Microstructure And Micromechanical Property Of Gels In Different Cementitious Systems

Gel is an important but complex hydration product in cementitious materials.Its composition,microstructure and micromechanical properties are closely related to the macroscopic properties such as strength,shrinkage and durability of cementitious materials.Therefore,it is very necessary to study the properties of gels in different cementitious systems from the micro-nano scale.This study has carried out systematic research on the gels in Portland cement?PC?system,PC-slag blended system and alkali-activated slag system.The main research contents and conclusions are shown as follows:?1?We have adopted backscattered electron image analysis,energy spectrum analysis and nanoindentation to investigate the distribution and characteristics of hydration products in PC system,PC-slag blended system and alkali-activated slag system.The influence of age and content of slag on the properties of hydration products has also been studied.The relationship between the micro-mechanical property and composition of the gels in different cementitious systems were discussed.The results suggest that the elastic modulus of outer hydration product?Op?in PC system and PC-slag blended system have positive correlation with their Ca/Si and Al/Si,which is related to the high elastic modulus crystal phases in Op.There is no significant correlation between the elastic modulus of inner hydration product?Ip?and its composition.There is also no significant correlation between the elastic modulus of the C-A-S-H gel and its composition in alkali-activated slag system.?2?The composition,microstructure and micromechanical property of the dark rim around slag were studied.The results show that the dark rim is mixture of C-A-S-H gel and layered double hydroxide?LDH?,which is formed by the in-situ reaction of the slag in this position.The dissolution and diffusion of Ca and Si in this part of the slag contributes to the low gray level of dark rim in BSE images.Based on the previous researches and our experimental results,the formation mechanism of the dark rim in the alkali-activated slag was proposed.?3?The microscopic creep behaviors of different gels in PC system and alkali-activated slag system were studied.The relationship between the microscopic creep behavior and composition of gels in these two systems was discussed.The results suggest that the creep displacement of gels in alkali-activated system is higher than that in PC system under the same loading condition,which could be attributed to the lack of crystal phases.The creep behavior model of the gel on the microscopic scale was proposed based on the experimental results.?4?A new method for in-situ observation of the leaching of cementitious materials was proposed.The leaching of PC system and alkali-activated slag system in deionized water,NH₄Cl solution,Na₂SO₄ solution and MgSO₄ solution were studied.The leaching behavior of different phases in these two cementitious systems were discussed.This new method can be used to intuitively compare the changes of cementitious systems before and after leaching,which can show the leaching of different elements in cementitious systems during leaching process more accurately.In the NH₄Cl solution,the Ca/Si decline of the C-A-S-H gel in the alkali-activated slag system is more obvious than that of the gels in the PC system,which may be related to the lack of calcium hydroxide in the alkali-activated slag system.The calcium leaching of the two cementitious systems in MgSO₄ solution is more obvious than that in Na₂SO₄ solution,mainly related to the substitution of Mg for Ca.Moreover,the Ca in the PC system is easier to be replaced by Mg in the MgSO₄ solution than that in the alkali-activated slag system.?5?The property and microstructure evolution of the PC system and the PC-slag blended system at elevated temperature were studied.The composition and microstructure change of the gels in these two systems at high temperature were highlighted.The microstructure evolution mechanism of gels in both systems at high temperature was proposed.The results suggest that the PC-slag blended system has better high temperature resistance than the PC system,which is related to the less CH content,denser packing of gels and better stability of gels at high temperature in PC-slag blended system.