The International Information Center for Geotechnical Engineers

Cement Additives for Permeation Grouting - Fly Ash

Fly Ash

Figure 7: Fly ash (Portland Cement Association)


Figure 8: Fly ash particles (University of Kentucky, 2014)



Fly ash (ASTM C618) is a by-product of the combustion of coal in power plants. In cement mixes, a portion of the cement can be substituted with fly ash due to its pozzolanic properties. Fly ash is a electrically precipitated powder produced from crushed coal. The fine (10 micrometer) particles are made up of silicate glass spheres containing silica, alumina, iron and calcium. The particle gradation is slightly more coarse than portland cement. Since fly ash is a waste product, it’s properties can vary by source (Weaver, 2007).

Fly ash comes in two different types: Class C and Class F. Class F is rather inexpensive and has pozzolanic characteristics but cannot set without a source of calcium (lime or cement). Class F fly ash is produced from anthracite or bituminous coal and cures slowly. Class C has both cementitious and pozzolanic characteristics, so it can set by itself without cement. It is produced from subbituminous or lignite coal and can be pulverized to improve hydraulic properties. If more than 15% by weight of cement is Class C fly ash, the grout can deteriorate due to expansive tendencies of the Class C fly ash. There should be no more than 10% carbon used in grouts containing fly ash since more water will be required (Weaver, 2007).

Typically for a fly ash/cement grout, 15-20% of the cement is replaced by grout but due to economic and environmental pressure, high volume fly ash/cement grouts (grouts containing > 55% fly ash) are being used with greater regularity. A few important property changes that occur from the use of high volume fly ash use include a decrease in flow time for low water/cement ratios, significantly increased stability for high water/cement ratios, a decrease in setting time due to the slow reaction of fly ash (shown below in Figure 9) and a reduction in modulus of elasticity (Mirza, 1999).

Figure 9: Effect of 60% fly ash on the initial setting time of portland cement and portland cement/fly ash mixtures (Mirza, 1999)


  • Often a cheap partial replacement for cement

  • Reduces heat generation during curing

  • Type F fly ash has sulfate resistant properties

  • Using fly ash in foam grouts can increase long term compressive strength, especially with type F fly ash (Henn, 2003)

  • Delays setting time and gain of strength

  • Provides chemical stability

  • Reduces permeability

  • Increases flowability/pumpability

  • Type C fly ash can increase water repellent characteristics (Weaver, 2007)

  • Reduces bleed water (Portland Cement Association)

  • Reduction of shrinkage upon drying (Mirza, 1999)


  • Reduced compressive strength (Mirza, 1999)

  • Increased setting time (Mirza, 1999)


Because of its typically inexpensive nature, fly ash is often used as a partial cement replacement for high volume applications such as soil, rock or oil well grouting (Mirza, 1999).

In the process of sliplining, briefly explained above in the foam grout section, significant amounts of fly ash are often used as a partial replacement of cement for foam grouts. Fly ash is primarily used to reduce cost, decrease shrinkage, increase flowability and manipulate other mechanical and chemical properties of a specific grout (Vipulanandan, 2000). An example of the shrinkage reduction of foam grouts from the use of class C fly ash can be seen below in Figure 10:

Figure 10: Reduction in shrinkage of a foam grout from the use of 50% fly ash replacement of cement (Vipulanandan, 2000)

Since fly ash is used as a cement substitute in many different concrete and grout mixes, it has positive environmental impacts. Of the 71 million tons of fly ash produced in 2004, about 40% of it was recycled by way of cement substitution and has the potential to save 10 million tons of carbon dioxide emissions annually (Portland Cement Association).

Case Study

Channel Tunnel Backfill Grouting in the UK (Gause)

When tunnels were constructed through the Lower Chalk Marl in the UK, there were many constraints on the grout used around the 20 mm gap between the concrete tunnels and the soil. The grout mix had to be pumped a long distance, but upon arrival at the construction location, must set quickly. Additionally, there was a lot of water in the soil which required the grout to be anti-washout and not bleed significantly. The grout also needed to gain a minimum 28 day strength of 8 MPa. After many tests, the ideal grout mix contained 50% Portland cement and 50% fly ash. It also contained superplasticizer and stabilizer to maintain pumpability and flowability. The large amount of fly ash was most likely used due to the excessively wet environment of placement. The anti-washout and anti-bleed properties were most likely met by the use of fly ash.


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