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Fly Ash, Slag, or Silica Fume?

When specifying your Concrete Mix Designs, are you aware of the cost implications of pozzolans or supplementary cementitious materials included in your mix?  Is fly ash, slag, or silica fume more costly and how much is needed to achieve a mix with the same 28 day strength?

In September 2020, Otto Schwarz, PE, SE, from Ryan Biggs | Clark Davis presented Concrete Mix Design for SE University.  Otto reviewed the composition and curing process of structural concrete and explained the function and roles of pozzolans or supplementary cementitious materials and admixtures.  He also advised on what to look for in a Concrete Mix Design Review and how to qualitatively predict the relative behavior of a concrete mix design.

Otto showed a visual which explains the Strength Activity Index for different pozzolans and supplementary cementitious materials and relates these to the costs for each material.  To hear Otto’s explanation, watch this short video.

As Otto explained, the material costs will obviously vary greatly depending on region, but in most cases, substituting fly ash or slag in place of silica fume will save on the overall cost of the mix.  Also, you can see the pound for pound replacement needed for each material to portland cement in order to produce a mix with the same 28 day strength.  Engineers should be aware of the cost implications and effectiveness when specifying pozzolans and/or supplementary cementitious materials in their concrete mix designs.