This invention relates generally to a process for producing foamed or cellular concrete molded articles by subjecting a molded article obtained from a slurry of quick-stiffening cement compound to high temperature and high pressure curing in which a controlled reactivity quicklime component is utilized in forming the slurry. The production of low density aerated autoclaved concrete is well established. Aerated autoclaved concrete is manufactured by mixing a silica rich material such as fine ground sand or fly ash, cement, a sulfate source such as gypsum, quicklime, a rising agent such as aluminum powder, and water. In a first chemical reaction, the quicklime reacts with the water to form heat and calcium hydroxide. The calcium hydroxide, in turn, reacts with the water and aluminum powder to form hydrogen gas which expands the concrete mix to about twice its original volume, or more. Similar to bread rising, the mix expands into a porous mass. After expansion has occurred, the porous mass is cut to a desired size and shape and is placed in an autoclave to build strength, rigidity and durability with the cement component serving to harden the mass. The autoclave is an airtight chamber that is filled with pressurized steam. During the autoclaving process, which is the formation of C--S--H gel tobermorite, typically 10-12 hours, a second chemical reaction occurs that gives the highly porous material its strength, rigidity and durability. The rate of reaction, or reactivity, of the quicklime with water and the subsequent reaction of calcium hydroxide and water with aluminum powder in the first chemical reaction is critical to the development of the required characteristics of the final product. In particular, a controlled reactivity quicklime is necessary for the development of uniform cell structure within the porous mass. At the present time, the reactivity of quicklime used in producing aerated autoclaved concrete is controlled or varied by varying the calcination parameters of the manufacturing operation which produces quicklime itself. By altering the temperature of calcination, the duration of calcination, and the type of calciner used, quicklime can be manufactured with a reactivity in a range from highly reactive for light-burned quicklime, to slightly reactive for hard-burned quicklime. This method to control the reactivity of quicklime for use in aerated autoclaved concrete requires a significant amount of time to set up and is effective only when producing large quantity of quicklime with a particular reactivity. In addition, variations in the quality of the quicklime can have adverse effects on the quality of the aerated autoclaved concrete. The present invention discloses an improved method for the production of aerated autoclaved concrete in which the properties of the aerated autoclaved concrete are controlled or varied by controlling or varying the reactivity of the quicklime component of the aerated autoclaved concrete mix. The present invention also discloses a method of producing an improved quicklime, for use in aerated autoclaved concrete, with a desired reactivity. The reactivity of the quicklime can be altered by the addition of certain chemical modifiers either prior to or simultaneously with the mixing of the aerated autoclaved concrete components. Alteration of the reactivity of the quicklime produces corresponding changes in the properties of the aerated autoclaved concrete. A decrease in the reactivity of the quicklime generally produces desirable changes in the properties of the aerated autoclaved concrete, such as a more uniform cell structure, lower density, higher strength and higher durability. The method of the present invention allows for the production of aerated autoclaved concrete of selected properties, without modification to conventional calcination processes and independent of the variability and quality of the quicklime. In the method of manufacturing an aerated autoclaved concrete material, a quick-stiffening mixture is prepared by combining a silica rich material, quicklime, a sulfate source such as gypsum, a rising agent, cement and water. The mixture is deposited into a mold and is allowed to form a stiffened body. The stiffened body is removed from the mold and is placed in an autoclave station in which it is steam cured at elevated temperature and pressure. The quicklime which is used to form the quick-stiffening mixture is modified with a chemical modifier to provide a desired degree of chemical reactivity in the quick-stiffening mixture. Preferably, the chemical modifier is selected from the group consisting of glycerol, glycols, lignosulfonates, amines and polyacrylates, metal sulfates, gypsum, sulfuric acid, phosphoric acid, carboxylates, sucrose and mixtures thereof. Most preferably, the chemical modifier is selected from the group consisting of sulfuric acid, gypsum, alkali and alkaline earth metal lignosulfonates, glycerol, ethylene glycol, diethylene glycol, triethylene glycol, monoethylene amine, diethylene amine, triethanolamine, polyacrylates, water and mixtures thereof. Examples of suitable polyacrylates include the alkali metal salts of polyacrylic acid, for example sodium polyacrylate (SPAL) and potassium polyacrylate.
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