Blended Cements Extend the Life Span of Concrete

Concrete mixes made of portland cement are porous and are susceptible to cracking especially during freeze/thaw cycles.  They can even be the cause of deterioration of rebar as moisture may seep in over time.  To reduce concrete’s permeability to water and prolong its life span, blended cements are used and offer an advantage of reducing the water-cement ratio (w/c), which one method of producing high-strength concrete. 

Blended cement as defined by the American Concrete Institute’s (ACI) Cement and Concrete Terminology is “hydraulic cement that is produced by intergrinding portland cement clinker with the other materials or by blending portland cement with the other materials or a combination of intergrinding and blending.”  Changes were made to blended cements in 2012 for special applications as well as for general use that have the approval of the ASTM.  

The ASTM’s C595 abstract on hydraulic blended cements describes “Type IS” as Portland Blast-furnace Slag cement and “Type IP” as Portland-pozzolan cement.  The C595 abstract states that, “They can also be described according to air-entraining, moderate sulfate resistance, moderate heat of hydration, high sulfate resistance, or low heat of hydration properties.”  Type I (SM) Slag-modified Portland cement has a standard that contains less than 25% slag by mass of the cement.  With the ASTM’s C595 change, it now permits Type IS Portland Blast-furnace Slag cement to contain between 25 and 70% by mass of the finished cement. 

 

By varying the proportions of the mix, blended cements are customized and produced to meet the demands of specific projects.  Furthermore, the ASTM goes on to say that, “cement shall undergo different tests to determine the following properties: chemical analysis, fineness by sieving, fineness by air-permeability, autoclave expansion, time of setting, air content of mortar, compressive strength, heat of hydration, normal consistency, specific gravity, water requirement, mortar expansions of blended cement and pozzolan, drying shrinkage, activity index with Portland cement, and sulfate resistance.”

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