The growing demand for new SCMs has been mentioned in many sustainability reports as a route to decrease the CO 2 output of the construction industry in the short term ( Scrivener et al., 2016 Monteiro et al., 2017 Habert et al., 2020 Pamenter and Myers 2021). For this reason, demand outstrips supply, resulting in high prices and lower clinker replacement rates on the level of a country or region than what is technically possible or environmentally optimal. On the other hand, ground-granulated blast furnace slags (GGBFS) from pig iron production are highly desired because of their beneficial influence on cement performance, durability, and sustainability. The sustainable transition of the energy sector, in a planned phase-out, away from coal combustion in most European countries, will result in a decreased production of fly ashes ( Taylor 2021). The cement industry is facing increasing shortages of traditional supplementary cementitious materials (SCMs) such as coal, fly ash, and blast furnace slag ( Juenger et al., 2019 Habert et al., 2020). Considering that a realistic cement replacement level by BR as an SCM could be 20–30 wt%, the entire annual BR production could be absorbed by the cement industry at the European and global levels. The production of cement in 2020 was 4,100 million tons globally ( Garside 2021), of which, 4.3% was in the EU for an estimate of 176 million tons ( CEMBUREAU 2020). Other possible applications are the production of iron and steel, the capping of landfills, road construction, and soil amelioration ( Power et al., 2011 Evans 2016) however, the cement sector is selected as the most promising target market for high-volume application of BR by the International Aluminium Institute ( IAI 2020). The utilization rate of BR is estimated at less than 3% ( Power et al., 2011 Evans 2016) and the most used application is in the production of Portland cement clinker as an addition to the raw meal ( IAI 2020). In Europe, more specifically, the production of alumina was approximately 10.1 million tons in 2020 ( IAI 2021) with an average BR/alumina ratio of 0.67 ton ( Evans 2016), resulting in an estimated 6.8 million tons of BR in 2020. In 2017, the global production of BR was estimated to be 159 million tons ( IAI 2020). The world’s average BR production is approximately 1.35 ton alumina ( Evans 2016), resulting in nearly 170 million ton production of BR in 2021. The world’s production of alumina was 134 million tons in 2020 ( IAI 2021). While the main product of these refineries is aluminum hydroxide (Al(OH) 3 or Al 2O 3.3H 2O), which results in alumina (Al 2O 3) after calcination, a solid residue is obtained, named bauxite residue (BR), also known as red mud. Most alumina refineries in the world use the Bayer process in which bauxite ore is leached with NaOH ( Power et al., 2011 Evans 2016). The production of alumina is associated with the co-production of bauxite residue (BR). During co-calcination, the sodium-containing phases reacted with kaolinite, delivering a supplementary cementitious material with high reactivity and low free-sodium content. The inherent reactivity of the bauxite residue mainly stemmed from the desilication products such as sodalite and cancrinite. The same was found for the positive effect on the leaching of heavy metals. This positive effect was also observed after calcination with 10 and 20 wt% of kaolinite. The negative effect on the workability that some BRs with higher content of free sodium exhibited was mitigated by co-calcination. Co-calcination reduced the mobility of heavy metals significantly. Most bauxite residues require only 20 wt% substitution by kaolinite to reach the reactivity and performance targets. The reactivity and contribution to strength development were shown to scale linearly with the kaolinite dosage. 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. SCMs with moderately high reactivities were obtained. Bauxite residues from eight alumina refineries were co-calcined with 30 wt% of kaolinite at 750☌. 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). 2KU Leuven Department of Materials Engineering, Leuven, Belgium. 1Sustainable Materials, VITO, Mol, Belgium. Arne Peys 1* Tobias Hertel 2 Ruben Snellings 1
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