Development And Application Of Liquid Alkali-free Accelerator

Liquid Alkalifree Accelerator (LAA), which is a kind of additives used forshotcrete, can reduce the setting time of concrete. Compared to conventional alkalipowder accelerators, LAA has such advantages as low alkali content, easy to beuniformly mixed with the admixture and low dust concentration. LAA has greatsignificance in minimizing the corrosions to the labors in tunnel construction,improving the strength of the concrete and avoiding dramatic fluctuation of the qualityof the shotcrete. Therefore, it becomes the focus of the present studies to develop LAAso as to replace the conventional accelerators.In the paper, the history of accelerator is presented, the state-of-art of acceleratoris described, the disadvantages of the conventional accelerators are analyzed and thenecessity to develop LAA is emphasized. Then, numerous theoretical analysis, testsand studies are made. Conclusions drawn are as follows:(1) The key to prepare effective LAA is to achieve stable solutions with highaluminium ion content. The conditions for the stable existing of aluminium ion insolutions are studied and the concept to develop solutions with high aluminium ioncontent is proposed.(2) The preparation of LAA is studied and it is concluded that the optimizedmixing ratio of LAA is as follows: the theoretical content of AlF3ranges from7%to8%, the content of Al2(SO4)3is43％±2％, the content of other admixtures is1.5%.The accelerator prepared according to the above-mentioned mixing ratio has goodstability. For most ordinary cement, the initial setting time is less than3min, the finalsetting time is less than5min, the1d compressive strength can reach about10MPa andthe28d compressive strength ratio is more than100%.(3) LAA SNJ-2(with basic mixing ratio) is developed. The raw materials forLAA SNJ-2is easily available, the production process of LAA SNJ-2is simple, thecost of LAA SNJ-2is low and the dosage of LAA SNJ-2is small (ranging from3%to5%). The adaptability of LAA SJN-2to the cement is studied and cement that can be well adapted to LAA SNJ-2is proposed: for P.O42.5cement, the reference indexes areas follows: the28d compressive strength≥48MPa, the specific surface area rangingfrom320m2/kg to350m2/kg. It is stated that the adaptability between the cementgrinding agent and the accelerator.(4) The acceleration mechanism of the accelerating ingredients is studied.Conclusions drawn are as follows: LAA compounded by Al2(SO4)3solution andAl-containing complex achieves its acceleration effect by means of producing Aft andC3AH6; The Al3+complex or Al complex in the solution plays the acceleration role,while the F-plays the dissolving role in the accelerator solution and plays retardingrole in the hydration system of the cement.(5) Regarding special cement, i.e., cement for which the basic mixing ratio of theaccelerator cannot be adapted, the mixing ratio of the accelerator is adjusted, themethod for the adjusting of the mixing ratios of accelerators for special cements isestablished, and LAA SNJ-3and LAA SNJ-4with good performance are prepared.The application of the accelerators mentioned above in the works demonstrates thatthese accelerators have such advantages as high early strength, high hour-strengthincreasing rate, low rebound and low strength loss.