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What are SCMs, and how do they work?

SCMs (supplementary cementing materials) include fly ash, slag, silica fume, and other natural pozzolans. In mix design, SCMs are incorporated by replacing a percentage of the cement with the SCM. SCMs create a less permeable matrix by disconnecting the pore structure, making the pore solution less able to move freely. A less permeable pore structure does not promote the ingress of outside contaminants or moisture. Each SCM has its own advantages and disadvantages.

Figure A4a.F1. Photo. PCA Image of SCMs. This figure is a photo of various supplementary cementitious materials.
Figure A4a.F1. PCA Image of SCMs.

Picture by PCA:

Fly Ash

Fly ash is the result of the coal burning process and is considered to be a "man made" pozzolan that has been used in concrete construction for more than 50 years. Fly ash is categorized into sub groups according to its chemical composition. In the U.S. fly ash is categorized by two classes defined by ASTM: F and C. According to Canadian Standards Classification (CSA), fly ash is categorized by three types: F, CI, and CH.

Canadian Standards Classification - CSA A23.5

Type F Low calcium < *% CaO
Type CI Intermediate calcium 8 - 20% CaO
Type CH High Calcium > 20% CaO

ASTM C 618 Classification

Class F Normally from bituminous and anthracite coal SiO2 + AI203 + Fe2O3 ≥ 70%
Class C Normally from sub-bituminous and lignite coal SiO2 + AI203 + Fe2O3 ≥ 50%

ASTM Class F generally equivalent to CSA Types F & CI
ASTM Class C generally equivalent to CSA Type CH

Figure A4a.F2. Classifications of Fly Ash.

The first work to demonstrate the effective use of fly ash was reported in 1952. There are hundreds of publications on fly ash and ASR that show the effect of fly ash depends on:

  • Amount of fly ash
  • Fly ash composition
  • Nature of reactive aggregate
  • Amount of alkali present in the concrete (e.g. from cement)

Typically, the amount of fly ash required to control ASR increases as:

  • Calcium content of the fly ash increases
  • Alkali content of the fly ash increases
  • Reactivity of the aggregate increases
  • Amount of available alkali in the concrete increases

Work done by Shehata and Thomas in 2000 based on ASTM C 1293 testing showed low calcium fly ash to be more efficient in controlling expansion than high calcium fly ash. This trend has been attributed to the fact that low calcium ashes reduce pore solution pH through a pozzolanic reaction that results in a diminished transition zone due to more C-S-H that binds significant amounts of alkalies (Na+ and K+).
There has been some testing by the University of Texas to suggest that higher calcium ashes, class C ashes, can control expansion due to ASR. However, the amount of ash required to do so successfully (more than 50% replacement of cement in some cases), produces its own unique set of problems and is not recommended.


Ground granulated blast furnace slag (GGBFS) is a by-product of the iron making industry. GGBFS is commonly referred to as slag or slag cement and has been used in concrete for more than 100 years. The first work to show slag having an effect on ASR was in 1952. There are many publications on slag and ASR that show the effect of slag depends on:

  • Amount of slag used
  • Nature of reactive aggregate
  • Amount of alkali present in the concrete (e.g. from cement)

Silica Fume

Silica fume is a by-product of the silicon metal and silicon-iron making industry. The use of silica fume for controlling ASR is less well-established as the use of fly ash and slag. In 1989, Oberholster showed increased alkali content of the concrete required greater percentage of cement replacement with Silica fume to effectively curb expansion due to ASR. In recent years, work by Thomas and Bleszynski (2000) proposed upper and lower bounds for the amount of silica fume required base on the alkali content of the concrete. However, their work indicated a need for more than the typical replacement by mass dosage in cases of extremely reactive aggregates. The problem with adding more than the typical dose of silica fume lies in the reduced workability and shrinkage problems that are created.

As an alternative, recent years have seen increased applications involving silica fume as part of ternary blends. Ternary cement blends, which contain three different cementing materials (portland cement plus two SCM) have been found to be very effective in controlling ASR, especially blends containing silica fume plus either Class F fly ash or slag. Relatively low levels of silica fume (3 to 5%) together with moderate levels of Class F fly ash (15 to 20%) or slag (25 to 35%) have been shown to be effective (Shehata and Thomas, 2002; Bleszynski et al., 2002).

Natural Pozzolans

Calcined shales and clays along with metakaolin are considered natural pozzolans. They are used to reduce permeability and increase strengths. Calcined clays and shales are used as cement replacements. Metakaolin is more often used as an additive to the cement content. ASR expansion is minimized due to the low permeability inhibiting the ingress of moisture.

Updated: 02/20/2015

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