| High strength and ultra-highperformance concrete with low water to cement ratio(w/c)is an important direction for the future development of concrete,however it has the characteristics of early self-desiccation and high autogenous shrinkage.Internal curing(IC)provides an important technical idea to solve the problem of early shrinkage and cracking of concrete with low w/c.However,the common internal curing agent usually has an irregular surface and water absorption and desorption in cement paste are quite complex and hardly controlled.Because of the lack of theoretical guidance about absorption and desorption mechanism,the actual usage of internal curing is not satisfactory,thus blocking the development of internal curing theory and applications.In this paper,based on the financial support from National "Twelfth Five-Year" Plan for Science & Technology Support Development Program of China(Project2014BAB15B01),a large spherical superabsorbent polymer(SAP)with special surface characteristic was used to study the moisture migration and exchange of SAP in cement paste,as well as the structure and nature of the interfacial transitionzone(ITZ)around SAP,and to reveal the effect mechanisms of water absorption and desorption of SAP on the micro-structure and performance of concrete with low w/c,thus obtaining the universal law of internal curing agent in concrete and guiding the practical applications of internal curing technology.The main work and innovations are as follows:The complex migration and state evolution of SAP in pore solution and cement paste werediscussed.The water absorption of SAP in the pore solution is not fixed,but will be potentially affected by the w/c and the hydration time.The low water entrainment ratio(defined as the ratio of introduced water to SAP content)leads to the water absorption behavior of SAP,which shows anadvance in exothermic peak,while the high water entrainment ratio leads to an early rapid release of SAP or the excess internal curing water,which shows the exothermic peak hysteresis and the water rich area around SAP.The water release proportion at different stages was quantified by the change of water content of SAP,thus the effective utilization rateof IC water was evaluated.Hereby,a final innovation was proposed by shortening the early stage release process controlled by osmotic pressure and extending the late release process controlled by the humidity difference,to improve the effective utilization of internal curing water stored in SAP.The effect of several potential factors on the desorption behavior of saturated SAP in concrete was studied by simulating the internal saline concentration,alkalinity and relative humidity.Results indicate that the desorption of SAP is mainly decided by osmotic pressure and humidity gradient,and the capillary force mainly plays a role of water transport.The desorption of SAP is mainly influenced by the osmotic pressure at early stage,and humidity gradient at late stage.Both of the two factors stay at a relatively low level between these two stages.At the same time,the desorption is not decided by pH value,and the factor that really works is the osmotic pressure.In addition,the water release of saturated SAP influenced by humidity shows the following trends: The water release rate indicates a clear linear trend as a function of time,and an approximate logarithm correlation exists between the water release rate and the relative humidity.Based on the above theories,a large spherical SAP was used to magnify theITZ.Based on the established release model of SAP in cement paste,the formation of ITZ and influence of SAP on moisture content,hydration,microstructure and microhardness of ITZ were investigated.The water release of SAP distinctly improve the moisture content of ITZ.The degree of hydration of ITZ is higher than that of matrix by about 5%,an equivalent of an increase in w/c by 0.04.This tendency is consistent with the result of BSE image analysis.The microhardness of ITZ around SAP is higher than that of matrix at 7 and 28 days.The results of moisture content,degree of hydration,microstructure and microhardness of ITZ indicate that a dense transition zone is produced in the presence of SAP.According to the results of nitrogen adsorption,The pore shape in ITZ is not affected by the existing of SAP.Capillary porosity of ITZ is increased at the early age,but reduced at the later age,both of which can be attributed to the desorption process of SAP.The fractal dimensions of ITZ were calculated by different fractal models,results indicate that both the surface fractal dimensions and mass fractal dimensions of ITZat 28 days are higher than that of matrix.A new perspective was gained according to the numerical calculation,results indicate that the calculated strength and permeability of ITZ are higher than that of matrix,and the shrinkage migration of SAP is not only because of the internal humidity compensation,but also related to the decrease of small capillary pores(<50 nm)which can play an important role on the shrinkage.The influence of absorption and desorption behavior of SAP controlled by different water entrainment ratio on the workability,mechanical property,electrical resistivity,pore structure,capillary pore pressure and autogenous shrinkage were systematically studied.The over-entrained internal curing water and real effective w/c during the mixing process were quantified.The over-entrained internal curing water is proved to account for the remarkable decrease of mechanical strength and electrical resistivity,as well as the remarkable increase of slump flow and porosity.However,the exist of over-entrained water seems not harmful to the mitigation of autogenous shrinkage and self-desiccation.Under the premise of satisfying the theoretical water requirement of internal curing,the state of SAP in cement paste can be stabilized by the design of water entrainment ratio of SAP near its equilibrium water absorption in the paste,thus minimizing the negative effect of absorption and desorption of SAP on the workability,strength,pore structure and ion transport of cement-based materials.According to the theory achievements above,the internal curing with SAP was practically used for the pavement construction in a hot summer to reduce the surface cracking.At the same time,a new porous concrete was designed and prepared through the replacement of normal aggregates by large spherical SAP.The spherical pore structure of this concrete can be designed and controlled by the size and content of spherical SAP,thus having many functions at the same time,such as controlled density,internal curing,heat insulation,noise reduction and so on.The mechanical strength of this concrete was proved deriving from the dense hydration layer around SAP.Some practical applications were also carried out successfully.This work will be meaningful to promote the development of SAP internal curing technology and accelerate the practical application of SAP internal curing inhigh performance concrete. |
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