|Ph.D Student||Shvartsman Asia|
|Subject||Properties and Microstructure of High-Performance|
Cementitious Systems with Metakaolin
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Konstantin Kovler|
The potential use of metakaolin manufactured from different sources of kaolin (a) as a component replacing Portland cement in concrete construction and (b) as a binder component in producing high-performance cementitious materials, is discussed.
The research focuses on a study of microstructure, hydration process and mechanical properties of high-performance cementitious materials made with metakaolin.
The effect of heating temperature on the dehydroxylation/amorphization process of kaolinite-based materials, such as natural and artificial kaolin clays containing different amounts of amorphous phase (metakaolin), has also been the subject of the investigation. The procedure for quantitative estimation of amorphous phase in the heat-treated kaolinite materials was developed. The process of dehydroxylation/amorphization of kaolinite was characterized by method combined differential thermal analysis with mass-spectrometry and X-ray powder diffraction. The influence of the heating temperature and content of the amorphous phase on pozzolanic activity was studied. The relationships between the chemical activity, Strength activity index and amorphous phase content were found and discussed. The thermal, mechano-chemical based on milling in the presence of special chemical admixtures and combined (thermal and mechano-chemical) treatments were applied for the activation of kaolin-based materials used as pozzolanic admixtures in different classes of cementitious systems: pastes and mortars. The effectiveness of two different types of activation was discussed. The effect of milling time, type and content of superplasticizer, and calcination temperature on the specific surface area, particle size distribution, morphology, flowability, compressive strength and pozzolanic activity of cementitious materials were studied. It was found that the heat treatment resulted in substantial increase of the Strength activity index. Strength activity index, specific surface area and fineness increased significantly with milling time. The change of kaolinite morphology during milling was also observed.
The compressive strength development metakaolin-portlandite paste as a function of microstructural change was studied as well. The kinetics of metakaolin - portlandite reactions, effects of portlandite/metakaolin ratio on both compressive strength and microstructural development were investigated. Portlandite - metakaolin pastes containing 25%, 50%, 65%, 80%, 90% and 95% of metakaolin were prepared at water/solid ratio of 1. The reaction kinetics was studied using DTA. The nature and quantity of hydration products were determined and the volumes of hydration products and porosity were calculated. A detailed microstructural investigation on the pastes was performed by scanning electron microscopy. The results indicate that the compressive strength development depends on portlandite/metakaolin ratio, volume and morphology of hydration products, such as gehlenite hydrate and calcium silicate hydrate (C-S-H).
In the additional series of tests the hydration process of metakaolin - Portland cement pastes was investigated by examining hydration products. The replacement percentage of metakaolinite was 8.4%, 15% and 20% by weight, respectively, and water to binder ratio equal to 0.33 in all the pastes. The relationships between microstructure, content of hydration products and strength development were obtained. The influence of the hydration products content on the development of strength is discussed.
Finally, effect of metakaolin on mechanical and physical properties, such as compressive strength, flexural strength, modulus of elasticity, drying shrinkage and chloride resistance, was investigated for high-performance concrete. Concrete cube specimens made with different content of metakaolin (10, 20, 40 and 60% by weight of binder) were tested.
The results obtained are important for optimization of large-scale industrial production of metakaolin and its use as active pozzolanic admixture in concrete.