Increase of bond strength at interfacial transition zone by the use of fly ash

The effects of the addition of silica powder, silica fume, fly ash, and hemihydrated gypsum to the interface of new-to-old paste on the enhancement of bond strength were investigated experimentally. Seven-day bond strengths for the specimens with pozzolanic materials added to the interfacial zone, except for silica powder addition, were slightly smaller than that for control without any additives, whereas the strength of all specimens with added pozzolanic materials were higher than that of the control at 28 days. It was suggested that increased effect of the bond strength at interfacial zone depended on the SiO2 and CaO contents in the additives; higher SiO2 and/or lower CaO contents were preferred. On the other hand, when a high CaO content fly ash was coated to the interface with some amount of hemihydrated gypsum, the bond strength enhanced significantly at 7 days, as well as at 28 days. This result suggests that the structure of the interfacial zone can be modified sufficiently by controlling the chemical components of the additives; even the use of fly ashes consisted of relatively high CaO and low SiO2. (C) 2000 Elsevier Science Ltd. All rights reserved.