By Kayla Hanson, P.E.
Editor’s Note: This is the third 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 second article in the series on slag cement, click here.
The third supplementary cementitious material on the list of the most commonly used manufactured SCMs is silica fume, also referred to as microsilica or condensed silica fume. According to Eckart Bühler, engineering services manager at Norchem Inc., silica fume is a very fine pozzolan that has been widely used in high-performance concrete applications for more than 30 years. It is ideally suited for precast concrete operations due to the high level of quality control involved in every step from material procurement to shipping and high-efficiency mixers.
“Advanced proportioning experience with silica fume can provide engineering sophistications from viscosity modification in highly flowable concrete mixtures to acceleration of production cycles with attaining one-day compressive strengths up to 10,000 psi,” Bühler said.
Typically, silica fume concrete is specified by the engineering community for added durability and overall lower life cycle costs in concrete structures.
Silica fume basics
Silica fume is a byproduct from the production of silicon alloys in an electric arc furnace. The materials are heated to more than 3,600 degrees Fahrenheit in a process that separates silica from the oxygenated silica fume. The process removes the oxygen from the silicon, separating the silica from the oxygenated silica fume. As it leaves the furnace, silica fume cools and condenses and is collected via filtration systems. Facilities collect the emanating silica vapor and process it for use in concrete.
On average, silica fume particles are about 1/100 the size of portland cement grains, measuring around 0.1 micrometers in diameter. The extremely small, spherical particles give silica fume a very high fineness and a larger surface area per volume, especially when compared with portland cement.
Small, uniform cementitious particle sizes are beneficial in concrete applications where optimized particle packing and density is crucial. Silica fume’s extremely fine particle size makes working with the material as produced impractical. It is primarily used in its densified form at a replacement rate of about 5% to 10% by total mass of cementitious material.
Like fly ash and slag cement, silica fume’s chemical makeup is dependent on the nature of the materials combined in the furnace. Silica fume created during silicon alloy production has a higher silica content, while silica fume produced as a result of manufacturing ferrosilicon alloys has a significantly lower silica content. Silica fume’s behavior in concrete is predictable, but still dictated by the material’s composition.
Production using silica fume
Silica fume reacts with calcium hydroxide produced by cement hydration reactions and produces more calcium silicate hydrate, which continues to add strength to the concrete matrix. When smaller quantities of silica fume are used as a replacement for portland cement, water demand may not increase. However, when replacement rates exceed approximately 5%, water demand will increase and workability can decrease. As a result, some mixes tend to develop a sticky consistency that could make finishing more difficult.
Silica fume also has a drastic effect on bleeding. Special care needs to be taken during placing, finishing and curing because mixes made with typical proportions of silica fume and a low water-cementitious material ratio may not bleed. As such, concrete made with silica fume has a higher tendency to develop plastic shrinkage cracks unless appropriate measures are taken to avoid mix water evaporation.
Like fly ash, silica fume will typically decrease air content because of an increased amount of finer cementitious material in the mix.
Setting time and strength development
When used at normal replacement rates, silica fume has little effect on setting time and little impact on heat of hydration. However, silica fume’s particle size and rapid pozzolanic reaction rate can help increase early strength gain. As with most pozzolans, concrete made with silica fume can continue to develop strength over time at a rate greater than ordinary portland cement concrete. The duration and extent of increased strength development is more drastic with silica fume than fly ash or slag cement.
SCMs increase durability
Concrete made with silica fume can dramatically increase corrosion resistance, decrease alkali-silica reactivity and increase resistance to sulfates. This increased durability and resilience can be attributed to decreased permeability and absorption. Silica fume is often used in high-strength concrete or high-performance concrete. In many mixes, compressive strengths in excess of 20,000 psi are possible.
A low water content and in turn, a low w/cm ratio, reduces porosity, or the ratio of the total volume of void space to the total volume of concrete. Lower porosity can lead to lower permeability, which makes it more difficult for any detrimental materials to enter and travel through concrete. All these factors help create a denser, less porous and less permeable product, which directly relates to increased durability.
Fly ash, slag cement and silica fume have little to no impact on concrete’s drying shrinkage and creep. Concrete’s behavior in these conditions is mostly affected by the relation between cementitious paste and water content. These SCMs also have no notable impact on freeze-thaw resistance.
Silica fume is often used as a mix design tool to increase concrete’s abrasion and impact resistance by strengthening the cementitious fraction of concrete to the level of the aggregate’s hardness. In this scenario, it is of benefit to decrease the total binder content and maximize aggregate content. Silica fume is more strength producing than portland cement, fly ash or slag cement. Therefore, the total binder content can be significantly reduced, achieving the highest possible psi per pound of binder. Higher compressive strength at minimized total binder volume assures maximized abrasion and impact resistance of concrete with any given aggregate.
Sustainable benefits and decreased life cycle costs
When designers choose materials for a project, they often look to use materials with lower environmental impacts and with recycled content. The use of SCMs in manufacturing concrete can help fulfill those goals.
Use of blended cement or the partial replacement of portland cement with industrial byproducts such as fly ash, slag cement, silica fume and other SCMs reduces the amount of clinker required per cubic yard of concrete. Because the clinker and cement manufacturing process requires so much energy, using less clinker reduces the resultant CO2 emissions and in turn, reduces the carbon footprint. For example, silica fume is designated by the U.S. Environmental Protection Agency as a recovered mineral component. This means it plays an important role in the sustainability of a structure by reducing the carbon footprint of concrete, according to Bühler.
When industrial byproducts are used, they not only provide a sustainable option through reuse, but also improve concrete properties while reducing cost.
Using blended cements or any appropriate partial replacement for OPC can result in cost savings. When a new component in a mix design provides a longer service life at a lower up-front cost and offers less maintenance and less cost throughout the product’s life, it may sound too good to be true. However, when used in appropriate applications, SCMs can make this scenario a reality.
Remember to use SCMs with care
Despite offering beneficial physical characteristics or contributing to concrete strength, SCMs must be used with care.
For instance, the benefits of one SCM may be offset or eliminated when incorporating an admixture or adding a third cementitious material. In another scenario, the fresh concrete may not behave as expected due to a fluctuation in the SCM’s source material or its manufacturing process. But a three-part blend of portland cement and two SCMs may bring out the best qualities in each component.
It’s important to weigh the benefits and drawbacks associated with each SCM and carefully proportion and adjust any mix design in accordance with the manufacturer’s recommendations, governing standards and specifications.
Kayla Hanson, P.E. is a technical services engineer with NPCA.