Refresh your knowledge of current cements used in the industry and learn about the new ASTM blended cement types, how they are specified, and updated permissible percentages for slag, limestone and pozzolans
Cement is our bread and butter. We know everything about the cement we use in our precast production plants and how hydration occurs. We can easily recite all the current cement types. But are you aware of the new blended cement types and allowable component percentages from ASTM? In this article you will find a short “Cement 101” refresher as well as what you need to know about ASTM’s significant changes for blended cements.
Cement 101 – a quick refresher
Cement, as defined by the American Concrete Institute (ACI), is “Hydraulic cement – a binding material that sets and hardens by chemical reaction with water and is capable of doing so underwater.” It further defines portland cement as “a hydraulic cement produced by pulverizing clinker formed by heating a mixture, usually of limestone and clay, to 1400 to 1600 C (2550 to 2900 F). Calcium sulfate is usually ground with the clinker to control set.” In the United States, precast concrete producers use cement that falls under these definitions, and most of these cements will conform to ASTM C150.1
Cement is the binding material that we use to produce concrete. The chemical reaction, called hydration, starts when water comes into contact with the cement. The reaction takes place at the surface of the cement particle where a fibrous growth takes place that connects the individual cement grains and also binds the aggregate at the same time. Most of this process takes place in the first 30 days, but it will continue as long as unreacted cement and water are available and there is space for the hydration product.
ASTM has three standards that address cement and cement types. ASTM C150 is the most common cement used and is familiar to most of us in the precast industry. The 10 current cement types listed in ASTM C150 are:
1. Type I – For use when the special properties specified for any other type are not required
2. Type IA – Air-entraining cement for the same uses as Type I, where air entrainment is desired
3. Type II – For general use, more especially when moderate sulfate resistance is desired
4. Type IIA – Air-entraining cement for the same uses as Type II, where air entrainment is desired
5. Type II(MH) – For general use, more especially when moderate heat of hydration is desired
6. Type II(MH)A – Air-entraining cement for the same uses as Type II(MH), where air-entrainment is desired
7. Type III – For use when high early strength is desired
8. Type IIIA – Air-entraining cement for the same use as Type III, where air entrainment is desired
9. Type IV – For use when a low heat of hydration is desired
10. Type V – For use when high sulfate resistance is desired
Some cements are noted with dual designations such as Types I/II or II/V, indicating that the cement will meet either type.
In most areas of the United States, Types I, II and III are the most commonly used. Certain areas, particularly for sulfur-rich California soils, may require Type V cement. With the general availability of air-entraining admixtures that can fine-tune air content, air-entraining cements are seldom, if ever, used today, and Type IV for mass concrete pours is rarely used. In instances where a Type IV cement may be required, fly ash and/or slag will be used in the mix with a Type I or II cement. These classifications are based solely on standard chemical and physical properties unless optional requirements are specified. There are no performance tests required to differentiate the durability properties of each cement type. For example, there are no tests required to show that Type V cement is, in fact, highly sulfate-resistant.
Readily available in North America, white portland cement is often used in architectural precast for its white color and to facilitate use of integral colors. Basically, white cement has the same properties as gray cement, except for color. Color depends on the raw materials used and the manufacturing process. Metal oxides (primarily iron and manganese) influence the color of the cement. White cement is manufactured to conform to ASTM C150 and CSA specifications. While Types I, II, III and V white cements are available, Types I and III are the most often used.
New ASTM blended cement designations
Blended cements governed by ASTM C595 pertain to four classes for both general use and special applications. Significant changes have recently been approved for this new 2012 specification. These changes will permit the interblending of naturally occurring limestone – both independently and in conjunction with pozzolan or slag, up to a maximum of 15%. There will now be four major classifications:
1. Type IS – Portland blast-furnace slag cement – up to 95% slag permitted
2. Type IP – Portland-pozzolan cement – up to 40% pozzolan permitted
3. Type IL – Portland-limestone cement – up to 15% limestone permitted
4. Type IT – Ternary blended cement3 – up to 70% of pozzolan + limestone + slag, with pozzolan being no more than 40% and limestone no more than 15%
How to name the new cement blends
There is an additional ASTM requirement to identify the type and amount of each component blended with the clinker. Thus, Type IP(15) designates a cement with 15% pozzolan and Type IS(25) is a cement with 25% slag. With the ternary blended cements, this requirement includes the addition of a letter designation for the component. Accordingly, a Type IT(S20)(P10) would be a cement with 20% slag and 10% pozzolan, with the larger percentage listed first. Where a component like pozzolan, for example, comes from two different sources, it would be identified as: Type IT(20P)(15P) and contain 20% from one pozzolan source and 15% of the other.
If air entrainment is desired, the letter A is added, so type IP(20)A would be a portland-pozzolan air-entrained cement. The letters MS will be added for moderately sulfate-resistant and MH for moderate heat of hydration. The letters HS designate highly sulfate-resistant cement, and LH stands for low heat of hydration. Thus, Type IT(S25)(P10)(MH)A is an air-entraining cement with a moderate heat of hydration containing 25% slag and 10% pozzolan. While these new designations can be quite lengthy, they are very descriptive and, as such, advantageous to the user. Somewhat simplified designations are permitted and will be encountered as described in ASTM C595. At the present time, limestone in excess of 5% is not permitted in MS and HS designations, pending results of further research.
As with the ASTM C150 cements, the blended cements are required to meet standard chemical and physical requirements. There are also chemical and physical requirements for the pozzolans, limestone and slag used to manufacture the blended cements. Blended cements are currently used in a small percentage of concrete, but this may change significantly in the future with the increase in green construction requirements.
ASTM performance specification
ASTM C11574, along with Canadian Standards Association’s CSA A30015, govern the following six classes of cement:
1. Type GU – Hydraulic cement for general construction, used when one or more of the special types are not required
2. Type HE – High early strength
3. Type MS – Moderate sulfate resistance
4. Type HS – High sulfate resistance
5. Type MH – Moderate heat of hydration
6. Type LH – Low heat of hydration
There is an additional option R for low-reactivity cements when used with alkali-reactive aggregates. ASTM C1157 cements have standard physical requirements but no standard chemical requirements. These cements are tested to verify that they meet the requirements for each type.
Source of material makes a big difference
For the precast concrete producer, the three main concerns with the finished product are required compressive strength, durability and appearance. Of these three, compressive strength is by far the easiest to measure and control. A subset of compressive strength is stripping strength, or the minimum compressive strength required to remove the concrete from the form. While cements will be of a certain ASTM type, actual concrete performance can vary greatly depending on the source of cement used. Some sources may provide very good stripping strength while the final compressive strength may not be as good as cements with a lower stripping strength. For such cases, the details of stripping time, temperature and preset time (for accelerated curing) must be well understood in order to make an informed decision about cement selection.
Again, depending on the source, if a supplementary cementitious material is added to the mix, the reaction with the cement can vary, and the greatest variation may be found in early strength development for stripping strength.
Without going through a detailed list of variables that can change relative to the cement source or type, it is safe to say that it is always a good practice to run trial tests to verify that you will obtain the performance you are expecting. Even within the same type, different cement sources do not perform exactly the same, but this does not mean that one cement is better than another. For example, if a particular cement results in lower stripping strength, this does not mean that the cement is “bad,” but simply that it is not ideal for your specific application.
Additional sources of information about cement are ACI 225R-99, “Guide to the Selection and Use of Hydraulic Cements,” and PCA’s “Design and Control of Concrete Mixtures.”
Terry Harris is manager of Technical Services, North America with Grace Construction Products, Cambridge, Mass.
1 Standard Specification for Portland Cement
2 Standard Specification for Blended Hydraulic Cements
3 “Ternary” means comprised of three parts. Ternary blended cements are multi-component cements made by blending portland cement (clinker + gypsum) with two complementary cementitous materials such as fly ash, slag, silica fume or pozzolans.
4 Standard Performance Specification for Hydraulic Cement
5 Cementitious Materials for Use in Concrete (MHb-LHb Blended Hydraulic Cement)