Thermodynamic Study on Gypsum Solidification Process

Thermodynamic Study on Gypsum Solidification Process - Shijiazhuang Jingjin Mechanical Equipment Installation Co., Ltd
I. Preface
Gypsum block is a new type of building material. Its production energy consumption is low, pollution is small, and it has the outstanding advantages of light weight, fire protection, heat insulation, sound absorption, small shrinkage, nailing, sawing, bonding, etc. It is an ideal material to replace red bricks, with broad development prospects. In China, gypsum blocks have been produced since 1981. Although it has been more than 20 years, gypsum production is still at a low technical level. Most manufacturers still use various single mode structures, manual and semi mechanized equipment. Although these equipment have low investment, quick start, and can quickly obtain economic benefits, it is difficult to gain advantages in the fierce market competition in the long run because of the simple equipment, high labor intensity, poor labor conditions, unstable product quality and performance, low production efficiency, and difficulty in forming economies of scale. We know from the actual production of the factory that in the automatic control design of the gypsum block production line, the data of temperature changes with time during the gypsum solidification process is required, especially the initial setting and final setting temperature, so that the time of gypsum staying in the molding mold can be reasonably determined, so that the production can achieve the purpose of energy saving and time saving.
The main raw materials for the production of gypsum blocks are gypsum powder, i.e. hemihydrate gypsum powder, additives, water, etc., and some other reinforcement materials can also be added. After the raw materials are measured and weighed, they enter the mixer. The mixed slurry enters the mold of the gypsum block continuous molding machine. While the molding machine rotates continuously, the slurry solidifies rapidly. When the predetermined final setting point is reached, it is taken out of the molding machine. As the temperature of gypsum block changes with time when it solidifies, if a temperature measuring point is set at an appropriate position on the mold of the molding machine, the solidification state of gypsum block can be judged according to the temperature change, so that it can take out the gypsum block from the molding machine at an appropriate time, thus realizing automatic production.
In view of the above problems, this paper discusses how to determine the initial setting point and final setting point of gypsum solidification process by measuring the relationship between temperature and time, and obtains the thermal effect data of gypsum solidification. These results have certain guiding significance for realizing continuous control of gypsum block forming process.
II. Theory
Semi hydrated gypsum (CaSO0.5HO) is soluble in water. It first dissolves into semi hydrated gypsum aqueous solution, and then hydrates to produce dihydrate gypsum (CaSO2HO). Due to the hydration reaction, the amount of solvent is relatively reduced. At the same time, the solubility of dihydrate gypsum is smaller than that of hemihydrate gypsum, and the dihydrate gypsum solution is supersaturated. When it reaches a certain supersaturation degree, dihydrate gypsum nucleates uniformly from the gypsum slurry, and then gradually grows to the critical size with stable crystal nucleus, and the gypsum slurry begins to thicken. In production, the time when gypsum slurry begins to thicken is the limit time for injection into the master mold, also known as the initial setting time, and the temperature measured at this time is its initial setting temperature. Subsequently, the crystal nucleus continues to grow and release a lot of heat (mainly from the heat of crystallization phase change); When the crystal grows into its inherent shape, the gypsum slurry will also become a porous solid with a certain hardness and strength. In actual production, it is possible to demould at the time, which is often called the final setting time. The temperature measured at this time is the final setting temperature. The initial setting time and final setting time can be determined through temperature measurement, so as to control the production process so as to realize the automation of production.
The chemical transformation process of hemihydrate gypsum dissolved in water is as follows:
CaSO0.5HO (s)  CaSO0.5HO (aq)                         （1）
CaSO0.5HO (aq) + 1.5HO(l)  CaSO2HO (aq)                   （2）
CaSO2HO (aq)  CaSO2HO (s)                          （3）
The total reaction formula is:
CaSO0.5HO (s) + 1.5HO(l)  CaSO2HO (s)                      （4）
According to the data provided in the literature, at 298.15 K and 101.325 kPa, 1 mol hemihydrate gypsum (solid powder) reacts with 1.5 mol water (liquid) to produce 1 mol dihydrate gypsum (solid powder), and the heat released is:
△H= (-2022.63 - (-1576.74) –1.5  (-285.83)) kJmol= -17.15 kJmol
The calculation formula of thermal effect (enthalpy change) in gypsum solidification process is as follows:
Under the experimental conditions, the gypsum solidification process in the system is an adiabatic heating process. At the initial state, all substances in the system are at the same lower temperature, and hemihydrate gypsum coagulates and emits heat. All substances in the system absorb heat to rise together and finally reach the final setting temperature.