| Mineral admixtures and chemical additives are widely used in the production of cementitious materials.A deep understanding of the influence mechanism of mineral admixtures and chemical additives on the structural build-up and rheological properties of cementitious materials is of great significance for the smooth realization of concrete pumping,pouring,3D printing and the preparation of ultra-high-performance concrete(UHPC).In this paper,the effects of the addition of different mineral admixtures on the hydration process of the composite cementitious materials and the dispersion state of particles in fresh paste were first measured,and then the development process of structural build-up and the change of rheological properties of common composite cementitious materials system and UHPC system with low water-binder ratio were studied respectively.The differential role and influence mechanism of different mineral admixtures were analyzed,and the interaction between polycarboxylate-based superplasticizer(PCE)and sodium citrate on the rheological properties of fresh paste over time was also studied.Based on the above research,the following conclusions can be drawn:(1)The process of structural build-up of the common composite cementitious materials system includes not only the stage of particle flocculation and overlapping to form a percolated network structure,the stage of linear growth of structural strength,but also the stage of exponential growth of structural strength.The transition time at which the phase angle tends to stabilize can be used to characterize the flocculation capability of the particles.The end time of the hydration induction period was characterized using the transition time at the end of the linear increase in the storage modulus.The nucleation and growth rates of hydration products during the accelerated hydration phase were characterized using the growth index during the third stage of development of the storage modulus.(2)High solid content and PCE content affect the structural build-up of UHPC system.The addition of low content of silica fume can improve the flocculation ability of the system,while the high content of silica fume is the opposite.The effect of adding fly ash microbeads,slag,ultra-fine slag and fly ash to enhance the flocculation capacity of the system increases sequentially.The structural strength of the UHPC system in the second stage of structural build-up does not increase in a completely linear fashion.(3)For the UHPC system,the ability of silica fume with the content of less than 15%,slag,fly ash microbeads and ultra-fine slag to reduce the dynamic yield stress of the paste increases sequentially,and the addition of fly ash or high content of silica fume will improve the dynamic yield stress of the paste.(4)The shear thickening mechanism of UHPC paste includes the formation of flocs,shear expansion and the transition from ordered state to disordered state.The shear thickening degree of paste with silica fume,fly ash,ultrafine slag,fly ash microbeads and slag increases in turn when the fluidity is equal.Under the influence of particle size,the apparent viscosity of paste with ultra-fine slag,fly ash microbeads,silica fume,slag and fly ash increases successively.(5)In the common composite cementitious materials system,the addition of silica fume and ultra-fine slag will increase the viscosity of the paste,and the addition of fly ash and fly ash microbeads will reduce the viscosity of the paste.The addition of slag has little effect on the viscosity of the paste.In the UHPC system,the addition of ultra-fine slag,fly ash microbeads and silica fume with a content of less than 15% will reduce the viscosity of the paste.Addition of slag and fly ash will increase the viscosity of the paste.(6)Citrate ions are preferentially adsorbed on the surface of cement particles,and PCE is delayed in adsorption.The combination of the two can control the time-dependent rheological properties of the cementitious materials paste. |
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