Study Of Metakaolin On Improving The Volume Stability Of Concrete

Based on the principle of volume stability of concrete, the measures used to compensate the volume shrinkage of concrete were explored. The paper presents studies on the optimum conditions of thermal treatment on kaolin, the principle and implementation methods to improve the volume stability of concrete with the metakaolin, and the applications of metakaolin in the grouting material. The main conclusions are as following:(1) For the concrete with high water to binder ratio, drying shrinkage is one of the major causes of the cracking of concrete under the conditions of non-load. The formation of ettringite in concrete with necessary conservation will contribute to the volume stability of concrete structures exposed to dry environment, which will improve the ability of anti-drying shrinkage of concrete and reduce the risk of shrinkage cracking. It's a commonly used and effective method to compensate the volume shrinkage of concrete.(2) Kaolins do not have the activity itself. Thermal treatment that the temperature increasing from 20℃to 700℃within an hour and then staying at 700℃for 4h is efficient for the kaolins. At these conditions the produced meatkaolin exhibits the highest pozzolanic reactivity. The order of factors affecting the pozzolanic reactivity as follows:thermal treatment temperature, holding time, cooling mode, heating mode.(3)The effect of MK is not only to enhance the compressive strengths of concrete but to reduce the drying shrinkage exposed to dry environment, the affect being greater at higher replacement levels. But there is no expansion be observed when MK is the only mixture material.(4)MK mixed with anhydrite will create some ettringite which dilate the concrete, and compensate the shrinkage of concrete exposed to dry environment, which make the shrinkage within 0.01%. But anhydrite will reduce the compressive strengths of concrete. Although calcium carbonate can inhibit the transform of AFt to AFm, it is no effect when the content of Al2O3 is excess. The exist of calcium carbonate can lower the expansion rate and value of concrete. The combined of MK and calcium carbonate can improve the volume stability and compressive strengths of concrete.(5)The grouting material mixed with MK is a material with high strength, good uniform, viscosity, and no bleeding. The expansion of this grouting material is about 0.25% from 1.5h to 3h after forming, and 0.021% from 3h to 24h. The compressive strengths of the grouting material is 40MPa at 3h,50MPa at 1d and 80MPa at 28d.