Study On Hydration Kinetics And Process Of MgO Expansive Agents And Cement Pastes

MgO shrinkage compensating concrete has been widely used in hydraulic mass concrete,but it is seldom used in industrial and civil buildings because of its poor stability due to its delayed expansion characteristics.The delayed expansion of MgO is closely related to the hydration process.The hydration rate and degree are the internal factors that determine the expansion performance of MgO concrete.There are many studies on the hydration process of MgO,but considering various factors in practical engineering application,such as temperature,water binder ratio,MgO activity,dosage and so on,there are few studies on the hydration kinetics of MgO expansive agent cement composite cementitious materials.In this paper,MgO cement composite cementitious materials are used as the research system.The physical properties of MgO expander are analyzed by laser particle size analysis and XRD.The hydration characteristics and kinetics process are studied by isothermal calorimetry.The hydration degree of MgO is analyzed by thermogravimetry and differential scanning calorimetry.The pore structure characteristics of hardened paste are studied by mercury injection method In addition,the experimental study on the expansion and mechanical properties was carried out.The results show that:?1?When the amount of MgO expansion agent does not exceed 12%,the Krstulovic model can be used to simulate the hydration kinetic process,and the simulation error increases with the increase of the MgO expansion agent content and the hydration temperature.?2?The addition of MgO expander will not change the hydration mechanism of the cementitious system,and the hydration reaction still goes through three stages:NG?I?D;the amount of MgO has the greatest influence on the ng process,the larger the amount of MgO is,the smaller the reaction rate K₁^?,and the longer the reaction time of the ng process;the greater the water binder ratio is,the greater the water binder ratio is,the smaller the K^?₂ value,the shorter the reaction time of I process is;the higher the activity of MgO expander,the lower the value of K₁^?,the longer the reaction time of ng process is;the higher the temperature is,the faster the hydration rate of each stage will be,and the hydration mechanism of the system will be changed.The hydration control mechanism of 45?is NG?I?D,and that of 65?is NG?D.?3?Research on the early?72h?hydration process of the MgO-cement composite cementitious material.The results show that the hydration release of the MgO-cement composite cementitious material after the M-type MgO expansion agent replaces part of the cement at 20?Both the heat rate and the exothermic energy decreased slightly.Compared with the blank group,when the M-type MgO content was 12%,the total exothermic heat of the composite cement material decreased by 2.67%;as the water-to-binder ratio and the activity of the MgO expansion agent increased,the compound The heat release rate and heat output of the cementitious materials increase,but the increase is not obvious;as the hydration temperature increases,the hydrated heat release rate and heat output of the composite cementitious materials increase significantly,and the second heat release peak is significantly advanced.Compared with 20?,at 45?and 65?,the total heat release increases by about 16%and 35%,respectively.?4?The research on the regularity of the hydration degree of MgO expansion agent in MgO-cement composite cementitious material with age over time is carried out.The results show that:MgO expansion agent has the fastest hydration rate before 3d and the hydration rate after 28d.Slow down,taking the M9-0.4-20 test group as an example,the degree of hydration of MgO expansion agent at 90d is 46.7%,and the degree of hydration at 3d and 28d is 60%and80%at 90d respectively;with the amount of MgO expansion agent added,Activity,water-to-binder ratio and hydration temperature increase,the degree of hydration of MgO at all ages will increase accordingly,of which the hydration temperature has the greatest influence on the degree of hydration of MgO expansion agent,followed by the amount and activity of MgO,the least The ratio of water to rubber.?5?A study on the law of the limited expansion rate of MgO-cement expanded mortar with age over time.The results show that:with the continued hydration of MgO expansion agent,the limited expansion rate of expanded mortar increases with age.The growth rate was the fastest in the first 28 days,then the expansion rate slowed down,but the slow expansion duration was longer,and the M-type MgO expansion stability period was around 75 days;the time-dependent change rule of the expansion development was basically consistent with the change rule of the hydration degree of the MgO expander However,the development of swelling slightly lags behind the degree of hydration.Taking the M9-0.4-20 test group as an example,the limiting swelling rates at 3d and 28d are 17%and 65%at 90d,respectively;with the amount of MgO swelling agent,activity,water The increase of the glue ratio increases the expansion of the expansion mortar at all ages,of which the amount of MgO has the largest influence on the limiting expansion rate,followed by the activity of MgO,and the least affects the water-to-binder ratio,and the degree of hydration with MgO varies with various factors The law is the same.The effect of temperature on the expansion is different from the degree of hydration.Increasing the temperature significantly increases the limiting expansion rate of the expanded mortar 3d,the expansion stability period is shortened,and the stable expansion amount is reduced.The stable expansion amount is the lowest at 65?.?6?A study on the pore structure and mechanical strength of MgO-cement composite cementitious materials with age changes over time.The results show that:compared with the blank group,the addition of MgO expansion agent makes the hardening slurry of each age The total pore volume,the most probable pore diameter increases,both the harmful and harmful pores increase,and the mechanical strength decreases;increasing the temperature will cause the harmful and harmful pores of the hardened slurry at all ages to increase;at 45?,90d The mechanical strength is reduced by about 7%compared with normal temperature curing,and about 18%at 65?.The expansion is not only related to the degree of hydration of the expansion source MgO,but also related to the internal pore structure of the hardened paste.Excessive temperature leads to the interior of the hardened paste The pore structure is coarsened,which makes the expansion mortar limit the expansion rate and reduce the strength.