The cement paste microstructure has a great impact on the strength, permeability, and volumetric stability (resistance to plastic shrinkage, drying shrinkage, and creep) of high-strength concrete. Factors affecting the microstructure of a paste are mixture proportions, temperature and humidity during curing, w/cm ratio, chemical and mineral admixtures, amount of shear during mixing, and the degree of overmixing (Nawy). Fly ash and blast furnace slag, in addition to reducing the heat of hydration at early ages, tend to reduce the effects of the preceding parameters on the formation of the paste microstructure. Silica fume has an accelerating effect on early hydration by reducing the effect of lignosulonate retarders. Thus, the microstructure formation will be faster than expected when using the retarders.

Table 7.1 (Nawy) classifies high strength concrete properties.

High-strength concrete has a compact, extremely low void structure resulting from occupation of pores with pozzolanic cementitious material. Thus, it has very low permeability. This helps to resist freeze-thaw attack, chemical attack, salt penetration, and corrosion of the embedded steel.
 

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