Study On Hydration Characteristics And Kinetics Of Composite Binder

Mineral admixtures have been widely used in modern concrete. Ground granulated blast furnace slag and fly ash are the most commonly used mineral admixture. The use of such materials leads to the decrease of mass fraction of cement in concrete, which has good economic and environmental effects. The difference of hydration activity between cement and mineral admixture leads to complicated hydration process and reaction mechanism of composite binder. The hydration of cement and pozzolanic reaction of mineral admixture occur simultaneously and may also influence the reactivity of each other. It is difficult to determine the reaction degree of cement and mineral admixture independently in hardened paste. Moreover, the hydration of composite binder often leads to a rise in internal temperature in concrete str ucture due to an exothermic process of binder and the low thermal conductivity of concrete. The elevated temperature will affect the hydration mechanism, and then the microstructure and mechanical properties of hardened composite binder paste. There is little information regarding the hydration kinetics of composite binder and reaction degree s of cement and mineral admixture independently in the literatures. Lack of deep understanding the internal relation among the reaction degree, the microstructure and the development of mechanical properties of composite binder cured at different temperatures.Slag(high activity), fly ash(moderate activity) and quartz powder(inactivity) were used in this paper. The hydration heat evolution rate and cumulative hydratio n heat of composite binder containing different content of slag, fly ash or quartz powder were measured at 25℃, 45℃ and 60℃ by isothermal calorimeter. Based on the hydration kinetics model, three hydration processes of composite binder, namely nucleation a nd crystal growth(NG), interactions at phase boundaries(I) and diffusion(D) were characterized, the relationship between hydration rate and hydration degree of composite binder was discussed in individual periods. The kinetics exponent, n, rate constant, K, and apparent activation energy, Ea, of hydration were calculated and analyzed. Compared to the cumulative hydration heat of composite binder containing quartz powder, the contribution of pozzolanic reaction of slag and fly ash to the cumulative hydration heat of composite binder was investigated and the reaction degrees of slag and fly ash were qualitatively evaluated at early ages. The hydration degree of composite binder was studied by measuring the non-evaporable water content of hardened paste. The development of later-age hydration properties was investigated by measuring the compressive strength of mortar and the pore size distrib ution of paste. The microstructure of hardened paste was observed, and the reac tion degrees of cement, slag and fly ash were independently determined by SEM-BSE imaging. Furthermore, the overall reaction degree of composite binder was obtained. The relationship between the overall reaction degree and the non-evaporable water content of composite binder paste was established.An increase in the content of slag or fly ash decreases the hydration heat evolution rate and cumulative hydration heat of composite binder, but the reduced amount of hydration heat is not proportional to the replacing ratio of slag. Increasing temperature increases the peak value of the second exothermic effect, shifts the peak forward and significantly increases the cumulative hydration heat of composite binder. The promoting effect of elevated temperature on the hydration of composite binder containing slag is larger than that of composite binder containing fly ash. For composite binder containing no more than 50% of slag and composite binder containing no more than 65% of fly ash, the hydration process is NG→I→D at 25℃ and 45℃, it becomes NG→D at 60℃. While for composite binder containing 70% of slag, the hydration process is NG→I→D at 25℃, which becomes NG→D at 45℃ and 60℃. The hydration kinetics model used in this paper is no lo nger applicable for composite binder containing 90% of slag owe to the variatio n of its hydration mechanism. The controlling mechanism of the hydration reaction of composite binder is mainly chemical reaction(i.e., NG process and I process) during the early hydration stage at low temperature, but it becomes mainly diffusion controlling(D process) at elevated temperature. The hydration reaction of composite binder transforms from NG process to I process at a relatively high degree of hydration, but from I process to D process at a low degree of hydration. High content of slag or fly ash has great effect on the hydration process of composite binder that increases the simulation error. Ea increases with increasing the content of slag or fly ash. The apparent activation energy of composite binder containing slag is larger than that of composite binder containing fly ash at the same replacing level. Temperature has significant effect on the hydration reaction of composite binder containing large volume of slag or fly ash. Addition of quartz powder decreases the exothermic rate and cumulative hydration heat, but it has little influence on the ending time of induction period and the appearing time of second exothermic peak, the hydration heat curves quickly level o ff. The contribution of slag reaction to the cumulative hydration heat of composite binder is greater that that of fly ash reaction. Elevated temperature increases the reaction degree of slag that leads to much heat released. But fly ash almost does not participate in the reaction at 25℃ and the reaction degree is still low at 45℃ and 60℃ at early ages.The early-age non-evaporable water content of composite binder paste is lower than that of Portland cement paste at 20℃ and 45℃ and it decreases with increasing the content of slag, but the non-evaporable water content of paste containing 30% of slag is higher than that of Portland cement paste at later ages. And it exceeds Portland cement paste at 60℃. Due to the low mass fraction of cement, the non-evaporable water content of paste containing 70% is lower than that of Portland cement paste. The paste containing fly ash has lower non-evaporable water content than Portland cement paste in the range of tesing temperature. Increasing the replacing ratio of quart z powder decreases the non-evaporable water content of paste containing quartz powder, whose increasing rate is high at early age, but the curves quickly level off. The elevated temperature decreases the non-evaporable water content of paste at later ages. Compared the non-evaporable water content of paste containing slag or fly ash to that of paste containing quartz powder, it is found that the reaction degrees of slag and fly ash are low at early age, especially for paste containing fly ash, whose non-evaporable water content is almost same as paste containing quartz powder. Elevated temperature has greater promoting effect on the pozzolanic reaction of slag or fly ash in composite binder paste than hydration reaction of cement in paste containing quartz powder. Elevated temperature advances the pozzolanic reactions of slag and fly ash and leads to high reaction degree, which contributes more to the non-evaporable water content, and the contribution of slag is larger than that of fly ash due to the high act ivity of slag at elevated temperature.The early-age compressive strength of composite binder mortar decreases with increasing the replacing ratio of slag or fly ash at 20℃, but the later-age strength is higher than Portland cement mortar when the replacing ratio is small. The strength of mortar conbtaining 30% of slag is higher than Portland cement mortar at 45℃ and 60℃. For mortar containing 35% of fly ash, the early-age strength is high at elevated temperature and the later-strength exceeds Portland cement mortar at 45℃, but the later-strength is lower than Portland cement mortar at 60℃. For mortar containing large volume of mineral admixture, the strength is lower than Portland cement mortar. Increasing the content of quartz powder significantly decreases the compressive strength of mortar. Elevated temperature promotes the pozzolanic reaction of slag or fly ash, which is benifical to the development of strength of mortar, but it has little effect on the development of strength of mortar containing quartz powder. Elevated temperature decreases the compressive strength of mortar at later ages.The pore size related to the most probable pore diameter increases with increasing the replacing ratio of slag or fly ash at 20℃ in early period. Addition of fly ash could significantly increase the porosity of paste at early ages. The pore size related to the most probable pore diameter of composite binder paste beco mes small at elevated temperatures and it is smaller than Portland cement paste at 60℃. At later ages, the pore size related to the most probable pore diameter of composite binder paste is obviously smaller than Portland cement paste. Early elevated temperature curing results in some large pores in hardened paste at later ages, which is not benifical to the development of strength. The hydration products generated by pozzolanic reaction of slag or fly ash could evidently refine the pore structure, and the refining effect of slag is higher than fly ash.Plenty of cement, slag or fly ash particles are observed in the hardened paste at room temperature in early period. The microstructure is loose with few hydration products and many pores. The amount of unhydrated particles decrease in hardened paste at later ages and the microstructure is improved. The reaction of binder particles is observed at elevated temperature at early ages and the microstructure is relatively dense. The amount of slag or fly ash significantly reduces at later ages, but the non- uniform distribution of hydration products leads to large pores in the paste.The reaction degree of cement in binder containing slag increases with increasing slag content in the range of tesing temperature. The similar regularity is observed for the reaction degree of cement in binder containing fly ash at 20℃, but it is lower than Portland cement at 45℃ and 60℃ in later period. Temperautre has little effect on the final reaction degree of Portland cement, but increasing temperature decreases the reaction degree of cement in composite binder. The reaction degrees of slag and fly ash increase with decreasing the replacing ratio of mineral admixture or increasing temperature. Compared to SEM-BSE imaging, the reaction degree of slag or fly ash determined by selective dissolution is low. Increasing the replacing rat io of slag or fly ash decreases the reaction degree of composite binder, but the ultimate reaction degree of composite binder containing 30% of slag is higher than Portland cement at elevated temperature. Increasing temperature increases the reaction degree of composite binder, especially the composite binder containing high amount of slag or fly ash. The non-evaporable water content cannot accurately characterize the reaction degree of composite binder. A linear relationship between the non-evaporable water content and the reaction degree of composite binder is established. The reaction degree of composite binder could be approximately determined by the non-evaporable water content of composite binder.Early elevated temperature curing cannot lead to low ultimate reaction degree of composite binder, but it will affect the water content, micromorphology and distribution of hydration products, the microstructure of composite binder paste is much coase and porous, which leads to the low final strength.