Reaction kinetics of basic clay components present in natural mixed clays

Abstract

The investigation reveals the reaction kinetics of three calcined phyllosilicates (metakaolin, metaillite and metamuscovite) in an alkaline solution which was prepared without the addition of clinker. The phyllosilicates were calcined at their individual optimal calcination temperature. Two test series without and with the addition of anhydrite were investigated. Calcite is present in all measurement series due to the impurity of portlandite. The reaction kinetics were investigated by means of isothermal calorimetry and in-situ x-ray diffraction (XRD). All measurement series show crystalline reaction products after 7 d in the absence of anhydrite. The addition of anhydrite leads to a first formation of ettringite within the first 13 h of reaction with all phyllosilicates tested. The water absorption capacity of phyllosilicates does not correlate with the specific surface area measure as BET-surface. Especially for metamuscovite, the high water absorption leads to a simultaneous formation of monosulfate and gypsum after 17 h. While metakaolin exhibits a significant concentration of dissolved alumina and silicon ions, the influences of metaillite and metamuscovite are less pronounced in that respect and exhibit reaction at a later stage. Nevertheless, it can be concluded that the reactivity of naturally occurring mixed layered clays cannot be reduced their metakaolin content but is due in addition to the contribution originating from other clays present in the mixtures. Metaillite and metamuscovite contribute even to the formation of hydrate phases at early ages of the reaction. Thus, the content and kind of phyllosilicates deserve more attention when used as supplementary cementitious material because of their high water absorption, their contribution to reactivity and their consumption of portlandite.