High yield spray application

A method for spray applying a settable composition, comprising:

• conveying a settable cementitious slurry comprising gypsum, Portland cement, or mixture thereof through a length of hose to a nozzle spray-orifice from which said conveyed cementitious slurry is spray-applied onto a substrate, said slurry further comprising at least one aggregate and a basic material;
• introducing an amount of acidic set accelerating agent into said hose-conveyed slurry at a distance “D” from said nozzle spray-orifice, said acidic set accelerating agent operative to react with said basic material contained within said slurry and to increase the yield thereof when spray-applied through said nozzle spray-orifice onto a substrate;
• monitoring the level of said acidic set accelerating agent in said conveyed slurry, at a distance less than “D” from said nozzle spray-orifice, by employing a sensor to detect changes in a physical property of said slurry corresponding to the amount of set accelerating agent being introduced into said slurry, said sensor comprising an assembly of electrodes in physical contact with said slurry, said electrode assembly comprising a first pair of electrodes operative to generate a voltage field within a portion of said slurry when a voltage source is connected to said first electrode pair, and a second pair of electrodes operative to detect voltage differences between said first pair of electrodes when a voltage meter is connected to said second electrode pair;
• and
• adjusting the amount of set accelerating agent introduced into said slurry if said voltage differences indicated by said voltage meter deviate from a predetermined value.