Modified Portland Cements | Common Types Of Modified Portland Cements

 

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, which are described in detail in feature posts; however, there is also hydraulic cement, which is 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 cement are described in the following sections.

 

Portland Pozzolan Cements

Portland pozzolana cement is a blend 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 cement 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 by-product 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 cement has both lower heats of hydration and lower rates of strength gain. On the other hand, they have enhanced resistance to sulfate attacks. 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 super sulfated 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 Cement

Expansive cement was 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 cement is 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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