Preparation And Mechanism Of Alkali-free Liquid Accelerator In Al₂（SO₄）₃ System

As China infrastructure projects enter a high-speed advancement stage,shotcrete concrete has gradually moved forward due to its fast construction speed and convenient construction,and are widely used in tunnel construction,road emergency repairs,support and protection projects.As an indispensable admixture in sprayed concrete,the accelerator has received widespread attention.Accelerators have an important influence on the setting time,early strength and rebound.At present,the accelerator is still in the research and development process.Alkaline powder accelerators have disadvantages such as the health of the constructors,the later strength of concrete,the amount of rebound,and the uneven dispersion during use.Therefore,it is of great significance to develop an alkali-free liquid accelerator.Through a large amount of literature,developing a specific experimental plan.Firstly,through literature review,the main accelerator components of alkali-free liquid accelerators are determined.Analyze the effect of the addition amount on the setting time and the compressive strength of the cement through the acceleration experiment of the single component.Taking the initial setting time,final setting time,1d strength,28d strength as factors to carry out orthogonal test L9（3⁴）,and obtain the best formula ratio;Glycolic acid as a stabilizer and polyacrylamide was used（PAM）as a thickener for compounding.The alkali-free liquid accelerator（AFA）was prepared through a certain order of addition at 70℃.AFA can exist stably without stratification at room temperature,and is colorless and transparent.When the addition amount is 7%,the initial setting time is 2.67min,and the final setting time is 5.76min,the 1d compressive strength ratio is 129.2%,and the28 d compressive strength ratio is 100.33%,which was in line with the GB/T35159-2017standard.Secondly,the relationship between the amount of AFA added and the setting time of the cement paste and the compressive strength of the mortar was studied,and the optimal addition amount was determined.The adaptability of AFA to cement,superplasticizer and the effects of water-cement ratio,temperature,mixing time and stirring time on the effect of AFA were studied by setting time test and compressive strength test.The test results show that the optimal addition amount of AFA is 7%;It has good adaptability to cements;when it is mixed with superplasticizers,the water-cement ratio or the addition amount can be adjusted appropriately to achieve the accelerating effect;When the water-cement ratio is increased,the amount of AFA can be appropriately increased;The amount of AFA can be appropriately increased when the temperature is lower than 20℃;Too long stirring time and too long mixing time will weaken the coagulation effect,the best stirring time is 20s,when the mixing time exceeds 10min,the accelerating effect can no longer meet the requirements.The effects of AFA on the hydration products and microstructure of cement paste were studied by the means of SEM and TG-DTG,And the influence of different Al³⁺/SO₄^2-（A/S）on cement hydration was discussed preliminarily.The result shows that AFA can accelerate the hydration of C₃A,promote the formation of early ettringite crystals,and make cement condensate and harden quickly,it also can promote the hydration of C₃S and short induction period,generate a large number of C-S-H gels.The generated C-S-H gels filled the pores to make the structure of cement paste more compact,and improved of the cement paste ultimately.With the ratio of A/S increased the setting time was shorten and the later strength was increased.