Pozzolanic and Cementing Activity of Raw and Pyro-Processed Saudi Arabian Red Mud (RM) Waste

Abstract

This paper investigates the composition, properties and reactivity of a red mud waste generated in Saudi Arabia with a view to find alternative materials to replace construction binders of high environmental impact. The phase transformation triggered by the sintering of the RM up to 1000°C is determined with thermal and X-Ray Diffraction analyses. Reactivity is investigated with chemical and physical methods including the Chapelle test, setting times, mechanical index and microscopy. The RM is clearly pozzolanic, and its activity is mainly due to the reaction of feldespathoids and the formation of zeolitic and feldspathoid-based hydrates. The positive effects of the thermal treatment are seen below 750°C, and include the loss of water in the zeolite/feldespathoids, and the destruction of the crystal structures of the clay minerals inherited form the parent bauxite. The negative effects of the thermal treatment are evidenced over 750°C, with a decrease in specific surface area, devitrification and crystal formation, whereby the active transition aluminas and the fedespathoids/zeolites (mainly cancrinite) transform into nepheline, tricalcium aluminate (C3A) and gehlenite. Despite the occurrence of nepheline, C3A and gehlenite in the RM sintered at 1000°C, the formation of pozzolanic hydrates that cause setting and strength development are greater at lower temperature. The optimum thermal treatment that enhances pozzolanic activity lies at c.400°C, as evidenced by the highest lime combination, the greatest mechanical index and the fastest set. The RM consists of gibbsite and boehmite, inherited from the bauxite, and cancrinite, chantalite and sodalite formed during the Bayer process. Feldespathoids have formed, instead of zeolites, due to the available silica and the high alkali content of the RM. The quick lime -CaO -, added twice during the refining process, has transformed the original goethite into hematite, and produced cancrinite. The Saudi RM has high SiO2 and high alkalinity, and an abundant specific surface area available for reaction. The chloride and carbon contents are low, and no environmental toxicity is inferred from its chemistry.

Investigation on Properties and Heavy Metal Ion Extraction of Thermally Activated Red Mud Incorporated Cement Mortar

“…In this study, a uniform contact time is considered in all the samples with each heavy metal ion. alumina compounds (Table 1), which is in agreement with the previous studies [24][25][26]. However, calcination beyond 700ºC causes decarbonation of calcite (calcium carbonate), and quick lime formation happens, adversely affecting the reactivity.…”

Pozzolanic and hydraulic activity of bauxite for binder production

“…Therefore, the thermal treatment of the bauxite increases the surface available for reaction at least up to 700 • C. The results indicate that, at a certain point after reaching 700 • C, the bauxite particles begin to agglomerate. A similar trend was obtained when sintering the red mud residue of this bauxite [25]. Other authors have also found similar trends in other bauxites.…”

“…The high pozzolanic index for bauxite when compared with RM and FA is largely due to the mineralogy of the bauxite including major kaolinite, gibbsite and boehmite. The major components of the bauxite's red mud are hematite, cancrinite -Na₆ Ca₂[(CO₃)₂| Al₆Si₆O₂₄]⋅2H₂O, gibbsite -Al (OH) 3 and sodalite Na 4 Si 3 Al 3 O 12 Cl [25] which, although reactive, combine less lime that the bauxite's aluminium phases in the short term.…”

“…RM-Red Mud residue. ** Alelweet et al, 2021[25]. XRD trace showing the crystalline phases the raw bauxite including gibbsite, kaolinite and boehmite with minor gypsum, rutile/anatase, nacrite and calcite.…”

“…MI of the bauxite compared with the MI of the bauxite's red mud -RM-(sintered at 300-750 • C) and other pozzolanic/cementing materials. * Alelweet et al, 2021[25]. + Walker and Pavia 2011[44].…”