| Accelerator is used in shotcrete technology because it can make cement paste set and harden quickly.With the continuous development of China’s western development strategy,a large number of tunnel projects have emerged to cope with the complex terrain in the west.Shotcrete is widely used in slope support and surrounding rock reinforcement projects because of its simple and convenient construction and high early strength.Accelerator is the most important additive in shotcrete,so it has been widely studied.The high alkali content of powdery alkaline accelerator is harmful to human body,and there will be problems such as large dust and unstable water-cement ratio when it is used.Therefore,the powdery alkaline accelerator has gradually withdrawn from the market and has been replaced by alkali-free liquid accelerator.However,although the alkali-free accelerator is no longer produced with alkalicontaining raw materials,in order to improve the stability and effect of the accelerator,some formulas introduce fluorine which is harmful to human body and environment.In order to solve these problems,a series of experiments are carried out in this paper,and a fluorine-free and alkali-free liquid accelerator(named AF-F)is developed,and its performance is verified by experiments.Taking aluminum sulfate,ferric sulfate,aluminum nitrate and triethanolamine as the main components to accelerate the setting and early strength,the effects of each component on the properties of cement were explored by adding them separately.The results show that aluminum sulfate can significantly reduce the setting time and improve the early strength of the paste,but the higher the dosage,the lower the later strength.It is suggested that the reasonable dosage of aluminum sulfate should be 3%-4%.When the dosage of ferric sulfate is low,it can shorten the setting time and improve the compressive strength,but when the dosage is large,the effect of reducing the setting time and improving the compressive strength are not significant,so it is suggested that the reasonable dosage is 0%-1%.Aluminum nitrate and ferric sulfate have similar trends,and the reasonable dosage is 0%-1%;Triethanolamine can significantly shorten the setting time of cement paste at a low dosage,and there is little difference in setting time when the dosage is 0.3%-0.4%,so the optimal dosage is0.1%-0.3%.After determining the rough content range,the content is refined,and the orthogonal experiment is designed to explore the influence of the overall formula on the initial setting time,final setting time,1d compressive strength and 28 d compressive strength of cement under the joint action of various components and other components.The results show that aluminum sulfate has the greatest influence on the 1-day compressive strength and 28-day compressive strength,and triethanolamine has the most significant influence on the initial setting time and final setting time.The mixing ratio of the mother liquor of the accelerator was determined according to various properties.The PH value of the accelerator was adjusted by using different amounts of phosphoric acid,oxalic acid and ethylenediamine tetraacetic acid,and finally 0.5%phosphoric acid was selected.The final formula of the accelerator is 50g aluminum sulfate,1g ferric sulfate,1.5g aluminum nitrate,3.5g triethanolamine and 44g water.After the mother liquor is synthesized,0.5%phosphoric acid is added.The accelerator in this formula is light orange and slightly viscous liquid.When the content of accelerator is 8%,the initial setting time is 3.7min,the final setting time is 8.7min,and the compressive strength is 11.2MPa on the 1st day and 43.7MPa on the 28th day,which is 6.4%higher than that of the control group with 0 dosage,and meets the relevant requirements of GB/T 359159-2017.The influence of different dosage of accelerator on the setting time and strength of cement is explored,and the optimal dosage of accelerator is 8% of cement quality.In order to explore the adaptability of AF-F,the influence of different amounts of fly ash and silica fume,different amounts of water reducer and cement from different regions on the accelerator was studied.In order to explore the influence of environment and other factors on AF-F,the properties of AF-F were tested at different environmental temperatures and different water-cement ratios,and finally compared with the existing accelerator products on the market.The results show that fly ash plays a retarding role and reduces the early and late strength.Silica fume can accelerate coagulation and early strength;Water reducer will reduce the setting speed,but the decrease is not significant when the dosage is small,and the low temperature environment has a great influence on the performance of AF-F.The overall adaptability of AF-F is good.Compared with the existing accelerators on the market,the performances of AF-F are balanced.With the help of XRD and SEM,the reasons why AF-F promotes the rapid setting of cement paste,and improves the 1d strength and the 28 d compressive strength are analyzed.The results show that Al ～(3+)and SO ～(42-)in AF-F quickly react with Ca ～(2+)and gypsum in cement after AF-F is added to cement,and finally,travertine is generated.These travertine crystals are filled in the water-filled pores of cement paste in a non-directional way and staggered and overlapped with each other to form a network structure.At the same time,a large amount of water and Ca2+ are consumed when producing travertine.The decrease of water content in cement paste weakens the fluidity of cement paste and accelerates the setting.This may be the reason why cement paste can quickly set and produce strength.When hydration continues,Ca ～(2+)is mostly used to generate travertine,resulting in low CH concentration and unable to form CH crystal nucleus.At this time,the C-S-H gel generated in cement is wrapped on the formed travertine grid,filling the gaps between cement particles and improving the later strength.Figure [62] table [29] reference [79]... |
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