Study On Hydration Behavior Of Cement-Fly Ash-MgO Expansive Agent Ternary Composite Cementitious System Under Continuous High Temperature Conditions

The problem of concrete cracking due to temperature has always been a hot topic in the engineering field.For mass concrete,due to its slow internal heat dissipation and long duration of high temperature,it is easy to cause concrete structure cracking,which will have a serious negative impact on the durability of concrete.MgO expansive agent has the advantages of less hydration water demand and stable hydration products,which can be used to compensate the cracking of mass concrete caused by temperature.At present,there are few studies on the shrinkage compensation ability of MgO expansive agent under continuous high temperature conditions.Therefore,this study takes the cement-fly ash-MgO expansive agent ternary composite cementitious system as the research object,and sets four continuous high temperature conditions（20°C,40°C,60°C,80°C,of which 20°C is the control temperature test group）.The hydration heat release characteristics,early hydration kinetics,expansion properties,mechanical properties,pore structure and hydration product content of the ternary composite cementitious system were studied.The main conclusions are as follows:（1）The change of temperature,fly ash content and MgO expansive agent content will affect the hydration heat release characteristics of the ternary composite cementitious system.With the increase of temperature,the early hydration heat release rate and heat release of the system increase,the time of the second exothermic peak appears in advance,and the peak value of the second exothermic peak increases.Under various continuous high temperature conditions,with the increase of fly ash content in the composite cementitious system,the early hydration heat release rate and heat release of the system decrease,the time of the second heat release peak is delayed,and the second heat release peak value is reduced.The effect of MgO expansive agent on the early hydration heat release characteristics of the system is similar to that of fly ash.However,at 60°C,with the increase of MgO expansive agent content in the composite cementitious system,the time of the second exothermic peak of the system gradually advances,and the heat release at 72 h gradually increases.（2）The Krstulovic-Dabic model can well simulate the hydration process of the ternary composite cementitious system.At 20°C and 40°C,the hydration reaction process undergoes three stages:nucleation and crystal growth（NG）,interactions at phase boundaries（I）and diffusion（D）.At 60°C,the hydration reaction process only undergoes two stages:nucleation and crystal growth（NG）and diffusion（D）.（3）The change of temperature,fly ash content and MgO expansive agent content will affect the restrained expansion rate of the ternary composite cementitious system mortar.Increasing temperature can significantly increase the early expansion rate and restrained expansion rate of mortar,and make the system enter the stable period of expansion in advance.At 20°C and 40°C,fly ash promotes the early expansion of mortar（20°C is before 14 d,40°C is before 7 d）,while at 60°C and 80°C,fly ash inhibits the expansion of mortar.With the increase of MgO expansion agent content in the composite cementitious system,the restrained expansion rate of mortar increases,and the influence of MgO expansion agent on the restrained expansion rate of mortar increases with the increase of temperature.（4）The change of temperature,fly ash content and MgO expansion agent content will affect the strength of ternary composite cementitious system mortar.Increasing the temperature will promote the development of the strength of the mortar before 28 d（including 28 d）,but it is not conducive to the development of its later strength（60 d and 90 d）.The incorporation of fly ash will reduce the strength of mortar,and the greater the content,the greater the reduction,but the increase of temperature will lead to the early volcanic ash reaction of fly ash,which has a certain contribution to the strength of mortar,and the higher the temperature,the greater the contribution of fly ash to strength.With the increase of MgO expansion agent content in the composite cementitious system,the strength of cementitious system mortar decreases,and the influence of MgO expansion agent on the strength of mortar increases with the increase of temperature.（5）The changes of temperature,fly ash content and MgO expansive agent content will affect the pore structure of the ternary composite cementitious system.With the increase of temperature,the total pore volume of the ternary composite cementitious system paste decreased,the pore volume and pore proportion of harmful pores and more harmful pores above50 nm decreased.At 20°C,with the increase of fly ash content in the composite cementitious system,the total pore volume of the ternary composite cementitious system paste,the pore volume and pore proportion of multi-harm pores above 200 nm gradually increase.However,at 80°C,the addition of 40%fly ash will reduce the total pore volume of the composite cementitious paste,especially the pore volume of harmful pores and multi-harm pores above50 nm.With the increase of MgO expansive agent content in the composite cementitious system,the total pore volume of the ternary composite cementitious system paste gradually increases,the pore volume and pore proportion of harmless pores below 20 nm gradually decrease,and the pore volume and pore proportion of harmful pores and multi-harmful pores above 50 nm gradually increase.The pore structure of the paste changes from dense to loose,and the higher the temperature,the greater the influence of MgO expansive agent on the pore structure of the paste.（6）The change of temperature,fly ash content and MgO expansion agent content will affect the content of hydration products in the cement-fly ash-MgO expansion agent ternary composite cementitious system.Increasing the temperature increased the content of Mg（OH）₂in the composite cementitious paste,but decreased the content of Ca（OH）₂ in the paste.Fly ash promoted the formation of Mg（OH）₂ in the early（3 d）paste at lower temperatures（20°C and40°C）,while inhibited the formation of Mg（OH）₂ at higher temperatures（60°C and 80°C）.With the increase of fly ash content in the composite cementitious system,the content of Ca（OH）₂ in the composite cementitious system paste decreases,and the influence of fly ash on the content of Ca（OH）₂ in the composite cementitious paste increases with the increase of temperature.With the increase of MgO expansive agent content in the composite cementitious system,the content of Mg（OH）₂ in the composite cementitious paste increases,but the content of Ca（OH）₂decreases,and the influence of MgO expansive agent on the content of Mg（OH）₂ and Ca（OH）₂in the composite cementitious paste increases with the increase of temperature.