Abstract: The percentage of entrapped air in mortar, plaster, concrete or a like building material which is prepared on a continuous basis is increased, preferably by at least 100 percent, by providing a mixing chamber with one or more barriers which oppose the progress of a mixture of solid and liquid constituents toward the outlet so that each increment of the mixture dwells in the mixing chamber for a relatively long interval of time which exceeds 15 seconds and can even exceed 40 or 50 seconds. The mixing chamber defines a sump for a supply of the mixture, and the rate at which the material leaves the chamber equals or approximates the rate of admission of constituents into the chamber so that the quantity of material which forms the supply is constant. Such supply is agitated by blades, paddles, vanes, grates, lattices or like devices to ensure continuous penetration of air into the material which dwells in, as well as the material which advances through, the mixing chamber.