Hydration Mechanism And Properties Of Cement-based Materials With Low Water-cement Ratio Upon Component Gradient Design

Low water-binder ratio is the main technical approach to achieve high strength and high durability of cement-based materials,and it is significant to improve the service life of ultra high-rise building,long-span bridge and other national key projects.Low water-binder ratio process can improve the compactness of cement hydration products,and improve the mechanical properties and permeability resistance of cement-based materials.However,the characteristic of low water-binder ratio(W/C < 0.23)and high cement consumption lead to the mixing water consumption of paste much lower than the theoretical water of cement hydration reaction,which is made the hydration degree of cement with high environmental sensitivity.low hydration degree of cement in matrix structure lead to unhydrated particles continuously hydrate,bringing volume instability during long-term;inconsistency of spatial distribution of hydration process lead to interface defects and surface brittle cracking.This paper is based on the financial support from China National key RandD program(2016YFC0701003-5).Based on design concept of hydration degree homogenization,the preparation method of cement-based materials with low water-binder ratio upon gradient components was proposed.Through the research of regulation and modification mechanism of mineral powders and polymers on cement-based materials with low water-binder ratio,the component design was built based on functional requirements of gradient layers.Preparation theroy exploration and process improvement of cement-based materials with high durability and low water-binder water could be achieved.The main works and innovations are presented as follows:(1)Establishment the matching method of composition optimization of cement-mineral powder materials based on compactness of paste.Exploration the hydration regulation mechanism of mineral powders on cement-based materials with low water-binder ratio.The rheological property and hydration process of cement paste,hydration products and properties of the hardened were studied under the synergistic effect of cement and mineral fines.Based on optimizing design of close packing with thickness boundary conditions of "water film" under slurry state of different mineral components powders,the hydration regulation mechanism of mineral powders on cement-based materials with low water-binder ratio was studied.The hydration activity coefficients of mineral powders were calculated by hydration activity coefficient method,the coefficient of hydration reaction at 28 d of silica fume,fly ash,wet fly ash and silica sol were 3.98,0.06,0.13 and 0.72,respectively.(2)Exploring the hydration mechanism of polymers modified cement-based materials with low water-binder ratio.The flow characteristic,physical changes and chemical reaction of polymers modified cement paste were studied.The hydration mechanism of polymers modified cement-based materials with low water-binder ratio was explored.In the paste state,physical adsorption and chemical reaction occurred between cement and polymers are the primary reason for paste agglomeration and flow degradation.The action mechanism of polymers on the cement hydration were revealed,and the early hydration reaction adsorption model of cement-PA was established.The preparation of polymers modified cement-based materials with low water-binder ratio and the relationship between structure and properties of hydration products were studied,which provided technical support for the preparation of polymer modified cement-based materials with low water-binder ratio.(3)Research of components gradient cement-based composite materials with low water-binder ratio(CGCC)based on homogenization of hydration degree.Using depth hydration degree experiment,the influence of water diffusion and temperature change of curing conditions on spatial distribution of hydration products were tested accuracy.Revealing the action mechanism of temperature field humidity field on hydration degree of cement-based materials with low water-binder ratio: during early ages,the water diffusion rate of plastic paste depends on environmental temperature,paste viscosity and the difference of internal and external humidity,resulting in that with the w/c decreasing or early curing temperature increasing,the spatial distribution difference of hydration degree increases;after initial hardening,the driving force of interface moisture transmit originated from capillary adsorption effect decreases,and the difference of hydration degree between inside and outside of cement materials with low water-binder ratio increases further.Through component design of internal and external gradient layer cementitious material(protective layer using high properties concrete modified with polymer,matrix layer using low clinker dense concrete),the preparation of CGCC was realized preliminary exploration.Compared with single component materials,CGCC could obviously reduces the clinker consumption,improves comprehensive hydration degree(increasing 38.6% at 90 d)and obtains the better volume stability and durability(permeability coefficient of chloride ion decreasing 30.5% at 90 d,early plastic shrinkage(3 d)and drying shrinkage(90 d)decreasing 22.5% and 24.3% respectively).Above research will improve the the hydration hardening mechanism and preparation of cement-based materials with low water-binder ratio,and provide technical support for performance optimization design of cement-based materials with low water-binder ratio and extension of civil engineering application.The research results are significance for improving the performance and durability of construction structure.