Who can resist placing their hands in wet cement?
Gypsum is a very soft mineral and the main naturally occurring form of calcium sulphate. Gypsum has many uses in the building industry, including drywall and concrete. When ground with clinker (lumps or nodules produced by heating limestone and alumino-silicate in a cement kiln), the resulting mixture is the basis for Portland cement, the most commonly used cement in construction work.
Stages of Gypsum
There are three basic stages. In its natural state, gypsum is called dihyrate, and has moderate solubility. When the gypsum is heated at temperatures above 105 degrees Celsius (221 degrees Farenheit), it becomes hemihydrate and is highly soluable. If the gypsum or hemihydrate is heated to a temperature above 170 degrees Celsius (338 degrees Farenheit), it become anhydrite, also highly soluable.
Why Use Gypsum?
Gypsum is used in concrete to prevent flash setting, or hardening of the concrete prematurely. The addition of the gypsum to the concrete mix does not just change its physical properties, but adds energy as well. This serves to raise the temperature of the mix to levels that prohibit bound moisture.
How Does Temperature Affect the Gypsum?
As discussed in Section 1, temperature changes the chemical makeup of gypsum. This affects the set time of the concrete differently. The more gypsum in the mix, the longer the set time for the concrete. However, if too much hemihydrate is used, then the concrete may false set (a set that doesn't permeate the mixture), resulting in fragile concrete.
Controlling the Temperature
The temperature of the mix needs to be constant to ensure proper setting, thereby providing the best product. As such, the ambient temperature must be maintained above 5 degrees Celsius (41 degrees Farenheit) until the concrete is fully dry. On the other hand, the temperature needs to be kept below 49 degrees Celsius (120 degrees Farenheit), to ensure proper dehydration of the gypsum and formation of strong concrete.
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