Title: Metakaolin in Concrete: The Role of Alumina

Author(s): Michael Thomas

Publication: Web Session

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Date: 10/17/2021

Abstract:
Data are presented from a number of studies on the durability of mortar and concrete containing metakaolin (MK), a calcined high-purity kaolin clay (kaolin: Al2O3·2SiO2·2H2O). Long-term data from concrete specimens exposed to the tidal zone on a marine-exposure site indicate that chloride penetration into concrete containing 12% MK is much less than that predicted based on permeability/diffusion measurements and comparisons with concrete containing silica fume with similar permeability and diffusion coefficients. The increase in chloride resistance of MK concrete is attributed to the significant increase in chloride-binding resulting from the use of MK which is high in alumina (approximately 45% Al2O3) and results in the formation of increased amounts of Friedel’s salt. The high alumina content of MK has also been credited for its efficacy in controlling damage due to delayed ettringite formation (DEF), a form of internal sulfate attack, with elimination of deleterious expansion at relatively low levels of replacement (for example, 10% MK). Unfortunately, the high alumina content of MK has been found to reduce the resistance of mortars exposed to external sulfate attack when the MK is used at moderate levels of replacement (10 to 15% MK) and this has been ascribed to the increased capacity for ettringite formation. At higher levels of replacement (= 20% MK), the sulfate resistance of mortars is increased by the use of MK. Reasons for this paradoxical behavior are discussed. Finally, data are presented from studies on alkali-silica reaction (ASR) which indicate that the alumina content has little impact on the ability of MK to suppress the alkalinity of the concrete pore solution and control expansion with reactive aggregates; these benefits are shown to depend predominantly on the silica content of the MK (approximately 55% SiO2).