Alkali Free Accelerators
Highly reactive amorphous aluminium hydroxide gel in powder form (50% Al) is used for production of alkali free accelerators used for sprayed concrete, by adding it to the saturated Aluminium Sulfate solution as a secondary step in the manufacturing process. The Amorphous aluminium hydroxide yields more available / soluble Al2O3, which in turn increases the speed of set when in contact with Portland Cement. Addition is typically nine parts alum: one part amorphous aluminium hydroxide.
Such solutions are not stable over long periods of time. They must generally must be stabilised with polycarboxylic acids or inorganic acids, or a mixture of both.