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Cement Additives for Permeation Grouting - Silica Fume

 Silica Fume

 

Figure 13: Silica fume (Portland Cement Association)

Introduction

Silica fume (ASTM C1240) is a by-product of the extraction of silicon or ferrosilicon manufacturing. The particles are composed of glassy spheres almost entirely made up of SiO2. The diameter of the particles ranges from about 0.1 to 0.15 micrometers and is generally condensed and precipitated to prevent the particles from going airborne. Silica fume can either be used as a replacement for cement within a grout mixture or as an additional component. As a substitution for cement typically only 4-10% by weight of cement is substituted (Weaver, 2007). Only small amounts of silica fume are substituted because it has been shown that 4-10% is the optimal amount to improve grout strength and elastic modulus. However, pumpability and strength are tradeoffs to grouts and especially important for silica fume since it does not greatly increase the strength of grout  as a replacement for cement (Akbulut, 2003). Using silica fume as a partial replacement for cement can also be a good way to create a lighter weight material, for a given compressive strength, silica fume can replace approximately 3 to 4 times its weight of cement (Aitcin, 1984).

Using silica fume as an additive as opposed to a replacing agent, can significantly increase the strength and decrease the permeability of a cement grout. This is the product of the silica fume reacting with the lime released by the hydration of the cement and creating a “compact secondary CSH”. In addition to increasing the strength and decreasing the permeability of a grout, this reaction increases the stability and decreases the threat of chemical attack (Aitcin, 1984). The use of varying amounts of condensed silica fume as an additive for a cement grout to increase strength can be seen below in Figure 14:


Figure 14: Comparison of compressive strength values for varying amounts of condensed silica fume (Aitcin, 1984)

Advantages

  • Greatly reduces permeability due to the small particle size

  • Improves grout stability

  • Improves durability

  • Water resistance (Weaver, 2007)

  • Resistance to chlorine penetration (Portland Cement Association)

  • Improves pumpability (Henn, 2003)

  • Increases chemical resistance  

  • Reacts rapidly for a pozzolanic material

  • Reduces and prevents lime leaching from the hydrated grout  (Aitcin, 1984)

Disadvantages

  • If added in a condensed form such as pellets, the pozzolanic reactivity is reduced and performance and efficiency of the grout is compromised (Weaver, 2007)

  • Expensive (Henn, 2003)

  • A superplasticizer may be required to counteract the increase in viscosity caused by partial replacement of cement with silica fume (Aitcin, 1984)

Applications

Due to its many advantages, listed above, silica fume as either an additive or a partial replacement for cement grouts, is applicable to many different construction applications. Because silica fume drastically increases the cohesive properties of a grout, it is an ideal additive for underwater grouts. The decreased permeability resulting from the use of a silica fume/cement grout makes it a useful additive for post-tensioning applications. Silica fume can also be an extremely useful additive to reduce or eliminate unwanted reactions in chemically harsh environments. The use of silica fume/cement grouts for oil well drilling applications slows or even stops gas leakage from the grouted well. Shotcrete, which is often used in underground applications such as tunnels and mining, also often uses a large quantity of silica fume to increase adhesion (Norchem, 2013).

Case Study

Compressed screw piles in British Columbia (Vickars, 2000)

In the 1990s in Vancouver, British Columbia, compression screw piles were grouted in soft, organic and unconsolidated soils. The use of grout around these piles improved the load capacity in the soft soils. Silica fume was used in the grout mix to minimize shrinkage, improve strength, improve corrosion protection of the piles, and bond well to the piles. The addition of silica fume also increased the density and flowability of the grout.

 

In one test site, an ungrouted pile driven to 15.8 m (52 ft) had a capacity of 169 kN while a grouted pile in the same location had a capacity of 320 kN driven to a depth of 13.7 m (45 ft). At the other test site, both grouted and ungrouted piles were driven to the same depth. However, the grouted pile had a capacity of 302 kN while the ungrouted pile had a capacity of 200 kN and the grouted pile deflected less at failure. In general, grouting improved the capacity of the piles at this site by about 60%.

 

 

 

 

 

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