bricks, a small amount of alumina or bauxite powder, co-milled with MgO
powder, was added to magnesia bricks. On firing, the co-milled MgO and
Al2O3 reacted to form in-situ spinel. Compared with silicate (CMS or M2S)
bonded magnesia bricks (see Chapter 6, Magnesia Refractories in this book),
this first generation of MgO–MA bricks had promising properties, including
higher hot mechanical strength and much better thermal shock resistance. The
early work also found that increasing Al2O3 content generally leads to better thermal
shock resistance. However, with increasing level of Al2O3, brick porosity
also increased and when the Al2O3 content was above 10 wt%, it was difficult
to make dense bricks. The improvement in thermal shock resistance with increasing
Al2O3 content was found to be related to the mismatch of thermal expansion
coefficients between MgO (13.5 1026) and MA (8.4 1026). Due to this
mismatch, microcracks form between MgO and MA grains during cooling, assisting
release of thermal stress and disrupting the microstructure, making crack
propagation more difficult. The increase of porosity with Al2O3 content is related
to the volume expansion (5 8%) associated with the spinel formation reaction
from MgO and Al2O3. For this reason, the amount of Al2O3 in the first generation
of MgO–MA bricks was limited to be ,10 wt%. The second generation of
MgO–MA bricks, developed in the mid-1970s, used a preformed synthetic spinel
grain to replace some MgO. The use of preformed MA grain instead of Al2O3
eliminated the thermal expansion caused by the formation of in-situ MA in the
first-generation bricks, so a large amount of spinel could be introduced without
significantly increasing the porosity. Nevertheless, in the second generation of
MgO–MA bricks, the matrix remained unimproved because no spinel fines
were present. To improve this, the third generation of MgO–MA bricks has
been developed recently which contain spinel aggregates and spinel introduced
in the matrix via direct addition of preformed spinel and/or alumina powders
to form in-situ spinel. The third generation of bricks thus combine the main
advantages of the first and second generations, so they show much better properties
such as higher hot strength, higher slag penetration resistance, and higher
thermal shock resistance and longer service life in many application areas. The
morphologies of in-situ generated spinel are a complex function of a range of factors
including local composition and temperature. Figure 10 shows several in-situ
spinel morphologies including the characteristic angular habit and a tuber-like
shape often seen in the presence of large amounts of liquid (21).