15234 Metakaoline

Metakaoline, generally called "calcined clay,'' is a reactive aluminosilicate pozzolan produced by heating kaolinite at a specific temperature regime. It may react with calcium hydroxide to form calcium silicate and calcium aluminate hydrates. The type of mineral admixture conforms to ASTM C618 class N pozzolan specifications. Metakaoline (Figure 15.5) typically has an average particle size of about 1.5 mm in diameter, which is between silica fume (0.1 to 0.12 mm) and Portland cement (15 to 20 mm).

As a new mineral admixture for producing HPC, metakaolinite can produce HPC of compressive strengths in excess of 110 MPa through replacing part of the cement in the mixture. Tests have shown that use of 5 to 10% by mass of metakaoline can produce HPC with performance characteristics comparable to those of silica fume concretes with respect to strength development, chloride ion penetration, drying shrinkage, and resistance to freeze-thaw cycles and scaling. Further tests are required to confirm these findings.

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