Study On The Sintering And Hydration Mechanism Of A C₄A₃$-C₂S Binary System

At present CSA cement has partly become an environmental substitute for Portland cement.Based on the main clinker minerals C4A3$ and C2 S of CSA cement,a C4A3$-C2 S binary system with different content of C2 S is designed and established in this paper.By means of XRD,RQPA,ICC,TG.FTIR,SEM and MIP,the formation,sintering mechanism,hydration products,hydration process and hydration mechanism of the C4A3$-C2 S binary system were systematically studied,aiming to solve the theoretical problems existing in the production and application of the C4A3$-C2 S binary system related materials.In terms of the sintering system optimization of C4A3$-C2 S binary system,the optimization and evaluation regulations were established by means of XRD,RQPA and ICC,and characterized and analyzed by using FTIR,TG-DTG and SEM-EDS.The results show that:In the C4A3$-C2 S binary system,the clinker mineral C4A3$ phase mainly exists in orthorhombic,and the main intermediate mineral phases are C3 A,C12A7 and C3AS3.When the design content of C2 S is not more than 45 wt.%,1350? and sintering for 1h is a relatively optimal sintering regulation;When the content of C2 S is greater than or equal to 65 wt.%,1300? and sintering for 2h is a relatively optimal sintering regulation.The intermediate phase content of the optimized system should not be greater than 5.8wt.%.In terms of exploration the sintering mechanism of C4A3$-C2 S binary system,the kinetics response relationship of the clinker minerals C4A3$ and C2 S in the system were studied by using the three-dimensional diffusion and crystal nucleation growth model.And the hydration heat release and the crystallite sizes evolution rules of the clinker were explored by ICC and RQPA methods.The results showed that the primary heat release in the initial hydration stage of C4A3$-C2 S clinker system was caused by the hydration of C4A3$ minerals.Increasing the sintering temperature can promote hydration and increase the cumulative heat release from hydration.However,when the sintering temperature is higher,the hydration induction period will be prolonged and the hydration rate will be reduced.Appropriate sintering time can balance the formation and decomposition of C4A3$ mineral in the binary system.In the C4A3$-C2 S clinker system,the formation kinetics of C4A3$ is more controlled by three-dimensional diffusion mechanism.However,the formation kinetics of C2 S in the C4A3$-C2 S clinker system neither focuses on three-dimensional diffusion nor nucleation.Thecrystallite size of ?-C2 S increases with the increase of sintering temperature and sintering times,the crystallite size of C4A3$-orthorhombic increases with the increase of sintering temperature,and first increases and then decreases with the increase of sintering times.The hydration process of C4A3$-C2 S binary system was studied by using CH phase existence traces as a main line.RQPA method as the dominant method and a combination of various characterization methods.The results showed that:the main hydration products of the system were CH,AFt,AFm,C2ASH8,AH3 and C-S-H.In the C4A3$-C2 S hydration system.the changes CH phase contents as follows:after curing for 3 days,CH phase only exists in the hydration sample with 95 wt.% C2 S content and the CH content is 3.6wt.%.After curing for 28 days,the hydrated sample with the 85 wt.% C2 S content presented a stable CH phase and the content is 1.1 wt.%.The CH phase also appeared in the hydration sample with the content of 95 wt.% C2 S.and the content of CH is 4.1 wt.%.In the hydration process,there are two critical points,AH3 phase consumed and CH phase generated and stabilized.The existence traces of CH phase can be described by the change of C2 S content in the system and the ages of hydration reaction.When the content of C2 S is less than 55 wt.%,the newly generated AH3 phase in the system has priority to react with C2 S to generate C2ASH8.When the content of C2 S is between 55 wt.% and 85 wt.%,the CH generated by the hydration reaction of C2 S will be consumed by the hydration product C2ASH8 in the system,failing to meet the conditions for the continuous crystallization and stable existence of CH.When the content of C2 S is greater than 85 wt.%,CH phase can be stable in the C4A3$-C2 S binary hydration system.The pH value of C4A3$-C2 S binary hydration system solution increases gradually to a trend of stabilization with the extension of hydration ages and the increase of C2 S content.The concept of the crystal/amorphous ratio in the hydration system is proposed.Meanwhile,the C4A3$-C2 S hydration system with gypsum(C$H2)addition was studied.The results show that in C4A3$-C2S-C$H2 hydration system,the dominant factors that affect the strength include the crystal/amorphous ratio in the hydration system,the structural defects of hardened cement phases,and the clinker phase and overall hydration degree in the hydration system.When the hydration degree is similar and the system hydration sample is not affected by the structural defects,the effect of the crystal/amorphous ratio of the hydration system on the macroscopic compressive strength of the sample is the most significant.At this moment,when the numerical range of the crystal/amorphous ratio is 0.8~1.4,the compressive strength of the 90 d samples in the hydration system is all greater than 50 MPa.In the C4A3$-C2 S hydration system,the change of the crystal/amorphous ratio is an important dominant factor.The added gypsum will dilute the volume of the cementitious materials,and the remaininggypsum particles will cause the structural defects of the hardened cement phases,thus leading to the reduction of strength.However,the crystal/amorphous ratio of hydration system can be changed by adding gypsum.And then,the crystal/amorphous ratio partly leads to the change of porosity at the micro scale,which in turn leads to the change of compressive strength at the macro scale.In the C4A3$-C2 S hydration system,the contribution rate of each hydration product to the strength follows the following order:AH3 gel?C-S-H gel> AFt> C2ASH8>AFm.The hydration kinetics response and hydration mechanism of C4A3$-C-S binary system were studied by using ICC analysis and Kondo equation.The results show that the hydration acceleration stage of C4A3$-C2 S binary system is controlled by two hydration mechanism factors,and theinitial acceleration stage is controlled by nucleation reaction.When the content of C2 S in the system increased to 75 wt.%,the hydration process in the corresponding hydration acceleration stage was changed to be controlled by the diffusion process.When the content of C2 S is low,the hydration process entering the deceleration stage is still controlled by the nucleation reaction.However,the hydration process in the stable stage is controlled by diffusion process.When the content of C2 S is high,the hydration reaction process is controlled by the diffusion process whether the hydration process develops to a deceleration stage or a stable stage.The total amount of hydration heat release of C4A3$-C2 S clinker system within 72 h did not exceed 500J/g.And with the increase of C2 S content,the hydration heat release rate decreased,and the hydration induction period was prolonged.Kondo equation has a good response to the hydration kinetics of C4A3$-C2 S binary system in the acceleration,deceleration and stable stage.Only in the early stage of hydration acceleration,the rapidly generated hydrat ionproducts covered the surface of clinker particles,which temporarily blocked the continuous hydration acceleration of clinker particles.This reduces the response of the Kondo hydration kinetics equation.