Preparation And Properties Of A New Alkali-free And Fluorine-free Liquid Accelerating Agent

Shotcrete is widely used in the support and reinforcement of tunnel engineering,among which the accelerating agent has the characteristics of rapid setting and hardening.The accelerating agent has become an indispensable admixture in the application of shotcrete.Liquid accelerator has the advantages of simple operation and fast construction speed,which has gradually replaced the powder accelerator in the past.However,most of the liquid accelerators on the market contain alkali or fluorine,among which the excessive alkali content is easy to lead to alkali aggregate reaction of concrete and the late strength of concrete is insufficient.On the other hand,fluorine compounds are highly toxic,endanger the health of staff and pose a very serious threat to the environment.In view of the above deficiencies,the purpose of this project is to develop a new non-alkali and non-fluoride liquid accelerator.The concentrate of alkali-free and fluorine-free accelerating agent was prepared by single component test and orthogonal test using polyaluminum sulfate as the main coagulant.By adding enhanced component and stable component to further optimize the mixture ratio,the optimal mixture ratio of the new alkali-free and fluorine-free liquid accelerator was determined as:polymer aluminum sulfate:diethanolamine:ethylene glycol:seven magnesium sulfate hydrate:nano silicon dioxide:QM stabilizer:dilute sulfuric acid:water=57:6:1:2:2:3:1:28,the detected solid content is 55%.When the content of the new alkali-free and fluorine-free liquid accelerator is 7%,the initial setting time and final setting time of the cement are 2min30s and7min25s,the compressive strength of the mortar at 6h,1d and 28d are 1.5MPa,12.4MPa and52.2MPa,and the compressive strength retention rate at 90d is 117.03%.The adaptability of the new non-alkali and non-fluoride liquid accelerator with cement,mineral admixtures and admixtures was studied.The new alkali-free and fluorine-free liquid accelerator has good adaptability with cement and mineral admixtures.Different cement manufacturers,different degrees of freshness,temperature,mixing and mixing time have significant influence on the setting time of liquid accelerator.Appropriate mineral admixtures can improve the compressive strength in later period.Compared with naphthalene high efficiency water reducing agent,polycarboxylic acid high efficiency water reducing agent has relatively little effect on the coagulation-promoting effect.The influence of a new alkali-free and fluorine-free liquid accelerator on the mechanical properties and durability of concrete was studied.Compared with the blank group,the compressive strength of concrete at 6h,1d and 3d can be significantly improved by adding the new alkali-free and fluorine-free liquid accelerating agent,and the strength retention rate at 56d is 108%.Compared with other liquid accelerators,the new alkali-free and fluorine-free liquid accelerators are more beneficial to the later strength maintenance of concrete.At the same time,it can improve the carbonation resistance,permeability resistance and frost resistance of concrete.The drying shrinkage of concrete was reduced compared with that of other alkali free liquid accelerator.By hydration heat,chemically combined water,XRD and other microscopic test,study the new non-alkali and non-fluoride liquid accelerator of cement hydration process and the influence of the microstructure of cement hydration products.Adding a new non-alkali and non-fluoride liquid accelerator can advance the time of the first and second exothermic peaks of cement hydration,by raising the concentration of Al³⁺and SO₄^2-in the liquid phase,and the calcium ions in liquid phase reaction and AFt crystal generated in great quantities.AFt crystal cross overlap each other,and consume a large amount of CH,further promote C₃A and hydrolysis of C₃S.The induction period of cement hydration can be eliminated by the addition of a new non-alkali and non-fluoride liquid accelerator.At the same time,accelerate the cement hydration process,so that the water purification mud in a short period of solidification.In the late hydration stage,the chemical binding water content of cement hardened slurry is increased to refine the structural pores.