Study On Hydration Characteristics And Kinetics Of MgO Expansive Agent,Fly Ash,Cement Composite Cementitious System

In view of the crack problem of mass concrete,adding MgO expansion agent and mineral admixtures such as fly ash to cement concrete is an effective measure to reduce the cracking of mass concrete.MgO expansion agent has not been listed in the national standard GB 23439-2017 concrete expansion agent,but compared with the traditional expansion agents such as calcium sulphoaluminate and calcium oxide,it has the advantages of less hydration water demand,stable expansion hydration products,and can compensate for the later shrinkage of concrete.At present,MgO expansion agent is mainly used in hydraulic mass concrete to compensate the shrinkage.The reasonable addition of mineral admixtures such as fly ash can reduce the hydration temperature rise of concrete,thus reducing the shrinkage cracks caused by temperature shrinkage of mass concrete.At present,there are few studies on the hydration process and kinetics of MgO expansive agent,fly ash,cement composite cementitious system,and there is no scientific understanding of the relationship between the degree of hydration,microstructure and macro performance development of composite cementitious system.In this paper,the MgO expansive agent,fly ash,cement composite cementitious system was studied.The factors such as the activity?M type and S type?and the content?6%,9%and12%?,the content of fly ash?20%and 40%?,the water binder ratio?0.3,0.4 and 0.5?and the hydration temperature?20?,45?and 65??were set.Through the hydration heat test,thermal analysis test and mercury porosimetry test,The hydration characteristics and kinetics of the composite cementitious system were studied.The results show that:?1?The addition of MgO expansion agent and fly ash reduces the early exothermic of the composite cementitious system,the reaction rate of each stage of hydration is reduced,and the early hydration temperature rise of mass concrete is reduced.At the same time,the addition of MgO expansion agent can effectively compensate the shrinkage of concrete.When the content of MgO expansion agent is less than 12%,the content of fly ash is between 20⁴0%,and the water binder ratio is 0.3⁰.5,the hydration process of ternary composite cementitious materials at 20?and 45?is NG-I-D,and krstulovic Dabic model can better fit the hydration process of composite cementitious system within the range of test variables.The addition of MgO and fly ash makes the whole reaction process more gentle.When the water binder ratio is 0.4 and 0.5,the effect of water binder ratio on the change of hydration mechanism is less.When the temperature increases,the kinetic parameters of hydration become larger and the hydration process is more intense.?2?The sensitivity of low activity MgO?S type?is higher than that of high activity MgO?M type?.?3?At 20?,the larger the content of M-type MgO expansion agent is,the larger the expansion rate of mortar block is.The higher the MgO activity is,the higher the early expansion rate of the shrinkage compensating mortar is,but the later expansion growth slows down obviously.With the change of water binder ratio,the restricted expansion rate of mortar increases with the decrease of water binder ratio.High temperature greatly promotes the early expansion of mortar specimens.?4?When the water binder ratio is 0.4 and 0.5,the increase of fly ash content can basically inhibit the expansion of mortar.?5?At 20?,the mechanical strength of the composite cementitious system decreases with the addition of MgO expansion agent.The higher the content of MgO expansion agent,the lower the strength of the composite cementitious system.The strength decreases obviously at12%content,and decreases 30.70%at 90 days.Before the age of 90 days,under the condition of 9%MgO content,the activity of MgO has little effect on the strength of mortar.With the increase of fly ash content,the early strength of composite cementitious system decreases.The improvement of curing temperature greatly promotes the development of early strength of the system,but the later strength of high temperature water curing mortar is lower than that of standard curing.?6?Under standard curing condition,with the increase of MgO content,the total porosity gradually increases.Under the condition of 12%MgO hydration,the pores larger than 200 nm increase obviously.The hydration of MgO results in the increase of pores in the range of 0.3²?m.With the change of MgO activity,the lower the activity of MgO,the larger the total porosity of the paste.Before 90 days,the porosity of the paste increases with the increase of fly ash content and water binder ratio.The porosity of the system decreases with the increase of the early curing temperature,but the refinement degree of the pores in the later stage is lower than that of the standard curing samples.?7?Under standard conditions,the higher the content of MgO,the higher the hydration degree of MgO,but the lower the content of Ca?OH?₂.28 days ago,the hydration degree of S-type MgO is lower than that of M-type MgO,and the content of Ca?OH?₂ is slightly higher than that of M-type MgO.The higher the content of fly ash,the lower the hydration degree of MgO,and the lower the content of Ca?OH?₂.The hydration degree of MgO increases with the increase of water binder ratio in 3 days and decreased in 28 days.The high water binder ratio promotes the hydration of the system in the early stage and decreases the effect on the hydration in the later stage.Under 65?water curing condition,fly ash reacts with alkaline products of mortar,namely Ca?OH?₂ and Mg?OH?₂,which consumes the expansible product Mg?OH?₂ and inhibits the development of expansion.