Mechanism Study Of Temperature Rise Inhibitor Affecting The Exothermic Process Of Cement Hydration

Solving the thermal cracking issue is of great significance to the modern concrete which has the characteristics of large cement content,structural complexity and extreme durability requirement.The existing solutions can be divided into three categories,namely adapt the construction materials,improve the construction process and adjust the structure design.Exclusive or compound use of the abovementioned solutions can,to some extent,control the change of temperature and stress field inside the concrete structure,and mitigate the risk of thermal cracking issue.Besides,chemical admixtures have been applied to mitigate the thermal cracking issue.The development of temperature rise inhibitor which can change the exothermic process of cement hydration and moderate the fast exothermic release during the main hydration peak sheds new lights on solving the thermal cracking issue.This study focuses on the effect of a starch-based solid type temperature rise inhibitor(abbreviated as S-TRI)on the exothermic process of cement hydration.The structure-function relationship and interaction mechanism of S-TRI are systemically studied by comparing the macro-test with micro-test,experimental observation with numerical simulation,kinetic research with thermodynamic calculation.The effect of S-TRI on the macro-properties of cement paste and temperature field inside concrete structure are also compared.Therefore,a theoretical basis for the application and ongoing development of temperature rise inhibitor is provided.This study further develops and investigates a sorbitol-based liquid type temperature rise inhibitor(abbreviated as L-TRI)which can achieve a similar effect on cement hydration like S-TRI but in a liquid form.Possible roadmaps controlling cement hydration through chemical admixtures are explored.The main results are as follows:The intrinsic properties of S-TRI and its influence on the exothermic process of cement hydration are studied.It is revealed that the consumption of S-TRI in the cement paste is comprised of dissolution and adsorption process.Limited dissolution of S-TRI particle in the cement paste is considered as the precondition to realize the effect of inhibiting temperature rise.Along the increase of the degree of starch acidification,S-TRI shows a better effect on depressing the main hydration peak.Dissolution property is the considered as the key factor affecting the depressing efficiency of S-TRI,the change of the degree of polymerization plays a minor role.The mechanism of S-TRI affecting the exothermic process of cement hydration during the first 24h is studied by multiple testing methods and an up-to-date hydration model–the Needle model.It is proposed that S-TRI partly inhibits the nucleation of C-S-H over its dissolution process to continuously depress the main hydration peak.The efficiency of S-TRI inhibiting nucleation of C-S-H can be affected by the adding mode or adding time.The higher the degree of coincidence between action time of S-TRI and nucleation process of C-S-H,the more the“effective inhibition”caused by S-TRI.With more than 0.1%(by weight of cement)S-TRI addition,a broader peak appears after the main hydration peak,namely the“second peak”.By comparing the effect of S-TRI on the exothermic process of various mono-phase systems and cement paste,it is concluded that C₃S contributes majority heat release during the second peak while C₃A plays an indispensable role on the formation of second peak.Blending together in-situ XRD analysis,SEM observation and GEMS calculation,the mechanism of S-TRI affecting the later hydration can be concluded as follows:the continuous formation of ettringite consumes S-TRI in the cement paste and invalidates its inhibition on C-S-H nucleation,once the depletion of S-TRI,the nucleation and growth of C-S-H restarts to form the second peak.The setting time of cement paste and development of mortar strength during the early age are inevitably delayed due to the inhibition caused by S-TRI on the cement hydration.With the appearance of second peak,the later mechanical strength is not significantly affected.It is revealed that,except the condition in which high alkali content invalidates the limited dissolution mechanism and the depressing effect,S-TRI has a strong robustness to inhibit temperature rise during the early age.In contrast,the appearance of the second peak is sensitive to the utilization condition.The effect of S-TRI on the exothermic process of cement hydration can effectively change the development of temperature rise in the concrete structure and lower the maximum core temperature,which helps to reduce the thermal cracking risk.To avoid the dispersion problem in the solid case and extend the application scope of temperature rise inhibitor,L-TRI is developed based on sorbitol and ethylene oxide.L-TRI owns weak adsorption ability while complexes with metal ions to disturb the aqueous environment.As a result,the growth rate of C-S-H needle during the acceleration period slows down dramatically while the duration of acceleration period prolongs significantly.Temperature rise caused by cement hydration is inhibited accordingly.Comparing the mechanism of S-TRI and L-TRI affecting cement hydration,it is supposed that,to achieve the depressing effect rather than a pure retarding effect,the window time and interaction strength of organic admixtures on cement hydration must fit the nucleation process of C-S-H.