Proper Dosage of Pozzolans in Concrete

Question:
When I’m using pozzolans in concrete countertops, what’s the best dosage rate?

Answer:
Generally pozzolans replace some of the cement in concrete. Because they are treated like cement, the total cement content in the concrete mix does not change. This keeps color formulas and admixture dosages consistent. However, simply adding pozzolans to a mix without reducing the total cement content can alter your colors and admixture proportions.

The pozzolan replacement dosage depends on which pozzolan you use. Fly ash and slag, for instance, often replace 20% to 40% of the cement, while silica fume generally only replaces 5% to 10%. The differences often have to do with the reactivity of the pozzolan and how it affects workability. Both fly ash and slag are not as reactive as silica fume, and both enhance workability. Silica fume, on the other hand, inhibits workability, sometimes described as “increasing water demand.” This is a troublesome description because adding water to increase workability often has other negative effects, such as altered color, lower strengths and higher porosity. Superplasticizers are the better choice and can help create a very workable concrete with a low water-cement ratio.

Here are some typical dosages for common pozzolans:

• Fly ash, Class F: 15% to 25%
• Fly ash, Class C: 15% to 40%
• Slag: 20% to 50%
• Silica fume: 5% to 10%
• Metakaolin: 10% to 15%
• VCAS (vitreous calcium aluminosilicate): 5% to 25%

Replacing large portions of the cement with pozzolans (especially the ones that are waste products) is a good way to make your concrete “greener,” or more sustainable, because the production of portland cement releases large amounts of carbon dioxide, a greenhouse gas. However, doing this sacrifices the early strength of concrete (usually that attained at 7 days or earlier). In general, the greater the pozzolan replacement, the greater the reduction in early strength. Most pozzolans only improve the concrete’s strength after a few weeks of curing.

To confirm this, I cast some concrete test cylinders using two identical concrete mixes. One contained gravel, sand, gray portland cement, water and a plasticizer. The other was identical, but 10% of the cement was replaced by VCAS, a popular white pozzolan used by many concrete countertop makers. Compression tests performed by an independent test lab showed that the 3- day compressive strength of the VCAS cylinders was 9% lower than the straight portland cement cylinders, and the 7-day compressive strength was 7% lower. I chose these times because early strengths are important for crack-free handling and processing of countertops. The 28-day strength of the VCAS concrete was undoubtedly higher than the plain portland cement concrete, but with countertops, long-term strength is not as important as early strength.

It’s important to realize that pozzolans work with portland cement but won’t work with CSA cement (calcium sulfo-aluminate). CSA does not generate calcium hydroxide, so there’s nothing for pozzolans to react with. (For more information, read The Pros and Cons of Using CSA Cements.)