Easy-to-use mullite and spinel sols as bonding agents in a high-alumina based ultra low cement castable

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Abstract

This paper deals with the preparation of colloidal suspensions of mullite and magnesium-aluminate spinel via a cheaper precursor of alumina sol. Viscosity, pH, solid content, DTA, TGA and XRD studies at different temperatures were performed to characterize those two sols. These mullite and spinel sols were used separately as bonding agents in a high-alumina based ultra low cement castable composition prepared by simple tapping technique and their performances have been compared in terms of bulk density, apparent porosity, cold crushing strength, flexural strength, slag corrosion, thermal shock, XRD and SEM reports. The results confirm that the mullite sol excels while the spinel sol degrades the refractory castable quality.

Introduction

Castables have been progressively gaining market share in all areas of refractory application in both ferrous and non-ferrous industries, replacing conventional concretes and ramming mixes [1], [2]. Mullite and magnesium-aluminate spinel are very much desirable phases in castables where improved hot strength, creep resistance, good thermal stability and slag penetration resistance are required. A wide variety of self-flow, no-cement and ultra low cement castables have been addressed by several authors [3], and colloidal stability software has also been developed to evaluate the castable properties [4].

Nowadays, a new bonding agent with superfine materials prepared from sol-gel route, appeared in the industry, which opened a new horizon for refractory technologists. The principle behind this bonding is formation of gel from a sol that surrounds the refractory aggregates and binds them through a network skeleton. It also reduces cement, and on subsequent heating develops strength via ceramic bonding by low temperature sintering and upgrades its high temperature properties [5]. These sol-gel binders differ from bulk sol-gel products because they only serve as bonding systems rather than the main body. Silica and alumina sols have been tried in this purpose although a huge literature information is not available. For alumina ceramics, boehmite has been suggested to be a binder and boehmite sol as a dispersant and the fabrication of several kinds of alumina products, as found in some literatures [6], [7], [8], [9], [10], is based on the use of this boehmite precursor. Mullite and mag–al spinel gels have been synthesized by several workers [11], [12] and found to be dependent on the intimacy of mixing of the respective ingredients. Starting from a cost effective boehmite sol precursor, mullite and spinel bonds can be conveniently prepared within the matrix of castables and as such a new generation of binders could be identified which are water compatible, inexpensive and capable to form low temperature bonding.

In this present investigation, an attempt has been made to synthesize the colloidal suspensions of mullite and mag–al spinel from a cheaper precursor like boehmite sol and the sols have been characterized with respect to viscosity, pH, solid content, DTA, TGA and XRD studies at different temperatures. These two sols were separately used as bonding agents to a tabular and fused alumina based ultra low cement castable composition and the samples were prepared by simple tapping method without vibration. A comparative study was done between these two kinds of sol-bonded castables with respect to bulk density, apparent porosity, cold crushing strength, flexural strength, thermal shock and slag corrosion resistance. X-ray diffraction analysis and scanning electron microscope studies of some fired samples were carried out to compare the effect of those sols on the physico-mechanical properties of castables.

Section snippets

Experimental

Alumina (boehmite) sol, prepared from the ammonia-nitrate route [13], having pH vale 2–4, particle size 14 nm and solid content about 5% was taken as the precursor material. Its viscosity, DTA, XRD, etc. have been reported [13]. To this boehmite sol, silica fume (CAB-O-SIL Fumed Silica, Cabot Corporation, USA, grade M-5, particle size 0.014 μm, S. A.=200 m2/gm) and MgCl2·6H2O (NEDMAG Industries) were slowly added in requisite amount with vigorous stirring (Fig. 1) to prepare mullite and mag–al

Viscosity

Fig. 2(a) and (b) show the viscosity vs. shear rate curves for mullite and mag–al spinel sols, respectively, and both of them indicate the same pseudoplastic nature. We may say therefore, more efficient particle packing reduces the range of sizes of interstices among particle [17] and the sols possess enough fluidity, which may help in casting the ULCC samples.

Solid content

The solid content was determined by the well-known ignition method and found to be 6 and 5%, respectively, for mullite and mag–al spinel

Conclusions

From the present study, it may be concluded that the easily prepared mullite sol, when used as an additive in ultra low cement-castable composition, offers a desirable combination of valuable properties. But the spinel sol is not compatible to that composition. It may further be suggested that this mullite-forming route may be exploited here due to the following reasons:

  • 1.

    Better homogeneity and control of composition is possible to result in an improved castables microstructure.

  • 2.

    High reactivity

Acknowledgements

The authors wish to express their sincere thanks to Dr. A. K. Banerjee, Officer-in-Charge, College of Ceramic Technology, for his help rendered at various stages during the course of experiments. The authors are grateful to Dr. S. K. Das, Refractory Division, Central Glass and Ceramic Research Institute, Kolkata for his valuable suggestions given during this work.

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