Physical and chemical properties of calcium silicate insulation products

1.Introduction to Calcium Silicate Thermal Insulation Materials

Calcium silicate insulation materials are divided into tobermorite(6CaO·5SiO2·6H2O)type and xonotlite(6CaO·6SiO2.H2O)type.The maximum operating temperature of tobermorite type materials is 650℃,synthesized by atmospheric steam curing;The maximum operating temperature of hard silica calcium stone material is 1000℃,and it is synthesized using dynamic steam pressure hydrothermal method.In general,the refractory industry only uses hard calcium silicate insulation products.Xonotlite is widely used in the cement industry because of its low Water of crystallization content,good temperature resistance,good heat insulation,safety and environmental protection.

At present,the country with the largest production and most advanced technology of ultra-light calcium silicate insulation materials in the world is able to mass produce ultra-light hard calcium silicate products with a density of 110kg/m2.In the late 1980s,the Chinese Academy of Building Materials Science successfully developed a hard calcium silicate product with a unit weight of 200kg/cm3.However,after a long time,there was not much improvement in this area.At present,there is still a significant gap between China and developed countries in terms of density and thermal insulation performance,which are the main performance indicators of hard calcium silicate products.

2.The relationship between pressure and temperature of H2O

The synthesis of calcium silicate products needs to be carried out under high temperature conditions.Therefore,it is necessary to use an autoclave process for hydrothermal synthesis reactions at temperatures exceeding 100℃.The figure shows the relationship between temperature and pressure inside the autoclave when the water solid ratio is 40.

Under normal pressure,water boils at 100℃.If you want to>To conduct a hydrothermal reaction at 100℃,it is necessary to obtain the required temperature by increasing the pressure in a closed container.In general,increasing the temperature is beneficial for increasing the reaction rate.However,increasing the temperature requires increasing the pressure,and increasing the pressure requires considering the pressure resistance of the equipment.Therefore,for safety reasons,the steam pressure temperature should be controlled within 230℃.

3.Dissolution of CaO and SiO2

When encountering water,the surface of lime particles will be hydrated immediately to generate Calcium hydroxide and give off a lot of heat energy.Due to the volume effect of reaction products,Calcium hydroxide disintegrates into water.Lime particles expose new surfaces and continue to undergo hydration.From this,until all the lime undergoes hydration.The solubility of Calcium hydroxide is not large,and decreases with the increase of temperature,as shown in the figure.

The solubility of silica increases with increasing temperature.For quartz,its solubility is very low at low temperatures,even at 150℃,its solubility is only 0.06g/L.After 150℃,the solubility increased rapidly,reaching 0.15g/L at 174℃.At 200℃,it is 0.24g/L,and at 240℃,it reaches 0.43g/L,as shown in the figure.

4.Phase Equilibrium of CaO-SiO2-H2O System

The CaO-SiO2-H2O system consists of a total of 12 minerals.Among them,minerals related to calcium silicate insulation products include CSH(A),CSH(B),tobermorite,and xonotlite.

CSH(B)is a poorly crystallized substance with a size of around 1um and a fibrous crystal shape.

CSH(B)can be prepared by reacting lime with amorphous SiO2,or by autoclaving lime with ground quartz sand at 125-175℃.In the CaO-SiO2-H2O system,a hydrate with worse crystallization than CSH(B),called C-SH gel,can also be generated.

CSH(A)is a type of elongated crystal that can be transformed from CSH(B).

Tobemullite(6CaO·5SiO2·6H2O)is a thin layered crystal with a size of approximately 2um or less.Tobemorite can be synthesized from lime and quartz powder at 130-175℃,but the time is about twice that of CSH(B).

In general,xonotlite(6CaO·6SiO2·H2O)is a fibrous crystal that decomposes at approximately 850℃.

Hard calcium silicate can be synthesized at 150-400℃using CaO and SiO2 that match its chemical formula.As the temperature increases,the synthesis rate accelerates.It takes 100 hours to synthesize well crystallized hard calcium silicate at 200℃.However,it only takes 5-10 hours at 300℃.The phase diagram of the CaO-SiO2-H2O system is shown in the figure.

From the figure,it can be seen that xonotlite can only exist at temperatures above 180℃.Therefore,in order to successfully synthesize xonotlite,a temperature above 200℃is required.