Study On The Performance Of C₁₂A₇-CaSO₄ Composite Cementitious Material Modification

Self-leveling materials are widely used because of their convenient and civilized characteristics.C₁₂A₇ modified CaSO₄-based self-leveling material is adopted to solve the disadvantages of the current gypsum based self-leveling material such as water resistance,poor wear resistance and low early strength.However,due to the rapid hydration of the two materials in the early stage,the viscosity is too large,resulting in the difficulty in removing bubbles,which affects the appearance and significantly reduces the strength in the later stage.In view of this situation,this topic on the basis of the original self-leveling mortar cementitious material formula,using heavy calcium powder,light calcium powder and fly ash to modify and optimize It will make the hydration products more reasonable to achieve better material performance.In this paper,the hydration characteristics of C₁₂A₇ with different amounts of gypsum were firstly studied.Then the hydration reactions between three modified materials and C₁₂A₇were respectively studied.After understanding the influence of modified substances on C₁₂A_7,the modified substances were mixed into the C₁₂A₇-CaSO₄ system to explore the role and mechanism of the modified materials in the binary system.The conclusions are as follows:（1）The hydration heat release of the C₁₂A₇-CaSO₄ system mainly concentrates on 2h to 6 h.When the gypsum content is 40%,C12A7 releases 129 J more heat per unit mass.The addition of gypsum can shorten the initial and final setting time to within 20 min.At a w/c of 0.40,after 28 days,a large number of AFt were formed due to the continuous reaction of gypsum,which would lead to cracking and destruction of the specimen.When w/c is 0.50,the compressive strength increased with the increase of the content and age,while when the gypsum content was lower than 30%,the strength shrank after 90 days due to the formation of SO₄-AFm phase in the system.（2）Adding heavy calcium powder to C₁₂A₇-CaSO₄increases the heat release rate of hydration,because of its"nucleation acceleration",the heat release per unit mass system increases.Although the early reaction of the system was dominated by C₁₂A₇-CaSO₄,the initial setting time and final setting time were prolonged by 2min and 3min,respectively,when 25%heavy calcium powder was added to 40%gypsum.At 40%gypsum,the strength of 15%heavy calcium powder reached 60.2 MPa at 28 d.The addition of heavy calcium powder inhibits the cracking of late specimens by increasing the actual water-binder ratio and increasing the porosity,and the effect is the best when the content of heavy calcium powder exceeds 15%.During the period from 28 d to 210 d,more than 1%AFt formation group would cause cracking of the specimen.Moreover,it would significantly shrink at 210 d.The addition of heavy calcium powder could form CO₃-AFm in the specimen,and at the same time,it could fill the void,thus inhibiting the strength decline.（3）Compared with the heavy calcium powder,the light calcium powder has a finer and neat microstructure,and needs a 0.5 water-binder ratio.It will react with C₁₂A₇ to form hydrated calcium carboaluminate.In the binary system,when the content is 25%,the strength of each phase is obviously enhanced.SO₄-AFm was generated at 20%gypsum content and the strength of AFt and CO₃-AFm in the system increased with the development of age when the light calcium powder was added.At 40%gypsum,the diffraction summit of AF in the later stage decreased due to decomposition,while the addition of light calcium powder stabilized AFt and avoided carbonization.（4）Fly ash accelerates the hydration and exothermicity of C₁₂A₇ and shortens the setting time.When the content of fly ash is 35%,a small amount of flakes of C₂ASH₈will be formed in 7 d,which will delay the time of phase transformation.However,due to its low activity,its mechanical properties will decrease with the increase of the content.When added to the binary,the reaction between fly ash and C₁₂A₇ will be inhibited,and it will generate AFt under the action of gypsum excitation.With the addition of fly ash,the strength increases with age,especially after 28 days.The hydration of fly ash will also reduce the basicity of the system,and the carbonization can be shown in TG-DTG.In conclusion,all the three materials can improve the 1d strength,and the light calcium powder is the most obvious.When the binary system is modified,the cementitious properties of the two kinds of limestone powder are related to C₁₂A_7,while the fly ash is related to CaSO₄.The composite product of CO₃-AFm+AFt was produced by the two kinds of limestone powder when the gypsum content was insufficient.The AFt content in the system had a great influence on the early strength,while CO₃-AFm would make the late strength develop continuously.When the amount of gypsum is sufficient,the activity of fly ash is more obvious after 28 days.