Tag Archives: Portland cement

Designing with Activated Fly-ash Cement Concretes

Photos courtesy CeraTech USA

Photos courtesy CeraTech USA

by Ivan Diaz-Loya, PhD

With evidence of climate change bringing awareness to greenhouse gas (GHG) emissions, the sticking point for concrete construction is the amount of embodied carbon in the material. Most of this embodied carbon comes from production of portland cement—the fine powder that hydrates and ‘glues’ sand and stone together to produce concrete.

Approximately 600 kg (1322 lb) of carbon dioxide (CO2) is released into the atmosphere for every ton of cement produced.1 This may not sound like much, but when the expected global cement output is 3.4 billion tons for 2013, it means roughly 2 billion tons of CO2—five percent of all manmade GHG emissions—will be released to the atmosphere as part of the cement production process.2

To bring this to the specifier’s perspective, when a cubic yard of portland cement concrete is specified to have a cement content of 249 kg (550 lb), that cubic yard of concrete already has around 149 kg (330 lb) of embodied carbon generated from cement production alone.

This figure shows typical shrinkage values of activated fly ash cement concrete tested per ASTM C157, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, at 50 percent relative humidity (RH).

This figure shows typical shrinkage values of activated fly ash cement concrete tested per ASTM C157, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, at 50 percent relative humidity (RH).

Concrete carbonation
Part of these emissions is recaptured throughout the concrete’s life when hydrated portland cement reacts with the CO2 in the atmosphere to form calcium carbonate—a process called carbonation. However, it is uncertain how much CO2 is reabsorbed into the concrete. Washington State University’s associate professor Liv Haslebach states in the article, “Concrete as a Carbon Sink,” published on Portland Cement Association’s (PCA’s) website, that although there have been numerous studies, no one has been able to put an accurate number to the amount CO2 recaptured though carbonation.3

A 2007 study by Claus Pade and Maria Guimares, “The CO2 Uptake of Concrete in a 100-year Perspective”, estimates the amount of CO2 recaptured through carbonation is between 33 to 57 percent over a century.4 Whatever the actual number, it only stands to reason by the time the hydrated cement in concrete fully carbonates, the structure will have reached the end of its service life, and another structure will be built using more portland cement concrete to replace the old one. Therefore, even in a perfect scenario where 100 percent of the CO2 is recaptured through carbonation and no new growth occurs, a revolving amount of CO2 is being maintained in the atmosphere as old structures decay and new ones are built.5

This graph shows moisture vapor emission rates (MVERs) of portland cement and activated fly-ash cement concrete tested per ASTMF 1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.

This graph shows moisture vapor emission rates (MVERs) of portland cement and activated fly-ash cement concrete tested per ASTMF 1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.

Alternative binder systems
PCA calls portland cement the “building block of the nation’s construction industry.” While portland cement will likely continue to be used for generations, there are alternative binder systems available for the production of concrete. One of these alternatives is fly ash—the fine powder filtered out from the flue gases resulting from the combustion of coal. Fly ash is typically rich in silica, alumina, and calcium oxide (CaO) in many cases. During the power production processes, fly ash moves rapidly from combustion to room temperature, forming tiny spheres composed mainly of reactive glass; the material has an appearance and fineness similar to portland cement.

This image shows placement of activated fly-ash cement concrete at the Port of Savannah—an application where concrete will be subjected to abrasion and loads of 55,791 kg (123,000 lb) from overhead mobile gantry cranes.

This image shows placement of activated fly-ash cement concrete at the Port of Savannah—an application where concrete will be subjected to abrasion and loads of 55,791 kg (123,000 lb) from overhead mobile gantry cranes.

Fly ash can be easily activated using a small amount of alkalis in the form of hydroxides or pH-neutral hydro-carboxylic acid salts to form a strong and durable binder that can replace portland cement in the production of concrete. Although fly ash has long been used to partially replace portland cement (sometimes up to 30 percent), total replacement of portland cement virtually eliminates all greenhouse gas emissions associated with the production of the cementitious binder in concrete.

Fly ash is typically produced from the combustion of coal to produce electricity, less than 35 percent of which is repurposed for beneficial use. The “2011 Coal Combustion Product (CCP) Production and Use Survey Report,” released by the American Coal Ash Association (ACAA), recorded only 23 million tons of fly ash were reused out of the 60 million tons produced in 2011.6 As such, approximately 37 million tons of fly ash could have been employed to produce activated fly-ash cement concrete instead of being sent to landfills. While some of this fly ash may have been lower quality, much of it could have been blended with high-quality material rather than disposed of.

Benefits of activated fly-ash cement
Activated fly-ash cement concrete not only offers environmental benefits, but also performance benefits, including:

  • improved volume stability;
  • mix design flexibility;
  • high early strengths;
  • low water vapor transmission at early ages; and
  • resistance to corrosive and high temperature environments.

Mix design flexibility
Concrete mixtures using activated fly-ash cement are proportioned in the same way as their traditional counterparts; in many cases, they even follow the same guidelines given by the American Concrete Institute (ACI). After all, it is still concrete with only the powder being different. The main difference is the addition of the activating liquid; at typically less than 24,815 mL/m3 (5 gal/yd3) of concrete, it is less than two percent of the total volume of the concrete mixture.

The type of activator may vary, but sodium hydroxide and sodium silicate are typically used in combination to activate low or moderately low calcium fly ash (i.e. <15 percent CaO). Hydro-carboxylic acid salts, which have a pH close to neutral, can be used for fly ash with higher calcium contents. Since portland cement mixtures can have a pH higher than 13, concrete practitioners are already accustomed to the level of alkalinity that fresh sodium hydroxide/sodium silicate-activated fly-ash mixtures can exhibit.

This graph shows the weight loss of portland cement and activated fly ash cement concretes exposed to different acids, tested per ASTM C267, Standard Test Methods for Chemical Resistance of Mortars, Grouts, and Monolithic Surfacings and Polymer Concretes.  Image courtesy American Concrete Institute’s (ACI’s) May 2013 Concrete International

This graph shows the weight loss of portland cement and activated fly ash cement concretes exposed to different acids, tested per ASTM C267, Standard Test Methods for Chemical Resistance of Mortars, Grouts, and Monolithic Surfacings and Polymer Concretes. Image courtesy American Concrete Institute’s (ACI’s) May 2013 Concrete International

Nevertheless, handling concentrated solutions, especially of sodium hydroxide, may be challenging to certain ready-mixed concrete operations because of its high alkalinity. Hydro-carboxylic acid salts on the other hand, are neutral by pH and produce fresh activated fly ash cement concrete with a pH of around eight. Once the concrete hardens, the pH raises to around 12, which is necessary to protect reinforcing steel from corrosion.

Hydro-carboxylic acid salts activate (or modify) high calcium fly ash by allowing the formation of a solution equilibrium, and the solubility of fractions of the fly ash that do not normally react when mixed with water alone. Then, when calcium aluminosilicate hydrates nucleate and grow, they produce a predictable set time and normal, but accelerated, strength development.

Volume stability
Another benefit of using fly-ash cement is the reduction of water necessary to achieve workable concrete mixtures compared to portland cement mixtures. This allows for fly-ash cement concretes with low water-to-cementitious (w/c) ratios, and the main reason the concrete exhibits low drying shrinkage. Drying shrinkage values of activated fly-ash cement concrete range from 100 to 300 millionths, while regular concrete values range from 400 to 800 millionths after 28 days. This important feature reduces the potential for cracking and can speed up construction times for concrete flatwork and foundations.

Vapor emission rates at early ages for floorcoverings
For floorcoverings and coatings applied over concrete, activated fly-ash cement offers a tremendous advantage given its inherent rapid drying. When flooring materials are applied over concrete with high moisture vapor emission rates (MVER), moisture can condensate beneath the floor covering and dissolve the alkalis in concrete, raising the pH. These conditions can break down the adhesive and cause failures in the flooring such as blistering and delamination. Months can pass from the time the concrete is placed until it has acceptable MVER levels (typically 1.45 kg/100 m2 [3 lb/1000 sf] daily) to apply a low MVER floor covering. This can leave installers with the choice of meeting the construction schedule or waiting for the concrete to dry. Activated high-calcium fly-ash cement concretes do not require wet curing and dry quickly, allowing flooring systems with the most stringent placement requirements to be placed in less than three weeks.

The picture shows the placement of activated fly-ash cement concrete for a high-temperature application at a steel fabrication business in Bakersfield, California. Photos courtesy CeraTech USA

The picture shows the placement of activated fly-ash cement concrete for a high-temperature application at a steel fabrication business in Bakersfield, California. Photos courtesy CeraTech USA

Resistance to corrosive acids
Activated fly-ash cement concrete also offers high chemical resistance, mainly due to a combination of two factors:

  • its low w/c ratio, which can be 40 to 50 percent less than typical portland cement mixtures, resulting in a denser concrete (approximately 80 kg/m3 [5 lb/y3] more than portland cement concrete) that restricts the ingress of destructive agencies; and
  • calcium in activated fly-ash cements is combined in a less soluble form than in portland cements.

Portland cement concretes contain a high proportion of calcium hydroxide, which is highly soluble in most acids. This causes the material to quickly decompose. Conversely, activated fly-ash cement concretes have no excess calcium hydroxide, making them more resistant to corrosive acids.

Resistance to heat and temperature changes
The reaction products of activated fly-ash cements decompose at higher temperatures than those of portland cement, making them able to withstand such conditions. Additionally, because the overall thermal properties of concrete also depend on the characteristics of the aggregate being used, it is important how heat travels into, and through, the material.

Many heat-related failures in concrete happen not because hydration products decomposed, but rather due to temperature changes in the structure, which create a tremendous amount of thermal stresses. An example is when a heated glass jar bursts when exposed to ice-cold water. The higher density of activated fly-ash cement generates two important changes in the thermal properties of concrete: higher heat capacity and thermal conductivity.

Although low thermal conductivity is one of the most sought-after features in most construction materials, it is not as desirable in more brittle (and thus thermal stress-susceptible) materials like concrete. Activated fly-ash cement concretes absorb heat more slowly, and dissipate heat more rapidly through its concrete structure than portland cement concrete because of thermal conductivity which effectively reduces the thermal stress on the concrete. For example, an activated fly-ash cement concrete slab raises its temperature more uniformly, having a lower temperature gradient between the top and bottom of the slab. These features are not only desirable in high temperature exposure applications, but also in regular concrete flatwork applications as they minimize the potential for curling and cracking caused by daily temperature fluctuations.

Placement of a pedestal base for a nitric acid absorber using activated fly-ash cement concrete in Tuscumbia, Alabama, to alleviate corrosion damage.

Placement of a pedestal base for a nitric acid absorber using activated fly-ash cement concrete in Tuscumbia, Alabama, to alleviate corrosion damage.

Photo 4

In the past, activated fly ash cement concretes were only accomplished in highly controlled lab conditions and were impossible for field applications. While this perception still remains for many, the reality has changed. Activated fly-ash cement are now being used as a one-for-one replacement for portland cement concretes in many applications.

In 2009, Missoula Federal Credit Union (MFCU) built the first Leadership in Energy and Environmental Design (LEED) New Construction (NC) v2.2 Platinum-certified building in Montana using 100 percent fly-ash concrete (i.e. portland cement-free) as the main building material. The concrete mixture incorporated recycled glass as aggregate, and was used to build the footings, foundation walls, floor slabs, interior components, column surrounds, and roof beams. Montana State University developed the concrete mixture and worked with local contractors and architects to design and build the new branch office for MFCU. Researchers at Louisiana Tech University have also been running field tests since 2008 to create confidence in activated fly-ash cement concrete.

These ingredients can be easily introduced to existing ready-mixed concrete plants. Activating liquids can be plumbed into the plants in the same way most admixtures are currently used and given most plants already have a dedicated silo for fly ash, there is no need for setting up additional storage for it. Leading designers and specifiers have started taking notice of the capabilities and advantages activated fly ash cement concretes provide to their clients.

In July 2012, the Georgia Ports Authority used activated fly-ash cement to replace part of the lane where overhead mobile gantry cranes operate in the Port of Savannah. The Federal Highway Administration (FHWA) is now looking at the potential use of activated fly-ash cement concrete in rigid pavements and other transportation structures by signing cooperative research agreements (CRADAs) with leading producers of activated fly-ash concrete.

From start to finish, the reconstruction of a military base’s “hush house” for engine testing was recently completed with activated fly-ash cement concrete to alleviate deterioration from high, intermittent temperatures.

From start to finish, the reconstruction of a military base’s “hush house” for engine testing was recently completed with activated fly-ash cement concrete to alleviate deterioration from high, intermittent temperatures.

Photo 5

The Department of Defense (DOD) has also been using activated fly-ash cement concrete regularly to the replace concrete slabs that receive impingement heat from vertical takeoff and landing aircraft. Military bases throughout the United States, and internationally, have successfully been using activated fly-ash cement concretes for more than a decade now in the construction and rehabilitation of surface and aviation transportation infrastructure.

This graph shows typical strength development curves of concrete using high calcium fly-ash activated concrete with hydrocarboxilic acid salts as binder.  [CREDIT] Data courtesy Patel et al’s, “Green concrete using 100 per cent fly ash-based hydraulic binder,” Proceedings of the 2012 Concrete Sustainability Conference (Seattle).

This graph shows typical strength development curves of concrete using high calcium fly-ash activated concrete with hydrocarboxilic acid salts as binder. Data courtesy Patel et al’s, “Green concrete using 100 per cent fly ash-based hydraulic binder,” Proceedings of the 2012 Concrete Sustainability Conference (Seattle).

Since activated high-calcium fly-ash cement relies on its hydraulic activity to harden and gain strength, it can be considered hydraulic—this a candidate for meeting ASTM C1157, Standard Performance Specification for Hydraulic Cement. Conversely, for fly ash with a lower calcium content—typically Class F per ASTM C618, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete—ASTM Committee C01 on Cement recently formed a task group to develop standard test method for testing compressive strength of alkali-activated fly ash and natural pozzolan cement. This test method is expected to be the base for the development of future specifications.

Conclusion
The current social and cultural forces shaping the construction industry are increasingly calling for materials in addition to meeting performance requirements and being produced with environmental responsibility. With regulations looming, it is difficult to predict what will happen in five, 20, or 50 years. On the bright side for fly ash, the EPA is now considering other options, none of which label the material as hazardous waste. 7

Activated fly-ash cement offers a solution by providing a value-added product while reducing the use of portland cement and associated emissions, and recycling a coal combustion product into beneficial construction materials, reducing landfill and disposal facilities.

Notes
1 For more information, visit www.globalcement.com/news/item/1806-decoupling-carbon-emissions-from-cement-production. (back to top)
2 Visit www.nature.com/news/green-cement-concrete-solutions-1.12460. (back to top)
3 Visit www.cement.org/tech/carbon_sink.asp. (back to top)
4 For more information, see Claus Pade and Maria Guimares’ “The CO2 uptake of concrete in a 100-year perspective” from the International Symposium on Sustainability in the Cement and Concrete Industry held in Lillehammer, Norway in 2007. (back to top)
5 For more, visit www.nrmca.org/greenconcrete/concrete%20co2%20fact%20sheet%20june%202008.pdf. (back to top)
6 Visit, www.acaa-usa.org/associations/8003/files/Final2011CCPSurvey.pdf. (back to top)
7 Visit www.epa.gov/epawaste/nonhaz/industrial/special/fossil/ccr-rule/ccrfaq.htm#3 (back to top)

Ivan Diaz-Loya, PhD, joined CeraTech Inc., in 2011 as a research engineer to help develop a better understanding of activated high-calcium fly-ash cement concrete. He holds a PhD in engineering from Louisiana Tech University, where his research focused on the material. Diaz-Loya is a member of American Concrete Institute (ACI) and ASTM where he chairs a task group developing a test method for compressive strength of alkali-activated fly ash cement mortars. He can be contacted by e-mail at ivan.diazloya@ceratechinc.com.

Specifying ‘Greener’ Concrete Infrastructure

All photos courtesy CTS Cement Manufacturing Corp.

All photos courtesy CTS Cement Manufacturing Corp.

by Eric Pascal Bescher, PhD, and Nick de Ocampo, LEED AP

Although ‘sustainability’ is a popular term, it remains an ill-defined and nebulous concept in the construction industry. This is especially the case with concrete, where it can be difficult to understand cost and environmental benefits.

There is no straightforward, simple definition of what sustainability means to the construction industry. Further, there are few tools or even metrics available to help decision-makers specify one construction material over another in relation to sustainability.

The sustainability of concrete, the most widely used material in the world, is still a difficult concept to quantify. However, the recently developed Product Category Rules (PCRs) for concrete are a step in the right direction.

This article focuses on the sustainability of materials used in civil infrastructure such as highways and bridges, but the concepts explored are also frequently applicable to materials used for building repair.

Defining sustainability
According to the U.S. Environmental Protection Agency (EPA), sustainability is based on the simple principle that everything needed for survival and well-being depends, either directly or indirectly, on the natural environment. Sustainability efforts ensure resources needed to protect human health and the environment remain available.

Panel replacement work on Highway 1 with rapid-setting cement in San Luis Obispo, California.

Panel replacement work on Highway 1 with rapid-setting cement in San Luis Obispo, California.

The sustainability of construction material is affected by the amount of non-renewable materials entering into its manufacturing and its lifespan before it has to be replaced and manufactured again. These non-renewable materials include sand, rock, and limestone for cement manufacturing.

Most sustainability or green voluntary certification programs—including Leadership in Energy and Environmental Design (LEED) and other multi-attribute certification programs developed by industry groups—have focused on energy, material and water conservation, indoor environmental quality, and site selection and development.

While these are all important aspects of sustainable building design and construction, lifecycle does not appear to be an integral part of this definition.

The Cement Sustainability Initiative—a project of the World Business Council for a Sustainable Development (WBCSD)—is a complex effort by 24 major cement-producers to look at the factors affecting concrete sustainability. It is still in its infancy and is attempting to deal with the many facts of concrete manufacturing. However, it cannot yet circumvent the fact portland cement technology and its carbon footprint has changed little since Joseph Aspdin obtained the patent in 1824.

A century-old technology
The portland cement industry in the 21st century is facing one fundamental fact of materials science: the technology is significantly dated. Although the industry has made great strides in the efficiency of limestone burning, the material’s carbon footprint has not changed significantly, and is not likely to any time soon. The industry is locked into a technology forcing it to emit large amounts of carbon dioxide (CO2). In order to make one ton of cement, approximately one ton of CO2 must be released into the atmosphere.

Given the global industry produced nearly 4 billion tons in 2012, portland cement production accounts for about five percent of total greenhouse gas (GHG) emissions worldwide.

Sustainability efforts have been generally limited to reducing the cement content in concrete or increasing the amount of filler, such as limestone, slag, or fly ash. While fly ash, blast furnace slag, and other pozzolans provide cementitious properties, such fixes are superficial and can only reduce the carbon footprint by a fraction; they do not address the basic, core issue of a 200-year-old chemistry As a result, it can be difficult to consider traditional portland cement concrete as a sustainable building material.

A greener solution
It is possible to produce ‘green’ cement that would tap less into natural resources?

Such ‘greener’ concrete is available with calcium sulfoaluminate (CSA) cement. This rapid-setting cement requires burning mixtures of limestone, bauxite, and gypsum at lower temperatures than portland cement—approximately 1482 C (2700 F) for portland cement versus 1232 C (2250 F) for CSA cement.

This lower burning temperature reduces the amount of energy and carbon dioxide emissions associated with portland cement production. CSA cement also requires less limestone, which is the primary source of carbon dioxide released during the chemical sintering process. CSA clinker is also easier to grind, reducing the energy needed during the milling process. A particularly interesting type of CSA cement is the CSA-belite (C2S) cement that provides some of the advantages of portland cement while lowering its carbon footprint.

CSA cement can play a significant role in improving the sustainability of construction materials by simply reducing the quantity of non-renewable resources used during manufacturing. The use of resources is just one part of the equation—durability, or lifecycle, is the second.

CSA-belite cements have been available for a few decades. They generally fall under ASTM C1600, Standard Specification for Rapid-hardening Hydraulic Cement. They are extensively used in runway or highway repair, (For example, the Seattle-Tacoma Airport repair discussed later in this article.) To date, more than 330 lane-miles of highway have been replaced in California using this material.

Quantifying sustainability
When selecting a building material—asphalt, portland concrete, or CSA concrete—sustainability should be easily quantifiable. One method for assessing the sustainability of a construction material could be to divide its lifecycle by the amount of non-renewable resources required in its manufacturing process.

Using this method, the Sustainability Index— developed by this article’s author Eric Pascal Bescher, PhD—would be defined as:

Sustainability Index (S) = Lifecycle
                                       Resources

In this equation, “lifecycle” refers to the durability of concrete in years. It is linked to fatigue life and other material properties such as shrinkage, cracking, and porosity.

The Seattle-Tacoma (Sea-Tac) runway mix design is a seven-sack CSA concrete tested at the University of Oklahoma to have a lifecycle of more than 80 years.

The Seattle-Tacoma (Sea-Tac) runway mix design is a seven-sack CSA concrete tested at the University of Oklahoma to have a lifecycle of more than 80 years.

“Resources” refers to the quantity of non-renewable resources used in concrete manufacturing (such as, but not limited to, carbon footprint). Finally, the Sustainability Index, as defined above, can have the unit of [m3*years/ton-CO2].

In this equation, sustainability is tied not just to resources used, but also to lifecycle. If the lifecycle of one cubic meter of concrete were infinite, it would be sustainable. However, if the entirety of our planet’s resources were required to manufacture the same amount of concrete, it would not be sustainable. The Sustainability Index helps to quantify the sustainability of producing and using one cubic meter of concrete in simple, measureable terms.

Further, LCA and EPD calculations, as defined by concrete PCRs, can be used as the numerator and the denominator, respectively, in the Sustainability Index equation.

The lifecycle of concrete
This straightforward index can help rate the sustainability of various materials and mix designs and helps decision-makers choose materials consistent with stated sustainability goals. It brings lifecycle into the equation, and a greater lifecycle decreases the burden on resources.

A 100-year-old pavement would have five times the Sustainability Index of a 20-year pavement, all other parameters being equal. At equal lifecycle, a mix design with half the carbon footprint would have twice the sustainability. This simple concept ties economic decisions to materials properties.

The concept also highlights the sustainability of concrete is, above all, a materials property. For example, lower shrinkage and lower porosity increase lifecycle. Doubling the lifecycle while halving the carbon footprint, quadruples the Sustainability Index. Using this approach, the sustainability of concrete is not such a nebulous concept—its basic definition becomes “the lifecycle of the material per unit of non-renewable resources.”

With this definition of sustainability for the concrete industry, decision-makers now have a tool allowing them to select between materials of different lifecycles made with different resources. Choosing a sustainable building material should be a matter of figures and numbers. The table in Figure 1 provides examples of the Sustainability Index for various mixes and materials in 0.7 m3 (1 cy) of concrete. Different mixes and materials have different sustainability indices. For example, the type of cement, cement content, or the addition of fly ash to a mix can affect its sustainability.

Concrete, water, porosity, and structure
Concrete is a highly porous material, with the amount of water used in the mix controlling its porosity. A highly porous concrete will have lower strength and shorter lifecycle. Also, as concrete shrinks as it ages, it causes cracks to develop. Any factor reducing concrete shrinkage decreases its tendency to crack. This, in turn, increases its lifecycle and therefore sustainability.

Generally, portland-based concrete shrinks because more water is needed to make a workable mixture than required for chemical hydration. Most of the shrinkage of portland cement concrete, and its lifecycle, are directly connected to the amount of water needed to make the mixture. Adding more water than needed to hydrate is one reason why the goal of 100-year pavement is still elusive.

CSA concrete permanently retains the mixing water in its crystalline structure, which makes it less prone to shrinkage or cracking. CSA concrete is less porous, stronger, and has lower shrinkage than portland cement concrete, which increases its lifecycle. CSA concrete’s lifecycle has been tested to be as long as 80-years versus 40-years for portland cement. Using the Sustainability Index, civil engineers can include lifecycle as part of the evaluation of the sustainability of a mix design.

Lifecycle implications
A long lifecycle is a significant advantage in many applications, such as highway rehabilitation. The United States is facing serious problems related to its deteriorating infrastructure as engineers and state Departments of Transportation (DOTs) are confronted with aging and decaying highways causing safety and transportation issues. When concrete ages and cracks, it must be replaced at a significant cost. Therefore, increasing the lifecycle of concrete is important to sustainability efforts.

As an example, many California freeways built in the 1950s and 1960s are now at the end of their lifecycle or past the point when they should have been rebuilt. Shutting down even a portion of a freeway for repairs has a tremendous impact on human activity and the economy. Many DOTs would be interested in a concrete that could last 80-to- 100-years, rather than the typical 20-to-30-year lifecycle of portland cement.

This analysis does not take into account carbon emissions from harvesting and transport of aggregates. It is assumed portland cement concrete has a 40-year design life, that calcium sulfoaluminate (CSA) cement concrete has an 80-year design life, and that reducing the cement content does not affect the design life. It is assumed that fly ash addition also has no effect on design life. Fly ash is not a zero carbon emissions material and the estimated carbon emissions are assumed to be 0.19-kg-CO2/kg-fly ash. This estimate is deduced from the annual consumption of coal in the United States, the carbon emissions from coal consumption, and the annual tonnage of fly ash produced in U.S. portland cement emissions data from www.CO2list.org.  Emissions data and lifecycle analysis of portland cement are taken from Bognacki et al.’s article “Increasing the Service Lives of Airport Pavements” in the January 2012 issue of Concrete International. It is proposed a maximum of 550-lb/cy of cement is required to achieve a design life of 40 to 50 years. It is also assumed that the Port Authority mix design can achieve a 100-year lifecycle due to lower water content and permeability.
This analysis does not take into account carbon emissions from harvesting and transport of aggregates. It is assumed portland cement concrete has a 40-year design life, that calcium sulfoaluminate (CSA) cement concrete has an 80-year design life, and that reducing the cement content does not affect the design life. It is assumed that fly ash addition also has no effect on design life. Fly ash is not a zero carbon emissions material and the estimated carbon emissions are assumed to be 0.19-kg-CO2/kg-fly ash. This estimate is deduced from the annual consumption of coal in the United States, the carbon emissions from coal consumption, and the annual tonnage of fly ash produced in U.S. portland cement emissions data from www.CO2list.org.
Emissions data and lifecycle analysis of portland cement are taken from Bognacki et al.’s article “Increasing the Service Lives of Airport Pavements” in the January 2012 issue of Concrete International. It is proposed a maximum of 550-lb/cy of cement is required to achieve a design life of 40 to 50 years. It is also assumed that the Port Authority mix design can achieve a 100-year lifecycle due to lower water content and permeability.

CSA concrete and LEED
CSA cement can play an important role in improving the sustainability of construction materials, mostly because its chemistry and materials science differ from the well-established standards of portland cement. It can play a significant role in the sustainability of concrete technology from the perspective of raw materials use, energy demand, carbon footprint, and pavement longevity.

A combination of low calcium content and low burning temperatures allows CSA cement to yield concrete with lower carbon footprints. When used as a shrinkage-compensating additive to portland cement, CSA can also improve sustainability through a decrease or total elimination of shrinkage, resulting in an increase in longevity.

Users of CSA cement can help a project earn points toward Leadership in Energy and Environmental Design (LEED) certification. Points can be obtained for energy and water conservation, reducing harmful greenhouse gas emissions, and reducing waste sent to landfills. For example, CSA cement can help a project earn up to 13 LEED points over a range of credits including recycled materials, heat island effect, and low-emitting materials.

CSA cement meets guidelines for LEED credits through its use of byproducts. Calcium sulfoaluminate requires aluminum oxide and calcium sulfates. These compounds can be introduced using byproducts of aluminum recycling or synthetic gypsum as raw materials. These byproducts eliminate the need for bauxite and gypsum mining.

The improved sustainability of CSA-based concrete is due to a combination of factors including:

  • lower emission of greenhouse gases;
  • decreased emissions of smog-producing nitrogen oxides;
  • use of recycled raw feedstock materials; and
  • longer lifecycle through higher strength, lower shrinkage, and lower porosity.

Rapid-setting and sustainability
There are several additional benefits that should be taken into account when considering CSA cement for a project. While portland cement can set in about three hours, it can take several days for it to reach structural strength. CSA cement achieves structural strength in three hours.

As a result, CSA concrete is used in time-sensitive pavement rehabilitation projects such as the Seattle-Tacoma (Sea-Tac) airport.

CSA cement was used for the overlay of the 79-year-old Lewis and Clark Bridge over the Columbia River running between Washington and Oregon.

CSA cement was used for the overlay of the 79-year-old Lewis and Clark Bridge over the Columbia River running between Washington and Oregon.

The Sea-Tac runway in Washington was shut down at midnight, old concrete removed, and fresh CSA concrete poured. By 6:00 a.m., planes were able to land. Another example is the use of CSA cement to rehabilitate aged concrete on freeways in California, where more than 300 lane-miles of pavement have thus far been replaced with an average replacement rate of 35 panels (i.e. 133.8 m3 [175 cy]) per eight-hour closure, and a maximum replacement of 203 panels (i.e. 776 m3 [1015 cy]) in 10 hours.1

There are substantial sustainability benefits to the rapid-setting characteristics of CSA concrete. For example, shorter closing times mean less fuel burned in traffic and lower economic impact on the travelling public. Additionally, it can be used in post-tensioned concrete buildings, or precast concrete work, where high, early strength means sooner post-tensioning and faster construction times.

Cost considerations
Cost is a significant factor in the selection of sustainable building materials. On a pound-for-pound basis, CSA cement could be approximately three times more costly than portland cement, or 1.2 times more expensive than portland cement accelerated with organic additives. This difference is a challenge for engineers and specifiers responsible for selecting a repair material. Does it make sense to select a cheaper, less durable material when the longer-lasting alternative is more expensive?

As a matter of accountability to the public who generally pays for these repairs, the overall benefits of a longer-lasting pavement outweigh the short-term impact on a budget. A less-expensive material that will need to be replaced in 20 years simply shifts the burden to future generations. Pavement management policies should make lifecycle its first priority over purchase price.

The Sustainability Index can be an important tool. It helps engineers and specifiers make decisions on cost and lifecycle considerations and environmental impact without being overly complex. A common misconception is cement chemistries and materials are all the same. This is not true, as evidenced by the differences between portland cement and CSA cement in terms of properties, cost, and sustainability. Understanding these differences and using the Sustainability Index is crucial to making the best decision for the project.

Conclusion
The industry must educate itself on the advantages of the technology for calcium sulfoaluminate cement to become widely used. There is always inertia in specifying a material that deviates from standards, especially if those standards were established in the 19th century.

When civil engineers have a choice of materials, they need a tool to differentiate between them. The Sustainability Index is a valuable tool to help decision-makers make informed, sustainable materials choices in tight fiscal times.

The construction industry should look to CSA cement with a fresh eye because of its impact on sustainable development and the improvement of infrastructure.

Notes
1 The Sea-Tac runway mix design is a seven-sack CSA concrete tested at the University of Oklahoma to have a lifecycle of over 80 years. (back to top)

Eric Pascal Bescher, PhD, has been adjunct professor in the Department of Materials Science at University of California, Los Angeles (UCLA) since 1998. He also joined CTS Cement as director of research in 1998, and is now vice-president for cement technology. Bescher holds a bachelor’s degree in materials physical chemistry from the University of Rennes, France, as well as a master’s and PhD in materials science and engineering from UCLA. He is the author of more than 33 scientific articles in the field of materials science and the holder of several patents. Bescher can be contacted via e-mail at bescher@ucla.edu.

Nick de Ocampo, LEED AP, joined CTS Cement in 2008 as a product development engineer to help promote the sustainable properties of calcium sulfoaluminate (CSA) cements. He holds a bachelor’s degree in materials science and engineering from UCLA. de Ocampo has developed LEED product evaluations for the company’s rapid-setting product line. He can be reached at ndeocampo@ctscement.com.