Research On Hydration Characteristics Of Portland Cement-silica Fume Composite Cementitious Material At Lower Temperatures

The physical and mechanical properties index of concrete construction in winter can suffer serious damages when the concrete suffers from early frost damage which is detrimental to the durability of concrete. Research on the early frost damage of concrete is one of the important problems that need to be solved. In this paper, concrete at low temperature was investigated beginning with the research on its early performances. Portland cement is used as the base component, the low temperature environment of 5℃ - 10℃ is selected, and sodium nitrite is added to maintain low temperature solution activity and silica fume as admixture. The mechanical properties, composition of hydration products, contents of hydration products, micro-morphology, pore size and hydration rate were used as the characterization parameters of the early hydration of the composite cementitious materials. Analytical methods for hydration characteristics of the low-temperature silica fume-Portland cement material under complicated factors were established. Dynamic research methods for the internal hydration process of the low temperature composite material were improved, in which the process starts from the initial reaction of the front end. So it can reveal the hydration mechanism of composite cementitious material system in low-temperature concrete, and lay the theoretical foundation for the research on low-temperature concrete. The main research contents of this paper are as follows:(1) Influence of different additions of silica fume (0,2%,5%,8%10%and 12%) on the hydration properties of the composite cementitious material was investigated. The results show that, adding silica fume can effectively improve the mechanical properties of the composite cementitious material at low temperatures. At the same time, the distribution of the hydration product is improved and the voids in the structure are filled by promoting the consumption of calcium hydroxide, the total porosity of the system decreases. This improvement effect increase more obviously with the increase of the addition of silica fume. When the addition of silica fume reaches 8%, the effect is the best. For example, the compressive strength can be increased by 16%, the total porosity be decreased by 7%. When the addition of silica fume is over 8%, the effect of the improvement is gradually weakened. Therefore, the amount of silica fume added to Portland cement should not be too large.(2) Influence of different temperatures (5 ℃,0 ℃,-5 ℃ and-10℃)on hyadration properties of the composite cementitious material was investigated. The results show that, the decrease of temperature can not only apparently decrease the compressive and flexural strengths of the composite cementitious material, but also decrease the Ca(OH)2 content of the hydration products and increase the total porosity and harmful pore volume of the material and lead to weak connection between crystals. Especially when the temperature decreased from 5℃ to 0℃, the above performances of the composite cementitious material changed obviously. The compressive strength at 7d was decreased by 29% and the harmful pore volume was increased by 27%. At the same time, it also can inhibit the volcanic ash activity of silica fume. The lower the temperature, the more obvious the situation is.(3) Influence of various solution environments (8.5% sodium nitrite solution and pure water) on hydration properties of the composite cementitious material was investigated. The results show that, the compressive and flexural strength of the composite cementitious material at the initial hydration stage (at 5℃)under absolute water solution environment are higher than that under sodium nitrite solution environment. However, the Ca(OH)2 content of each material with different additions of silica fume that in sodium nitrite solution after 14d of hydration is higher than that in the absolute water solution. Compared with the material in the absolute water solution, the hydration products in the sodium nitrite solution are thin and of large particles with heterogeneous connection, large total porosity and small structural compactness. Although large amount of sodium nitrite leads to increasing the surface tension of solution, it can maintain the activity of water at 5℃ ～ 10℃, which is beneficial for the hydration of the composite cementitious material, especially, the kinetic studies.(4) The dynamic characteristics of hydration reaction of the composite cementitious material under various conditions in the environment with salt solution was investigated by measuring the total hydration heat of the material at low temperatures. The reaction rate constants, reaction order n and apparent activation energy Ea were calculated by the preferred hydration kinetics equations and computer programs and further to establish hydration kinetics models of the composite cementitious material at low temperatures. The results show that, at low temperature, the hydration process of Portland cement-silica fume composite cementitious materials can be divided into three stages, namely, NG, I and D. The reaction order n of the composite cementitious material with different addition of silica fume is a non integer between 1.2-1.6, indicating that the hydration reactions of the composite cementitious materials at low temperature are complex elementary reactions. Adding silica fume can decrease the total heat release rate of the hydration, increase the hydration heat rate of the acceleration period, and decrease the order of reaction n and increase the hydration rate constant K at different stages. However, the decrease of the temperature can prolong the hydration period of the system, and the second exothermic peak is decreased, and the order of reaction n and the rate constant K value decrease with the decrease of the temperature. And the differences of apparent activation energy of the composite cementitious system with different additions of silica fume at different hydration stages are smaller.In this paper, the low-temperature hydration characteristics of the Portland cement-silica fume composite cementitious material under complicated factors were analyzed from the macro-micro perspectives, of which the hydration properties and microscopic mechanisms investigations with different additions of silica fume, various curing temperatures and different solution environments were conducted. Meanwhile, the combination of macro performance testing and microstructure analysis, combination of qualitative analysis and quantitative analysis of the hydration products of the system, and combination of analysis of the hydration process of the system and the establishment of the dynamic equation were used. The investigation was conducted at constant low temperatures in the whole process and the dynamic data was collected from the initial hydration stage. Hydration kinetic models of the Portland cement-silica fume composite cementitious material based on Krstulovic-Dabic equation at lower temperatures were determined. The hydration mechanisms of hydration control factors, dynamic characteristics and hydration kinetics of the composite cementitious material at low-temperature environment are disclosed, which provide theoretical supports for the research on low-temperature concrete.