Green building is a measurable and cost-effective solution to the complex challenges of climate change. The building sector accounts for 38 percent of carbon dioxide (CO2) emissions in the U.S. per year, and therefore represents a significant portion of the greenhouse gas emissions that affect climate change.
Supplementary cementitious materials (SCMs) and blended cements offer sustainable and performance advantages for those who build and occupy structures of all kinds. Their use results not only in more durable, high-performance concrete but also in lower energy consumption and greenhouse gas emissions. In fact, for every ton of clinker (a product of the cement kiln process of making Portland cement) replaced, CO2 emissions are reduced by approximately 0.8 ton. Since SCMs are used as a substitute for clinker, they reduce the amount the material used to produce cement and also the fuel required to produce the clinker.
The most common SCMs are slag cement (a by-product of the iron-manufacturing process), fly ash (a coal combustion by-product from power plants), and silica fume (a by-product of manufacturing silicon metals and ferro-silicon alloys). SCMs can be utilized in concrete mixes either as a separate component or as a constituent of blended cement. Cementitious blends have many properties that contribute to sustainable design—their use can result in stronger, longer-lasting concrete and reduced consumption of nonrenewable raw materials and emission of greenhouse gases, in addition to turning by-products from other industries into resources that would otherwise be disposed of in landfills.
Considering that concrete is the most widely used construction material, cementitious blends can have a major impact on the environment and make a significant contribution to achieving sustainable building goals, such as those prescribed by the LEED green building rating system. SCMs contribute to LEED credits in the following categories: Sustainable Sites, Energy and Atmosphere, Materials and Resources, Indoor Environmental Quality, and Innovation in Design.
SCMs impart a wide range of exceptional properties to concrete, which are particularly sought after by construction professionals. While the advantages of concrete in its plastic state are quite numerous (workability, finishability, etc.), perhaps the greatest benefits of SCMs can be seen in the hardened properties.
Strength: SCMs contribute to the strength gain of concrete. Typically, slag cement and fly ash will lower early strengths (one to 14 days) but will significantly improve long-term strengths (28 day and beyond), depending upon the proportions and materials used . For example, Class F fly ashes tend to have a slow strength-gain curve contributing mainly to the strength beyond 28 days, whereas silica fume contributes primarily to the three-to-28-day strengths. Both compressive and flexural strengths can increase markedly at 28 days and beyond with the addition of SCMs.
Permeability: SCMs can significantly extend the life of concrete by reducing the permeability of concrete to the ingress of chlorides and other aggressive agents, especially at later ages. Silica fume has a very profound effect, exhibiting as much as a five-fold reduction in permeability.
Alkali-silica reaction (ASR): SCMs can effectively prevent excessive expansion and cracking of concrete due to ASR. The amount of slag cement required depends on the nature of the slag cement, the reactivity of the aggregate, and the alkali loading of the concrete. In most cases, 50 percent slag cement is sufficient with highly reactive aggregates. The amount of fly ash required typically is in the range of 15 to 55 percent, depending on the chemical composition of the ash, reactivity of the aggregate, and the alkali loading of the concrete. Generally, Class F ashes are much more effective in controlling expansion due to ASR than Class C ashes. Silica fume can control ASR; however, the amount required generally results in poor constructability. Blends of slag cement and silica fume, as well as blends of fly ash and silica fume, have a synergistic effect in mitigating expansion due to ASR, while producing a very workable concrete.
Sulfate attack: Concrete containing SCMs generally offers superior resistance to sulfate attack; they lower the permeability, thus restricting the ingress of sulfate-bearing ions. In a number of cases, they also reduce the compounds that can react with sulfates to form deleterious compounds. Typically, slag cement, silica fume, and Class F fly ashes are very effective in improving sulfate resistance; the effectiveness of Class C fly ashes is very dependent on the ash chemistry and the replacement level, however.
Thermal stress: If the temperature differential between the concrete’s surface and its interior is too high, cracking and loss of structural integrity can result. Utilizing high replacement levels of slag cement and/or fly ash in properly proportioned mixes can reduce the peak temperatures as well as the rate of heat generation. Reducing the heat of hydration of the mix can moderate the development of thermal stresses within the concrete and prevent cracking.
NEXT GENERATION CEMENTS
While the performance benefits of SCMs are quite numerous, their effect on the trend toward ever more green construction practices cannot be overstated. In most cases, blended cements can be substituted on a one-to-one basis for Portland cement. Various organizations, including the American Concrete Institute and the Slag Cement Association, offer detailed recommendations that specifiers can consult to determine whether and how to specify such substitutions.
The environmental movement in general, and the LEED program in particular, will continue to provide a strong incentive for developing and specifying even more innovative, sustainable building solutions. One proven method to reduce greenhouse gases per ton of cement is to use interground limestone. Widely used in Europe for a number of years, Portland limestone cement (PLC)—produced by intergrinding Portland cement clinker and limestone in quantities greater than 5 percent—represents one such promising approach for the North American market. In Canada, up to 5 percent limestone has been permitted in Portland cement since 1983. ASTM allowed the use of the same amount of limestone in Portland cement in 2004, with AASHTO following suit in 2007. These changes will ultimately reduce energy consumption by 11.8 trillion Btu and carbon dioxide emissions by more than 2.5 million tons per year.
In response to growing pressures to reduce the clinker content in cement, the Canadian Standards Association (CSA) introduced a new classification of cement in 2008, which is Portland limestone cement containing up to 15 percent limestone. Based on a number of trials and considerable testing, PLC with up to 15 percent limestone can produce equivalent performance to Portland cement in concrete, including strength, durability as well as other properties. Such an increase in limestone content typically reduces clinker content by an additional 10 percent. When combined with 40 to 50 percent SCM, the effective reduction in clinker content is greater than 50 percent.
Finding new ways to further reduce energy needs and CO2 emissions is a top priority for cement companies, and solutions are continually being developed that minimize the amount of clinker while maximizing the use of alternative materials. With this next-generation class of cement, customers can use the same amounts of SCMs in their mix while also replacing up to 15 percent of the Portland cement with inter-ground limestone. Manufacturers can provide technical assistance to help develop or modify specifications, and most can provide detailed test results, quality control records and additional support to specifiers. We have a responsibility to reduce the carbon footprint of the building sector.
- Bruce Blair is vice president of product performance and marketing at Lafarge Cement. He has over 30 years of experience in the concrete construction industry and is a member of various committees within key standards organizations, including ASTM, ACI, CSA, PCA, and SCA.