Hydration kinetics, microstructure and physicochemical performance of metakaolin-blended cementitious composites

Abstract

This paper presents a comprehensive investigation on influences of metakaolin on the early and later performances of cementitious composites. Systematic measurements from hydration to physicochemical performance and thermodynamic modeling based on ternary compatible phase diagram were carried out. Different fineness of metakaolin with d50 of 25, 15.3 and 3.37 μm was adopted to replace ordinary Portland cement. The replacement level of metakaolin varied at 5 %, 10 %, 15 % and 20 %. Effects of metakaolin in view of pozzolanic activity and filling capacity on the microstructural formation of cementitious composites were quantified based on the Frattini test and the strength activity index. It is found that a higher addition or specific surface area of metakaolin decreases the fluidity of cementitious composites and meanwhile the setting time was delayed. The filling effect of metakaolin on cement hydration works primarily in the early 3 d while pozzolanic effect plays a major role afterwards. The relationship between particle size of MK and its pozzolanic activity becomes pronounced for metakaolin content greater than 10 %. The presence of metakaolin contributes to strength development mainly at late age. Finer particle size of metakaolin does not ensure a pronounced filling effect, which depends on the content of metakaolin. The metakaolin with high fineness d50 = 3.37 μm enables to promote cement hydration. The decrease of metakaolin particle size results in a higher content of C-A-S-H and high alumina-rich hydrates, as demonstrated by both experiments and simulation.