Performance of Lime Stabilization on Extremely Alkaline Red Mud Waste under Acidic Environment Manas Chandan Mishra 1 ; Karra Sateesh Babu 2 ; N. Gangadhara Reddy 3 ; Partha Pratim Dey 4 ; and B. Hanumantha Rao 5 Abstract: Excessive pH (>11) and inconsistent engineering properties because of the chemicomineralogical composition of red mud warrant devising an alternate approach to render it more environmentally benign. In this study, the performance of lime stabilization coupled with oxalic acid, which acts as a neutralizing agent, was explored systematically to improve the long-term strength and to reduce the alkalinity of red mud. The mechanisms governing the behavior of additives along with their effects were investigated by examining the mineralogical, elemen- tal, and morphological characteristics. The results suggest an effect of sodalite and alumina on the inconsistency in dry unit weight and claylike behavior of red mud. A reduction in pH was found to be instrumental during early phases of lime stabilization and in precipitation of cementitious hydration products, leading to strength gain in the long term in red mud. Moreover, the addition of lime and oxalic acid was found to yield more encouraging results than lime treatment alone, and hence this is recommended to be used for red mud stabilization. The obtained results were substantiated by comparison with code provisions. DOI: 10.1061/(ASCE)HZ.2153-5515.0000448. © 2019 American Society of Civil Engineers. Author keywords: Red mud; Lime stabilization; Oxalic acid; Alkalinity; Hydration reactions; Long-term strength. Introduction A large quantity of highly alkaline bauxite residue, referred to as red mud (RM), is generated in Bayer’ s process and the production of alumina (Al 2 O 3 ) from bauxite ore. High pH, low compressive strength, low compactibility, dispersion, and leaching are a few is- sues associated with RM (Nikraz et al. 2007; Borra et al. 2015; Alam et al. 2018; Reddy et al. 2018). Its use is negligible at present (Sutar et al. 2014), but a few applications in the construction in- dustry as a building material or in geotechnical applications as a geomaterial have been developed with a vision to promote its large-scale use (Pontikes and Angelopoulos 2013; Samal et al. 2013) after neutralization and stabilization (Wang et al. 2008). Based on its low compressive strength, studies categorize RM as a soft soil, which needs to be stabilized (Gordon et al. 1996; Reddy and Rao 2016). Generally, lime is a widely considered additve for stabilization of soft soils. Naturally available quicklime and hydrated lime have found their way into the science of soil stabilization since ancient times (Dash and Hussain 2011), and have been used successfully for treating a variety of geomaterials (TRB 1987; Mitchell and Soga 2005; Ciancio et al. 2014). Different soils respond to lime treatment in different ways: lime sometimes simply alters their consistency limits, modulates soil strength and compactibility, affects the swell- ing behavior, or changes dispersive characteristics (Du et al. 1999; Al-Rawas et al. 2005; Umesha et al. 2009). This may be attributed to the versatility and heterogeneity of the material to be treated, diversity in mineralogical composition and content percentage, and the vast area of applications of the soil types for different engineering purposes (Cherian and Arnepalli 2015; Tahasildar et al. 2017). Nagaraj et al. (2014) evaluated the effect of lime on the long-term strength and durability of red earth and sand mixture to understand stepwise changes in the soil during the treatment procedure. Com- paring the results for lime and cement treatment, they reported that strength gain in lime stabilized soil continues beyond a duration of 2 years, in contrast to that of cement, which gains maximum strength within 6 months. Such reports of the effect of time and curing on the efficacy of lime stabilization technique invoke the necessity to inves- tigate these factors by conducting detailed experimental study. Few mechanisms have been proposed by researchers to explain these re- sponses, namely carbonation, cation exchange capacity, presence of sulfates, cementation, flocculation-agglomeration reaction, pozzo- lanic reaction, and pH changes (Eades and Grim 1960; Diamond and Kinter 1965; Jacobson et al. 2003). Based on the results of previous investigations, lime may pro- vide a viable solution to stabilize the RM. Lime is an alkaline material, and its efficacy as an additive during treatment depends on the alkalinity of the parent material to be treated (Hoak et al. 1944). However, the effect of pH as a factor affecting the stabili- zation of RM has not been investigated when lime is used as an additive. Furthermore, the effect of curing time on lime stabiliza- tion is another factor to be scrutinized in detail. Source materials/ soils with pH more than 8.5, such as RM, have been recommended not to be used for construction purposes (IRC 2001, 2002). 1 Research Scholar, School of Infrastructure, Indian Institute of Technology Bhubaneswar, Bhubaneswar 752050, India. Email: mcm10@ iitbbs.ac.in 2 Formerly, Postgraduate Student, School of Infrastructure, Indian Insti- tute of Technology Bhubaneswar, Bhubaneswar 752050, India. Email: ksb10@iitbbs.ac.in 3 Research Scholar, School of Infrastructure, Indian Institute of Technol- ogy Bhubaneswar, Bhubaneswar 752050, India. ORCID: https://orcid.org /0000-0002-1302-1017. Email: gn11@iitbbs.ac.in 4 Assistant Professor, School of Infrastructure, Indian Institute of Technology Bhubaneswar, Bhubaneswar 752050, India. Email: ppdey@ iitbbs.ac.in 5 Assistant Professor, School of Infrastructure, Indian Institute of Tech- nology Bhubaneswar, Bhubaneswar 752050, India (corresponding author). Email: bhrao@iitbbs.ac.in Note. This manuscript was submitted on November 19, 2018; approved on April 4, 2019; published online on June 6, 2019. Discussion period open until November 6, 2019; separate discussions must be submitted for individual papers. This paper is part of the Journal of Hazardous, Toxic, and Radioactive Waste, © ASCE, ISSN 2153-5493. © ASCE 04019012-1 J. Hazard. Toxic Radioact. Waste J. Hazard. Toxic Radioact. 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