| It is an important guarantee of cement-based materials’ service life and safe operation for the realization of China’s "13th five-year plan" and "Belt and Road Initiative" development strategy. When the influence of cement-based materials composition on degradation behavior is studied, it is found that alkali, as a trace component, can control early shrinkage and cracking of cement-based materials to a large extent, affecting the safety and durability of the structures seriously. So, the alkali content is strictly limited as a degradation component. With the diversification of modern cement-based materials and the increasing deterioration of service environment, alkali source is in varied forms and becomes more and more difficult to control. At present, alkali is generally converted to the equivalent Na20eq (Na2Oeq=Na2O+0.658K2O) in the study of the alkali influence on degradation problems, without distinguishing the own influence of Na+ and K+. Meanwhile, there are many combining types of Na+ and K+ with other anions, such as ROH, R2SO4, RCl(R represents Na and K) and so on. All those factors will bring much uncertainty on analyzing and controlling of early shrinkage and cracking of cement-based materials. Therefore, it is necessary to study the effect of different alkalis on early shrinkage and cracking of cement-based materials, to make sure that modern cement based materials could get the long service life under different deterioration environment.This study was sponsored by project supported by the national basic research program of China (973 program) -"Basic research on the long service life cement-based materials and structures under harsh environment-Microstructure formation and control of long service life cement-based materials under harsh environment" (Grant No.2015CB655101) and project supported by the national natural science foundation of China -"Studies on microstructure diversity of nanoparticles in hydraulic structural concrete and performance control" (Grant No.51379163). Differential impacts of different kinds of alkalis on the shrinkage and cracking properties of cement-based materials were studied, and the mechanism was systematically investigated with the help of modern material testing technologies, like isothermal calorimeter, mercury intrusion porosimetry(MIP), non-contact resistivity tester, high resolution solid nuclear magnetic resonance (NMR), scanning electron microscopy(SEM) and nanoindentation technique. Moreover, the effect of active sandstone powders with different specific surface areas on the suppression of ASR was studied, and the key factors of suppressing ASR by new type admixture were revealed based on the ability of alkali dissolution and alkali binding of sandstone powders. The results of this thesis would provide theoretical basis and technologic support for the effective control of deterioration behavior of shrinkage, cracking and durability of cement-based materials. The main conclusions can be concluded as follows:1. Through studying the composition, migration form and microscopic mechanical properties of the Al phases and Si phases in the hydration products of cement-based materials, the mechanism of shrinkage and cracking of cement-based materials caused by Na+ or K+, when the type of alkali was ROH, were revealed systematically. The results also provided the atomic scale mechanical parameters for modifying the shrinkage and cracking model of the cement-based materials.K+ was more likely to promote the early shrinkage of cement-based materials than Na+, including chemical shrinkage, autogenous shrinkage and drying shrinkage. Based on the analysis of the shrinkage stress, the mechanism of that was K+ could promoted the further refinement of the pore structure of the cement-based materials, lead to greater shrinkage stress of cement matrix; Based on the analysis of main deformation phase C-S-H, the mechanism of that were K+ was more likely to promote the increase the content of C-S-H and the degree of polymerization of C-S-H, and reduce the content of AFt that could restricted deformation of cement matrix.At the same concentration, it can be verified that K+ could increase cracking sensitivity of cement mortar twice than Na+, which was largely determined by the shrinkage ratio. But more important micro mechanism lies in ① K+ was more likely to reduce the bond strength between C-S-H clusters, which make C-S-H clusters more easier to separate from each other; ② K+ was more likely to increase the substitution rate of Al atoms in the C-S-H chain, which reduced the mechanical properties of C-S-H; ③ K+ was more likely to increase HD C-S-H/LD C-S-H ratio, which reduce the extensibility of cement matrix.2. Through studying hydration and hardening process, pore structure, hydration products electrical properties and migration form of the Al phases and Si phases of cement-based materials, the mechanism of shrinkage and cracking of cement-based materials caused by different type alkalis, such as ROH, R2SO4 and RCl, were revealed systematically. The results also provided technical support for controlling reasonably the shrinkage and cracking of cement-based materials under different deterioration environment.According to the influence of different type alkalis on shrinkage and cracking of cement-based materials, the order from large to small was RCl>R2SO4>ROH. Based on macroscopic and microscopic analysis, for the shrinkage properties, the reason of that was related to the difference of cement hydration process and microstructure development, which reflected in the difference of pore structure refinement degree, conversion efficiency of AFt to AFm, Al substitution rate and the degree of polymerization of C-S-H; For the cracking properties, expect for shrinkage of cement-based materials, it was needed to pay attention to the different influence of different type alkalis on the electrical characteristics of C-S-H surface, which make the ion adsorption on the C-S-H surface change, affecting the bonding characteristics of C-S-H clusters.In addition, the effects of Na+ and K+ on shrinkage and cracking of cement-based materials containing different type alkalis were different. For the shrinkage properties, K+ was more likely to increase shrinkage of cement-based materials containing ROH, R2SO4 or RCl than Na+. However, for the cracking properties, when the alkali was ROH, K+ was more likely to increase cracking of cement-based materials than Na+, but when alkalis were R2SO4 or RCl, Na+ was more likely to increase cracking of cement-based materials than K+. The reason of that was mainly related to the difference of cement hydration process and microstructure development of cement-based materials containing different type alkalis. Results showed that the early structure of cement-based materials develops, the easier cement-based materials crack.3. Taking sandstone powders with certain alkali content as new admixture to suppress expansion of cement-based materials causing by ASR, the influence of replacement mode, replacement level and specific surface area of sandstone powders on internal alkali content of cement-based materials was studied. Based on the activity, alkali dissolution and alkali binding properties of sandstone powders, the relationship between parameters of sandstone powders and effect of suppressing expansion causing by ASR was established, and the mechanism of suppressing expansion causing by ASR was also revealed. The results also provided theoretical basis for utilizing reasonably a new type admixture to suppress expansion causing by ASR.The effect of sandstone powders on suppressing expansion of cement-based materials causing by ASR decreased first and then increased as increment of specific surface areas, the specific surface area causing the highest expansion was 610m2/kg. Under certain specific surface area and replacement level, sandstone powders could suppress ASR effectively. The mechanism of that was though alkali dissolution of sandstone powder was increase as increment of the specific surface area, which could increase alkali concentration in the pore solution of cement-based materials, meanwhile the hydration activity of sandstone powder was improve as increment of the specific surface area, which could improve alkali binding ability of sandstone powder by increasing the C-S-H content and the C-S-H polymerization degree. When alkali binding ability was greater than alkali dissolution ability, sandstone powders could reduce the expansion of cement-base materials mixed with alkali activity aggregate, and vice versa. Besides, at the same replacement level of sandstone powders, suppression effect by replacing sand was higher than that by replacing cement.4. Comparative analysis of the influence of fly ash or slag on hydration process, pore solution characteristics, Ca/Si ratio and structure of C-S-H of cement-based materials. From the micro-to-macro point of view, the mechanism of early age shrinkage and cracking of cement-based materials partially replace by fly ash or slag was explored. The results also provided theoretical basis for choosing reasonable measures to restrain early age shrinkage and cracking of cement-based materials under different deterioration environment.Under the low-alkalinity condition, when the replacement level was 30%, fly ash and slag could both effectively decrease the early shrinkage and cracking of cement-based materials. While under the high-alkalinity condition, when the replacement level was 30%, fly ash only could effectively inhibit the shrinkage and cracking of cement-based materials containing the alkalis of ROH or R2SO4, slag did not have the ability to inhibit the shrinkage and cracking of cement-based materials. Based on the alkali adsorption effect, the reason of that was compared with slag, fly ash was more likely to increase the content of C-S-H with low Ca/Si molar ratio, which increased the ability of cement matrix to adsorb alkali, reducing the effect of alkali. Based on the alkali-activated effect, the reasons of that were under high alkalinity, compared with fly ash, slag was more likely to promote the process of hydration, the degree of pore structure refinement and the development of elastic modulus of cement-based materials, resulting in shrinkage stress increasing and accumulation of shrinkage stress accelerating. Thus, shrinkage and cracking sensitivity of cement-based materials partially replace by slag were increased by alkali.However, it should be noted that the effect of RCl on shrinkage and cracking of cement-based materials partially replace by fly ash or slag system was largely determined by its effect on hydration of cement, which means that conventional method of inhibiting shrinkage and cracking by the alkali of RCl was still open to discussion.In addition, for the effects of Na+ and K+, fly ash or slag could increase the ability to inhibit shrinkage and cracking of cement-based materials containing high Na+. The main reason was that the secondary pozzolanic reaction of fly ash or slag produced more C-S-H with low Ca/Si molar ratio that the adsorption capacity of Na+ was greater than K+, reducing the effect of Na+. |
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