Influence of silica flour–silica fume combination on the properties of high performance cementitious mixtures at ambient temperature curing

• Silica flour–Silica fume combinations improved the early age compressive strength.
• Increase in fineness of silica flour improved the early-age strength of HPCM.
• Combinations of silica fume and silica flour reduced permeability of HPCM.
• Higher fineness and dosage of silica flour increased drying shrinkage of HPCM.
• RCP and permeable voids of HPCMs showed a good correlation.

The compressive strength, chloride ion permeability, bulk density, volume of permeable voids, and drying shrinkage of high performance cementitious mixtures (HPCM) containing both silica fume (SFU) and silica flour (SFL) were studied under ambient-temperature curing. SFU was used as cementitious material in addition to cement; SFL was used as filler material replacing part of the fine aggregate. The use of SFL in combination with SFU increased the early-age compressive strength of HPCM, but decreased the later-age compressive strengths. Based on the results of both thermo-gravimetric analysis and loss on ignition tests, this phenomenon was attributed to the improved hydration of cementitious materials by SFL at early ages, but inhibited continued hydration at later ages. The addition of SFL combined with SFU resulted in further reduction of chloride ion permeability and the volume of permeable voids. The addition of SFL slightly increased the drying shrinkage. Among SFLs with different fineness, the finest SFL which densified the microstructure of HPCM most resulted in the highest drying shrinkage. For HPCM cured under ambient-temperature the use of SFU alone will suffice when used at sufficient dosages; and the addition of SFL along with SFU presents advantages mostly in improving the early-age compressive strengths.