Volume 39, Issue 4 (July 2011)
Effect of Cement Kiln Dust and Rock Dust as Mineral Fillers on Bulk Specific Gravity of Fine Aggregates
The bulk specific gravity (Gsb) of aggregates is a critical parameter in the design of asphalt mixes. In the Superpave volumetric mix design, Gsb is used to determine the amount of asphalt binder absorbed by aggregates and the percentage of voids in the mineral aggregate. Problems with the current test methods of measuring the Gsb of fine aggregates, i.e., AASHTO T84 or ASTM C128, have been reported in some previous studies as well as in the current study. These standard test methods remain questionable under certain conditions: (1) When rough and angular as well as small and varying particle size distributions of fine aggregates are present and (2) when water reactive mineral fillers (passing No. 200 sieve) are added to fine aggregates. This study examined the addition of two selected mineral fillers, namely, Cement Kiln Dust (CKD) and rock dust, on the overall Gsb of fine aggregates by employing the AASHTO T84 and the CoreLok-Aggplus test procedures. When using the AASHTO T84 test method, the overall Gsb values reduce for both the additives. When using a newly developed CoreLok method, the Gsb values increased for rock dust up to 10 %, beyond which a reduction is observed. With CKD, the Gsb values show an increase for up to 6 % addition of it and then a decrease is observed. The data suggest that the AASHTO T84 test method may not be applicable in the presence of CKD due to the chemical reactions in the presence of water and the formation of cementitious products. A possibility of chemical reaction due to the presence of limestone in the rock dust and its fine gradation influence the results of the AASHTO T84 test method. Thus, use of the CoreLok method appears more appropriate in the presence of such reactive materials. To further analyze the problem, the specific surface area of fine aggregates and mineral fillers was measured using water vapor and nitrogen adsorption methods. This study also reveals that angular fine aggregates with a high angularity and rough surface texture pose significant difficulty in measuring Gsb by the AASHTO T84 test method.