The rate of densification of 0.0–2.0 wt.% zircon-doped calcined dolomite samples (CaO-MgO) was studied by means of linear shrinkage and bulk density as a function of firing temperature (1400–1700°C) and soaking time (1–4 h). The resistance to hydration and to slag attack of selected dense samples were investigated in relation to their thermal equilibrium and microstructure. The nature, chemical composition and zonation of the phases and the microstructure of the products were characterized by X-ray fluorescence (XRF) and X-ray diffraction (XRD), as well as scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). A mercury intrusion porosimeter was also used to investigate pore-size distribution. The modular system of the quaternary phase diagram MgO–CaO–C2S–C4AF was used in the determination of the thermal equilibrium data, and the solid and liquid phases that coexist at ~1300 and 1400°C. The rates of hydration and attack by steel-slag were also measured using steam-curing under atmospheric pressure and pill-test.
The studied Egyptian dolostone is composed mainly of dolomite with minor amounts of calcite, while the dopant zircon sand is contaminated with rutile grains. The 0.5 wt.% zircon-doped dolomite sample gives maximum bulk density and linear shrinkage after firing for 4 h at only 1400°C. This is attributed to enhancement of homogeneous discontinuous grain growth of the main free CaO and MgO phases, achieved by crystallization of a Ca-zirconate (s.s) along grain boundaries and at grain triple points. The Ca-zirconate crystallizes by cooling of an evolved liquid phase, together with ZrO2-bearing Ca-silicate and Ca,Mg-alumino-titanate solid solutions. The crystallization of these phases in a matrix of CaO and MgO grains leads to decreased penetration of the viscous ZrO2-rich molten slag through the doped sample, compared to un-doped one. The dense doped sample also gives relatively higher hydration resistance than the un-doped sample on steam curing under atmospheric pressures up to 8 h.
The addition of 0.5 wt.% zircon, slightly contaminated with rutile, is therefore sufficient to enhance the densification parameters as well as the slag and hydration resistance of the dolomite-based refractories.