Using Metakaolin vs. VCAS

Question:
What’s the difference between VCAS and metakaolin, and can they be used interchangeably in concrete countertop mixes?

Answer:
Both VCAS and metakaolin are manufactured mineral admixtures. VCAS is a trademarked name for vitreous calcium alumino-silicate, and metakaolin is an amorphous alumino-silicate. VCAS is manufactured using post-industrial ingredients, while metakaolin is manufactured using mined kaolin clay.

VCAS and metakaolin are pozzolans, meaning they react with the calcium hydroxide generated during cement hydration. This increases the strength of the concrete while decreasing efflorescence, porosity and concrete alkalinity (read Benefits of Metakaolin As a Cement Substitute).

Both pozzolans are often used as cement replacements, although they could be used as cement additions. When used as a cement replacement, VCAS tends to have a more positive effect on workability, allowing for better flow and even a slightly reduced water-cement ratio. Metakaolin, on the other hand, tends to have a higher water demand, which means it decreases workability slightly when compared to a pure portland-cement-based mix. Typically more superplasticizer is needed to maintain workability with metakaolin, while VCAS usually allows for a slight decrease in superplasticizer.

Both metakaolin and VCAS increase the strength of concrete relative to plain cement-based concrete. However, very early strengths (after 3 days) are somewhat higher with metakaolin than VCAS. After 7 days, the strength of both metakaolin and VCAS mixtures begins to surpass the strength of plain cement-based concrete. In summary, metakaolin and VCAS can be used interchangeably (or even together) to boost longer-term strength. VCAS gives the edge in workability while metakaolin provides greater early strength. Both are white (or nearly white), so pigmented concrete remains true to the desired color.