The composition and structure of the interfacial zone be-tween aggregate and portland slag cement paste as well as the orienta-tion coefficient of portlandite(I_CH) have been studied by means ofXRD and SEM. The results show that the main components in the in-terfacial zone are C-S-H gel, portlandite and AFt as well as unhy-drated clinder particles. Compared with portland cement, there aremore C-S-H gel and less portlandite in the zone and the portlanditecrystallizes badly with lower I_CH in a narrower region. Afer adding5.0% silica fume to the cement, the orientation of portlandite in thezone disappears; afer 90d, the bond strength between # 425 portlandslag cement paste and aggregate approximately attains that between #525 portland ciment paste and aggregate.
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