Influence of the Thermal History of Granulated Blast Furnace Slags on Their Latent Hydraulic Reactivity in Cementitious Systems

Together with about 1180 million tons of hot metal (2017) about 380 million tons of blast furnace slag are produced yearly worldwide. Most of it (about 280 million tons) is quenched forming the glassy granulated blast furnace slag (GBS). For more than 130 years, this by-product is used as a clinker substitute in cement and concrete. Many approaches exist to evaluate the reactivity of ground granulated blast furnace slag (GGBS) in cementitious systems based on glass content, chemical composition, fineness, etc. But all approaches fail to define a suitable tool for judging an unknown GBS in a way that its strength contribution can be predicted. Only a rough differentiation might be possible. Compared to other parameters influencing GBS reactivity, the thermal history and the glass structure are mostly not investigated so far. However, from thermodynamic and kinetic points of view it is obvious that the thermal history of the slag should have a significant influence on glass structure and reactivity. The basic idea of the research project was to use analytical techniques being already established, for example, lime–soda–silica glasses. The differential scanning calorimetry and viscosity measurements have been combined for GBS characterization in order to measure the fictive temperature Tf (glass transition temperature on cooling during industrial quenching process) and to calculate retroactively the unknown cooling rates of liquid slags. Using these methods in combination with classical cementitious tests, it was possible to verify a correlation between the thermal history of GBS and its reactivity in a cementitious system.