The Standard for Portland Cement
Portland cement is used in almost every type of construction project from streets and sidewalks to skyscrapers. Revisions to C150/C150M, Specification for Portland Cement, have kept it relevant in a changing marketplace with focused attention on both sustainability and user needs.
Discussion in a joint American Association of State Highway and Transportation Officials/ASTM harmonization task group has resulted in revisions that make C150 more effective for cement manufacturers, who hope to reduce environmental impacts, and for users, including a major user group — state departments of transportation. As part of this effort, parallel changes have been adopted by AASHTO in its portland cement standard, AASHTO M85, which is used by some U.S. state departments of transportation.
One revision includes the creation of a new cement type, Type II(MH), with requirements for moderate heat generation and moderate sulfate resistance. The chemical reactions that cause concretes to gain strength also generate heat. In hot parts of construction seasons, or in massive concrete structures, this can cause excessive temperature rise in concrete, which can shorten service life. This revision provides another tool for concrete designers to use for specific applications.
For concretes exposed to soils or groundwater with moderately high sulfate contents, it might be most efficient to use Type II(MH) cements during hot parts of the construction season, and traditional Type II cements for cooler parts. Precast concrete manufacturers, which fabricate structures or parts of structures in a factory-like setting, after which they are assembled on-site, often need sulfate-resistant cements but may find that moderate heat cements can hurt production rates. For them, Type II cement is often a more efficient choice.
The manufacturing of portland cement generates carbon dioxide due to fuel used to create the high temperatures necessary to form cementitious phases, and the calcination of limestone, a primary raw material. Although significant increases in process efficiency have been realized, continued industry efforts to reduce the carbon footprint of cement production have led to changes to the standard, in 2004, to permit up to 5 percent uncalcined limestone as a cement ingredient and more recently, to establish clearly defined provisions for use of up to 5 percent inorganic processing additions. Both of these proposals were intensely studied prior to balloting to assure no loss of performance. Although other requirements of the specification will usually limit use of these ingredients to less than the maximums allowed, the reductions in CO2 emissions and energy savings are appreciable and have become an integral part of industry efforts to reduce environmental impact.
The task group’s success in developing consensus on specific technical revisions was rooted in a shared set of statements of what a cement specification should provide its users. Task group members agreed that C150 should ensure concrete performance to the extent possible in a specification for a single ingredient (cement) of a multicomponent material (concrete). The standard must provide methods to show compliance, that is, requirements should be measurable or verifiable.
The specification should also provide a consistent material and, whenever possible, use simple and reliable testing and sampling methods. Flexibility for optimizing available natural resources and manufacturing technology and accommodating user requirements is also necessary, as this supports sustainability goals and provides utility for users.
Finally, the standard should help ensure communication between buyers and sellers, which is a primary goal of standards — to facilitate commerce. Improvements to ASTM C150 based on these principles have resulted in an increase in usefulness to users and producers and the ability to provide more environmentally sustainable concrete construction.
Paul Tennis, manager of cement and concrete technology for the Portland Cement Association, Skokie, Ill., is a member of Committees C01 on Cement and C09 on Concrete and Concrete Aggregates, and he is current chair of Subcommittee C01.10 on Hydraulic Cements for General Concrete Construction.