Research On The Composition, Properties And Mechanism Of Chloride-free And Alkali-free Liquid Accelerator

Sprayed concrete is used for the construction of tunnel supporting, minestrengthening, road repairing and waterproofing. It plays an important role in nationalinfrastructure construction. Accelerator is an essential key material in preparingsprayed concrete. The development direction of accelerator is chloride-free andalkali-free liquid accelerator to meet the requirements of concrete durability and wetspaying. But the present problems existing in the production and application of liquidaccelerator are poor stability, high dosage and long-term strength loss of concrete.Liquid accelerator production accounted for just3%of total accelerator in China in2013. Therefore, a breakthrough of liquid accelerator in technology is neededurgently.A kind of liquid accelerator characterized of environmental friendly,chloride-free and alkali-free was prepared through orthogonal experiments. Relevantproperties of the accelerator were studied. Mechanism of action of the accelerator wasalso discussed. The specific research contents are as follow:(1) Preparation of ASA. The raw materials included polyaluminium sulfate,magnesium sulfate, alkanolamine, inorganic acid and stabilizer. The acceleratingeffect of each component to cement was studied respectively. Stability parameter,initial setting time and final setting time were used as assessment indexes inOrthogonal experiment. The accelerating effect was better when the proportion ofpolyaluminium sulfate, magnesium sulfate, hydramine, inorganic acid, stabilizer andwater was48:14:10:1.5:1.0:25.5. The prepared chlorine-free and alkali-free liquidaccelerator (named ASA in this paper) with solid content of51%and density of1.41g/cm3was0.037%of alkali content,0.014%of chlorine ion content and about3.0of pH value.(2) Properties of ASA. When5%of ASA accelerator was added, the initialsetting time of cement paste was4min10s and the final setting time was8min40s. The1d compressive strength of mortar was7.23MPa, increasing by68%compared withcontrol sample, the28d compressive strength ratio of mortar was108.3%, meeting therequirement of qualified product in JC477-2005. When the dosage of ASA acceleratorwas7%, the initial and final setting time of cement paste were decreased to2min40sand4min40s respectively. The1d compressive strength of mortar was increased to8.56MPa, increasing by99%compared with control sample, and the28d compressivestrength ratio of mortar is103%.(3) Stability of ASA. Stability parameter d was presented to evaluate the stabilityof liquid accelerator based on the Turbiscan Lab instrument. Based on the analysis ofstability mechanism, a raw material with higher Al2O3/SO3value, a stabilizer thatwas able to react with Al3+forming a soluble complex ion and a method of adjusting pH were used to solve the problems of poor stability of liquid accelerator effectively.The stable period of ASA was more than6months, while the compared samplesappeared to have the precipitation or crystallization phenomena in some extend after1~2months.(4) Performances of cement paste with ASA. The setting time of cement pastewith ASA was influenced by fineness and freshness of cement, the type and dosage ofgypsum and mineral admixture in cement. When the water cement ratio is lower than0.35or the dosage of ASA is more than7%, the compatible of ASA and water reducerwas good. Testing results of setting time were influenced respectively by the factors ofroom temperature, water cement ratio, mixing time of cement-water-ASA system andthe time when to add ASA.(5) Mechanical and durability properties of concrete with ASA. Compared withcontrol concrete, the1d compressive strength of concrete with alkaline acceleratorincreased by70.6%, while the28d compressive strength decreased by16.8%. Theshrinkage of concrete increased31.29%at the age of120d, both frost resistance andpermeability resistance were decreased. While the1d compressive strength and28dcompressive strength of concrete with ASA increased by43.5%and10.2%respectively, the shrinkage of concrete increased8.35%at the age of120d, the frostresistance was decreased but the permeability resistance was increased, comparedwith the same control concrete.(6)Mechanism of action of ASA. XRD, DSC-TG, ESEM and MIP were used toanalysis the hydration products and microstructure of hardened cement paste withASA, compared with control hardened cement paste. It showed that the fast settingand high early compressive strength of cement paste with ASA were due to Al3+andSO42-provided by ASA reacted with dissolved Ca2+in the liquid phase instantlyforming a large amount of ettringite, those ettringite netted to each other leading fastsetting of cement paste. The precipitated ettringite in the liquid phase didn’t cover thesurface of C3A wholly, resulting in the continued hydration of C3A. The consumptionof Ca2+in liquid phase promoted the hydration of C3S, the generated C-S-H gel filledin the pore, making the microstructure denser and increasing the compressive strengthof cement paste.