Effect of Silica Fume Agglomerates on ASR-Related Expansion of ASTM C1567 Mortar Bars

When densified silica fume (DSF) is used in the ASTM C1567 test method and mixed as prescribed in the standard, the silica fume agglomerates (SFAs) may not break up. The lack of dispersed silica fume can result in significantly increased expansions of C1567 mortar bars relative to the same mix containing well-dispersed silica fume. Testing was performed to determine the difference in expansions between C1567 mortar bars prepared with as-received DSF and those prepared using different methods to break up and disperse the silica fume. The potential of the SFAs to act as reactive silica particles that can contribute to expansion of the mortar bars was also investigated. Petrographic examinations (ASTM C856) were performed on the bars following the testing. Point count analyses (ASTM C457) were performed to determine the volume of the SFAs in the tested mortar bars. Recommendations are made regarding modifications to the C1567 test method when using silica fume.