Use of Cement Bypass Dust as Filler in Asphalt Concrete Mixtures

Cement bypass dust (CBPD) or cement kiln dust is a by-product of the manufacture of portland cement. It is generated during the calcining process in the kiln. Lime (CaO) constitutes more than 60% of CBPD composition. Other compounds include ${\mathrm{SiO}}_2\text{,}$ ${\mathrm{Al}}_2{O}_3\text{,}$ ${\mathrm{Fe}}_2{O}_3\text{,}$ $K_{2}O\text{,}$ ${\mathrm{Na}}_{2}O\text{,}$ ${\mathrm{Cl}}^{-}\text{,}$ etc. Oman Cement Company generates about 25,000–30,000 t of CBPD every year. Some CBPD is recycled back again with the clinker. However, most of the material is disposed of on-site without any further reuse or reclamation. As such, research was carried out to investigate potential reuses of CBPD in the Sultanate of Oman. This paper will present the results of a study that investigated the use of CBPD as a filler in asphalt paving mixtures. Initially, the effect of adding either lime or CBPD (0, 3, 5, 7, 10, and 15%) on binder properties (penetration, ductility, and softening point) was investigated. Then three different asphalt concrete mixtures were prepared using lime (control), and 5 and 13% CBPD substitution for lime or fine aggregate. The mixtures were subjected to Marshall testing. Results indicate that the substitution of 5% CBPD for lime will essentially produce the same optimum asphalt binder content as the control mixture (4.5%, by weight of aggregate) without any negative effect on asphalt concrete properties (stability, flow, voids in total mix, voids in mineral aggregate, and voids filled with asphalt). However, the use of 13% CBPD for lime and fine aggregate will require a higher optimum asphalt binder content of 5.7%, by weight of aggregate. This will produce an uneconomical mix. Thus, 5% CBPD substitution for lime or cement would be the optimum used in asphalt paving mixtures.