The aerated expansion of aerated concrete is due to the chemical reaction of aluminum powder in an alkaline solution, and this reaction is performed in a specific environment of aerated concrete with rheological properties. The gassing reaction of aluminum powder is manifested as the expansion of slurry volume. The change of the elastic-viscous-plastic properties of the slurry itself is manifested in the macroscopic view of the gradual thickening and coagulation of the slurry. These two time-varying processes exist in the same system. Just stable. Therefore, the factors that affect these two processes must also affect the stability of the pouring process. In order to analyze the stability of the pouring process, we must first understand the main factors affecting the above two processes.
1. Factors Affecting Gas Speed
(1) Gas characteristics of aluminum powder.
Due to differences in production processes and quality control, aerated aluminum powder or aluminum powder paste for aerated concrete, the products of different production plants, and even different batches of products in the same factory will not always be completely the same. The actual gas evolution characteristic curve exhibited by the aluminum powder paste in use will have different shapes and form different corresponding relationships with the ultimate shear stress curve of the slurry. Figure slightly,
The speed of aluminum powder gas generation is basically within the range required by the process, and the slurry expansion rate falls within the range of the shaded surface in the figure. In this case, although there is a short concentrated gasification period in the early stage of aluminum powder gas generation, It becomes slow, and there is more aluminum powder left to gas when the slurry is close to thickening. In this case, the gas generation process in the later period is hindered, and radon and bubbling may occur, and the pouring is not stable enough.
The aluminum powder gas is less concentrated, the gas curve rises gently, and a large amount of gas is generated after the slurry is thickened. In this case, the slurry expands slowly and the radon is severe in the later period. Even the slurry cannot expand normally. Bubbles perforate and merge inside the slurry, bubbling or even sinking. The above phenomenon occurs mainly due to the poor composition of the aluminum powder particles. Although there are some fine particles, there are more large particles, or some low-activity particles are mixed. If the aluminum powder is too fine, the gas generation time will be greatly advanced. In this case, if the slurry is too thin. Poor gas holding capacity, severe bubbling, or even boiling may occur. In some cases, a large amount of gas may appear in the mixer, and its pouring stability will be more affected.
(2) Slurry temperature
The reaction speed of aluminum powder is closely related to the temperature. The higher the temperature is, the faster the reaction proceeds. At higher temperatures, the dissolution rate and solubility of the medium solution to the reactants and reaction products increase correspondingly, which will undoubtedly facilitate the reaction. The relationship between the reaction speed and temperature of aluminum powder gas can be determined experimentally. The higher the temperature, the earlier the reaction start time. On the contrary, the reaction progresses slowly and the time is prolonged. It can be seen that by changing the temperature of the slurry, the gas generation and thickening process can be coordinated to a certain extent. Of course, this can only be within a certain range, and it is impossible to adjust infinitely, just as the slurry thickening speed is adjusted. It is impossible to adapt to the speed of exhalation without limitation. If the temperature is increased too much in order to adapt to a thickening slurry, for example, the temperature of the slurry is increased to 60-70 degrees, in most cases I am afraid that the gas will be generated during stirring, and it will also inevitably promote the The pulp will thicken faster and the effect will be counterproductive.
(3) Stirring time
The time for stirring the aluminum powder mainly affects the gas generation speed and its consistency with the thickening of the slurry from two aspects. On the one hand, the timing of the aluminum powder input into the slurry; on the other hand, the stirring time required for the aluminum powder in the slurry. . The former mainly adjusts the timing of the aluminum powder to start gas by regulating the contact time of the aluminum powder with the alkaline solution, and the latter mainly regulates the gas speed of the aluminum powder from the length of the stirring time of the aluminum powder. Under appropriate consistency conditions. Obviously, the right time should be chosen. Too early, the slurry is too thin, too late, the slurry is too thick, which is not good for aluminum powder gas generation and slurry expansion.
(4) Alkali concentration
The higher the alkali concentration in the slurry. The faster the aluminum powder reacts, the faster the aluminum powder evaporates in the sodium carbonate solution than in the lime solution. When sodium hydroxide is added to the solution, the reaction speed of aluminum powder will be greatly accelerated. Therefore, some factories often reserve some sodium hydroxide solutions to adjust the gas speed.
(5) Lime
In the slurry with lime as the main calcareous material, the digestion of lime generates Ca (OH) 2. Therefore, the amount of A-CaO in the lime and the digestion temperature directly affect the gas generation rate. Therefore, when lime is used as a calcium material, lye is generally not added.
(6) Cement varieties
The hydration rate of cement and the heat of hydration affect the gas generation of aluminum powder. If the cement contains more chromate, it will oxidize the surface of aluminum powder and make the gas reaction slow. In this case, you can use A small amount of ferrous sulfate. However, when the amount of lime is large, the effect of cement is small.
(7) Plaster
Gypsum will significantly delay the aeration process of aluminum powder. According to the experiment of Beijing aerated concrete factory, in cement-lime-sand aerated concrete, when the amount of gypsum is about 3 times the weight of aluminum powder, the speed of aluminum powder aeration Will be extended 5-8 times. If more gypsum is used, the gas generation of aluminum powder will be more severely suppressed. Of course, this relationship does not always develop in direct proportion. When the amount of gypsum reaches more than 6 times the amount of aluminum powder, the inhibitory effect reaches a maximum degree. Therefore, in aerated concrete using gypsum as a regulator, whether the amount of gypsum is appropriate is not only related to the performance of the product, but also an important factor affecting the aeration process.
(8) Additives
Some admixtures also have different degrees of influence on the gas evolution process of aluminum powder. For example, in a chemical degreasing agent, the aluminum powder can be degreased at a water temperature of 20-30 degrees, and the normal gas speed can be obtained. At a water temperature of 50 degrees, although it can also degrease, at the same time, it makes the gas speed slower than normal.
Water glass and some strong oxidants can inhibit the gas generation of aluminum powder. Additives such as triethanolamine and ethylene glycol can also play a role in promoting the gasification of aluminum powder while suppressing lime digestion. The water reducing agent NNO is the opposite.
(9) Water-to-material ratio
The water-material ratio of aerated concrete slurry has an indirect effect on the aeration process of aluminum powder. The water-to-material ratio is too small, and the slurry is too thick, and its ultimate shear stress is bound to be too large, so the bubbles are not easy to grow and push the slurry to expand, and the gas generation process is slow or even blocked. When the water-to-material ratio is too large, the viscosity of the slurry is too small, the gas retention is poor, the gas is easy to rise and escape, and the formed bubbles are also easy to merge and rupture, which also adversely affects the gas generation process.