Study On The Adaptability Of Alkali-free Accelerator To Cement

Shotcrete technology in complex tunneling environment is a difficult issue in the field of infrastructure construction.As the core admixture of shotcrete,accelerator can shorten the setting time,increase the strength and reduce the rebound of shotcrete.Various kinds of cement used in the project in the process of preparation of clinker,gypsum and admixtures of different types and proportions,resulting in the same accelerator for different mineral composition of the cement effect there are differences.If the poor adaptability makes the effect of alkali-free accelerator is weakened,and make the late concrete performance worse.Therefore,accelerator and the adaptability of the cement has been the focus of research,user concerns.And due to the different geographical environment in the project caused by the construction process there are high and low temperature environment,the influence of temperature on the hydration of cement should not be underestimated.In this study,microscopic analysis such as heat of hydration,XRD,TG and MIP are used to verify the macroscopic test results and establish the constitutive relationship.The adaptability of alkali-free accelerator with different cements,the effect of different kinds of admixtures mixed into cement on the adaptability of alkali-free accelerator with cement and the effect of temperature on the adaptability of different cements and cements containing different mineral admixtures with alkali-free accelerator were investigated.The results show that:The strength performance of Jinyu cement is the best and Jinchuan cement is the worst after the addition of alkali-free accelerator.It can be seen from the cement composition that the elemental sulfur content of Jinchuan cement is low,and the addition of accelerator accelerates the consumption of gypsum in Jinchuan cement,and the under-sulfate reaction will hinder the hydration of cement after the gypsum is depleted,making its hydration slower and less hydrated.The content of aluminum and sulfur elements in both Jinyu cement and Esheng cement is more than that of Jinchuan cement,and the hydration rate is faster and the degree of hydration is higher generating more hydration products AFt,CH and C-S-H gel yield,which are lapped and interspersed to make the microstructure of Jinyu cement and Esheng cement more dense and thus stronger.Comparing the effect of alkali-free fluorinated accelerator and alkali-free fluorinated accelerator,the mixing of alkali-free fluorinated accelerator has a negative effect on the 1d and 28d strength of the cement,although it shortens the setting time of the cement.The1d strength of cement was improved by alkali-free fluorine-containing rapid setting agent and most of the cement had higher 28d strength retention.The effect of different types and amounts of admixtures incorporated in the preparation of cement on the adaptability of the accelerator to the cement was further investigated.Compared with the blank group,all three admixtures shortened the setting time of the cement,and the cement with fly ash showed poorer strength performance.The addition of 5%-15%silica fume and 10%mineral powder led to the improvement of both early and late strength,and this effect was more significant with the addition of silica fume dosing.The morphological effect,activation effect,and microaggregate effect of the three admixtures led to the improvement of cementite microstructure,thus improving the durability of the cement.The effects of three temperatures,20℃,40℃and 60℃,on the adaptability of two accelerators with different cements and alkali-free and fluorine-free accelerators with cements containing different admixtures were investigated.The increase of temperature promoted the hydration of each cement,refined the pore structure of each cement and increased the degree of hydration.The strength of each cement mixed with two accelerators increased with the increase of temperature,and the enhancement effect on the age of 6h and 1d was more obvious,and the effect of temperature on its strength enhancement weakened with the age growth.The slower the hydration rate of the cement itself,the more obvious the effect of high temperature.However,the higher the temperature,the earlier the conversion of AFt generated in the cement to AFm at 7d age,and the hindering effect of AFm generation on the hydration of C₃S is more obvious than that of AFt,which slows down the hydration of the cement at the later stage.