Study On The Effect Of Admixtures On The Hydration Characteristics Of Ordinary Portland Cement Under High Temperature Environment

The cement hydration reaction is easily affected by many factors,and the reaction process is complex and changeable,and the influence mechanism of high temperature environment on cement hydration reaction is more complex.Both admixtures of urea and maltodextrin will change the hydration reaction process of cement,but the mechanism of their influence on cement hydration reaction at high temperature is not clear.Therefore,this thesis first uses isothermal calorimeter(TAM air)to explore the influence of urea and maltodextrin on the hydration thermodynamics of cement at five different temperatures,and analyzes the changes of cement hydration kinetics under different working conditions based on the Krstulovic-Dabic hydration kinetic model.Secondly,the influence of the dosage of the two admixtures on compressive strength and microscopic pore characteristics at five different temperatures was clarified.Finally,the variance analysis of the above data was carried out,and the influence of temperature,admixture and its interaction on the hydration characteristics,compressive strength and microscopic porosity of cement was further quantitatively evaluated.The response mechanism between the hydration characteristics of cement and its mechanical properties and pore characteristics under different temperature environments of the two admixtures was clarified.The research results are as follows:(1)Both urea and maltodextrin compounds reduce the hydration rate of cement,and the reduction ability of maltodextrin is slightly stronger than that of urea.Both admixtures can avoid concentrated heat release of cement and reduce cement cracking.The increase in curing temperature will cause the peak of the first hydration rate to increase first and then decrease,reaching a peak at 60 °C.After the curing temperature exceeds 60 °C,the hydration heat release rate will have a second peak.An increase in curing temperature leads to a gradual increase in the peak second hydration rate.However,the admixture does not cause a second peak in the hydration rate.The influence of temperature on cement hydration is more obvious than that of admixtures.(2)The two admixtures of urea and maltodextrin do not fundamentally change the hydration kinetic process of cement,and both are NG-I-D processes.With the increase of temperature,the hydration kinetic process will undergo fundamental changes,and when the curing temperature rises to 60 °C,the hydration kinetics will change from NG-I-D process to NG-D process,not experiencing I process.In addition,both urea and maltodextrin will increase the hydration kinetic parameter n,and the cement hydration reaction is difficult to carry out after the addition of urea and maltodextrin.Temperature will reduce the hydration kinetic parameter n value.Temperature can promote the hydration reaction of cement and reduce the difficulty of hydration reaction.(3)Both urea and maltodextrin compounds will reduce the early compressive strength of cement mortar(3d and 7d),and the reduction effect of maltodextrin is slightly stronger than that of urea.But with the extension of the curing age,the compressive strength of mortar specimens mixed with urea or maltodextrin will return to the level consistent with the compressive strength of pure cement mortar when the curing period is 28 d.The compressive strength of cement mortar specimens under different dosages first increases and then decreases with the increase of curing temperature,and reaches a peak at 60°C curing temperature.(4)The increase of curing temperature will cause the porosity and average pore size value of cement slurry to decrease first and then rise,and the porosity is lowest when the curing temperature is 60 °C.Both urea and maltodextrin increase the porosity and average pore size of cement slurry,but the reduction effect of maltodextrin is slightly stronger than that of urea.(5)The influence of temperature on hydration characteristics and average porosity exceeds that of urea or maltodextrin admixtures.The effect of urea on hydration characteristics is different from the effect of maltodextrin on hydration characteristics.Urea has the greatest effect on the peak hydration exothermic rate,while maltodextrin has the greatest effect on the timing of the peak hydration rate.The interaction of temperature with urea or maltodextrin admixtures has a more pronounced effect on hydration characteristics than compressive strength and average porosity.