Investigation of Microstructure Properties and Early Age Behavior of Cementitious Materials Containing Metakaolin

Metakaolin is one of the most attracting gradients of high and ultra high performance concretes which are used in different types of buildings such as nuclear waste storages, impact-resistant military buildings, fire-resistant high-rise buildings and reinforced concrete bridges. The metakaolin induced concretes are very dense and more homogeneous than normal strength concrete. However, these properties cause significant increase in brittleness of hardened concrete and change of volume during the hardening of these materials. In this work, cement pastes were produced at three different water/binder (w/b) ratios of 0.42-0.35 and 0.28 for which cement has been replaced by metakaolin with at different weight fractions of 8%, 16% and 24%. 48 hours volumetric autogenous shrinkage measurements showed that in pastes with 0.42 w/b ratios autogenous shrinkage reduced with increasing metakaolin content. In mixtures with 0.35 and 0.28 w/b ratios, on the other hand, it has been seen that the autogenous shrinkage increased with increasing the amount of metakaolin. Using thermo-gravimetric analysis and mercury porosimetry measurements, it has been found that with increasing metakaolin content the amount of calcium hydroxide decreased and critical pore size reduced. It has been determined that as a result of the pozzolanic reaction the amount of fine gel pores increased and those of the coarser pores significantly decreased at 28 days. One year linear autogenous shrinkage measurements showed that in pastes with 0.42 w/b ratios the autogenous shrinkage increased with increasing metakaolin content, while it decreased in pastes with lower w/b ratios.