Editor’s Note: This is the first article in a year-long series explaining common raw materials used in precast.
By Claude Goguen, P.E., LEED AP
Precast concrete manufacturers are always refining mix designs and production techniques to make high quality concrete. Luckily, modern technology provides many useful resources to accomplish this goal. Scientists have developed chemical admixtures that can reduce water demand, configure air bubbles to address freeze-thaw concerns, control shrinkage and protect steel from corrosion. In this article, we will look at mineral admixtures, most commonly referred to as supplementary cementitious material (SCM). These products did not originate in high-end laboratory test tubes but rather in landfills and nature.
Mount Vesuvius supplied the Romans with the volcanic ash they used to construct aqueducts, sewers and buildings that still stand today. While the volcanic ash industry may no longer be focused on its potential as an SCM, we are fortunate to have other types of pozzolans at our disposal. A pozzolan is a siliceous material (or a blend of siliceous and aluminous materials) that will chemically react with calcium hydroxide (CH) in the presence of moisture to form calcium silicate hydrate (CSH), which is the compound responsible for quality concrete. This pozzolanic reaction differs from the hydration of ordinary portland cement (OPC). OPC is a hydraulic material, meaning it reacts chemically with water, and forms CH and CSH. CH is more of a byproduct in concrete and is prone to reactions with aggressive elements that may cause durability issues. Therefore, exchanging the CH formed by cement hydration for CSH makes the use of fly ash very appealing.
Using fly ash also enhances the economy and sustainability of the product by replacing a portion of OPC. That replacement can be up to 25% (by mass) of total cementitious materials, although higher levels have been used. Fly ash can be used as a separate SCM or used in a blended cement.
Fly ash particle size varies from less than one micrometer to more than 100 micrometers. Fly ash is primarily silicate glass containing silica, alumina, iron and calcium, but also includes magnesium, sulfur, sodium, potassium and carbon. The specific gravity of fly ash generally ranges between 2.0 and 2.8, lower than the specific gravity of OPC, which is 3.15. Consequently, we substitute up to 1 1/2 pounds of fly ash for every pound of OPC. To compensate for this additional material in a fixed volume of concrete, the amount of fine aggregate may be reduced to accommodate the additional volume of fly ash.
ASTM C618, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete,” classifies fly ash into two types, based on calcium content. Class F fly ash has a low-calcium content (5%-10%) while Class C fly ash has a higher calcium content (10%-30%).
Class F fly ash has a carbon content less than 5% but sometimes as high as 10%. Class C typically has a carbon content of less than 2%.
How fly ash impacts fresh concrete
Using fly ash usually reduces water demand versus the same slump or spread concrete while using just OPC. However, at higher fly ash proportions, the water demand can increase.
Due to its spherical nature, fly ash tends to enhance workability of concrete and reduces bleeding. Fly ash can also impact air entrainment efficiency. Generally, when fly ash is added to the mix, more air-entraining admixture is required to achieve a specific air content because high carbon content will soak up the air entraining and can result in lower air contents. Because the impact on air entraining depends on carbon content, the effect is less evident with Class C fly ash than Class F.
Fly ash use can also slow down setting times, which can be problematic for reaching stripping strengths. This can be offset by using an accelerating admixture or a Type III cement.
How fly ash impacts hardened concrete
Strength gain with fly ash is similar to that of concretes made solely with OPC. However, there can be some additional early age strength gain, especially with Class C fly ash. The conversion of CH to CSH also helps to reduce permeability, which increases concrete’s durability in many situations, including high sulfate environments. Additionally, the added CSH from using fly ash can also chemically bind with the alkalis in the concrete and help resist alkali silica reaction (ASR).
Slag cement, originally known as ground granulated blast-furnace slag (GGBFS), is a byproduct from the production of iron. The blast furnace is used to refine iron ore into iron and the resulting components from the heating of ingredients are iron and molten slag. The iron is used to produce steel and the molten slag is converted to a cement-like material by rapidly cooling it with water. This rapid cooling creates glassy granules, which are ground into a fine powder. Slag is not a pozzolan like fly ash, but rather a nonmetallic hydraulic cement. Fortunately, it does consume CH by binding alkalis in its hydration products.
Slag cement is generally ground to less than 45 microns and has a specific gravity in the range of 2.85 to 2.95. Slag is generally used in higher percentages than fly ash, commonly constituting between 30% and 45% of the cementitious material in concrete, while some comprise as high as 70% or more of the cementitious material in a mix. ASTM C989, “Standard Specification for Slag Cement for Use in Concrete and Mortars,” classifies slag by its increasing level of reactivity as Grade 80, 100 or 120. Grade 80 has a low activity index and is used primarily in mass structures because it generates less heat than OPC. Grade 100 has a moderate activity index and is most like OPC with respect to cementitious behavior and is readily available. Grade 120 has a high activity index and is more cementitious than OPC.
How slag impacts fresh concrete
Slag cement usually decreases water demand between 1% and 10%, depending on dosage. Like fly ash, slag cement will generally enhance the workability of concrete. However, its effect on the required dosage rate of air-entraining admixtures is variable. The impact of slag cement on bleed rates will depend on its fineness. Concretes containing ground slag of comparable fineness to that of the cement tend to show an increased rate and amount of bleeding than OPC concretes, while slag ground finer than cement generally reduces bleeding.
Similar to fly ash, the use of slag cement can slow down setting times. This can also be offset using an accelerating admixture or perhaps a Type III cement.
How slag cement impacts hardened concrete
The strength gain of slag concrete may increase compared to the same mix using only OPC. Slag cement in concrete will create a denser matrix, reducing permeability and enhancing durability when exposed to aggressive chemicals. This is especially true in high sulfate environments where some studies indicate concrete with ground slag has a sulfate resistance equal to or greater than concrete made with Type V sulfate-resistant OPC.1 Slag cement can also reduce the potential for ASR by consuming alkalis in the hydration process and reducing their availability.
Metakaolin is a natural pozzolan just like volcanic ash. Modern use of natural pozzolans dates back to early 20th Century public works projects, such as dams, where they controlled temperature rise in mass concrete and provided cementitious properties. In addition, natural pozzolans were used to improve resistance to sulfate attack and were among the first materials found to mitigate ASR. The most common natural pozzolans used today – calcined clay, calcined shale and metakaolin – are processed materials, which are heat treated in a kiln and then ground to a fine powder.
Metakaolin is considered a special calcined clay and produced by low temperature calcination of high purity kaolin clay. The product is ground to an average particle size of about 1 to 2 micrometers. The specific gravity of metakaolin is about 2.5. Metakaolin is used in special applications where very low permeability, very high strength or both are required.
In these applications, metakaolin is used more as an additive to the concrete rather than a replacement of cement. Typical additions are around 10% of the cement mass. The reactivity of metakaolin is based on chemical composition and reactive surface. Highly reactive metakaolin has become available as a considerably reactive pozzolanic material in concrete.
How metakaolin impacts fresh concrete
Metakaolin generally has little effect on water demand at normal dosages; however, higher dosages can significantly increase water demand. Like fly ash and slag cement, metakaolin will generally enhance workability of concrete; yet, its effect on the required dosage rate of air-entraining admixtures is minimal. Metakaolin has little effect on bleeding and setting times. Metakaolin concrete tends to exhibit better finishability compared to other SCMs due to its creamy texture.
How metakaolin impacts hardened concrete
Metakaolin’s reaction rate is rapid, significantly increasing compressive strength even at early age, which can allow for earlier stripping. Mixes with metakaolin at 8% of the total cementitious materials have produced concrete compressive strength increases of more than 20% in one day and 40% at 28 days.2
Air-entrained concrete containing about 10% of metakaolin by mass will withstand ingression of chloride ions and increases durability to repeated cycles of freeze-thaw.
Metakaolin in concrete tends to reduce the size of pores, which, consequently, leads to higher density and more resistance to aggressive chemicals. Furthermore, metakaolin improves concrete resistance to ASR and sulfate attack.
Sustainability considerations of fly ash, slag cement and metakaolin
The use of SCMs such as fly ash, slag cement or metakaolin in manufacturing precast concrete can contribute to the sustainability of a product or project. Using SCMs helps in terms of minimizing waste of resources and energy during construction. The use of blended cement or the replacement of OPC with industrial byproducts such as SCMs reduces the amount of clinker required per cubic yard of concrete. Less cement in the precast means less embodied energy and represents CO2 emissions. When industrial byproducts such as fly ash and slag are used, they not only provide a sustainable option because of their reuse, but also improve concrete properties while reducing cost. Metakaolin has added advantages of lowering the processing temperature, providing a smaller embodied energy and reducing greenhouse gas emissions.
While manufacturers continue their quest to make high quality concrete, they must consider the available options in their areas and be open to experimenting using different ingredients such as the ones discussed in this article. Using one or a combination of SCMs may significantly enhance durability of the precast concrete product and even provide economic benefits.
Claude Goguen, P.E., LEED AP, is NPCA’s director of technical education and outreach.
1. (ACI 233 and Detwiler, Bhatty, and Bhattacharja 1996).
2. Justice, J. M. and Kurtis, K. E., “Influence of Metakaolin Surface Area on Properties of Cement-based Materials,” ASCE Journal of Materials in Civil Engineering, September 2007, Vol. 19, No. 9, pp. 762-771.