| Supplementary cemetitious materials (SCMs) have been an integral part of ultra high performance fiber reinforced cementitious composites (UHPFRCC), in which silica fume (SF) is widely utilized. However, the quality and output are both unstable as silica fume is an industrial byproduct collected from metallurgical industry, so it can not meet the needs of the development of concrete engineering. The search for new high quality SCMs is imminent. Metakaolin is manually manufactured by calcination of kaolin clay, possessing high pozzolanic reactivity and stable quality. People abroad began to study the application of metakaolin in concrete in 1980s. The effects of metakaolin (MK) gradually draw attention in recent ten years, and the research on utilization of MK in concrete has already been one of a focuses. Although MK has been extensively studied as a cementitious material in cement based materials, few studies have been carried out on the performance of cementitious composites employing MK under very low water to binder (w/b) ratio.The following two aspects of research on metakaolin were carried out in the thesis. The first one was comparing the pozzolanic reactivity of MK with silica fume at 3d,7d,28d and 56d reaction age; the second one was the effects of MK, by which cement or SF was partially replaced, on fresh state performance, mechanical performance, microstructure and cement hydration of UHPCC. The following conclusions can be concluded:Pozzolanic reactivity test result shows that the pozzolanic reactivity of MK is higher than SF, especially at 28d. The pozzolanic reactivity of either MK or SF is not high within 7d, whose reactivity indexes are 36.9 and 22.6, respectively.The fresh properties test results show that, due to the irregular particle morphology of MK, fluidity of mortar decreases with the increase of MK content. MK can efficiently restrain the sedimentation of steel fibers in mortar, and the distribution of steel fiber of mortar with 10% MK content is the best.The mechanical properties test results indicate that, for mortars without steel fibers, the flexural strength decreases with the inclusion of MK, whereas the 10% MK content mortar shows the highest compressive strength. For steel fiber reinforced mortars, MK increases both flexural strength and compressive strength. Considering two series mixes, the 20% MK dosage mortars have higher flexrual toughness and fracture energy. However, as the poor workability of fresh mortar containing 20% MK,10% MK content is the most promising mix in practical engineering. In the condition of 85℃ steam curing for 3 day, the highest compressive strength of UHPCC reached 210.4 MPa in this study. In this mixture, w/b ratio is 0.17, and the dosage of SF, fly ash and MK is 10%,20% and 10%, respectively. Steel fiber volume fraction was 3%, and sand to binder ratio (s/b) was 1.The micro-properties test results show that porosity about 10% is the primary reason why cement-based materials have ultrahigh performance. The addition of MK into cement-based material increases the total porosity, but refines the pore size; XRD Rietveld quantitative analysis of cement hydration of all mixes indicates that cement hydration degree is increased with the addition of MK, and the cement hydration degree is 60% approximately. As high pozzolanic reactivity, MK consumes most of Ca(OH)2 in paste; Nanoindentation test result shows ultrahigh density C-S-H content increases in mortar sample with MK compared to mortar without MK. Just because of the effects of MK on refining pore size, homogenizing steel fiber distribution and strenghening bonding strength between matrix and steel fiber, ultrahigh perfirmance cementitious material have more excellent properties with the incorporation of MK. |
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