The formation of geopolymer concrete from bauxite-refining residue and fly ash: Application in sustainable concrete production

Bauxite-refining residue is generated in the processing of bauxite into alumina using the Bayer process, and fly ash is a by-product of burning pulverized coal in an electricity generating. Bauxite-refining residue (red mud) and its’ concentrated sodium aluminate solution (Spent Liquor) are known to have properties of high alkalinity and fly ash consisting of fine powder are easily dispersed into the surrounding environment, such as aquatic, air and soil which may lead to ecological problems. This research synergistically incorporates red mud, fly ash, and the Spent Liquor from the Bayer process along with sodium-silicate solution to produce a geopolymer based material which can be used as building materials. This research focused on the sustainable use of red mud as an additive material with fly ash to form concrete. A series of three laboratory trials using of 10 – 30% dried and crushed red mud addition on the geopolymer formation reaction, and 70% red mud as combined with Spent Liquor was carried out. An improvement in setting time and compressive strength was observed with red mud addition at all trial conditions for 7, 14, and 28 days. The structural characterization revealed that the rate of reaction of red mud was dependent on the caustic soda (NaOH) concentration. However, the development of mechanical properties was related to the collaborative effect of NaOH concentration, solubility of silicates and the iron oxides presence. Based on standards for concrete, the compressive strength achieved in all series is suitable for paving blocks, slabs, driveway, backfill, retaining blocks etc. The production of these concrete materials and compressive strength test meets the requirements of the Australian Standards’: AS 1012.1- 2014, AS 1012.3.1, AS 1012.8.1-2014, and AS 1012.9-2014 respectively. Leaching of toxic metals were within permissible limits, but further research should be carried out for the identification of ongoing reactions