Improvement of high-strength gypsum production technology

The purpose of the study, conducted on the basis of the Department of Building Materials of the Perm Polytechnic Institute in the conditions of an operating enterprise manufacturing low-welding gypsum binder, was to study all available quality factors of high-strength gypsum and find the most effective way to develop the technology of this production.

It should be noted that there are no plants for the production of high-strength gypsum binders of the G-20 brand and higher with high productivity, for example, 30 thousand tons per year or more, in our country and in Europe. Meanwhile, the research results obtained made it possible to issue a technical task for designing, and then carry out the construction of the corresponding workshop at the Perm Gypsum and Gypsum Products Plant, which served as a research platform.

To solve these problems, the researcher adopted a working hypothesis, which consists in paying attention to the following factors that determine the quality of high-strength gypsum::

• Use raw materials-gypsum stone, partially or completely enriched by removing clay and sand from it during the development of the deposit, although with the preservation in limited quantities of other accompanying impurities from limestones, magnesites, dolomites, anhydrite, etc.;
• Take hydrothermal treatment of raw materials in an autoclave as a basis in the production technology of high-strength gypsum and determine its optimal increase in steam pressure with a corresponding increase in temperature;
• Use the fractional composition of gypsum crushed stone with the maximum possible minimum of its specific surface area so that during hydrothermal treatment in an autoclave secondary dihydragen is formed in the smallest quantities, as a harmful component that reduces the strength of the finished product;
• Convert residual dihydrates and newly formed inclusions of secondary dihydrates after hydrothermal treatment, drying and grinding with repeated careful heating at 110-170 °C and atmospheric pressure, to calcium sulfate semihydrate, which in small quantities is practically harmless to high-strength gypsum;
• To obtain a compacted powdery mixture of coarse-grained alpha-semihydrate with a small amount (up to 0.5-1.0%) of fine-grained gypsum;
• Use efficient heating methods with minimal heat and electricity consumption for drying, grinding and increasing the uniformity of powdered high-strength gypsum-then, with strict compliance with these conditions, the possibility of mass production of high-strength gypsum alpha-modification of semi-hydrate of improved quality (not lower than G-20) from raw materials of grades I-III is guaranteed, and the technology of its production will be very effective in technical and economic terms with the maximum possible use of equipment that previously produced low-welding gypsum binders, while maintaining its high productivity.

Samples of gypsum stone from 9 deposits delivered from various regions, including the Samara Region,were selected as raw materials for the research. The gypsum stone of these deposits differs in different composition, different morphology of the crystalline intergrowth, as well as the amount and type of impurities. Alpha-semihydrate production in laboratory conditions was carried out in an autoclave with a capacity of 18 liters with electric heating, equipped with an automatic pressure regulator and a temperature indicator.

In order to optimize drying modes and reduce heat energy consumption, a fundamentally new pulse-vacuum drying method was developed specifically for the study in the same autoclave where gypsum stone was processed, without overloading dehydration products. The essence of the method is that drying was carried out at a temperature of 110-150°C by alternating exposure to infrared radiation and vacuum at a pressure of 0.04-0.065 MPa until the separated water is completely removed.

The structural and genetic characteristics of gypsum stone and gypsum binders were studied by microscopic analysis using a polarizing microscope, and thermal programming, dynamic weighing during heating, spectral analysis, and other precise methods were also used.

Considering that the quality and quantity of alpha-hemihydrate can be significantly affected by both the fractional composition of natural stone and hydrothermal treatment modes, studies were conducted in the laboratory to develop optimal technological parameters for the production of high-strength gypsum binder.

Analysis of the obtained data indicates a significant influence of the particle size distribution and autoclaving modes on the decomposition of CaSO4•2H2O and the speed of this process, as well as on the content of residual hydrate water in the product. At a low steam pressure (0.2-0.3 MPa), the process of cleavage of the crystallization ore takes a long character and does not end with the selected duration of stone processing. The drying process is also extended, since the initial temperature is not high. At a pressure of 0.7-0.9 MPa, the isothermal exposure is 6-4 hours, respectively, and drying is intensified. The quality of hydrothermal treatment products is also increasing.

The maximum yield of alpha-semihydrate was recorded with a minimum content of secondary dihydrate when processing gypsum stone with a piece size of 90-120 mm, when the steam pressure is 0.7 MPa, and the duration of isothermal exposure is 6 hours. The presence of calcium sulfate dihydrate in the product of hydrothermal dehydration of gypsum stone in an amount of up to 4-5% leads to a loss of up to 48% of the strength indicators of the gypsum binder.

Pulse-vacuum drying at a temperature of 125-135 °C without overloading the dehydration product from the autoclave increases the strength of the wilt by 8-19% compared to combined drying. Hardening occurs due to a smaller amount of secondary calcium sulfate dihydrate remaining (mainly on the surface of the pieces).

Secondary heating to 120-140 °C of the ground dehydrated and autoclave-dried product is an effective operation with the conversion of the remaining secondary dihydrate to alpha-semihydrate. The conversion of this dihydrate to calcium sulfate alpha-semihydrate at this stage of technology relatively improves the setting time, volume expansion, water demand and other indicators of the binder. The presence in the finished product of high-strength binder alpha-semihydrate in the amount of 4-5% does not significantly affect its physical and technical parameters, and with smaller amounts (0.5-1%) it has a positive effect on the quality and structure of hardened artificial gypsum stone, in particular, due to the compaction of powdered and dough-like gypsum.

Applying the technological techniques mentioned above, it is possible to avoid the need to use any chemical additives that strengthen and compact the microstructure, which, as a rule, are scarce, expensive and complicate the technological process. Thus, the practical application of the research results makes it possible to organize efficient production of high-strength gypsum and significantly increase its production.