Influence of Ultrafine Metakaolin in Improving the Compressive Strength and Durability Properties of Concrete

This research reports on the effects of using ultrafine metakaolin to improve the durability properties of concrete. The particle size of ultrafine metakaolin was obtained using high-energy milling with 2 h of grinding. An experimental investigation was carried out by replacing portland composite cement with 5 % and 10 % ultrafine metakaolin. The results show that the optimum amount of ultrafine metakaolin in concrete mixes is 10 % by weight. For instance, the compressive strength of the concrete specimens with 10 % ultrafine metakaolin (UM-10) improved the compressive strength by 123 % at 3 days, 85 % at 7 days, and 53 % at 28 days, respectively. The results from water sorptivity and volume permeable void tests also confirmed the significant effects of ultrafine metakaolin in terms of it improving the durability resistance and increasing the density of the concrete. The water sorptivity value of the UM-10 sample was found to be 76 % and 68 % lower compared to the control concrete (PC). The percentage of the volume of the permeable voids in UM-10 was also reduced at about 70 %, indicating the modification of the pore size distribution because of the ultrafine sizes of the metakaolin. The results were confirmed by microstructure analysis such as scanning electron microscopy/backscattered electron image, X-ray diffraction, and differential thermal analysis/thermogravimetric analysis conducted on hardened cement paste with and without ultrafine metakaolin. It was concluded that the addition of ultrafine metakaolin influences the pore modification and reduces the calcium hydroxide (CH) content, thus initiating the formation of calcium silicate hydrate gel as one of the beneficial effects of amorphous ultrafine metakaolin that results in the enhancement of the performance of the cement concrete composites with ultrafine metakaolin.