Preparation And Performance Research Of High Strength Liquid Alkali-free Flash Setting Admixture

Liquid alkali-free flash setting admixture is a kind of concrete admixture which can make the shotcrete condense and harden quickly in short time.With the improvement of requirements of the quality of shotcrete and construction environment friendly,liquid alkali-free flash setting admixture will replace the traditional alkaline flash setting admixture,and it is the direction of future development of flash setting admixture.Currently,the problems of most of the liquid alkali-free flash setting admixture are poor stability,high dosage and high cost,and it restricts the application of liquid alkali-free flash setting admixture.Therefore,the preparation of a superior performance of the liquid alkali-free flash setting admixture is very necessary.In this paper,with the setting time and compressive strength as the index,composition design of mother liquor of flash setting admixture was optimized through one-component coagulation test and orthogonal test.then mother liquor of flash setting admixture was remixed with stabilizer and tackifier,high strength liquid alkali-free flash setting admixture（hereinafter referred to as HFA）was synthesized by the specific process.The adaptability of HFA to cement,mineral admixtures,water-reducer and the effects of water-cement ratio,temperature,mixing time and stirring method on the effect of HFA were studied by coagulation time test and compressive strength test.With the shrinkage as the index,the influence of HFA on the volume stability of cement mortar was studied by free shrinkage and strength test.The effects of HFA on anti-freeze-thaw corrosion resistance and anti-dry corrosion resistance of cement mortar were studied by freeze-thaw corrosion test and wet-dry corrosion test.The effect of HFA on the hydration process in cement paste was studied by means of the concentration of Ca²⁺,the determination of chemical bound water and the determination of resistivity.The effects of HFA on the hydration products and microstructure of cement paste were studied by the means of XRD,SEM and TG-DSC,and the mechanism of HFA was discussed.In the process of preparation of HFA,a kind of polyaluminum sulfate enriched with Al2O3 was synthesized,and a new methods of stability and solid content evaluation of HFA were proposed.The solid content,density and lkali content of HFA are 42.7%,1.37g/cm-3and 0.13%.When 7% of HFA was added,Its performance indicators are to meet the first-class requirements of "Flash setting admixture for shotcrete" of national standards,and the 28 d compressive strength ratio can be increased by 111.6%.HFA had good adaptability to most cements and,mineral admixtures,water-reducer.compared with commercially available flash setting admixtures,the HFA has the advantages of low dosage and good stability,and it can improve the volume stability and durability of cement mortar.HFA can accelerate the hydration of C3 A,promote the formation of early ettringite crystals,and make cement condensate and harden quickly,it also can promote the hydration of C3 S 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.