Decoupling of durability indicators and ASR mitigation caused by agglomeration in densified silica fume

Silica fume (SF) is widely used to improve concrete strength and durability; however, in practice it is often supplied in densified form, producing agglomerates that may not fully disperse during mixing. Although coarse SF agglomerates can act as localized sources of reactive silica and promote alkali–silica reaction (ASR), their effect on concrete mechanical and durability performance remains unclear. This study investigates whether the agglomeration of commercially densified SF can decouple conventional durability indicators from ASR mitigation. Three densified SF grades, moderate, large, and very large were evaluated. Concrete and mortar performance was assessed through compressive strength, water-permeability, chloride-ion penetrability and sulfate resistance. ASR susceptibility was examined by accelerated mortar bar testing, and damage was characterized by petrography and SEM. All SF mixtures improved conventional performance, but ASR response was strongly dependent on agglomerate size and dosage. These findings identify agglomerate-size distribution as a key qualification parameter for commercially densified SF.