°Cd Rheological and mechanical properties of bauxite residue as hot mix asphalt filler Rajan Choudhary a *, Abhinay Kumar a , Ghazali Rahman b a Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam-781039, India b Department of Civil Engineering, Integral University, Lucknow, Uttar Pradesh-226026, India Received 15 August 2019; received in revised form 15 September 2019; accepted 25 September 2019 Abstract Bauxite residue (red mud) is a highly alkaline solid waste generated in the process of production of aluminium metal from bauxite ore. More than 5.5 million tons of red mud is generated in India. The aluminium industries, worldwide and in India, have expressed strong need for dedicated research to explore large-scale utilization of red mud. Towards this end, the objective of this study is to investigate the use of red mud as a filler in hot mix asphalt (HMA). Stone dust, a traditionally used filler material, is used as the control filler. For a comprehensive evaluation approach, the present investigation is conducted at three levels: filler, mastic and mixture characterisation. Red mud and stone dust fillers are characterised using scanning electron microscopy (SEM), pH, Rigden voids, methylene blue value, particle size analysis, and specific surface measurement. Mastic rheology is studied using the dynamic shear rheometer by temperature sweep and multiple stress creep and recovery (MSCR) tests. Finally, dense-graded bituminous concrete (BC) mixes are designed by the Marshall method followed in India, at different filler contents of both filler types. Mixture performance is investigated in terms of indirect tensile strength and Cantabro abrasion loss. Results show that addition of both fillers improved the viscoelastic response of the base binder. Red mud mixes showed higher resistance to disintegration and higher tensile strength than the control mixes with stone dust filler. Based on the findings of this study, red mud showed good potential for use as a filler in dense graded bituminous mixes. Keywords: Filler; hot mix asphalt; red mud; waste utilization; mastic rheology; MSCR 1. Introduction Highway infrastructure in India has expanded substantially in the last few decades, both in terms of capacity and dimensions. India has the world’s second largest road network after the US with a total of 5.47 million km of roads spread throughout the length and breadth of the country [1]. Qualitatively, however, 61 percent of India’s roads are paved, and the topmost quality of roads—the national highways and expressways—constitute only 2 percent of the road network but carry nearly 40 percent of the total road traffic. Realizing the urgent need to upgrade, construct, and widen major highways to international standards, the Govt. of India has embarked upon various road development programs. About 95 percent of Indian roads are asphalt pavements, consisting of bituminous courses underlain by granular base and sub-base courses on a compacted subgrade. Large scale highway * Corresponding author E-mail addresses: rajandce@iitg.ac.in (R. Choudhary); abhinay.kumar@iitg.ac.in (A. Kumar); erghazali@gmail.com (G. Rahman). Revised version of a paper presented at 15 th World Conference on Transport Research (WCTR), Bombay, Mumbai, India, 26-31 May 2019. construction demands a huge amount of construction materials, particularly stone aggregates and mineral fillers, which typically constitute 95 percent of an asphalt mixture by weight. At present, India ranks 4th in the world in terms of bauxite production. Bauxite is one of the most important minerals produced in India and the main ore for the production of aluminium metal. About 22 million tons of bauxite is produced in India annually [2]. The first step in production of aluminium is the extraction of alumina (aluminium oxide) from the bauxite ore through the Bayer process. Under the process, the bauxite ore is washed, crushed and dissolved in caustic soda (sodium hydroxide) under the conditions of high temperature and pressure. The result of this process is formation of sodium aluminate. The sodium aluminate is processed further through filtration and calcination to yield alumina (aluminium oxide). The substances insoluble in caustic soda are left behind as residue, called the ‘bauxite residue’ or ‘red mud’ [3-4]. Presence of iron oxide imparts the characteristic reddish brown color to the residue. Red mud is the major waste material generated from the aluminium industry and is associated with serious environmental concerns. Red mud is one of the largest industrial by-products with global levels estimated at around 3000 million tons in 2010 with ISSN: 1996-6814 DOI: https://doi.org/10.1007/s42947-019-0074-4 Chinese Society of Pavement Engineering. Production and hosting by Springer Nature International Journal of Pavement Research and Technology Journal homepage: www.springer.com/42947 International Journal of Chinese Society of Pavement Engineering