Upon consideration of the SiO2-Al2O3 phase diagram, Figure 12.27, for each pair of the following list of

Question:

Upon consideration of the SiO2-Al2O3 phase diagram, Figure 12.27, for each pair of the following list of compositions, which would you judge to be the more desirable refractory? Justify your choices.
(a) 20 wt% Al2O3-80 wt% SiO2 and 25 wt% Al2O3-75 wt% SiO2
(b) 70 wt% Al2O3-30 wt% SiO2 and 80 wt% Al2O3-20 wt% SiO2