Publish Time: 2024-12-24 Origin: Site
The process control of the steam curing of autoclaved aerated concrete slabs directly affects the product quality. Based on the universal problems of AAC slabs in actual production, this paper provides a preliminary analysis of the causes of defects and discusses prevention measures.
AAC slabs are construction components with structural properties and made with reinforcement to meet specific requirements, Due to the various advantages of the product, manufacturers will give priority to production expansion with AAC slab equipment or reserve the space required for installation of AAC slab equipment.AAC slabs have numerous advantages including less market competition, wide range of applications, high added value to production, high construction efficiency, significantly low cost, energy saving properties, energy saving properties, environmental protection and recycling, etc.
In comparison to AAC blocks, the production of AAC slabs has stricter requirements on raw materials.process ratios and process control, Autoclaved curing is necessary for aerated concrete to obtain strength and other properties, and it is a specific method for achieving hydrothermal synthesis of products. it is not only related to the performance of the products, but also to the improvement of production efficiency and energy consumption of factories, This paper focuses on the optimal autoclave curing system for AAC slabs and presents the main equipment and operational procedures of autoclave curing. Based on field production experience, the author summarizes problems occurring in production to then provide improvements towards increased quality and production stability of AAC slab products.
Thermophysical process of autoclave curing
Heat exchange process
In the autoclave, in addition to heat exchange with the AAC material, the steam also carries out a series of heat exchanges with the kettle, the condensate, the steam car, the bottom plate, etc, The higher the heat transferring efficiency, the faster the heating of the AAC, and the shorter the time to reach a uniform temperature inside and outside the material. In the whole autoclave curing process, the heat transfer inside the AAC is mainly accomplished bu the migration of condensate water and the infiltration of steam.so that the air permeability of the AAC cake has a great influence on the heat transfer inside the material. The main factor affecting the air permeability of AAC are the constituent materials used in the mix, for aerated concrete of the same material composition, the fineness of the components affects the air permeability.In addition, the solids content in the raw material and the proportion of waste used in the batching process, to some degree, have an influence on the permeability of the AAC cake.
Thermal equilibrium
In the autoclave curing process, the source of heat in the kettle is generally saturated water steam (a small number of enterprises use a certain degree of superheated steam), and the heat consumption steps are as follows:
1. heating up of the AAC material and the moisture contained in it,2. heating up of the kettle,
3.heating and steaming base plate and steaming car
4.residual air in the heating kettle,
5.heating condensate water in the kettle
6. the kettle body dissipates heat to the environment.
The actual steam consumption is related to the autoclave curing system adopted, the environmental temperature difference, the timeliness of condensate discharge in the kettle, the amount of steam taken out in the process of condensate discharge, the reactivity of raw materials and the thermal insulation measures of the autoclave, The hydrothermal reaction of AAC is of exothermic nature, Theoretically, when autoclave curing reaches constant temperature, the pressure and temperature of the autoclave should not decline, but in fact, due to the heat dissipation of the autoclave kettle body and the leakage from pipes and valves, it is difficult to keep up the pressure and it is often necessary to properly compensate the pressure in the initial stages of constant temperature.
Thermal expansion and contraction of products
During the autoclave curing process, expansion and contraction of the AAC cake volume will occur due to the temperature changes, The heat deformation of the product is not uniform and in the heating stage.the surface temperature of the product rises first, so the deformation occurs earlier compared to the inside of the product, in contrast, when cooling, the internal temperature of the product decreases at a lower rate than the surface of the product, The uneven temperature distribution results in uneven thermal deformations, if a product is subjected to sharp temperature changes during autoclave curing, it may be damaged because of excessive temperature stress.
In the AAC cake, in addition to solid substances, there is a lot of liquid and gas, The internal pressure of the mass is dependent on whether the water is in a gaseous or liquid stage, which is mainly related to expansion and contraction of steam and air. If the AAC cake has sufficient strength, this does not pose a threat to the product, because the pores in the product are not completely closed, and hence the water and air mixture can move through the connected capillary holes or be expelled, The internal pressure differential is thus alleviated, A destructive effect on the product only occurs when the strength of the product is too low, or if the internal pressure difference increases too fast.