Effect of increasing the Blaine fineness of Metakaolin on its chemical reactivity

Abstract

Metakaolin (MK) is widely used as a partial substitute for cement in concrete, mainly due to its high reactivity. MK exhibits simultaneously a chemical reactivity known as pozzolanicity and a physical one, principally linked to the material's fineness. The chemical reactivity of MK depends on its chemical and mineralogical compositions, its dehydroxylation/amorphization degree but also on its fineness. The present study attempts to clarify the effect of increasing the Blaine fineness of MK on its chemical reactivity assessed by chemical tests of Frattini, modified Chapelle and saturated lime and the 28-day compressive strength. Thermal treatment of Kaolin at 800 °C for 5 h was used to obtain MK; however the uncalcined kaolin was used, at the same proportion and fineness, as an inert material for comparison purposes. It has been observed that chemical tests are more sensitive to fineness of MK than mechanical test. Chemical reactivity increases with fineness in the case of chemical tests, however, it does not appear to be influenced in the case of compressive strength test, since the strength activity index remains unchanged although the Blaine fineness increases from 4290 to 8220 cm2/g. It has been concluded that the fineness of MK has promoted its lime consumption in the case of chemical tests, but factors such as MK content, amount of reactive phases and the depletion of fine particles did not allow improving the chemical reactivity in the case of compressive strength test.