ASR in Concrete — PART 4

By Jon S. Belkowitz, PhD

Introduction to Nano Silica

Nano silica has been produced since the early 1950s but has only been used in concrete for the last two decades. Despite the early availability of nano silica to the concrete and construction industry, little has been used on commercial job-sites. Early on during the commercial application of nano silica in the concrete industry, misuse and lack of understanding of the importance of particle size and surface area when dealing with these nanoscale pozzolanic materials resulted in concrete failures. Unfortunately, these early failures hindered the use of nano silica in U.S. markets and resulted in a less than favorable foundation for future use.

Although nano silica is used at a much lower quantity than standard pozzolanic materials such as Class F fly ash and silica fume, it offers several distinct advantages over standard pozzolanic materials when used in cement composites and concrete: 1) early strength development, 2) increased cement efficiency 3) concrete densification, and 4) increased resistance to chemical degradation. Nano silica dispersions, like standard pozzolanic materials, are characterized by their particle size (measured by diameters), particle distribution, silica purity, and percent replacement of cementitious material. Subsequently, the main hydrate of both pozzolanic materials (nano silica and Class F fly ash) yield a C-S-H similar to that produced from the cementitious reaction of di-calcium silicate (C2S) [5].

While nano silica dispersions offer particular benefits over standard pozzolanic materials, when using these particles in cement composites and concrete in laboratory and commercial applications, the distinct properties of nano silica require attention. Particularly, nano silica dispersions are extremely prone to agglomerating from poor dispersion and over-saturation based on the size, surface area of the particle used, and the proportion of ordinary Portland cement (OPC) replacement. This phenomenon has been recognized to have a higher sensitivity with smaller nano silica particles [6, 23].

Nano silicas in concrete experience pozzolanic reaction, a microstructural enhancement that increases the density of the hydrated cement matrix (HCM), as a result, increases the strength and durability of concrete. Furthermore, the use of nano silica provides an excessive amount of free silica breaching the pessimum for alkali-silica reactive (ASR) gel polymerization and expansion (a form a concrete chemical degradation). Ultimately, nano silica brings a much greater amount of free silica content than standard pozzolanic materials to enhance the microstructure and macroscopic properties of the concrete.

References

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10. Bentz, D. and Stutzman, P. Evolution of Porosity and Caclium Hydroxide in Laboratory Concretes Containing Silica Fume. Cement and Concrete Research, 1994. 24: p. 1044–1050.

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21. Garcia-Diaz, E., D. Bulteel, Y. Monnin, and P. Fasseu. ASR Pessimum Behavior of Siliceous Limestone Aggregates. Cement and Concrete Research, 2010. 40: p. 546–549.

22. Helmuth, R. Fly Ash in Cement and Concrete. U.S.A.: Portland Cement Association, 1987: p. 85–138.

23. Said, A., M. Zeidan, M. Bassuoni, and Y. Tian. Properties of Concrete Incorporating Nano-Silica. Construction and Building Materials, 2012. 36: p. 838–844.

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