Setting properties gypsum

The production of commercial building materials, e.g., portland cement and stucco, utilizes calcium sulfate in the form of the dihydrate or hemihydrate.f Gypsum is added in the production of portland cement to control set. Properties of cement can be adversely affected by the formation of hemihydrate during the grinding process. In the production of stucco, the hemihydrate is the preferred form. Consequently, investigators have adapted thermal analysis methods for estimating the quantities of each phase in these products. 

In another process water is introduced into the hot calcined gypsum mass in a kettie to reduce the temperature of a portion of the mass to below the boiling point of water. The mass is then reheated (2). Stabilized setting and water demand properties are claimed as are water demand levels below those attainable through aridizing. 

In addition to the four compounds discussed above, the final Portland cement may contain gypsum, alkali sulfates, magnesia, free lime and other components. These do not significantly affect the properties of the set cement, but they can influence rates of hydration, resistance to chemical attack and slurry properties. 

Hydrates quickly, responsible for strength of cement in early stage setting time can be controlled by addition of gypsum Responsible for strength in all stages Responsible for final strength Little effect on physical properties... 

The uncontrolled variation in the gypsum-hemihydrate or soluble anhydrite ratio (G-H or A) that occurs during production, storage and transportation of cement can cause compatibility problems with certain superplasticizers, particularly a reduction in workability. The extent of the reduction on workability properties is dependent on the G-H ratio (ranging from 80 20% to 20 80%), level of C A and alkalis present in the cement and the fineness of the cement. For the most reactive type of cement with a high content of both C A and alkalis, a reduced G-H or A ratio affects the yield stress, while piastre viscosity is not much affected. These effects are much less pronounced for a less reactive cement, but with the lowest G-H or A ratio, false set can occur [130-132], An increased fineness of cement increases the effect of the G-H or A ratio. The effect has been found to be most pronounced with melamine-based superplasticizers [130, 133],... 

The Portland clinker used should contain a high amount of tricaldum silicate, preferably more than 45%. This is necessary as the hydration of this phase produces the calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the clinker minerals is mainly responsible for the setting and initial strength development of the cement, as the reaction rate of the fly ash is rather slow. The lydration of the ash contributes to strength only at longer hydration times, but also affects other properties of the hardened material. The calcium sulfate added in the form of gypsum or anhydrite serves to control the setting of the fresh paste in a similar way as in plain Portland cement. 

Dahnay P, Smith A, Chotard T, Sahay-Tumer P, Gloaguen V, Krausz P (2010) Propoties of cellulosic fiber reinforced plaster influence of hemp or flax fibers on the properties of set gypsum. J Mater Sci 45(3) 793-803... 

Portland cement is made from limestone or other natural sources of calcium carbonate, clay (a source of silica), alumina, ferric oxide, and minor impurities. After grinding, the mixture is fired in a kiln at approximately 2500°F (1371 C). The final product is ground to a fineness of about 10 xm and mixed with gypsum to control setting. In order to produce a concrete mix of the desired consistency and mechanical properties, it is combined with sand, stone, and water. 

Iwo-component adhesives that are used for household applications differ quite strongly as to their properties. For instance, there are epoxy resin adhesives that are very well suited for bonding glass, china, ceramics and metals. They set ss-clear, are dishwasher-safe, and therefore exceptionally well suited for repairing dishes or similar household articles. Usually, the two components resin (epoxy) and hardener (amine or mercaptan) are filled in aluminum tubes or plastic double syringes and are mixed before application according to the directions for use. Other two-component adhesives are based on meffiacrylate and contain a powder hardener (dibenzqyl peroxide in gypsum). These adhesives bond materials such as stone, plastics, ceramics, and metal ... 

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