PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 229

cement is hydrated lime. Adding a pozzolan to portland cement generates an
opportunity to convert this free lime to a cementitious material.
Tables 6.9 and 6.10 summarize the effects of supplementary cementi-
tious admixtures on fresh and hardened concrete. These summaries are
based on general trends and should be verified experimentally for specific
materials and construction conditions.

■ 6.11.7 Specialty Admixtures

In addition to the admixtures previously mentioned, several admixtures are
available to improve concrete quality in particular ways. The civil engineer

T ABLE 6.9 Effect of Supplementary Cementitious Admixtures on Freshly Mixed Concrete

Quality Measure Effect

Water Requirements Fly ash reduces water requirements. Silica fume increases water

requirements.

Air Content Fly ash and silica fume reduce air content, compensate by

increasing air entrainer.

Workability Fly ash, ground slag, and inert minerals generally increase

workability. Silica fume reduces workability; use superplasticizer
to compensate.

Hydration Fly ash reduces heat of hydration. Silica fume may not affect

heat, but superplasticizer used with silica fume can increase heat.

Set Time Fly ash, natural pozzolans and blast furnace slag increase set

time. Can use accelerator to compensate.

T ABLE 6.10 Effect of Supplementary Cementitious Admixtures on Hardened Concrete

Quality Measure Effect

Strength Fly ash increases ultimate strength, but reduces rate of

strength gain. Silica fume has less effect on rate of
strength gain than pozzolans.

Drying shrinkage and creep Low concentrations usually have little effect. High

concentrations of ground slag or fly ash may increase
shrinkage. Silica fume may reduce shrinkage.

Permeability and absorption Generally reduce permeability and absorption. Silica

fume is especially effective.

Alkali-aggregate reactivity Generally reduced reactivity. Extent of improvement

depends on type of admixture.

Sulfate resistance Improved due to reduced permeability.