Improving MgO-containing castable refractories without excessive expansionPublished on August 5th, 2011 | By: Eileen De Guire
Much of the research we report on can be compared to thoroughbred racehorses: it’s elegant, confident, stunning and promising. Now and then, though, it’s good (and maybe comforting) to take the time to learn about the research activities of the stalwart workhorses of the ceramics industry. Thus, I share with you a recent paper out of Brazil on a new binder for castable refractories (dry materials that can be combined with water to form a homogeneous mass that can be cast in formwork similar to concrete). Brazil has a huge steel-making and non-ferrous industry, so refractories also are a big business there. Likewise, refractories-related innovations are much sought after.
Refractory castables that contain magnesia are of important because of MgO’s high refractoriness and resistance to alkaline slags. Other oxides are in the cement (or binder, the authors’ preferred term) formulations, the most common compound being calcium aluminate, which is versatile and easily processed. Additional common binder materials include hydratable alumina, silica and alumina colloids and phosphates.
However, there are two drawbacks to adding MgO to binder formulations. First, there is a significant density mismatch between MgO and its hydrate, Mg(OH)2, which sets up compressive stresses that are damaging in the castable structure, an effect referred to as “apparent volumetric expansion.” A side effect problem is that hydrated MgO can be difficult to disperse well during mixing, which can lead to explosive spalling when the hydrate decomposes. The second drawback is that reactions between the magnesia and other oxides can result in fewer refractory oxide compounds.
In the work reported by Saomão, Bittencourt and ACerS member Victor Pandolfelli, the goal was to develop a magnesium base binder such that the hydration reaction would harden the castable refractory quickly while avoiding the problems of apparent volumetric expansion.
The approach was to evaluate a range of precursor MgO compounds obtained through partial calcinations of Mg(OH)2. Four particle size ranges were studied, and calcined MgO additions to calcium aluminate of 3- and 6-mass percent were evaluated. The magnesium hydrate precursors were calcined at temperatures ranging from 50-1000ºC and added to calcium aluminate cement in two formulations, 3- and 6-mass percent. Four initial MgO particles size ranges were studied. Samples were vibration cast and cured under uniform conditions and dried at temperatures ranging from 50-1000ºC. The porosity and compressive strengths of samples were measured.
The researchers found that particle size was important—smaller particles hydrated quickly and without excessive expansion—and the extra volume could be accommodated within the porosity of the castable. The Mg(OH)2 decomposes between 380-650ºC, and the release of water vapor and crystallographic changes create cracks and pores, increasing the specific surface area. The precursor calcination temperature that yielded the best balance between the reactivity of the remaining Mg(OH)2 when mixed in the calcium aluminate cement and the surface area generated by decomposition was found to be 600ºC. The only sample that had strength comparable to the MgO-free control sample had a 3-mass percent addition of Mg(OH)2 precursor that had been calcined at 600ºC and dried at 1000ºC, which showed that within a narrow range, magnesia can be part of a castable binder formulation.
While this research is not exciting or edgy, there could be big payoffs if maintenance shutdowns and refractory failures are minimized.
The thoroughbreds are great fun to watch, but the workhorses pay the bills.
Paper: R. Salomão, L.R.M. Bittencourt, V.C. Pandolfelli, “A Novel Magnesia Based Binder (MBB) for Refractory Castables,” Supplement to Interceram 60 (2011) 
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