Research On Preparation And Performance Of Lightweight Concrete Mixed With Metakaolin And Slag

High performance concrete is a kind of materials which are widely used in construction projects. In order to lower the weight of concrete and to meet the engineering demand, people have prepared high performance lightweight concrete. Because of its advantages including lightweight, high strength, good durability and fire-resistant, the high performance lightweight concrete have been used in high-rise buildings and bridges. The shrinkage is one of defects of high performance lightweight concrete with low w/b ratio. Many domestic and foreign researchers utilize the pozzolanic materials to reduce shrinkage and to improve the performance. Metakaolin is a kind of pozzolanic materials with high reactivity. The effects of metakaolin and silica fume to improve the concrete properties are very close which are much higher than fly ash and ground granulated blast-furnace slag(GGBFS). The price of metakaolin is lower than that of silica fume. Moreover, metakaolin can also reduce the shrinkage of concrete. To sum up, it's very meaningful that using metakaolin in lightweight concrete to substitute silica fume.On the other hand,fresh water and desalinated seawater are often used in coastal and oceanic concrete projects. The transport and desalination of water increase the cost. Seawater is rich in thousand kinds of salts. Some salts can promote hydration of cement, but some other salts will lead to the destruction of concrete structure. Most of salts will be separated out after desalination. Seawater contains quantities of chlorides; these chlorides accelerate the corrosion of steel and destroy the concrete structure. The chloride binding ability of metakaolin is very strong. The problem whether in the lightweight concrete mixed with seawater system, metakaolin could eliminate the harmful effect of seawater but remain the good effect deserves our study.Consequently, this article prepared two kinds of lightweight concrete with addition of metakaolin and GGBFS, one mixed with fresh water and the other mixed with seawater. The addition of GGBFS can reduce the cost and improve the packing structure of concrete to make it denser. On the basis of ensuring the work performance of concrete, this article studied the effect of metakaolin and GGBFS on performance and microstructure of lightweight concrete, combined effect of metakaolin, GGBFS and seawater, and analyzed feasibility of using seawater, metakaolin and GGBFS to prepare concrete by a series of testing method including XRD, SEM, DSC-TG and MIP etc.The main results of this paper are as follows:1. This article combines mechanical performance with microstructure, and proves that addition of metakaolin or GGBFS both can improve the performance of lightweight concrete, the effect of metakaolin is better. Addition of metakaolin and GGBFS is the best. The optimal mixing amount is 10% metakaolin and 10% GGBFS. The experimental indicates that metakaolin and GGBFS improve the structure of interfacial transition zone(ITZ), and enhance the bonding between aggregates and cement pastes. The strength of ITZ increases and the mechanical performance of lightweight concrete get enhanced.2. Comparing with lightweight concrete mixed fresh water, the microstructure of concrete mixed with seawater is better, strength of ITZ is higher, and the mechanical performance of lightweight concrete is better. The experimental indicates that seawater can improve the mechanical performance of concrete and don't affect the reactivity of pozzolanic minerals.3. Testing results shows that mixing seawater in lightweight concrete would introduce large quantities of chlorides. Some of chlorides react with hydration products and form Friedel's salt. The addition of metakaolin and GGBFS will increase the amount of Friedel's salt. And the amounts increase with the pozzolanic mineral content. Seawater can promote the hydration of cement, and increase the content of hydration products. Metakaolin and GGBFS can consume a lot of CH to promote the hydration of cement.4. The chloride resistance ability of lightweight concrete without pozzolanic minerals is very low. Addition of metakaolin or GGBFS both can enhance the chloride resistance ability of concrete. Addition of metakaolin and GGBFS is better. Seawater will degrade the chloride resistance ability and degrade the durability of concrete. Addition of metakaolin and GGBFS can improve the chloride resistance ability of lightweight concrete dramatically. They can eliminate the harmful effect of seawater.5. The shrinkage of lightweight concrete without pozzolanic minerals is very large. Addition of metakaolin and GGBFS both can reduce the shrinkage of concrete. And addition of metakaolin and GGBFS is better. The effect of metakaolin is much better than that of GGBFS. When addition content of metakaolin is up to 10%, the effect of GGBFS on shrinkage is very minor.