CEMENTITIOUS PROPERTIES OF STEELMAKING SLAGS
2011
The present study is directed towards the use of
steelmaking slags as raw material for sulphoaluminate belite cement
(SAB). Another important objective was to characterise the
cementitious properties of phases in ladle furnace slag (LFS)
specifically the calcium aluminates. Mayenite (C12A7) is considered
one of the most important calcium aluminate in LFS, and since
comparatively limited data on the kinetic properties of this phase
are available, it was decided to study C12A7 more closely with
regard to both particle size and temperature sensitivity. The
behaviour of high-temperature reactions of tested SAB mixtures was
investigated using thermogravimetric analysis coupled with a
quadrupole mass spectrometer. Mineralogical observations were
carried out with x-ray powder diffraction (XRD) and scanning
electron microscopy (SEM). The results proved that steelmaking
slags have the potential to work as raw material, since
sulphoaluminate (C4A3 S ) along with polymorphs of dicalcium
silicate (C2S) and ferrite phase (C4AF) were detected after firing
at 1200oC in an air atmosphere. The hydration properties of the
specimens were analysed through conduction calorimetry, and
compressive strength of specimens hydrated for 2 and 28 days. The
compressive strength was in accordance with that suggested in the
literature for slow hardening SAB cement. Both mixtures tested
behaved the same with regard to heat development as well as the
amount of ettringite (AFt) formed during the first 24 hours of the
hydration. The formation of AFt was characterised with both
differential scanning calorimeter (DSC) and XRD. The
crystallographic distribution in LFS samples was quantified using
Rietveldanalysis. Calorimetric studies were performed at 20, 25 and
30°C in order to calculate the activation energy of hydration and
thereby to suggest a kinetic model for tested compositions within
this temperature interval. In addition to heat of hydration,
compressive strength tests were completed on mortar prisms of LFS,
and LFS in a blend with ground granulated blast furnace slag
(GGBFS) which hydrated for 2, 7 and 28 days. Both compositions
reached acceptable early strengths, whereas, after 28 days
hydration, the blend was superior to neat LFS. Related activation
energy was according to the Avrami-Erofeev model determined to 58
kJ/mol for the LFS and 63 kJ/mol for the blend. Corresponding
calorimetric studies at the same temperatures were performed on a
fine and coarse size fraction (Fraction A and Fraction B) of a
synthesised C12A7. The purity was confirmed by XRD, and the
hydraulic behaviour was investigated in excess water with respect
to the dissolution. The apparent activation energy was calculated
to 33 and 79 kJ/mol, respectively, for Fractions A-B using the
Avrami-Erofeev model. From the model, it was also concluded that
the acceleration period can be ascribed to a phase-boundary
controlled mechanism. The principal calcium aluminate hydrates
obtained were C2AH8 and C2AH7.5, and it was further observed that
C12A7 is accompanied by an anomalous…
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