Aggregate/mortar interface: influence of silica fume at the micro- and macro-level

Microstructural investigations and chemical analyses were conducted by utilizing scanning electron microscopy and energy dispersive X-ray analyser in the interfacial zone between three types of cylindrical aggregates (sandstone, limestone and granite) and two types of mortar matrices (plain mortar and 20% silica fume mortar). The strength of the interface was determined experimentally by means of aggregate push-out test. Interfacial strength was then related to the interfacial thickness and the amount of silica at the interface. It was found that the higher the silica concentration in the interface, the thinner the relative interfacial thickness and the higher the interfacial bond strength as a result of the densification role of silica fume. The interfacial bond strength was found to increase as the aggregate size decreased, irrespective of the type of aggregate and the mortar matrix. This size effect is shown to be statistical in nature and described by Weibull theory.