Study On The Production And Performance Of High-strength Concrete With Metakaolin

As a kind of mineral admixture, metakaolin has extensive sources, stable components and similar activity with silica fume. Therefore, it exists a great potential of using metakaolin as mineral admixture instead of silica fume in the production of high-strength concrete. The influence of the calcining temperature, holding time and the rates of heating and cooling to the activity of metakaolin was studied through the orthogonal test. At the same time, some other activity evaluation methods were conducted to get a rational calcining process. In addition, the compounds of metakaolin with fly ash, slag and modified kaolin were studied and the improvement mechanism of the compounds to the strength and workability of concrete was analyzed too. On the basis of the researches above, concrete with the strength of 100MPa was prepared. The microstructure of high-strength concrete with metakaolin was researched systematically using XRD, SEM and microhardness measurements. The main conclusions are as following:(1) The activity of metakaolin is related to the components and calcining process of kaolin. The effect order of the calcining factors to the activity of metakaolin is calcining temperature, holding time, cooling rate and heating rate. When the kaolin used was calcined at 700℃for 2 hours, quickly heating and cooling were conducted, the activity of the metakaolin was the highest.(2) Although the strength of concrete with metakaolin was increased, the workability was decreased because of the layer structure, high specific surface area and water demand of metakaolin. The strength and fluidity of mortar were improved when metakaolin was incorporated together with fly ash and slag. The compressive strength at the age of 28 days of the mortar containing 10% metakaolin,10% slag and 5% fly ash was 36.2% higher than the control mortar. Meanwhile, the compressive strength at the ages of 7 and 28 days of the mortar containing 5% metakaolin and 3% modified kaolin were 32.9% and 22.7% higher than the control mortar respectively.(3) The mechanism performance of concrete can be improved significantly with the incorporation of metakaolin. The compressive strength of concrete containing 15% metakaolin, prepared at the w/b ratio of 0.25 reached 100MPa at the age of 28 days, but the workability was not satisfied. The slump of concrete containing 10% metakaolin,10% slag and 5% fly ash reached 205mm, while the compressive strength of the concrete at the age of 28 days was 105.4MPa. (4) The interspace of kinds of particles in concrete can be filled by metakaolin. The content and the orientating arrangement of Ca(OH)2 at ITZ decreased with the incorporation of metakaolin in high-strength concrete, while the density and strength of the hydration products were enhanced too. The microstructure of high-strength concrete was improved significantly by metakaolin.