Blended Cement

Blended cement is obtained by mixing OPC with mineral admixtures or additives like fly ash, slag or silica fumes. Blended cements are now being considered superior as compared to conventional OPC category of cements.

Blended Hydraulic Cements

When cement manufacturers intergrind or blend portland cement with fly ash or natural pozzolans or slag cement, or create a ternary combination of SCMs, the blended cement is specified under ASTM C595 (AASHTO M 240), Standard Specification for Blended Hydraulic Cements. These materials are classified as follows:

Type IP(X) to indicate a portland-pozzolan (P) cement in which "X" denotes the targeted percentage of pozzolan expressed as a whole number by mass of the final blended cement. Thus, Type IP(15) is cement that contains 15 percent pozzolan.
Type IS(X) to indicate a portland-slag (S) cement in which "X" denotes the targeted percentage of slag cement expressed as a whole number by mass of the final blended cement. Thus, Type IS(25) is cement that contains 25 percent slag cement.
Type IT(AX)(BY) to indicate ternary (T) blended cement in which the "A" refers to the type of SCM (either "P" for pozzolan or "S" for slag cement) that is present in the larger amount by mass and the "B" refers to the SCM (again, either "P" for pozzolan or "S" for slag cement) that is present in the lesser amount. The "X" and "Y" refer to targeted percentage of mass for constituent "A" and "B" respectively. For example, a material designated as Type IT(S25)(P15) contains 60 percent portland cement, 25 percent slag cement, and 15 percent pozzolan. If the percentages of the SCMs are the same, the pozzolan material is listed first, i.e., Type IT(P15)(S15). Two different pozzolans can also be blended together to create a Type IT(PX)(PY).
Typical replacement rates for blended cements are 15 to 25 percent for Type IP and 30 to 50 percent for Type IS. A Type IT might have 15 to 30 percent slag cement and 10 to 20 percent pozzolan, although these can vary significantly depending on the characteristics of the specific SCMs.

In addition to the above designations, blended cements can be further labeled with the following suffixes:

"A" to indicate air-entrained material.
"MS" or "HS" to indicate moderate or high sulfate resistance.
"MH" or "LH" to indicate moderate or low heat of hydration.
Although it has been a more common practice for the concrete supplier to blend cement with SCMs at the concrete plant, when the SCMs are interground or blended by the cement supplier under ASTM C595 (or AASHTO M 240), there is a greater level of quality control over the final product with less potential for unforeseen interactions and incompatibilities (Taylor et al. 2006). In addition, the use of ASTM C595 (AASHTO M 240) blended cements helps avoid the potential for proportioning mistakes that can occur in the field. However, this does limit the concrete supplier's ability to adjust the SCM content in response to changing conditions (e.g., cooler weather).

         *It reduces water demand and therefore water-cement ratio can be reduced.
          *It improves workability for the same water content.
           *The blended cements are finer as compared to OPC, therefore the permeability of concrete is less. This results into improved durability.