength of the hardened pastes. GBFS enhanced the development of compressive strength, while the high-activity aluminates in SS enhanced the early-stage compressive strength. Thermodynamic simulations and experimental results confirmed that reactive aluminates and sulfates led to the formation of expansive hydration products AFt and AFm, with volume expansion peaking around 10 d. As the hydration reaction progressed, the number of aluminates participating in the reaction gradually increased, promoting the formation of C-A-S-H, hydrogarnet, and hydrotalcite, as well as the transformation of AFt into AFm. Comprehensive analysis suggested that within the GBFS–SS–FGDG system, the proportion of FGDG should not be less than 10 %, the content of GBFS should be controlled between 50 and 57.5 %, and the content of SS should not exceed 37.5 %. This study revealed the hydration mechanisms within the GBFS–SS–FGDG system, emphasizing the critical roles of each component.