| Summary: | The incorporation of additives to the clinker or to the raw materials stream is a common
practice in cement manufacture. However, steel slag, unlike its ironmaking parent the blast
furnace slag, it is not a conventional admixture for cement. Currently most steel slags are slow
cooled rendering stable crystalline compounds with minor hydraulic value. Nevertheless, if steel
slags would be quenched and granulated, the resulting glassy product might display increased
hydration and strength development potential. The use of steel slag in cement could contribute to
important savings for both cement and steelmaking industries and provide a solution for the
environmental problems linked to CO₂ emissions and costs of transport and disposal. The
purpose of this research is to explore the thermodynamics and kinetics of steel slag hydration in
an effort to produce a cement additive, or a more promising material of near Portland cement
composition.
An important criteria used in the assessment of slags as potential cements is the presence
of a glassy phase. At present, it is not very clear why glass enhances the hydration process.
However, it is known that the free energy of formation for glasses is less than for crystals so that
glasses are easier to hydrate compared to crystalline materials. In the particular case of steel slag,
the glassy phase would have to contain high amounts of iron. Steel slags are known to display
iron levels approximately 10 times higher than Portland cement and commonly used blast
furnace slags. However, the effect of high Fe₂0₃ levels on the setting and strengthening of
cement paste is not clearly understood due to the fact that most cement additives do not present
this characteristic.
The present work looks at the progress made in recycling steel slag as cement additive,
the complexity of the hydration process in slags, the possibilities of improving the hydration
potential of slags at laboratory and industrial level, and the problems that still need to be
addressed. However, the focus is on the glassy phase present in quenched steel slag and its
influence on the hydration rate. B-SEM, Image Analysis, XRD techniques and a series of
isothermal calorimetric experiments on synthetic as well as oxidized industrial steel slags vis-avis
Portland cement assist in this endeavor.
Temperature is a thermodynamic and kinetic factor modifying the enthalpy of hydrate
formation (heat of hydration) and accelerating the hydration reaction. Hydration tests were
carried out at temperatures ranging from 25 to 70 °C to determine the heat release, the rate of
reaction and the apparent activation energy for steel slag and Portland cement hydration. The
kinetics of hydration were explored in synthetic steel slags in both amorphous and fully
crystalline form. The mechanism of hydration for both amorphous and crystalline steel slag was
found to be a combination of nucleation and growth and diffusion, with higher reaction rates for
the glassy slags. The higher reactivity in the glassy slags was explained by a lower activation
energy when compared to the crystalline parent. Also, it was confirmed that slags have higher
activation energies than Portland cement. As expected, and comparably to granulated blast
furnace slags, quenched steels slags exhibited a significant hydraulic potential and hydrated at
longer times, thus being expected to contribute to the late (> 5-180 days) strength development in
slag - cements with compression strength superior to pure Portland cement.
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