Liquid concrete accelerating mixtures and methods for use thereof

#1. Field of the Invention

#This invention relates to concrete accelerating additives and, in particular, to a liquid accelerating additive ideally suited for shotcreting applications.

#2. Brief Statement of the Prior Art

#The wet concrete accelerating additive of the invention comprises a mixture of alumina and sodium and potassium aluminates. The overall composition of the additive is from 15 to about 40, preferably from 20 to 30, weight percent alumina, from 10 to about 25, preferably from 12 to 20, weight percent of each of sodium and potassium, expressed as the oxide, and the balance being water, present in an amount from 10 to about 60, preferably about 40 to about 50, parts. The liquid additive can be prepared by admixing in the proper proportions alumina and sodium and potassium hydroxides followed by heating of the mixture to a sufficient temperature and for adequate time to prepare a stable liquid. Typically, the mixture is heated at a temperature from 215.degree. to about 275.degree. F. from a period of from 15 to about 120 minutes. Preferred conditions include a temperature from 220.degree. to about 240.degree. F. and a time of heating from 20 to about 45 minutes.

#If desired, the composition can be heated for an extended period of time, longer than the afore-indicated preferred time and/or at higher than the afore-indicated preferred temperature. The result of this intensified heating is to increase the viscosity of the liquid additive to a level where the liquid acquires a semi-solid consistency. In some instances, this may be preferred, e.g., when it is desired to incorporate in the composition an additional quantity of alumina which exceeds the solubility limits for alumina. The increased viscosity of the mixture resulting from the more intense heating produces a liquid product in which the alumina can be suspended in a supersaturated condition and retarded against sedimentation by the liquid's high viscosity. Accordingly, heating to the maximum of the aforeindicated temperatures and/or for the maximum of the aforeindicated times may be desirable in such instances.

#The liquid accelerating additive can be incorporated in the concrete at a concentration from 0.5 to about 10, preferably from 1 to about 5, weight percent of the weight of cement of the concrete. The liquid accelerating additive can be incorporated in the concrete by admixing the liquid additive with the wet concrete while pumping or spraying the wet concrete in a shotcreting application. Because the additive is in a liquid state and is readily water soluble and/or dispersible, the material can be readily blended with the concrete without any extensive mixing time. Alternatively, the liquid additive can be blended with the water employed in preparation of the wet concrete and can thereby be admixed with a premixed blend of the dry solids for the concrete comprising, chiefly, Portland cement and aggregate.