Influence And Mechanism Of Shrinkage Reducing Admixtures On Shrinkage Reduction Of Cement Based Materials

Volume deformation of concrete, related to moisture migration by internal cement hydration or moisture change with external environment, is the main reason of cracking and other defects. The concrete surface defects such as microcracking can easily form for the inadequate curing condition. On the other hand, for high performance concrete, characterizing by high cementitious material content and lower water/cement ratio, the autogenous shrinkage is often remarkable. It seems that shrinkage cracking of concrete was inevitable in practical engineering for those reasons. The present methods for mitigating the shrinkage cracking of concrete, such as optimizing the raw materials and the composition of concrete, enhancing the early-age moisture curing, applying expansion agent, fiber, etc. However, all have some limitation. In recent years, many researches indicated that the shrinkage and shrinkage cracking can be significantly reduced by incorporating shrinkage reducing admixtures (SRA) or water absorbent media into cement-based materials. By significantly reducing the surface tension of the pore solution then resulting in the reduction of drying or autogenous shrinkage of cement based materials, SRA is adapted to most concrete project. However, mixing the SRA directly into the fresh concrete has adverse impacts to cement hydration and early-age strength. Moreover, the dosage of SRA is usually high which will increase the cost of concrete. Therefore, the investigation of practical SRA applications, in order to reduce the cost of SRA and its adverse influence, is useful to promote the applications of new materials such as SRA and other new technologies. It can also provide a more effective and practical method to control shrinkage cracking of concrete.For those reasons stated above, several shrinkage reducing technologies related to the application of SRA was proposed in current study, such as coating the SRA independently(SRA-Coat) or combined with curing compounds(SRA+CCP) onto the surface of cement based materials and mixing the LWA saturated with SRA (SRA-LWA) as an internal curing agent of cement mortar. Meanwhile, the function and mechanism of the technologies were also investigated. The results are as follows:Firstly, the influences of SRA on the solution, cement hydration and basic physical properties of cement mortar were researched. By comparing the basic properties of different SRA concentrations, the surface tension of solution can be reduced significantly as the increasing of SRA concentration and the presence of SRA accelerate the evaporation rate of solution when the SRA concentration was less than 5%. However, the surface tension of solution reach to the balance, and viscosity increased linearly, evaporation rate decreased significantly with the increase of SRA concentration when the SRA concentration was more than 5%. The results of the setting time, content of no-evaporation water, hydration products show that, the influence of SRA on cement hydration indicated that the delay of setting time and hindrance to hydration process. The pore of 30-100nm and total porosity will increase significantly as the SRA dosage up to 3%. In lower w/c mortar, the shrinkage can not be reduced further as the SRA dosage increasing, but the strength remarkably reduced. Though the presence of SRA can decrease the moisture loss of cement mortar, it has little influence on shrinkage reduction.Secondly, the influence of SRA-Coat on shrinkage reduction of cement mortar was investigated. The results show that the 50% concentration SRA solutions obviously reduce the drying shrinkage of cement mortar. Moreover, it can enhance strength of cement mortar and concrete by 3%-11% and 0%-12%, respectively. On the contrary, if the SRA are directly mixed into the cement mortar, with recommended dosage of 2% by cement weight, the strength will loss 0-12%. Further more, the resistance of plastic cracking and carbonization of concrete coating with SRA are also enhanced. The effects of SRA-Coat combined with curing compounds (SRA+CCP) on the properties of cement mortar and concrete are also investigated. The results show that, comparing to SRA-Coat, the SRA+CCP can remarkably improve the ability of shrinkage reduction, moisture maintenance and strength enhancement of concrete. Other sides, the SRA+CCP obviously improve the chloride-penetration resistance and lower the water absorbability.Finally, SRA-LWA is proposed as an internal curing agent in current study. The properties of cement mortar cured with SRA-LWA internally are studied. Deformation, physical and other measurements include drying/autogenous shrinkage and strength of cement mortar. The result indicated that, compared to traditional internal curing with water saturated light-weight aggregate(Water-LWA), the cement mortar incorporated with SRA-LWA shows more effective ability of shrinkage reduction, even the dosage of SRA-LWA is only half of Water-LWA, and no marked effect of strength is observed as the SRA-LWA introduced into cement mortar for internal curing. At the same time, the introduction of SRA is also less than the recommended dosage in mortar. In the other hand, the shrinkage reduction of SRA-LWA is equivalent to mixing the same dosage of SRA, but the adverseness of SRA on cement hydration and early age strength is remarkably avoided.By comprehensively analyzing the effect of SRA on cement hydration, the effect of coating SRA with higher concentration on the strength of cement mortar, and combining the character of SRA solution migrated from LWA and the effect of SRA-LWA on the cement hydration, it is deduced that the SRA solution reserved in SRA-LWA is probably released into the hardened cement paste after final setting occurred. Meanwhile, fewer-defects and denser microstructure of ITZ are generated in SRA-LWA systems.