Process and performance of hard calcium silicate products (calcium silicate board)

1. Process flow of hard calcium silicate products

In general, the process flow of calcium silicate insulation board made of hard calcium silicate (6CaO · 6SiO2 · H2O) is to first calcine limestone to obtain lime; Secondly, lime, crystalline silica powder, water and additives (such as zirconium oxychloride, Strontium nitrate) are mixed and well mixed; Next, use an autoclave with a stirrer to synthesize a hard calcium silicate slurry; Then, fibers are added, pressed and filtered into steel molds, and dried to obtain insulation products. The process flow of preparing hard calcium silicate insulation products using dynamic hydrothermal method is shown in the figure.

In order to improve performance, amorphous silica powder can be partially or even completely replaced by crystalline silica powder. In order to improve production efficiency, the hard calcium silicate slurry added with fibers can also be vacuum dehydrated, extruded, and then mechanically pressed into shape. Some important aspects of calcium silicate board process control include the selection of calcium and siliceous raw materials, reaction temperature and time, and stirring speed.

2. Production of calcium based raw materials

In order to produce calcium silicate boards with good thermal insulation performance, it is necessary to synthesize xonotlite crystal fibers in a hydrothermal reaction and make them intertwine to form chestnut shell like or algal like hollow secondary particle spheres. Therefore, it is necessary to control the activity of lime. If the satin burning is not complete, the limestone cannot be fully decomposed, and the slurry cannot form hard calcium silicate as planned. If the calcination is excessive, some dead burned lime cannot be hydrated for a while, which will also affect the synthesis of xonotlite and the growth of chestnut shell shaped secondary spherical particles. The figure shows the hydration rate of lime calcined at different temperatures.

The figure shows that the lime calcined at 900~950 ℃ dissolves quickly and has good activity; The digestion of 1000 ℃ calcined lime is slightly slower and its activity is better; The digestion time of 1050 ℃ calcined lime is longer and the activity is significantly reduced. Since the decomposition of limestone is a strong Endothermic reaction, and there is more or less temperature difference in the furnace, 20% of CaCO3 in limestone fired at 900 ℃ is not decomposed. So, the optimal forging temperature is about 1000 ℃.

3. Selection of Siliceous Raw Materials

Siliceous raw materials have an important impact on the production and performance of hard silica based thermal insulation materials. The activity of siliceous raw materials determines the difficulty of the reaction and affects the morphology of xonotlite crystals and secondary particles.

According to research, amorphous siliceous raw materials have high activity, easy reaction, and low requirements for process parameters. However, the crystallinity of hard calcium silicate formed is high, and the primary fibers are relatively coarse and short, making it difficult to form secondary chestnut shell like hollow spherical particles. After drying, the product has a high density. The activity of crystalline siliceous materials is low, the reaction difficulty is high, and the process requirements are high. The crystallinity of the primary fibrous particles formed by hard calcium silicate is not high, and the primary fibers are slender needle shaped, which is easy to intertwine and form secondary chestnut shell shaped hollow spherical particles. After drying, the density is low. Therefore, in order to reduce density, it is necessary to choose crystalline silica as the raw material. The problem of low reaction activity can be solved by increasing reaction temperature, prolonging reaction time, and using chemical additives.