Properties of fly ash-modified cement mortar-aggregate interfaces

This paper investigates the effect of fly ash on strength and fracture prop erties of the interfaces between the cement mortar and aggregates. The mort ars were prepared at a water-to-binder ratio of 0.3, with fly ash replaceme nts from 15 to 55%. Notched mortar beams were tested to determine the flexu ral strength, fracture toughness, and fracture energy of the plain cement a nd fly-ash modified cement mortars. Another set of notched beams with morta r-aggregate interface above the notch was tested to determine the flexural strength, fracture toughness, and fracture energy of the interface. Mortar- aggregate interface cubes were tested to determine the splitting strength o f the interface. It was found that a 15% fly ash replacement increased the interfacial bond strength and fracture toughness. Fly ash replacements at t he levels of 45 and 55% reduced the interfacial bond strength and fracture toughness at 28 days, but recovered almost all the reduction at 90 days. Fl y ash replacement at all levels studied increased the interfacial fracture energy. Fly ash contributed to the interfacial properties mainly through th e pozzolanic effect. For higher percentages of replacement, the development of interfacial bond strength initially fell behind the development of comp ressive strength. But at later ages, the former surpassed the latter. Stren gthening of the interfaces leads to higher long-term strength increases and excellent durability for high-volume fly ash concrete. (C) 2000 Elsevier S cience Ltd. All rights reserved.