Abstract: The effect of composition and dosage of admixture that consist of fly ash and slag powder on workability and mechanical properties of box girder C50 concrete were studied by using fly ash/slag powder as a composite replacing part of the cement amount, and the microstructure of concrete was observed by SEM.The results show that the optimum mass ratio of fly ash to slag powder is 1∶2 in admixture.The workability of C50 concrete is improved, but the compressive strength of 3 d, 10 d, 28 d, 56 d decreased when dosage of admixture increased from 0% to 30%.It can be observed by SEM that the amount of hydration product C-S-H gel are less in the systems that admixed some of fly ash and slag powder.Then the bonding strength of interface of concrete decreased, and resulted to compressive strength reduced.

Key words: fly ash/slag powder;composite admixture;concrete;workability;mechanical property;microstructure