Aluminum lactate role in improving hydration and drying behavior of MgO-bonded refractory castables

Abstract

Developing MgO-bonded castables is still an important subject for refractory producers and end-users based on the expansive character of the in-situ Mg(OH)2 formation. Considering that magnesia undergoes hydration when exposed to water and the generated hydrated phase needs to be properly accommodated in the resulting microstructure to inhibit the generation of cracks, it is very important to find out alternatives to control/change the MgO hydration reaction rate, which may help to optimize the permeability and green mechanical strength of the castables. Therefore, fast and safer drying of such refractories can be carried out when adjusting Mg(OH)2 generation with proper additives. This research investigated the use of aluminum lactate (AL) as a likely additive to change the hydration and drying behavior of vibratable castables bonded with different MgO sources (dead burnt, caustic or fumed one). Firstly, XRD, TG and DSC measurements of magnesia-based aqueous suspensions were evaluated to identify the AL effect on changing the hydration reaction products during the curing and drying steps. After that, Al2O3-MgO refractories were prepared and their flowability, curing behavior, cold flexural strength, apparent porosity, permeability and explosion resistance were evaluated. The results indicated that, instead of Mg(OH)2, Mg6Al2(OH)16(OH)2.4.5H2O/ Mg6Al2(OH)16(CO3).4H2O was the main hydrated phase identified in the AL-containing compositions. Due to this change in the hydration behavior of the refractories, the mixtures prepared with dead-burnt or magnesia fumes plus organic salt presented a longer setting time. Besides that, crack-free samples with improved permeability and green mechanical strength could be obtained when adding 0.5 wt.% or 1.0 wt.% of aluminum lactate to the tested castable compositions. Consequently, 1.0 wt.% of the selected additive favored the design of refractories with enhanced properties and greater spalling resistance, as no explosion could be observed even when subjecting the prepared samples to severe heating conditions (20°C/min).

Crack-free caustic magnesia-bonded refractory castables

“…The samples kept at 110°C for 24h presented a small increase in the peaks' intensity of the hydrated phases. Based on these results and some data reported in previous works [33,[35][36][37], the incorporation of aluminum hydroxyl lactate into MgO-containing compositions leads to the generation of hydrotalcite-like phases on the magnesia particle's surface, which inhibits the brucite formation, as pointed out in Fig. 2.…”

“…Caustic magnesia has the potential to be used as a binder due to its higher reactivity (as it is produced by thermally treating MgCO3 in the range between 600-1300°C), however the faster hydration reaction of this oxide may result in the castable's disintegration during the initial processing steps [20,22,31,32]. Aiming to minimize such a negative effect, previous investigations [18,27,28,[33][34][35] reported that adding organic compounds (i.e., acetate, carboxylic acids, etc.) to MgO-containing compositions might change the morphology and nucleation rate of the formed brucite crystals to better fit their growth in the resulting microstructure.…”

Hydroxylation Studies on High-Solid Load Magnesia Aqueous Suspensions

“…In macroscopic structures, such forces produce significant volumetric expansion that usually ends up cracking and crumbling the material 1,2,6,8,10,11,[43][44][45][46] . Several solutions have been designed towards preventing such damages by hampering MgO hydroxylation 2,7,12,32,45 , changing the morphology of its products 10,[30][31][32]43,46,47 , or generating a surrounding microstructure for accommodating the extra volume produced 6,8,9,43,44,48 .…”

“…In such systems, the balance between MgO being dissolved and the precipitation of Mg(OH) 2 may be affected by the large quantity of water to be heated in the spaces amongst solid particles. Although such aspects were deeply studied in dilute aqueous suspensions, such as refractory castables (10-50 vol% of solids for selfflow 1,3,4 and 50-80 vol% for vibrate ones 49,50 ), they remain unexplored for more concentrated systems such as refractory mortars (80-95 vol% of solids) 6,8,12 and pressed pellets and bricks (above 95 vol% of solids) 1,2 . Even though these classes of pre-shaped refractories contain little or no water in their original formulations, hydroxylation reactions can occur due to contact with atmospheric moisture 14,15 , hydraulic cementing agents 9,10 , or layers of spray-applied refractory concrete for maintenance repair 1,2 .…”

“…Magnesium oxide (MgO), or magnesia, is a key raw material for steelmaking due to its high refractoriness (above 2800ºC melting point) and resistance to molten slag corrosion [1][2][3][4][5] . Nevertheless, the deleterious effects on the reactions of magnesia in contact with water, i.e., its expansive hydroxylation behavior, are equally important and well-documented [6][7][8][9][10][11][12] .…”