Pozzolan dosing for concrete countertop mix designs

I recently got the following question regarding my blog post “Mix design for cast in place concrete countertop in Cayman“:

Q: How much pozzolan can you safely add?

First, see my article on what pozzolans are and what they do.

The most common pozzolans for concrete countertops are:

  • VCAS
  • Metakaolin

I recommend and sell VCAS because it is white, recycled and enhances workability. Purchase VCAS here.

Dosage of VCAS is:

  • For precast mixes, 15% cement replacement
  • For GFRC mixes, 20% cement replacement

Dosage of pozzolans depends upon the particular pozzolan selected:

PozzolanDosageEffects
Fly ash: Class C15% to 40%Increased workability. Early strength suffers with higher replacement doses
Fly ash: Class F15% to 25%Increased workability. Early strength suffers with higher replacement doses
Slag25% to 50%Increased workability. Early strength suffers with higher replacement doses
Silica fume5% to 10%Significantly decreased workability
VCAS5% to 25 %Increased workability
Metakaolin10% to 15%
Decreased workability

The mix I used for the cast-in-place gazebo in Grand Cayman was based on a standard island 6-sack mix (564 lbs cement per cubic yard of concrete).

  • To this mix I added 140 lbs of a recycled glass pozzolan similar to VCAS per cubic yard. This translates into adding about 25% VCAS.
  • When you add up the cement and VCAS (700 lbs), the equivalent cement replacement is 20%: There are 140 lbs VCAS per 700 lbs total cementitious material (cement+VCAS).
  • 20% is a very common amount for glass-based pozzolans when used as a cement replacement (or 25% when used as an addition, like I did).

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