There are two types of organic explosion-proof fibers: natural and artificial. Most of them currently used are artificial. Its explosion-proof mechanism is that during the baking process of the castable, the organic fibers evenly mixed in the castable shrink, melt or carbonize when heated, forming tiny long and narrow pores, thereby increasing air permeability. It is worth pointing out that after adding organic fibers, there is no chemical reaction in the castables, the construction process is easy to control, and there is no danger. However, the use of organic fibers will worsen the fluidity of the castable and affect its strength.
When studying the air permeability of fiber-containing refractory castables, it was found that the mechanisms of action of natural and artificial fibers are different. The air permeability of green castables containing synthetic fibers and the air permeability measured at room temperature were not improved by the addition of fibers. Castables containing PP, PET and Nomex fibers show even lower air permeability compared to fiber-free materials. In the case of PP samples, increasing the fiber content by 50% cannot improve the air permeability in the unburned stage. These results indicate that, under the experimental conditions employed, voids filled by synthetic fibers in green castables are not breathable neither through the loose fibers nor through the fiber-matrix interface. In fact, these dense fibers fill the voids in the castable material that were originally filled by the porous matrix, thereby reducing the air permeability constants k1 and k2. Different results were observed in the case of using natural fibers. Comparing with the fiber-free sample, the air permeability constant of jute fiber without calcining increased, k1 increased by 33%, k2 increased by 177%, similarly, in the fiber-containing sample The observed increase in air permeability k1 by 28% and k2 by 121% in castables made of plain fibers can be explained by the fact that these fibers naturally contain some pores at room temperature due to their swelling properties. , which may be beneficial to the dehydration process as it will provide more channels for water to be released from the structure in the early heating stages.
Regarding the impact of fiber on the air permeability of pre-fired cast iron, the researchers found that the change in air permeability depends on the type and amount of fiber added. Another important factor to consider is how to remove fibers from the castable during heat treatment. Ideally the fibers should break down only into gaseous products, leaving fully open breathable channels. However, depending on the chemical composition of the fiber and the heat treatment regime used, it is impossible to completely burn the fiber, and solid residues will also be produced as decomposition products. As a result, these solid particles create resistance to the flow of air and steam, blocking the pores and hindering the improvement of breathability. After materials are charred and burned, they produce large amounts of solid inorganic residues, which is further complicated by the presence of oxygen, especially in the absence of oxygen. Natural organic fibers, such as jute and cellulose fibers, may not be a suitable environment for rapid and complete decomposition in an environment rich in water vapor but not rich in oxygen. This may explain why these fibers do not achieve satisfactory breathability after heat treatment when compared with PET and Nomex, despite the same fiber loading. Overall, synthetic fibers offer the greatest improvement in breathability.
In addition to metal aluminum powder and organic explosion-proof fibers, adding organic gas-generating agents to castables is also a commonly used method today. The gas-generating mechanism of organic gas-generating agents is that the gas-generating agent decomposes with cement and water to generate a certain calcium salt, and the calcium salt decomposes to produce gas. Since organic gas-generating agents do not react as violently as metal aluminum powder, the castables are not prone to bulging and fine powder floating problems, and organic gas-generating agents will not cause differences in gas-generating speed and volume due to differences in construction and maintenance ambient temperatures. Big changes so construction is easier to master. The most commonly used organic foaming agent is azodicarbonamide (C2H4N4O2). The principle is that azoamide, which is originally insoluble in water, generates calcium salt and dissolves in the coexistence of high alumina cement and water. The calcium salt then changes with temperature and time. Gases such as N2 are generated due to changes in the temperature, forming fine opening pores of about 10 μm. Azodicarbonamide increases the porosity of castables because N2, CO2, NH3, etc. are generated during the initial stages of curing and drying.
(5) Effect of curing conditions on breathability
The influence of curing conditions on the air permeability of castables is mainly due to the influence of curing temperature and curing time on the type and quantity of the binding phase. For aluminate cement, when cured at different temperatures, the hydration rates of cement minerals are different, and the hydrates formed are different. Their chemical composition, crystal form, stability and density are also different, so the permeability of the castables is also different.
(6) Changes in air permeability during baking
In the early stage of baking, when the baking temperature is lower than 110°C, the free water in the pores of the matrix gradually evaporates, and the air permeability also changes accordingly. Nagai et al. experimentally demonstrated that this change in air permeability is due to changes in water content in the pores. As the baking temperature changes, the hydrated bonded phase will transform and decompose, and the breathability will also change. Each hydration-bound phase has a different crystal form and density, creating different fluid channels during the transformation process. For aluminate cement-bound castables, the metastable binding phase CAH10 has a low density, resulting in low air permeability. After transforming into a stable phase C3AH6 with a higher density, the air permeability can be increased; at the same time, as the temperature increases and Over time, the alumina gel transforms into bayerite, which also improves breathability. When the hydrated binding phase removes chemically bound water, it will increase the porosity of the matrix, thereby increasing the air permeability. This increase in air permeability is closely related to the type and amount of binding agent. Thermal expansion is a microstructural change that directly affects the air permeability of castables. It will cause changes in the pore structure, thereby affecting the air permeability. Thermal expansion depends on the thermal expansion coefficients of the aggregate and matrix and the shape of the pores. Air permeability measurement tests using high-temperature-treated specimens measured the individual influence of thermal expansion of the castables on air permeability during baking. The rearrangement of structural pores caused by thermal expansion causes the pore diameter to decrease and the air permeability constant to decrease. Compared with the Darcy flow air permeability constant k1, the non-Darcy flow air permeability constant k2 decreases more.