What are SCMs and how can you use them to your advantage?
Part II: Hydraulic SCMs
By Adam D. Neuwald
The Resource Conservation and Recovery Act (RCRA) requires U.S. agencies using federal funds to purchase products composed of the highest percentage of recovered materials practical. In response, the Environmental Protection Agency (EPA) created the Comprehensive Procurement Guideline (CPG), which identifies various items that are or can be made with recovered materials. The items are broken down into eight categories. Manufactured concrete products are an ideal solution for the following categories: construction products, landscaping products, park and recreation products, transportation products and miscellaneous products.
The EPA’s Comprehensive Procurement Guideline recommends the use of various supplementary cementitious materials (SCMs) in the production of concrete when adhering to the guidelines set forth in the RCRA.
The government of Canada has also played a strong role in promoting sustainable development. The preamble of the Canadian Environmental Assessment Act states “The Government of Canada seeks to achieve sustainable development by conserving and enhancing environmental quality, and by encouraging and promoting economic development that conserves and enhances environmental quality.”
Both the U.S. and Canadian governments are promoting the use of supplementary cementitious materials in the production of concrete.
SCMs can be divided into two categories based on the type of reaction they undergo: hydraulic or pozzolanic. Hydraulic materials react directly with water to form cementitious compounds, while pozzolanic materials chemically react with calcium hydroxide (CH), a soluble hydration product, in the presence of moisture to form compounds possessing cementitious properties.
In addition to meeting government requirements, supplementary cementitious materials are often used to reduce cement contents and improve the workability of fresh concrete, increase strength and enhance durability of hardened concrete. Part I of this article addressed the use of pozzolanic SCMs in addition to presenting a brief overview on the use and history of SCMs. As a supplement to Part I, the following is a brief overview of two of the more common hydraulic SCMs used in the manufactured concrete products industry as well as a review of blended cements and the benefits associated with the use of these materials.
According to the Slag Cement Association, a record 3.1 million metric tons of slag cement were shipped for use in concrete and construction applications in 2003. This figure represents both slag cement shipped as a separate product (conforming to ASTM C 989) and as a component of blended cement (conforming to ASTM C 595).
Slag, as commonly used in the manufactured concrete products industry, is often referred to as Ground Granulated Blast-Furnace Slag (GGBFS), which refers to the manner in which the material is processed. Slag is a byproduct from the production of iron and must be periodically removed from the blast furnace. The chemical properties of the material are controlled by the input materials used in the production of iron, while slag’s physical properties are influenced by the manner in which the molten slag is cooled.
To form a hydraulic cementitious material, the slag must be rapidly quenched, or cooled, producing a reactive amorphous vitreous glass that is ideal for use in concrete construction. Quenching with water is the most common process. High-pressure water jets are used at a water-to-slag ratio of about 10 to one by mass. The blast furnace slag is broken up and instantaneously cooled, producing slag with a high glass content. Slag can also be cooled utilizing the pelletization process, which uses less water. During pelletization the slag passes over a vibrating table where it is expanded and cooled by water jets. The material then passes over a rotating drum that throws the slag into the air where it is further cooled, producing spherical glassy pellets of various sizes as illustrated in Figure 1. The resulting material from both of these processes is generally referred to as granulated blast-furnace slag (GBFS).
Larger pellets, which are porous and partially crystalline, are often used as lightweight aggregate, while smaller pellets are ground to produce blended cements meeting the requirements of ASTM C 595, “Standard Specification for Blended Hydraulic Cements,” or ground separately for use as a supplementary cementitious material meeting the requirements of ASTM C 989, “Standard Specification for Ground Granulated Blast-Furnace Slag for use in Concrete and Mortars.” Granulated blast-furnace slag is often ground finer than portland cement, thereby increasing its reactivity at early ages. The grinding of GBFS requires more energy than the grinding of portland cement clinker. Thus, GGBFS is often similar in price to portland cement. (From here on, the term “slag” will refer to ground granulated blast-furnace slag. The price of slag is also influenced by transportation costs. A majority of slag granulation and grinding facilities are centered around the U.S. iron industry, which is concentrated east of the Mississippi River.
There are numerous factors simultaneously contributing to the cementitious performance of slag, including chemical composition, glass content, fineness, alkali concentration within the hydrating system and temperature during the initial stages of hydration. ASTM C 989 merely sets a numerical limit on the amount of sulfide sulfur (S) and sulfate (SO3) present while classifying the material based on its performance in comparison to a reference cement meeting the requirements of ASTM C 150. The slag activity index (SAI), as defined in ASTM C 989, is a ratio of the compressive strength of a 50-50 slag blend mortar to the compressive strength of a reference cement mortar at seven and 28 days. The slag is then given one of the following designations: Grade 80, Grade 100 or Grade 120. A Grade 120 slag can be expected to produce higher 28-day compressive strengths than a concrete produced with just the reference cement, while a Grade 80 slag can be expected to reduce the compressive strength of a concrete when compared to the reference cement.
By now you may be asking yourself: Why use slag if it’s roughly the same price as cement and may even produce lower compressive strengths? One reason is that you may be required by the Resource Conservation and Recovery Act to incorporate the use of SCMs when bidding on federally funded projects. In addition, you may come to realize the numerous advantages associated with the use of slag. Improved workability, reduced water contents, reduced permeability, alkali silica reaction (ASR) mitigation and improved resistance to chemical attack can all be reasonably expected when slag is used as a supplementary cementitious material.
Slag has been found to improve the workability and cohesion of a concrete mix. The glassy surface characteristic of slag creates smooth slip planes within the paste, which increases workability, while the presence of an impervious coating of amorphous silica and alumina absorb little if any water during the initial mixing period. The water content can often be reduced by 3 percent to 5 percent when slag is used as a cement replacement. Research has also shown that concretes produced with slag require less energy to adequately consolidate, leading to less wear and tear on forms and vibration equipment used for both wet-cast and dry-cast production operations. Bleeding can also be expected to decrease for slag ground finer than portland cement, while the opposite can be expected for coarser slag.
There are various factors that must be considered when designing a concrete mixture containing slag. From a production standpoint, proper measures must be taken to ensure the concrete gains sufficient strength to be stripped and handled the following day. Fortunately, slag is extremely sensitive to temperature, and adequate strengths typically can be obtained by using slag activators, accelerators or elevated curing temperatures. The reaction of slag is a two-part reaction, first reacting with the available alkalis in the system and later with the reaction product calcium hydroxide (CH). The alkalis present in portland cement are typically sufficient to activate the hydration of slag. In addition, the solubility of alkali hydroxides from portland cement will increase as the curing temperature is increased, promoting the early reaction of slag. The hydration of slag produces calcium silicate hydrate (CSH), the same reaction product formed during the hydration of portland cement.
Research has found that a 40 percent to 50 percent cement replacement by mass with slag will produce the greatest 28-day strength. Typical dosage rates for slag in the manufactured concrete products industry range from 25 percent to 50 percent by mass, depending on production operations, strength requirements and durability requirements. Slag can greatly improve concrete’s resistance to sulfate attack, but replacement levels may need to be as high as 50 percent when using a Type I cement. The conversion of calcium hydroxide to calcium silicate hydrate throughout the concrete’s pore structure will reduce the overall permeability, preventing the ingress of harmful sulfates. In addition, the tricalcium aluminate (C3A) content of the overall system will be reduced, since less cement is being used. Research has established a clear correlation between the C3A content of portland cement and its susceptibility to sulfate attack, which is why Type II and Type V cements, which contain lower C3A contents, are often specified.
Slag can also be used to mitigate concrete expansion caused by reactive aggregates. While slag will not prevent the expansion of the aggregates themselves, it will greatly reduce the expansion of concrete containing highly reactive aggregates. Research has shown that replacement levels between 35 percent and 60 percent have been used successfully to reduce expansion caused by alkali-silica reaction (ASR). According to the American Concrete Institute (ACI), slag’s effect in preventing or reducing the detrimental expansion of concrete containing reactive aggregates can be attributed to the following: reduced permeability, change of the alkali-silica ratio, dissolution and consumption of the alkali species, direct reduction of available alkali in the system and reduction of calcium hydroxide needed to support the reaction.
Slag can also be used to prevent the corrosion of reinforcing steel by decreasing the permeability of the concrete and ultimately reducing the penetration of chloride ions and the depth of carbonation, which ultimately promotes the corrosion of reinforcing steel. The use of slag creates additional calcium aluminate hydrates within the system, improving concrete’s chloride binding effect, thus immobilizing chloride ions and reducing the potential for corrosion.
Slag can be purchased in bulk or smaller quantities and should be stored in the same manner as cement. Because slag is a hydraulic SCM it should be kept dry to prevent hydration of the material. Trial batches should always be cast, and mix proportioning should be done in accordance with ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete.” Concrete mixes with high percentages of fine particles tend to become “sticky” when finishing, so the coarse-to-fine aggregate ratio may need to be increased slightly. It should also be noted that concretes containing slag may turn a greenish-blue color within the first few days after casting. The color will fade as the concrete oxidizes with the atmosphere. The color will remain if the concrete is continuously exposed to water or sealed prior to oxidation.
Class C Fly Ash
As a quick review, fly ash is a byproduct from the combustion of coal used in electric power plants. It is a fine residue of mineral impurities that melt and recrystallize within the air stream moving through the combustion boiler. The material is then collected from exhaust gases using electrostatic precipitators or filters. The composition of fly ash is controlled by the chemical composition of the coal used by the power plant. Class C fly ash, as defined in ASTM C 618, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolans for Use in Concrete,” is typically found west of the Mississippi River where lignite or subbituminous coals are used.
The chemical composition analysis as presented in ASTM C 618 does not address the nature or reactivity of the material and is merely presented as a quality control measure. Class C fly ashes have higher calcium contents and lower carbon contents than Class F fly ashes. Class C fly ashes, which contain calcium-rich glassy phases, are considerably more reactive than Class F fly ashes, meaning Class C fly ashes have both pozzolanic and hydraulic properties and often exhibit higher reaction rates at early ages. This may be advantageous for use in the manufactured concrete products industry, making it possible to meet production schedules.
The carbon content (as measured by loss on ignition) of Class C fly ash is often less than 1 percent and will not have an adverse effect on the use of air-entraining admixtures in comparison to a Class F fly ash. Lower dosage rates of air entraining admixture may even be expected when using a Class C fly ash. Class C fly ashes often contain higher percentages of particles finer than 10 µm, contributing to improved workability while reducing the potential for bleeding in fresh concrete.
Although the preceding information expresses the advantages of using a Class C fly ash over a Class F fly ash, one must be aware of the shortcomings of Class C fly ash. Research has suggested that some Class C fly ashes will actually reduce concrete resistance to sulfate attack, especially when used with cements having a high tricalcium aluminate (C3A) content. In addition, fly ashes containing high concentrations of sulfur, as measured by SO3, should be checked for the potential for efflorescence. Although efflorescence is not a structural concern, it may cause problems in architectural products. One must also be cautious when using fly ash that contains calcium oxide (CaO) in the form of free lime, especially in low water-to-cement ratio mixes. If there is not enough moisture present to hydrate the free lime prior to initial set, delayed hydration may cause detrimental volume changes.
Fly ash typically has a lower density than portland cement, thus its bulk density should be used when ordering and checking inventories. Fly ash will require about 30 percent to 40 percent more storage space when compared to an equivalent mass of portland cement. Because fly ash is spherical in shape, the material tends to flow quite easily. Storage bins should be separate from those used for cement and other materials. Bins should be maintained and checked for flaws that may lead to cross contamination or loss of material and positive shut-off valves should be installed to prevent over-batching.
Blended Cements, Ternary Blends and Quaternary Blends
Blended cements have been used for the production of concrete for more than 100 years. Blended cements are produced by intergrinding portland cement clinker with various SCMs or by blending portland cement with varying quantities of SCMs. The advantage of blending materials is that each material is ground to its optimum fineness prior to blending. Blended cements may contain quantities of slag, fly ash, silica fume or natural pozzolans depending on local availability. The advantage of using blended cements is that they can be designed for a specific application. By varying proportions of the blend, attributes such as increased sulfate resistance, ASR mitigation or strength gain can be attained. Chemical admixtures such as air-entrainment admixtures or accelerators/retarders can also be incorporated. Blended cements allow producers with limited storage capacity the ability to capitalize on the numerous advantages associated with the use of SCMs.
The disadvantage of using blended cements is that you minimize the flexibility to change your mix design to accommodate various product lines or project specifications. The use of blended cements is more common throughout Europe, while North American producers tend to incorporate SCMs separately during batching at the mixer. This, however, requires additional storage bins or silos.
Ternary systems consist of portland cement and two additional SCMs, while quaternary systems contain portland cement and three SCMs. Before discussing the use of multiple SCMs we will quickly review some of the more common pozzolanic SCMs covered in Part I of this article.
Silica fume is an industrial byproduct of silicon metal or ferrosilicon alloy production. Silica fume particles are extremely small and spherical in shape resulting in enhanced particle packing and, ultimately, higher compressive strengths and enhanced durability. Silica fume is typically used as a 5 percent to 10 percent replacement for cement. High-range water-reducing admixtures are often used to offset increased water demand and improve particle dispersion for concretes containing silica fume. Other pozzolanic SCMs include Class F fly ash and raw or processed natural pozzolans such as shale, clay, metakaolin and rice husk ash (RHA). Natural pozzolans are either ground and used in their natural state or require calcining. Calcining is the process of altering the composition or physical state by heating a material below the temperature of fusion. Metakaolin and RHA are highly reactive and often used in the same manner and proportions as silica fume.
There are numerous advantages to using multiple SCMs in blended cements or as a separate additive in ternary and quaternary systems. According to the Portland Cement Association (PCA), silica fume can be used to offset the early strength gain associated with the use of Class F fly ash, while fly ash and slag can also be used to increase the long-term strength gain of silica fume concrete. Silica fume can also be used to reduce the levels of fly ash or slag required for sulfate resistance and alkali silica reaction mitigation. Fly ash, and to a lesser extent slag, can be used to offset the increased water demand associated with the use of silica fume. Overall, the use of multiple SCMs will greatly improve the resistance of concrete-to-chloride ion penetration.
There are numerous benefits to incorporating the use of supplementary cementitious materials into your mix design. Not only will SCMs allow cement contents to be reduced, they will also improve both the fresh and hydrated properties and performance of your concrete. Plus, by using SCMs, you are helping to preserve and protect our environment. The use of byproducts such as fly ash, slag or rice husk ash in the production of concrete keeps these materials from the land fill. In addition, the use of SCMs may also lead to a reduction in cement consumption, further helping our environment. The production of a ton of cement releases roughly a ton of carbon dioxide into the atmosphere.
We must continue to investigate and use environmentally friendly materials that improve the strength and performance of manufactured concrete products.