The glass structure of granulated blast furnace slag and its effect on reactivity

World-wide granulated blast furnace slag (GBS) is used for cement and concrete production. About 280 Mt are produced yearly. Thus, regarding availability and reactivity GBS is the most important SCM. Many attempts exist to predict the latent hydraulic property of GBS, e.g. based on chemical composition and glass content. However, a general method could not be found. The reason is that beside chemistry and glass content additional factors influence the GBS reactivity, too. From thermodynamic and kinetic points of view it is obvious that thermal history of the slag (blast furnace, slag runner, cooling process) should have a significant influence on the glass structure, too. Thus, GBS glasses of the same chemistry can have a different resistance against corrosion (= different reactivity).

The basic idea was to adopt characterisation methods being well-established e.g. for soda-lime glasses for a wide range of GBS being produced in wet or dry processes and covering the whole range of industrial GBS. The cementitious properties (strength, heat of hydration) varied enormously while the physical properties (fineness, mixture etc.) were kept constant. So far known for the first time methods like Differential Scanning Calorimetry and viscosity measurement have been combined for GBS characterisation in order to measure e.g. the fictive temperature (glass transition), to calculate retroactively cooling rates of liquid slags and to describe the correlation with cementitious properties. The investigations are as well an important contribution to understand the different reactivity of GBS as a basis for optimisation processes regarding the chemical composition and/or the cooling technology.

Author: A. Ehrenberg, J. Deubener, H. Bornhoeft, N.R. Sarcos, D. Hart

Section: Science and production

Keywords: granulated blast furnace slag, secondary cementitious material, cement, glass, fictive temperature, enthalpy, heat of hydration, strength development