| The water-binder ratio is an important factor in affecting the cement-based material performance.There are difference for the hydration characteristics and mechanism of cementitious system with multiple admixtures under the condition of low water-binder ratio.In order to clarify the hydration properties of the composite cementitious system at low water-binder ratio,and provide a theoretical basis for the rationality of the high-performance concrete mix,this study takes the composite system with cement-slag-fly ash as the research object,and it reveals the influence rule of different low water-binder ratios(0.2,0.25,0.3)and different compound contents of slag-fly ash(30%,40%,50%)on the hydrating capacity and mechanism of the composite system.Main research works and conclusions are as follows:(1)Test the hydration heat evolution of composite cementitious system with the isothermal calorimeter method.The results indicate that: When the water-binder ratio is as low as 0.2,the slag and fly ash mixing will have little impact on the heat release of the system and the early and late hydration rates in certain mixing amount;decrease of the water-binder ratio will increase the early accumulated heat release and maximum rate of heat release;the promotion or inhibiting effect on the cement’s early hydration of the pozzolanic reaction with the slag and fly ash varies from the change of the water-binder ratio.(2)Krstulovic-Dabic model is established based on isothermal calorimetry and the model parameters are calculated.The results indicate that: When the water-binder ratio is low,if it is decreased,it would change the mechanism of hydration reaction.When the water-binder ratio is 0.3 and 0.25,the hydration reaction mechanism of the composite cementitious system is NG-I-D.When it is 0.2,the mechanism is NG-D.The mixing of slag-fly ash will weaken the phase boundary reaction control and decrease the hydration rate at each reaction stage,but it will not change the hydration mechanism.(3)The chemically bound water content of each group was measured with the high temperature burning method.The results indicate that: the hydration degree is significantly increased with age.The early hydration degree of the composite cementitious system is low,but the growth rate of the late hydration degree is higher.The reaction degree of the hardened cement paste under the low water-binder ratio is positively correlated to the water-binder ratio.When the water-binder ratio is changed,the superposition effect varies from the different compound contents.(4)X-ray diffraction,scanning electron microscopy and transmission electron microscopy are adopted for the research of the 28 d hardened pastes’ hydration products and microstructure.The results indicate that:When it is hydrated to 28 d age,the pozzolanic reaction of the slag and fly ash mixing is more significantly shown in the samples with the higher water-binder ratio for the CH consumption.C-S-H morphology varies greatly in different water-binder ratios.When the water-binder ratio is at 0.2,C-S-H morphology is spherical with higher compactness,and C-S-H is fibrous when it is 0.3.(5)Mortar strength is measured for the composite cementitious system.The results show that: The the mixed slag-fly ash can alleviate the mechanical loss with the increase of water-binder ratio to a certain extent.The relationship between the growth rate of bound water and strength varies with the water-to-binder ratio.The positive effect of slag fly ash on the hydration of 0.2 water binder ratio system is in the early stage,and that of 0.25 and 0.3 water binder ratio system is in the late stage. |
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