Effect Of Temperature Rising Inhibitor On Hydration Behaviour Of Cementitious Materials

Despite the annual consumption of more than 10 million m3 mass concrete in China,thermal cracking remains a problem in projects such as dams,long-span bridges,harbor constructions and skyscrapers,thus,controlling temperature cracks is of great significance for ensuring service safety and improving durability of mass concrete.Apart from the cooling pipes,large amount of mineral admixture addition and other temperature reduction methods,a starch-based chemical admixture(Abbreviated as TRI)and expansive agent are utilized to reduce temperature peak caused by cement hydration heat and compensate for shrinkage stress,the key science issues involved are the effect of TRI on hydration behaviour of cement and expansive agent.Based on the background,technical methods and scientific issue,this study focuses on the following four aspects: the first is the effect of TRI on the basic properties of cement pastes,the effect of TRI on induction period and the mechanism of action were ascertained.The second is the effect of TRI on the temperature of cement pastes,the effect of TRI on acceleration period and the mechanism of action were proved.The third is about the effect of TRI on the volumetric deformation of cement pastes,the effect of TRI on autogenous shrinkage of cement pastes was studied.The fourth is the effect of TRI on volumetric deformation of cement pastes containing expansive agent.On the basis of the four parts of experimental research,the following innovative research results have been obtained:(1)The study reveals that the same total amount of TRI in cement paste extends the induction period,depending on the amount of dissolved TRI and the predissolved method:dissolved in Na OH > dissolved in water.It has been found that TRI has no influence on the dissolution process of cement particles and the solid-liquid interface energy in C-S-H nucleation process,but the size of C-S-H particles can be controlled at 50 nm.The mechanism of the effect of dissolved TRI on induction period during cement hydration can be concluded that TRI inhibits the growth of C-S-H.The C-S-H nucleation growth process continues after the continuously generated C-S-H consumes the dissolved TRI.This conclusion was verified by the phenomenon the prolonged induction period almost disappeared when dissolved TRI was adsorbed by synthesized C-S-H.(2)With the increase of TRI content,the hydration rate of cement hydration in acceleration period is decreasing.The solid TRI sample significantly reduced the growth rate of C-S-H,and the dissolved TRI completely inhibited the growth of C-S-H.The mechanism of the effect of dissolved TRI on acceleration period during cement hydration can be concluded that solid TRI first dissolved in the pore solution during cement hydration,then partially adsorbed on the formed C-S-H surface to inhibit C-S-H growth,thereby reducing the hydration rate of cement.Additionally,this conclusion was confirmed by the combined effect of TRI and synthetic C-S-H or limestone on cement hydration.(3)Cement pastes with TRI exhibited significantly lower shrinkage strain when compared to that of the reference sample with increases in TRI.By testing the factors related to shrinkage stress and expansion stress and using effective phase saturation coefficient and crystallization pressure,the shrinkage reduction mechanism is proposed that TRI reduces growth rate of CH and AFt phase which results in larger crystallization,the pressure causes expansion to counteract the autogenous shrinkage strain.(4)The OPC-EA mixture with TRI showed much higher expansion strain as compared to the reference sample and a decreased shrinkage of OPC-EA cement pastes.It can be concluded that 0.2% TRI has less influence on the stiffness of OPC-EA matrix and delayed the hydration of the expansive agent to produce large expansion strain,while 0.4% TRI has a great influence on the stiffness of the OPC-EA matrix and also delayed the hydration of the expansive agent.The mechanism of action of TRI on the expansion agent is that the addition of TRI inhibits the growth process of ettringite and delays the hydration of CSA.The mechanism is verified by the volumetric deformation of cement-based materials containing pure CSA and TRI.(5)The effect of TRI on the nucleation-growth process of AFt was studied by classical nucleation-growth theory.The similar crystal-solution interfacial energies indicate that TRI has little influence on the nucleation of ettringite.The initial slopes of conductivity curves decreased from 0.07323 μS/cm/min to 0.0053 μS/cm/min,indicating that TRI significantly reduces the growth rate of AFt.Furthermore,the lengths of the ettringite crystal in the solution with 0.05 g or more dissolved TRI were kept constant at 500 nm.The results of XRD and FT-IR experiments indicate that TRI will adsorb on the surface of the synthesized AFt crystals to reduce its growth rate.In summary,this study reveals the influence and mechanism of TRI on hydration behaviour of cement and expansive agent to achieve effective control of temperature process and stress development in mass concrete.Moreover,it provides a theoretical basis for reducing the thermal cracking risk of mass concrete and promoting the application of TRI in mass concrete construction.