  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 the MgO hydration reaction rate.
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 (20C/min).
