Abstract: To investigate the effect of adding silica fume (SF) and fly ash (FA) on the strength and shrinkage performance of cement slurries at various curing ages, five different composite cement slurries (water-binder ratio of 0.29) with different content of SF and FA were designed.The hydration heat release characteristics and pore structure composition of different composite cement slurries were characterized by calorimeter and mercury porosimeter tests, and the changes of hydration heat release and porosity with the increase of SF and FA content were analyzed. The relationships between compressive strength and pore structure, hydration characteristic and shrinkage strain were also established. Results show that FA greatly reduces the early compressive strength of cement pastes, while it is beneficial to reduce the autogenous shrinkage strain and dry shrinkage strain at the same time. The 3 d compressive strength of the cement pastes can be improved obviously by adding SF. However, the autogenous shrinkage strain at 3 d and dry shrinkage strain at 28 d of cement pastes increase significantly once SF content exceeds 10% (mass fraction). The addition of SF advances the start and end time of the induction period of cement hydration, and increases the hydration reaction order and the constant value of reaction rate at each stage, which leads to a significant increase of total hydration heat release amount and heat release rate of cement-silica fume in comparison with cement-fly ash system. SF and FA not only effectively improve the internal pore structure of cement slurries, but also increase the proportion of gel pores and reduce the proportion of large pores. It is found that the total hydration heat release amount of composite slurries at 72 h has positively correlation with 3 d autogenous shrinkage strain, and porosity has negatively correlation with compressive strength of composite slurries.

Key words: silica fume, fly ash, strength, shrinkage performance, hydration heat, pore structure