The term supplementary cementing materials (SCMs) -sometimes called “mineral admixtures”- refers to a broad range of materials that are widely used in concrete in addition to Portland cement.
An SCM may be defined as “a material that, when used in conjunction with Portland cement, contributes to the properties of the hardened concrete through hydraulic or pozzolanic activity, or both”.
Thus, SCMs include both pozzolans and hydraulic materials.
SCMs that are commonly used in concrete today include fly ash, ground granulated blast furnace slag, silica fume, and a wide range of natural pozzolans, such as volcanic ash, calcined clay or shale, and diatomaceous earth.
Fly ash, silica fume, or slag are often mandatory in the production of high-strength concrete; the strength gain obtained with these supplementary cementing materials cannot be attained by using additional cement alone.
These supplementary cementing materials are usually added at dosage rates of 5% to 20% or higher by mass of cementing material.
Some specifications only permit use of up to 10% silica fume, unless evidence is available indicating that concrete produced with a larger dosage rate will have satisfactory strength, durability, and volume stability.
The water-to-cementing materials ratio should be adjusted so that equal workability becomes the basis of comparison between trial mixtures.
For each set of materials, there will be an optimum cement-plus-supplementary cementing materials content at which strength does not continue to increase with greater amounts and the mixture becomes too sticky to handle properly.
Supplementary Cementing Materials undergo chemical reactions in concrete, and the products of reaction are cementitious in nature; that is, the products help bind the components of the concrete together in the same manner as the reaction (or hydration) products of Portland cement.
Consequently, SCMs are considered to be part of the cementing material component of the concrete, and they should be included in the calculation of the water-to-cementing materials ratio, W/CM, of the concrete.
As such, SCMs should be differentiated from finely divided mineral fillers, such as ground limestone or quartz, which are generally chemically inert in concrete and are not considered to be part of the cementing material.
Blended cements containing fly ash, silica fume, slag, or calcined clay can be used to make high-strength concrete with or without the addition of supplementary cementing materials.
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