Effective inhibition of Cr(VI) in the Al2O3-CaO-Cr2O3 refractory castables system through silica gel assisted in-situ secondary phase tuning

Cr2O3 is the best-known corrosion resistant refractory materials under high temperature extremely harsh environment. However, the formation of toxic, carcinogenic & water-soluble Cr(VI) compounds during its application remained a major predicament. Here, we have introduced a facile route to fabricate the Al2O3-CaO-Cr2O3 refractory castables to mitigate Cr(VI) using basic silica sol (pH similar to 9) via in-situ secondary phase modification. Basic silica sols 0.25, 0.49 and 0.98 wt% (CaO/SiO2 ratio of 5.8, 2.9 and 1.45 respectively) were doped by keeping the castables properties reasonable. Addition of silica drastically reduces the mid-temperature (500-1100 degrees C) formation of Cr(VI) phase (CaCrO4), while relatively higher temperature Cr(VI) phase (hauyne, Ca4Al6CrO16) totally disappeared from 900 to 1300 degrees C. However, at 1300-1500 degrees C, silica addition promote the solid solution phase formation like Ca(Al,Cr)(12)O-19 and (Al,Cr)(2)O-3 where chromium exists as +3 oxidation state. Also, silica addition reduces the dimensional tolerance of the castables at a higher temperature associated with the secondary phases (dehydration of cement phases and expansive formation of CaAl12O19). A comparable mechanical strength (up to 156 MPa) and a remarkable Cr(VI) reduction (up to 94%) could be achieved upon silica addition. (C) 2019 Elsevier Ltd. All rights reserved.