Blended Cement
As lower cost alternatives to Portland Cement, blended cements are increasingly being used in a great many construction applications. Blended cements are made by inter grinding Portland cement with another material. Fly ash, blast furnace slag, and condensed silica fume are all common components of blended cements, and each of them results in a different type of blended cement with an unique set of properties.
Blended cements are not a new idea. As far back as Roman times, pozzolanic materials such as volcanic ash were being added to cements in order to create long lasting strong concretes. To see the evidence of how long wearing these cements were, one merely needs to visit Rome and see some ancient cements still standing their ground against time and the elements.
Modern blended cements have been slow to gain popularity for a variety of reasons. For a long time there were no real standards for blended cements, so it was hard to source a reliable product. In many cases the additives which were blended with Portland cement to create blended cements actually weakened the end product and made it less reliable due to unforeseen alkali aggregate reactions.
More recently however, blended cements have come into their own as increasing care has been taken to develop high quality, high performance blended cements which far outperform traditional cements at a far lower price.
Why Blended Cements?
Portland cement is the most widely used cement on the globe, and there are several types of Portland cement which can successfully be used in almost any construction application, so why use blended cements at all?
Here are a few reasons why blended cements are being both developed and used more and more frequently.
Blended cements:
• Are cheaper than Portland cement.
• Are more long lasting and durable than Portland cement.
• Have higher strength tolerances than Portland cement.
• Have less drying shrinkage than Portland cement.
• Can, unlike Portland cement, be made to the buyer’s specifications.
• Are more environmentally friendly than Portland cement, as they are comprised of recycled materials.
Blended cements have been developed to both lower the cost of cement, and also to combat problems which are traditionally encountered with OPC (Ordinary Portland Cement) such as susceptibility to sulfates, cracking, steel corrosion, and susceptibility to freezing failure and carbonation.
Warning: As will be discussed in the section on blended cement standards, there are a myriad of ways in which cements are classified and standardized. There are relatively few blended cements available on the market which meet the specifications of the most recent and reliable standard, ASTM C 1157, but it is recommended that purchasers seek out cements which do meet this standard, as those which do not may be entirely unsatisfactory.
Blended Cement Standards
There are a plethora of cement standards around today, and most of them are wildly confusing to those who are not experts in the field. With three or four different standards, and several subtypes within each standard, it is easy to become quickly confused.
However, finding good quality blended cement is not as difficult as it might seem at first, due to the inception of a new standard specifically designed for blended cements. This is the ASTM C 1157 Standard, and it is recommended that buyers wishing to purchase blended cements check for compliance with this standard first before going any further with their investigations.
Unlike previous standards, which were based on the composition of the cement, ASTM C 1157 is based on performance, making it more reliable and easy to understand than previous standards. There are only two major subtypes within this standard, blended cements which meet the ASTM C 1157 standard, and blended cements which meet the standard and also will not react with alkali-reactive aggregates. These types of cement will be marked with [R].
Various types of cement under the ASTM C 1157 standard are defined by the compressive strength which they attain at the standard one, three, seven and twenty eight day marks.
Fly Ash Pozzolanic Blended Cements
Blended cements made with fly ash are pozzolanic cements. Pozzolanic materials are silica or alumina based materials which are not cements on their own, but which react with calcium hydroxide at room temperature to form cementious compounds. A relatively popular component of blended cements, fly ash in the form of coal ash is often used as an additive in blended cement production, especially in China and the United States.
Blast Furnace Slag Blended Cements
Blast furnace slag is a material which is high in silicates. When silicates are combined with Portland cement they improve the workability of concrete, and create a product which has low permeability, and does not allow alkali aggregate reactions to take place.
Condensed Silica Fume Blended Cements
A waste product from silicone production, condensed silica fume is perhaps the best additive of the three substances commonly used to create blended cements. Silica fume blended cements are high performance cements which have very little permeability and very high strength. Because silica fume is so effective at increasing the performance of cement, small amounts of silica fume may also be added to other types of blended cement as a secondary additive to develop the overall quality of the cement.
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