Beschreibung:
The present article investigates the potential of co-calcination with kaolinite as a sector-wide solution for the transformation of bauxite residue into an effective supplementary cementitious material (SCM). Bauxite residues from eight alumina refineries were co-calcined with 30 wt% of kaolinite at 750°C. SCMs with moderately high reactivities were obtained. Mortars with 30 wt% replacement of Portland cement (CEM I) by co-calcined bauxite residue had relative strengths of 73 ± 4%, 87 ± 4%, and 88 ± 2% after 2, 7, and 28 days compared to a CEM I reference mortar. The reactivity and contribution to strength development were shown to scale linearly with the kaolinite dosage. Most bauxite residues require only 20 wt% substitution by kaolinite to reach the reactivity and performance targets. Co-calcination reduced the mobility of heavy metals significantly. The negative effect on the workability that some BRs with higher content of free sodium exhibited was mitigated by co-calcination. This positive effect was also observed after calcination with 10 and 20 wt% of kaolinite. The same was found for the positive effect on the leaching of heavy metals. The inherent reactivity of the bauxite residue mainly stemmed from the desilication products such as sodalite and cancrinite. During co-calcination, the sodium-containing phases reacted with kaolinite, delivering a supplementary cementitious material with high reactivity and low free-sodium content.