By Kayla Hanson, P.E.
Editor’s Note: This is the second article in a series detailing the types of supplementary cementitious materials (SCMs) available and the role they play in concrete’s strength. To read the first article in the series on fly ash, click here.
Today, both producers and consumers are on an endless hunt for fast, affordable, high-quality and green solutions. Whether it’s newer concepts like Amazon Dash for instant ordering; car-share programs in congested cities; or simply growing popularity of existing ideas like reduce, reuse and recycle, people everywhere and in every industry are playing a part. The precast industry is no different. This outlook and proactive behavior has resulted in new solutions to manufacturing challenges, even if the solutions come from existing materials like slag cement.
According to the Slag Cement Association, some of the quantitative benefits of using slag cement or ground granulated blast-furnace slag in concrete include improved workability and finishability, higher compressive and flexural strengths, improved resistance to aggressive chemicals, improved durability and reduced life cycle costs.
For this reason, Phil Lapp, plant manager at Faddis Concrete Products in Downingtown, Penn., said in 2016 his company started using slag cement instead of fly ash to manufacture 90% of their products. He said many departments of transportation are making slag cement a requirement for mitigating alikali-silica reactivity, a form of concrete deterioration that occurs when certain aggregates react with alkalis in portland cement. No difficulties were noticed after the switch, and he wants to leave producers with one important message.
“Don’t be afraid of it,” he said.
Slag is a byproduct of smelting iron ore as part of the steel manufacturing process. Iron and many other types of metal ores found organically are impure and contain traces of other materials or metals. Heating iron ore in excess of 2,700 degrees Fahrenheit melts the metal and separates the pure iron from the impurities in the ore. Together, the impurities and the other materials in the furnace create slag. Molten slag floats above the denser molten iron. The liquid slag is isolated and quenched in water to quickly cool it, resulting in a hard, rock-like substance with a surface resembling glass.
The cooled and dried slag is ground into a fine powder, resulting in small, rough, jagged particles. Portland cement may then be blended directly with it to create slag cement. Slag can be used in its powdered form and can be added to the mixer during batching as a separate cementitious component.
Slag cement will undergo pozzolanic reactions when it is combined with both water and an activator such as calcium hydroxide (CH) or sodium hydroxide. The pozzolanic reactions commence as soon as sufficient CH or sodium hydroxide is produced from the cement hydration reactions. As slag cement, water and CH react, CH is consumed and calcium silicate hydrate is created, adding strength and durability to the growing paste matrix.
Like other supplementary cementitious materials that are produced as industrial byproducts or co-products, slag cement’s chemical composition varies depending on the parent materials that are used in producing the iron and other substances in the kiln from which the slag originated. Slag’s behavior in concrete is determined primarily by its chemical makeup.
Slag characteristics and replacement rate
Ground slag particles are approximately the same size as or slightly smaller than portland cement particles. Most slag grains are less than 45 micrometers in length or diameter, which gives the particles a greater surface area per unit volume compared to portland cement. Despite the material’s angularity and rough texture, incorporating slag cement into a concrete mix can reduce water demand due to the smaller particle sizes. Similar to how optimal particle packing among well-graded aggregates can help increase concrete’s strength and durability by reducing the number and size of void spaces, the same is true of the components that make up concrete’s paste. Incorporating finer cementitious particle sizes, such as with using partial slag cement replacement, enables the cement and SCM grains to pack closer together. To an extent, this can help reduce water demand. In cases where SCM particles are drastically finer than cement (for example, silica fume), the total surface area of material is significantly greater than cement, resulting in increased water demand.
Slag cement is typically used at a replacement rate between 30% and 50% by mass of cementitious material. However, in some unique scenarios, slag cement can account for up to 80% of the total cementitious material.
An advantage of slag cement is its consistency and uniformity in manufacturing. This can lead to enhanced predictability and repeatability.
Concrete made with slag cement generally has a decreased water demand but an increased level of workability. According to the Portland Cement Association, “The effect of slag cement on bleeding and segregation is generally dependent on its fineness. Concretes containing ground slags of comparable fineness to that of the portland cement tend to show an increased rate and amount of bleeding than plain concretes, but this appears to have no adverse effect on segregation. Slag cements ground finer than portland cement tend to reduce bleeding.”
Concrete’s air content is typically unaffected by slag cement. A cementitious material’s carbon content can cause changes in the fresh concrete’s air content, but slag is devoid of carbon. The lack of carbon helps lower fluctuations and unpredictable variability in entrained air.
Setting, strength development and durability
Since pozzolans extend the hydration process, concrete made with slag cement can exhibit a lower heat of hydration, slower set times and lower early age strength. However, at 28 days the strength results displayed by slag cement concrete can exceed those of ordinary portland cement concrete. Concrete made with pozzolanic SCMs can continue to gain strength well beyond 28 days, although the rate of strength development steadily slows over time. Slag cement concrete not only boasts increased compressive strength, but also improved flexural strength. This is largely attributed to increased paste density and improved paste-aggregate bond.
Slag cement concrete’s lower heat of hydration is beneficial in mass concrete applications and in cases where avoiding excessive concrete temperatures or cold joints is a concern.
Concrete made with partial slag cement replacement also generally exhibits decreased permeability, absorption and alkali-silica reactivity. Slag cement usage in concrete also helps boost corrosion resistance and resistance to sulfates.
Use with care
When adding slag cement to a mix design, it’s imperative to not only consider the positive results the new material will have, but also how the material could impact the behavior of other staple mix components and how other ingredients might affect the slag cement. As with any SCM, it’s also important to consider the type of product slag cement will be used in, the environment the product will be exposed to, and the conditions the concrete will face during placing, finishing and curing.
Kayla Hanson, P.E. is a technical services engineer with NPCA.
Portland Cement Association, Design and Control of Concrete Mixes, 16th Edition
Slag Cement Association, slagcement.org