Introduction

High performance concrete (HPC) is a modern construction material that has gained significant popularity due to its excellent properties, such as high strength, durability, and reduced permeability. HPC is specifically designed to meet the demands of modern construction projects by offering enhanced performance characteristics. One of the essential components that contribute to the improved performance of HPC is the use of mineral admixtures. In this blog, we will discuss the role of mineral admixtures in HPC, with a special focus on silica fume.

What are Mineral Admixtures?

Definition and Importance

Mineral admixtures, also known as supplementary cementitious materials (SCMs), are finely divided materials that are added to concrete mixtures to improve their performance. These materials are derived from natural sources or industrial by-products and are used to replace a portion of cement in concrete. The use of mineral admixtures has several advantages, such as:

• Enhanced durability and performance of concrete
• Reduction in cement consumption
• Lower environmental impact and carbon footprint
• Cost-effectiveness

Some common types of mineral admixtures include fly ash, ground granulated blast furnace slag (GGBFS), silica fume, and metakaolin.

Silica Fume in High Performance Concrete

Overview

Silica fume, also known as microsilica, is a by-product of the production of silicon and ferrosilicon alloys in electric arc furnaces. It consists of very fine spherical particles that are rich in silicon dioxide (SiO2) content, which typically ranges between 85% and 98%. Due to its unique properties, silica fume has become an essential component of high-performance concrete mixtures.

Properties of Silica Fume

Silica fume exhibits several properties that make it a highly effective mineral admixture for HPC, such as:

• Particle size: Silica fume particles are extremely fine, with an average diameter of about 0.1 micrometers, which is 100 times smaller than cement particles. This small particle size helps in improving the packing density of concrete, leading to reduced permeability and increased strength.
• Pozzolanic activity: Silica fume exhibits high pozzolanic activity, which means it reacts with the calcium hydroxide (a by-product of cement hydration) to form additional calcium silicate hydrate (CSH) gel, the primary binding agent in concrete. This reaction contributes to enhanced strength, durability, and resistance to chemical attacks.
• High silica content: The high SiO2 content of silica fume makes it an excellent mineral admixture, as it actively participates in the hydration process and improves the microstructure of the concrete.

Advantages of Using Silica Fume in HPC

Incorporating silica fume into high-performance concrete mixtures offers several benefits, including:

• Increased compressive strength: Silica fume can significantly enhance the compressive strength of concrete by improving its microstructure and increasing the CSH gel formation. Research has shown that concrete mixtures containing silica fume can achieve compressive strengths of over 100 MPa.
• Improved durability: The use of silica fume in HPC leads to a denser and more impermeable microstructure, which enhances the durability of the concrete. This results in increased resistance to aggressive chemicals, chloride ingress, and sulfate attacks.
• Enhanced resistance to abrasion and erosion: The dense microstructure of HPC containing silica fume provides better resistance to abrasion and erosion, making it suitable for applications such as industrial floors, bridge decks, and hydraulic structures.

• Reduced alkali-silica reaction (ASR): Silica fume’s pozzolanic activity can help mitigate the risk of ASR in concrete. ASR is a deleterious reaction between the alkalis in cement and reactive aggregates, leading to the formation of a gel that swells and causes cracking in the concrete. By consuming the calcium hydroxide and reducing the alkalinity of the concrete, silica fume helps to minimize the potential for ASR.
• Improved bond strength: The use of silica fume in HPC can lead to improved bond strength between the concrete and reinforcement bars, which is crucial for the structural performance of reinforced concrete elements.

Optimum Silica Fume Content in HPC

The optimum silica fume content in high-performance concrete mixtures can vary depending on the specific requirements of a project. Generally, the silica fume content ranges between 5% and 15% by weight of the total cementitious materials. However, a higher silica fume content can be used for special applications where extreme strength and durability are needed. It is essential to properly adjust the water content and superplasticizer dosage in HPC mixtures containing silica fume to ensure adequate workability and ease of placement.

Other Mineral Admixtures for HPC

While silica fume is a highly effective mineral admixture for high-performance concrete, it is not the only option available. Other common mineral admixtures used in HPC include:

• Fly ash: A by-product of coal combustion in power plants, fly ash is known for its pozzolanic properties and its ability to improve the workability of concrete mixtures. It is often used in combination with silica fume to optimize the performance of HPC.
• Ground granulated blast furnace slag (GGBFS): GGBFS is a by-product of the iron and steel industry and exhibits both cementitious and pozzolanic properties. When used in HPC, GGBFS can contribute to improved workability, reduced heat of hydration, and increased long-term strength.
• Metakaolin: Produced by calcining purified kaolinite clay, metakaolin is a highly reactive pozzolanic material that can significantly enhance the strength, durability, and resistance to chemical attacks in HPC.

Conclusion

Mineral admixtures, particularly silica fume, play a crucial role in enhancing the performance of high-performance concrete. By improving the microstructure of concrete, silica fume contributes to increased compressive strength, durability, and resistance to chemical attacks. The use of mineral admixtures in HPC not only improves the performance of the material but also promotes sustainability by reducing cement consumption and utilizing industrial by-products.