Study On The Alkali-free Liquid Accelerator Of Aluminum Sulfate System Modified By Nano-SiO₂ And Fluorosilicon Slag And Its Action Mechanis

Alkali-free liquid accelerator is an indispensable key material in the construction of shotcrete,which plays an important role in the rapid condensation and hardening of shotcrete.At present,the main component of alkali-free liquid accelerator is aluminum sulfate.The effect of single use of aluminum sulfate is not ideal,and fluoride is often introduced to improve its rapid setting effect.However,fluoride compounds often have an adverse effect on cement strength.Therefore,in order to reduce the adverse effect of accelerator on cement strength,this paper uses nano-SiO₂,fluorine silicon slag and other substances to modify aluminum sulfate alkali-free liquid accelerator to obtain an alkali-free liquid accelerator that has no obvious adverse effect on cement strength.Firstly,the mother liquor of alkali-free liquid accelerator was prepared by using aluminum sulfate,fluorosilicic acid,aluminum hydroxide and other raw materials,and modified by nano-SiO₂ to prepare nano-SiO₂ modified alkali-free liquid accelerator.The experimental results show that the use of nano-SiO₂ modified accelerator can promote cement condensation and increase compressive strength.The alkali-free liquid accelerator modified by 3%nano-SiO2 can reduce the initial setting time and final setting time of cement to 3 min 15 s and 5 min 45 s respectively,and the compressive strength of 1 d is increased to 10.2 MPa,which is 162%of the accelerator without nano-SiO2 modification.The compressive strength of 28 d reaches 50.5 MPa,and the compressive retention rate is 99.4%.However,when the content of nano-SiO2 in the accelerator reaches 4.5%,the ettringite will change into a fine needle shape,which will lead to the deterioration of the accelerator effect.Secondly,in this paper,the alkali-free liquid accelerator of aluminum sulfate was modified by using the solid waste fluorine silicon slag produced in the production process of wet-process phosphoric acid,and an alkali-free liquid accelerator LAFS-1that can significantly enhance the early strength of cement was prepared,which provided a new scheme for the resource utilization of fluorine silicon slag.The experimental results show that the addition of 8%LAFS-1 can reduce the initial setting time and final setting time to 2 min 30 s and 11 min 50 s,respectively.The compressive strength of 1 d and 28 d can reach 200%and 102%of the cement strength without accelerator,which meets the requirements of national standard GB/T 35159-2017 for alkali-free liquid accelerator.The addition of LAFS-1 can quickly generates a large amount of AFt in the cement paste.The SiO₂ introduced by the fluorine silicon slag will consume calcium hydroxide and generate hydrated calcium silicate to promote cement hydration.Finally,due to the high energy consumption and pollution of the direct use of fluorine silicon slag to modify the alkali-free liquid accelerator,this paper first uses sodium hydroxide and aluminum hydroxide to alkali-treat the fluorine silicon slag to obtain an alkali-treated fluorine silicon slag with smaller particle size and more stable fluorine element.The experimental results show that the alkali treatment can reduce the particle size of the fluorine-silicon slag,the particles become small flake particles,and Al F₃,Na₂SiO₃ and other substances are generated.After drying at 105°C,the fluorine in the alkali-treated fluorine silicon slag does not escape and remains in the solid material.Then the alkali-treated fluorine silicon slag was used to modify the alkali-free liquid accelerator to prepare the alkali-free liquid accelerator LAFS-2.The experimental results show that the addition of LAFS-2 can significantly reduce the setting time of cement and improve the early compressive strength of cement,and has no obvious adverse effect on the later compressive strength.At 8%dosage,the compressive strength of 1 d cement can reach 12.4 MPa,which is 159%of the strength of cement samples without accelerator.The compressive strength of 28 d can reach 47.6MPa,and the compressive retention rate is 102%.Meet the requirements of national standards for alkali-free liquid accelerators.The incorporation of LAFS-2 can also quickly generate a large amount of AFt,and the silica in the alkali-treated fluorine silicon slag can promote cement hydration,thereby improving the compressive strength.