| Cement-asphalt(Cement-Asphalt,hereafter called CA)system is a new type of civil engineering material,which is prepared by the modification of cement-based material by asphalt emulsion.It overcomes the shortcomings of easy cracking of cement-based materials and insufficient strength of asphalt,which is widely used in the cushion layer of high speed railways,semi-flexible pavement and repairing material.As a kind of organic-inorganic composite,CA paste is composed of cement,asphalt emulsion,water and superplasticizer,which is an important part of CA system and is also closely related to the development of CA system performance.As an organic material,asphalt emulsion would exert a direct impact on the early-age hydration behavior and the formation of microstructure of cement paste,thus further affecting the workability,mechanical properties and durability of CA paste.Therefore,it is of great significance to study the early-age hydration behavior,hydration products and microstructure evolution of CA paste.In this paper,the influence of cationic asphalt emulsion on the early-age hydration process of cement paste was studied,and the retardation mechanism of cationic asphalt emulsion on cement hydration was disclosed.In addition,the influence of cationic asphalt emulsion on the formation of early-age hydration products and the evolution of microstructure of cement paste was also investigated.The main conclusions could be obtained as below:1.The process of cement hydration was delayed by cationic asphalt emulsionThe influence of cationic asphalt emulsion on cement hydration was studied by setting time,conductivity and chemically bound water.The results showed that:with the rise of A/C(the mass ratio of cationic asphalt emulsion to cement),the setting time of cement paste was prolonged;the conductivity of cement paste increased with the rise of A/C,meaning the ion dissolution from cement particles being hindered;the content of chemically bound water decreased with the rise of A/C,implying the hydration degree of cement being reduced by cationic asphalt emulsion.The influence of cationic asphalt emulsion on the strength of CA mortar was investigated,the result showed that:cationic asphalt emulsion significantly reduced the compressive and flexural strength of cement mortar.The obtained results mentioned above indicated that the process of cement hydration was inhibited by the cationic asphalt emulsion.The retardation mechanism of cationic asphalt emulsion on cement hydration was revealed:(1)Cationic asphalt emulsion with positive charge was adsorbed on the surface of cement particles with negative charge via the electrostatic attraction,which hindered the contact between cement particles and water,and thus resulted in the inhibition of ion dissolution from cement particles;(2)The cationic asphalt emulsion gradually gathered and formed a layer of asphalt membrane on the surface of cement particles,which enveloped the cement particles after demulsification,the water transfer to the surface of the cement particles was further hindered.Additionally,the organic asphalt membrane was not beneficial to the precipitation of inorganic ions on the surface of the cement particles,that is,the formation of hydration products was inhibited.2.The formation of cement hydration products and microstructure were hindered by cationic asphalt emulsionThe influence of cationic asphalt emulsion on cement hydration product Ca(OH)2 was studied by TG and XRD.The results showed that:the hydration product Ca(OH)2 was inhibited by cationic asphalt emulsion,the content of Ca(OH)2 decreased with the rise of A/C.The influence of cationic asphalt emulsion on cement hydration product C-S-H gel was studied by FTIR、XPS and NMR.The FTIR test showed that:with the rise of A/C,the peak value of the stretching vibration of the Si-O bond was shifted to the lower wave number,and meanwhile the peak strength and peak area decreased,which proved that the polymerization of silicon-oxygen tetrahedron was hindered by cationic asphalt emulsion;XPS test showed that the addition of cationic asphalt emulsion decreased the binding energy of Ols and the amount of bridging oxygen,which thus could be deduced that the formation of long chains of C-S-H gel was hindered;the solid NMR test showed that:the cationic asphalt emulsion reduced the average length of the straight chain of C-S-H gel,the degree of polymerization of silicon-oxygen tetrahedron in C-S-H gel was decreased obviously.The above experimental results proved that:the formation of hydration product C-S-H gel was inhibited by cationic asphalt emulsion,and its content decreased with the rise of A/C.The variation law of ultrasonic wave velocity of CA mortar was studied by ultrasonic test,the test result showed that:the ultrasonic wave velocity of CA mortar decreased with the rise of A/C,which indicated that the compactness of CA mortar was reduced by cationic asphalt emulsion.The mercury injection test proved that the average pore size shifted towards to the bigger value and the cumulative pore volume of the CA paste increased with the increase of A/C,further indicating that the hydration process of cement paste was inhibited by cationic asphalt emulsion,which was in good agreement with the result of ultrasonic test;from the SEM observation,it can be clearly seen that:the hydration products of CA paste were significantly less than that of cement paste,resulting in the inhibition of the formation of microstructure,and the coverage of the asphalt membrane on the surface of hydration products led to the formation of organic-inorganic microstructure.3.The hydration process and pore structure of CA paste was characterized by low-field NMR technologyThe low-field NMR technology characterizes the cement hydration process by monitoring the transformation from evaporable water to non-evaporable water in the paste.The experimental result showed that the content of evaporable water in cement paste increased with the rise of A/C,indicating that the consumption of evaporable water was reduced with the addition of cationic asphalt emulsion,namely the hydration degree of cement was dropped with the rise of A/C at the same age.The low-field NMR technology could be adopted to probe the pore structure by measuring the relaxation time of H1 in water molecules in the CA paste pore under the condition of water saturation.Compared with the method of mercury injection to measure the pore structure of cement paste,the low-field NMR technology can realize the continuous and non-destructive detection of CA paste.The result showed that:with the rise of A/C,the maximum capillary pore size and the porosity increased,further indicating that the process of cement hydration was retarded by cationic asphalt emulsion. |
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