What Are The Different Types Of Modified Portland Cements?

Increasingly, modern concretes contain a blend of Portland cement and other cementitious materials. When other materials are added to Portland cement at the time at which the concrete is batched, they are referred to as mineral admixtures; however, there are also hydraulic cements, which are produced either by forming other compounds during the burning process or by adding other materials to the clinker and then intergrinding them.

The common types of such modified cements are described in the following sections.

Portland Pozzolan Cements
Portland pozzolan cements are blends of Portland cement and a pozzolanic material. The role of the pozzolan is to react slowly with the calcium hydroxide that is liberated during cement hydration.

This tends to reduce the heat of hydration and the early strength but can increase the ultimate strength of the material. These cements tend to be more resistant to sulfate attack and to the alkali–aggregate reaction.

Portland Blast-Furnace Slag Cements
Ground granulated blast-furnace slag (GGBFS), which is a byproduct of the iron and steel industry, is composed largely of lime, silica, and alumina and thus is a potentially cementitious material. To hydrate it, however, it must be activated by the addition of other compounds.

When the GGBFS is to be activated by lime, the lime is most easily supplied by the hydration of the Portland cement itself. Slags may be present in proportions ranging from 25 to 90%. They react slowly to form C–S–H, which is the same product that results from the hydration of the calcium silicates.

In general, because they react more slowly than Portland cement, slag cements have both lower heats of hydration and lower rates of strength gain.

On the other hand, they have an enhanced resistance to sulfate attack. When the GGBFS is to be activated with calcium sulfate (CaSO4), together with a small amount of lime or Portland cement, the material is known as supersulfated cement.

This cement is available mostly in Europe, where it is used for its lower heat of hydration and its resistance to sulfate attack.

Expansive Cements
Expansive cements were developed to try to offset the drying shrinkage that concrete undergoes. This is particularly important when the concrete is restrained against contraction or when it is to be cast against mature concrete in repair situations.

In both cases, severe cracking may occur as a result of the shrinkage. Expansive cements are based on the formation of large quantities of ettringite during the first few days of hydration; however, they are little used today, in large part because it is very difficult to control (or predict) the amount of expansion that will take place for a particular concrete formulation.

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