Study on the behavior of cement matrices with alkaline and alkalifree set accelerators at low temperature
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Document typeMaster thesis
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Sprayed concrete is widely used as structural support for underground constructions. Although the sprayed concrete technology advanced significantly over the past years, questions continue to arise regarding its performance, efficient use and optimized design. One of the main subjects that requires further research is the behavior of sprayed cement matrices containing accelerators at low temperature, since the application of sprayed concrete may be hindered in regions with cold weather due to the difficulties to fulfill the requirements of fast setting and early age strength development under such conditions. Understanding the behavior of set accelerator at low temperature can provide useful information for predicting mechanical strength evolution and identifying the most efficiency mix proportions for real practice and constructions. In this context, a study of Vicat test and calorimetry test covering the main factors, which affect the behavior of cement matrices, including temperature, types of cement, types of set accelerator and content, was launched. About the Vicat test, four different temperature degrees: 20⁰C, 15⁰C, 10⁰C, 5⁰C were compared, for cement matrices, two types of cement: CEM I and CEM II were used, four types of set accelerator: ALNA73, AKF63, AKM55, AKF125 were added, with two dosages: 5% and 7%, excepting for mixes with ALNA73 which using a typical 3% content for alkaline based type. The composition of cement matrices for calorimetry test is the same with Vicat test, however experiments for mixes at 5⁰C was unable to reach due to some technical problem. The factor temperature; results from Vicat test present that low temperature delay the setting time of cement matrices in absolute terms, the alkaline accelerator, ALNA73 shows a general better performance over alkali-free set accelerators for cement matrices at low temperature. In relative terms alkali-free accelerator AKM55 and AKF125 are the most efficient accelerators, since they experience a general least efficiency index value when temperature reduced. If the parameters, heat flow and energy released in the hydration process are considered, low temperature affects the initial reaction of accelerator with cement, moreover delays the formation of hydration products, resulting the hydration reaction need longer time to finish, even now the hydration of cement and accelerator could occur independently. The factor type of set accelerator and content; the results from Vicat test and calorimetry test indicate that mixes with accelerator AKM55 show an abnormal behavior at low temperature. Increasing the content of accelerator used in cement matrices at low temperature could shorten setting time in absolute terms, whereas the increasing of content may not lead to a reduction in the loss of efficiency. The factor type of cement; generally, the types of cement does affects the behavior of cement matrices, whereas no clear tendency shows which type presents a better performance.
DegreeMÀSTER UNIVERSITARI EN ENGINYERIA ESTRUCTURAL I DE LA CONSTRUCCIÓ (Pla 2015)
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