Effects Of Silane Modified Admixtures On Properties Of Ultra-high Performance Concrete

Ultra-low water-to-binder ratio,high cement content,mineral admixtures and fibers are prominent features of modern ultra-high performance concrete.However,the low water-binder ratio and high admixture will result in high viscosity and poor working performance of cement paste.Silane as hydrophobic agent and dispersant,and the hydroxyl groups generated by its hydrolysis will condense with the hydroxyl groups on the surface of the admixture,so that silane is adsorbed to the surface of the admixture.The hydrophobicity of silane will hinder the adsorption of water on the surface of the admixture,thereby increasing the lubricating water in the fresh concrete.The dispersibility of silane will make the admixture particles adsorbed with silane dispersed in the fresh concrete.Therefore,silane can be pre-adsorbed on the surface of admixture to improve the workability of ultra-high performance concrete.The main research contents and conclusions are as follows:(1)Through the test of slump expansion and rheological properties of ultra-high performance concrete,it was found that the yield stress and plastic viscosity of fresh concrete were greatly reduced after silane modified admixture,thus the slump expansion of concrete was increased,and then the working performance was improved.By comparing the slump expansion of fresh and concrete after 1 h of fresh mixing,it was found that the silane modified admixture also reduced the loss of slump expansion of ultra-high performance concrete over time.At the same time,the same silane has different modification effects on different admixtures,that is,there is a certain matching relationship between different silanes and different admixtures.(2)Through the test of the mechanical properties of ultra-high performance concrete,it was found that silane modified admixture has a great influence on the strength of ultrahigh performance concrete.The four silanes with non-alkyl groups all negatively affected the strength,and the modification of silica fume and fly ash by γ-methacryloxypropyltrimethoxysilane even gave concrete 1d almost no strength,and The strength of each age is much lower than that of the ordinary group.Alkyl silanes have a relatively small negative impact on the strength of the concrete,and both methoxy silanes have less impact on strength than ethoxy silanes.(3)Through the X-ray diffraction,non-evaporative water and hydration heat tests of concrete paste,it was found that silane modified admixture did not cause the generation of a new crystal phase and affect the type of hydration products.The hydrophobicity of silane will hinder the diffusion of water and its hydroxide to the surface of silica fume and fly ash,thus will affect the pozzolanic reaction between the Mineral admixture and the cement and changing the hydration heat release of the concrete.(4)Through the mercury injection and open porosity test of concrete mortar,it was found that the silane modified admixture has a certain influence on the concrete pore structure.The four non-alkyl silane modified admixtures increased the porosity of concrete and increased its harmful pores.The ethoxysilane modified admixture made concrete more harmless and harmful pores than blank group.The methoxysilane modified admixture only has a great influence on the small pores of ultra-high performance concrete,which increases the number of pores smaller than 50 nm.