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:
I recommend VCAS by Vitro Minerals because it is white, recycled and enhances workability.
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:
|Fly ash: Class C||15% to 40%||Increased workability. Early strength suffers with higher replacement doses|
|Fly ash: Class F||15% to 25%||Increased workability. Early strength suffers with higher replacement doses|
|Slag||25% to 50%||Increased workability. Early strength suffers with higher replacement doses|
|Silica fume||5% to 10%||Significantly decreased workability|
|VCAS||5% to 25 %||Increased workability|
|Metakaolin||10% 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).