Compactness of the cement microstructure versus crack bridging in mortars reinforced with amorphous cast iron fibers and silica fumes

Both mechanical compaction and addition of pozzolanic silica fumes can provide low permeability interfacial transition zones around the fibers which reinforce a mortar matrix. This paper deals with the controversial effect of achieving a higher matrix compactness and its influence on the fracture behaviour of a mortar reinforced with amorphous cast iron fibers. Test were conducted in uniaxial tension on notched composite mortar prisms in order to plot load versus crack opening curves and evaluate the bridging energy provided by the fibers across a single opening crack. These measures were correlated to SEM observations of the microstructure of fiber/mortar interface, depending on the compaction energy and/or the mortar composition. It is relatively difficult to establish a compromise between ductility and high performance in terms of durability for the material system tested. Indeed, fibers were pulled out of low compactness mortars exhibiting large porous interfacial transition zones (ITZs) along the fiber surface. These zones mainly comprised fibrillous CSH, ettringite and large portlandite crystals. Conversely, when the ITZs around the fibers where filled with compact CSH, resulting from the pozzolanic reaction between silica fume and portlandite, no fiber slippage was observed, but the reinforced mortar broke in a quasi-brittle manner.