Study On The Effect And Mechanism Of Temperature Rise Inhibitors On Cement Pastes Blended With Fly Ash Or Slag

Thermal cracking is a major problem faced by mass concrete.By mixing supplementary cementious material and starch-based temperature rise inhibitors（abbreviated as TRI）,the heat flow during the main hydration peak can be effectively reduced（summarized as a“depressing effect”）;the cumulative heat release can be slowed down;and the cracking risks of concrete caused by the hydration temperature rise can be decreased.In order to use TRI in engineering practice,this study investigates the influence of TRI on the properties of pure cement and cement-fly ash/slag blended systems including the hydration heat release,compressive strength and setting time.The strength development of different systems with TRI is explained by the influence of TRI on the reaction degree and pore structure of hardened pastes.Finally,the mechanism of its effect on cemen-fly ash/slag systems is revealed by the dissolution and adsorption behavior of TRI and the influence of fly ash/slag on the depressing effect of TRI.The main results obtained are:The influence of TRI on the macroscopic properties of pure cement and cement-fly ash/slag blended systems is studied.The depressing effect of 0.2%TRI on pure cement system is better than that of cement-fly ash/slag blended systems,but a better depressing effect of 0.3%TRI is found on blended systems.The setting time is extended and the early-age strength is reduced due to that TRI inhibits the early hydration of cement.The strength loss of pure cement system is less than that of the cement-fly ash system,but greater than cement-slag systems.The strength loss of pure cement and cement-slag systems gradually decrease with the hydration age,and similar strength level as the blank sample is reached at 60d.While the strength loss of cement-50%fly ash does not decrease with hydration age,and there is still a 42.6%strength loss when hydrated for 120 days with 0.3%TRI.The strength development is explained by analyzing the influence of 0.3%TRI on the reaction degree and pore structure of pure cement（PC）,cement-50%fly ash（50FA）and cement-50%slag（50SL）blended systems.With 0.3%TRI,the decrease of reaction degree of PC is less than 50FA and greater than 50SL at early ages.The loss of reaction degree of PC and50SL gradually lowers with the continuous hydration,and is similar as the blank sample at 60d.However,the reaction degree of 50FA still reduces 11.6%,which is in good agreement with strength results.Selective dissolution experiments show that limited amount of fly ash reacts at7d,and the reaction degree of fly ash with 0.3%TRI decreases from 6.8%to 2.7%at 60d compared to the blank sample,which is a large decrease;the reaction degree of slag only decreases about 2%at 7d and 60d.The Ca（OH）₂content shows that cement hydration is inhibited more seriously in blended system with the same TRI dosage,and the loss in the three systems is 50FA>50SL>PC.No negative influence of 0.3%TRI on the pore structure of PC and 50SL hydrated for 60d is observed.However,the pore structure of 50FA is coarsened,which affects the compressive strength seriously.The mechanism of TRI affecting the above-mentioned properties and reaction degree of different systems is revealed.Pore solution experiments show that the p H is reduced by adding fly ash/slag to reduce the dissolution of TRI solid particles;and the filler effect of fly ash/slag promotes the formation of early hydration products,which will weaken the depressing effect of TRI.Adsorption experiments show that the adsorption of TRI on cement is greater than that on fly ash and slag,which increases the TRI content on per unit cement particle in blended systems to strengthen the depressing effect.There is a competition between the weaken and strengthen effect:with lower TRI content,the first effect above mentioned dominates and with higher TRI content,the second effect dominates.The competitive adsorption of TRI on cement particles leads to a greater loss of cement reaction degree in blended systems than pure cement paste.The inhibition effect of TRI on cement hydration in 50FA is enhanced,so that the hydration product Ca（OH）₂ stimulating the pozzolanic reaction of fly ash is reduced,which has an indirect impact on the reaction of fly ash.The reaction of slag is not affected by the reduction of Ca（OH）₂ due to its high activity,which can make up for the decrease in compressive strength caused by the competitive adsorption of TRI on cement.